Technical field

The present invention belongs to the field of antimicrobial compositions and antimicrobial photodynamic therapy, and more specifically to hydrophilic phenalenones and their use in said field. It relates more specifically to new yellow-fluorescent phenalenone photosensitizers for applications in antimicrobial photodynamic therapy (aPDT).

The present invention relates to new hydrophilic phenalenone compounds and their use as a medicine, in antimicrobial photodynamic therapy, and more specifically in the treatment of various diseases, such as acne vulgaris, periodontis infections, rosacea, ptyriasis versicolor, molluscum contagiosum, genital herpes, tinea pedis, endodontics infections, viral keratitis, COVID-19 or influenza. The invention also relates to the method of synthesis of new hydrophilic phenalenone compounds according to the invention.

In the description below, references between [ ] refer to the list of references at the end of the examples.

Technical background

Antimicrobial resistance is a major worldwide threat that requires effective actions and innovative long-term solutions from researchers, policy-makers and society as a whole [1,2]. In a report published in 2019, the Interagency Coordination Group on Antimicrobial Resistance estimated that at least 700,000 deaths globally a year involved drug resistance with gloomy predictions in the decades to come if no actions are taken [3],

Antibiotic misuse and overuse for human and animal applications have accelerated the emergence of virulent and antibiotic-resistant pathogens which are difficult to eradicate with the current medicines [4,5]. The discovery of efficient and long-term viable therapeutic solutions, particularly against drug-resistant bacterial pathogens, is urgently required to tackle the challenge of antimicrobial resistance.

Amongst the possible strategies, antimicrobial photodynamic therapy (aPDT) has many advantages to foster innovation in the development of new therapeutic solutions [6-8], Antimicrobial photodynamic therapy involves three key components for killing bacterial pathogens: a photosensitizer, a light source and oxygen [9], Upon irradiation, the triplet excited state of the photosensitizer is populated from singlet excited state via intersystem crossing (ISC) [10,11]. In type II photosensitized oxidations, the triplet excited state of the photosensitizer is quenched by ground-state oxygen leading to the formation of singlet oxygen, ¹ O2, via energy transfer while type I photoreactions is based on electron transfer processes with the excited photosensitizer leading to radical species involved in the oxidative processes [12] The biocidal reactive oxygen species formed during photodynamic therapy can react with a variety of biomolecules such as nucleic acids, lipids and proteins leading to the death of pathogens [13,14].

This non-selective mode of action makes aPDT less keen to induce resistance and this is an attractive feature in the context of antibiotic resistance [15] The design of photoantimicrobial agents has attracted siderable attention from the scientific community over the past decades

However, for applications in antimicrobial photodynamic therapy, the photosensitizer has to fulfill some criteria:

(i) strong absorption of visible light with efficient production of reactive oxygen species;

(ii) high photostability under aPDT conditions;

(iii) non-toxicity to the human host and high selectivity towards microbes;

(iv) appropriate water solubility; and

(v) cost-effective and straightforward synthetic processes to prepare the photosensitizer [7,17]. Based on these considerations, phenalenone is an attractive scaffold to build type II photoantimicrobial agents owing to a high quantum yield of intersystem crossing, high energy transfer efficiency from the triplet excited state of phenalenone to form singlet oxygen from ground-state oxygen and its photostability [18-21], As a result, the combination of the phenalenone motif with cationic substituents such as ammonium or pyridinium [22-28] and phosphonium [29] has led to the emergence of hydrophilic photosensitizers with exciting antibacterial activities. The introduction of the cationic substituents plays a dual role by increasing the solubility in aqueous media and by improving the interaction of the photosensitizer with the negatively charged bacteria cell wall [30],

For example, the patent application WO2012/113860 [43] discloses novel photosensitizers which inactivate microorganisms more efficiently with a compound having the formula (a): where R’ ¹ through R’ ⁸ , which can be identical or different from each other, each are selected from of the group comprising hydrogen and the radical - (CH2)k-X, where k is a whole number from 1 to 20 and where X is an organic radical comprising a) at least one neutral, protonizable nitrogen atom and/or b) at least one positively charged nitrogen atom, under the provision that at least 1 radical R’ ¹ through R’ ⁸ is not hydrogen, and one of these compounds is known in the field as SAPYR [27-28],

For example, the patent application WO2018/167264 [44] relates to a photosensitizer composition comprising one phenalene-1 -one of formula (a), which ensures easy application to preferably different types of surfaces and, in particular, at the same time exhibits good antimicrobial activity after exposure to electromagnetic radiation of suitable wavelength and intensity. The exemplified compounds comprise only one substituent, either on position R’ ¹ or R’ ² in formula (a).

Despite great accomplishments in the search for phenalenone photoantimicrobials, some challenges remain to be addressed. For instance, synthetic strategies towards functionalized phenalenone photoantimicrobials remain limited and most of the photosensitizers absorb in UVA regions which lowers their interest for medical purposes [31],

In addition, the development of phenalenone photosensitizers exhibiting a proper balance between fluorescence and intersystem crossing rates have not been reported for antibacterial applications [32], New compounds with such molecular architectures could pave the way to theranostic agents which would have a therapeutic action (singlet oxygen production) and would allow a diagnostic by fluorescence imaging.

Detailed description of the invention

Applicant has developed new hydrophilic phenalenone compounds that solve all of the problems listed above.

To address the limitations associated with known phenalenone- based photosensitizers such as those described in WO2012/113860 or WO201 8/167264 [43,44], the Applicants surprisingly found out the compound object of the present invention is particularly efficient in antimicrobial therapeutic and non-therapeutic application, while in the meantime, it does not have the problems associated to known phenalenone derivatives.

The present invention relates to new hydrophilic phenalenone compounds, their synthetic methods and their applications, such as their use as a medicine or to clean surfaces from fungi, bacteria, viruses, parasites or chemical compounds. The hydrophilic phenalenone compounds of the invention present the advantage to be easily synthesised, in mild conditions and present an antimicrobial activity. They also have the following advantages:

- they can be used as blue-absorbing dyes;

- they possess a modularity of substituents borne by the phenalenone framework which allows blue-absorption, non-aggregation properties and introduction of cationic substituents at 8-position on the phenalenone skeleton;

- they can be used as yellow-fluorescent theranostic agents;

- they are photostable; and

- they possess antibacterial activities under light.

The main features of the photosensitizers include a quaternary ammonium or heterocyclic onium (e.g pyridinium) unit for increasing the hydrophilicity and for improving the interaction with the bacteria cell wall, and an alkoxy group enabling blue light absorption. In some applications, the Ar group helps preventing aggregation between the photosensitizers.

Photophysical measurements have demonstrated that the photosensitizers produce singlet oxygen upon irradiation under blue light and exhibit yellow fluorescence.

Antibacterial photodynamic inactivation studies revealed promising phototoxicity activities of the photosensitizers against Gram-positive (such as Staphylococcus aureus) and Gram-negative (such as Pseudomonas aeruginosa) bacteria. The invention thus relates in a first aspect to a compound of formula I: Formula I wherein,

- R ¹ represents a Ci to C12 linear or branched, saturated or unsaturated, hydrocarbon chain, optionally bearing a substituted or unsubstituted 4- to

26-membered aryl or heteroaryl moiety,

- Ar represents a substituted or unsubstituted 4- to 30-membered aryl or heteroaryl moiety,

- R ² represents a Ci to C3 linear or branched alkyl chain, - X’ represents an anion,

- R ³ which independently of one another may be identical to or different from each other, respectively represent:

• a Ci to C10 linear or branched alkyl chain, optionally bearing a substituted or unsubstituted, saturated or unsaturated, aromatic or non-aromatic, 4- to 26-membered cyclic or heterocyclic moiety,

• a Ci to C26 linear or branched hydroxyalkyl group, and/or

• at least two R ³ groups form, together with the nitrogen atom, a substituted or unsubstituted, saturated or unsaturated, aromatic or non-aromatic, 4- to 10-membered heterocyclic moiety. The compounds of formula I comprise a phenalenone ring. As meant herein, a phenalene-1 -one ring is of formula II: Formula II

The compounds according to the invention are phenalenone derivatives where the position 6 has been grafted with -OR ¹ , the position 7 by Ar and the position 8 by -R ² [N(R ³ )s] ⁺ .

An important feature of photosensitizers according to the invention is the presence of an alkoxy group (preferably methoxy) on the phenalenone framework leading to UV-Vis. absorption in the blue region [31] which is suitable for the treatment of accessible infections (i.e. skin, mouth, eyes) [33] or the eradication of bacterial biofilms on materials. Another important feature is the presence of the Ar group (preferably phenyl) on the phenalenone framework leading to an additional diversity site. Finally, compared to the known compounds, the ammonium or pyridinium group is not on the position 2, which allows to increase the hydrophilicity and the ability of the phenanelone to interact with the bacteria cell wall. Surprisingly, the combination of these three groups, on three particular positions on the phenalenone rings could not have been anticipated by the person skilled in the art.

It is meant by “aryl”, a group derived from arenes by removal of a hydrogen atom from a ring carbon atom; arenes being monoyclic and polycyclic aromatic hydrocarbons (IIIPAC). According to the invention Ar may comprise from 4 to 30 carbon atoms, preferably 4 to 15 carbon atoms, more preferably 6 carbon atoms (e.g. phenyl group). Heteroaryl are the class of heterocyclic groups derived from heteroarenes by removal of a hydrogen atom from any ring atom; an alternative term is hetaryl. Heteroarenes being heterocyclic compounds formally derived from arenes by replacement of one or more methine (-C=) and/or vinylene (-CH=CH-) groups by trivalent or divalent heteroatoms, respectively, in such a way as to maintain the continuous TT-electron system characteristic of aromatic systems and a number of out-of-plane n-electrons corresponding to the Huckel rule (4 n+2) (IUPAC).

Advantageously, R ¹ may represent a Ci to C12 linear or branched, saturated or unsaturated, hydrocarbon chain, optionally bearing a substituted or unsubstituted 4- to 26-membered aryl or heteroaryl moiety. Preferably, R ¹ may represent a Ci to Ce linear or branched, saturated or unsaturated, hydrocarbon chain, optionally bearing a substituted or unsubstituted 4- to 10-membered aryl or heteroaryl moiety. The aryl or heteroaryl moiety may be substituted by -OR ⁴ , in which R ⁴ represents a Ci to C3 alkyl chain, preferably Me. More preferably, R ¹ may be chosen from a Ci to C4 saturated or unsaturated, hydrocarbon chain, a benzyl group (i.e Ci hydrocarbon chain bearing a phenyl group) and a methoxybenzyl group (i.e Ci hydrocarbon chain bearing a methoxy substituted phenyl group). For example, R ¹ may represent a group chosen from methyl, ethyl, isopropyl, n- propyl, n-butyl, n-propenyl, n-propynyl, benzyl and methoxybenzyl.

Advantageously, R ¹ may be chosen from the following groups: wherein * represents the point of attachment to the phenalenone ring. Preferably, R ¹ may be chosen from the following groups: wherein * represents the point of attachment to the phenalenone ring. Advantageously, Ar represents a substituted or unsubstituted 4- to 30- membered aryl or heteroaryl moiety, preferably a substituted or unsubstituted 4- to 15-membered aryl or heteroaryl moiety. The aryl or heteroaryl moiety may be substituted by one or two, identical or different, groups chosen from halo (e.g. Cl, Br, I), -N3, -OR ⁴ , -NO2, -R ⁴ , -CN, -CHO, - COR ⁴ , -CO2R ⁴ , phenyl, 2-R ⁴ CeH4 and 2-pyridyl, in which R ⁴ represents a Ci to C3 alkyl chain, preferably Me. Preferably, Ar is chosen from a substituted or unsubstituted 4- to 15-membered aryl or heteroaryl moiety. Ar may represent a group chosen from substituted or unsubstituted phenyl, naphtyl and anthracyl group. Advantageously, Ar may be chosen from the following groups:

Y ¹ H. Me. Ph. 2-McC,,H ₎ . 2 -pyridyl wherein * represents the point of attachment to the phenalenone ring.

Preferably, Ar may be chosen from the following groups:

Y ¹ = H, Me, Ph, 2 Mc< „l I .. 2-pyridyl wherein * represents the point of attachment to the phenalenone ring.

Advantageously, R ² represents a Ci to C3 linear or branched alkyl chain. Preferably, R ² is -CH2-.

Advantageously, R ³ which independently of one another may be identical to or different from each other, respectively represent:

- a Ci to C10 linear or branched alkyl chain, optionally bearing a substituted or unsubstituted, saturated or unsaturated, aromatic or non-aromatic, 4- to 26-membered cyclic or heterocyclic moiety,

- a Ci to C26 linear or branched hydroxyalkyl group, and/or

- at least two R ³ groups form, together with the nitrogen atom, a substituted or unsubstituted, saturated or unsaturated, aromatic or non-aromatic, 4- to 10-membered heterocyclic moiety.

Preferably, the substituted or unsubstituted, saturated or unsaturated, aromatic or non-aromatic, 4- to 26-membered cyclic or heterocyclic moiety may be substituted by one or two, identical or different groups, chosen from halo (e.g. Br), -N3, -NO2, -CN, -OR ⁴ , -R ⁴ , phenyl, 2-R ⁴ CeH4 and 2-pyridyl, in which R ⁴ represents a Ci to C3 alkyl chain, preferably Me. The R ³ groups may be independently chosen from methyl, hydroxyethyl, Ci to C4 N- alkylpyridin-2(1 H)-one, substituted or unsubstituted benzyl group (i.e. Ci alkyl chain bearing a substituted or unsubstituted phenyl group), substituted or unsubstituted naphthalen-2-ylmethylgroup (i.e. Ci alkyl chain bearing a substituted or unsubstituted naphtyl group) and substituted or unsubstituted anthracen-2-ylmethyl group (i.e. Ci alkyl chain bearing a substituted or unsubstituted anthracyl group) or the R ³ groups may form, together with the nitrogen atom, a substituted or unsubstituted heteroaryl group, preferably a pyridinyl group.

Advantageously, R ³ may independently be chosen from the following groups:

Y ² = H, Me, Ph, 2-MeC ₆ H ₄ , 2-Pyridyl wherein * represents the point of attachment to the nitrogen atom. Preferably, R ³ may be chosen from the following groups: wherein * represents the point of attachment to the nitrogen atom. Advantageously, the R ³ groups may form, together with the nitrogen atom, a substituted or unsubstituted pyridinyl group that may be represented as follow: wherein * represents the point of attachment to the phenalenone ring.

Advantageously, X’ represents an anion. X- may be chosen from Cl’ , Br, I’, PF ₆ ’, BF ₄ ’, TSO’.

Advantageously, the compounds according to the invention may be chosen from the following compounds:

Advantageously, for each of the above listed compounds, the anion Cl- or I- may be different, and chosen from Cl’, Br, I’, PFe’, BF and TsO’.

The invention also relates to a pharmaceutical composition comprising a compound according to any of preceding claims, a solvent, preferably water, and optionally at least one pharmaceutically acceptable excipient. Advantageously, the solvent may be water, and/or an organic solvent chosen from DMSO, MeOH, EtOH, MeCN and DMF.

Advantageously, the pharmaceutically acceptable excipient may be chosen from:

- polyols (e.g. propylene-glycol or glycerol),

- oils (from plant origin),

- waxes (e.g white wax, lanolein or carnauba wax),

- mineral oils (e.g. hydrocarbon based oil, vaseline or paraffine) and silicone oils,

- sugars and hydrophilic polymers (e.g saccharose, lactose, sorbitol, starch, gums such as arabic and tragacanth, alginates, cellulose such as cellulose and methylcellulose),

- proteins such as gelatin,

- synthetic polymers (e.g. polyvinylpyrrolidone or P.E.G.),

- mineral compounds (e.g. silica, talc, titanium oxide and various silicates such as kaolin, bentonites),

- anionic, cationic and/or non-ionic surfactants, and

- liposomes.

The invention also relates to compound or composition according to the invention, for use as a medicine. The medicine may be an antimicrobial agent or a theranostic agent.

The invention also relates to compound or composition according to the invention, for use in antimicrobial photodynamic therapy. It is meant herein by “antimicrobial photodynamic therapy” (aPDT), a method that involves the use of light with appropriate wavelength to kill microorganisms treated with a photosensitizer drug. aPDT could be a useful adjunct to mechanical as well as antibiotics in eliminating periopathogenic bacteria.

Advantageously, the compound or composition according to the invention may be for use in the treatment of various diseases or infections, such as for example acne vulgaris, periodontis infections, rosacea, ptyriasis versicolor, molluscum contagiosum, genital herpes, tinea pedis, endodontics infections, viral keratitis, COVID-19 or influenza. This list should not be considered limitative and various other diseases or infections may be treated with the compound or composition according to the invention.

Advantageously, the compound or composition according to the invention may be for use in odontology. For example, it may be useful in the treatment of oral candidiasis, tooth decay and periodontitis.

On another aspect, the invention also relates to an antimicrobial composition comprising a compound according to any of preceding claims 1 -5, a solvent, preferably water or an organic solvent chosen from MeOH, EtOH, DMSO, DMF and MeCN and optionally at least one excipient.

The invention also encompasses a non-therapeutic use of a compound or composition according to the invention, as an antimicrobial agent.

The compound or composition according to the invention may be used for the non-therapeutic eradication of microbial biofilms from surfaces. It may preferably be used for the non-therapeutic treatment of surfaces. The surfaces may be surfaces of medical tools or any apparatus/tool/device found in a medical environment. The compound or composition according to the invention may also be used in paint for building or any surfaces that requires the elimination of microorganisms to be safely used. The compound or composition according to the invention may thus also be used for application for non-therapeutic application of antimicrobial photodynamic therapy in the food industry. For instance, the compound according to the invention may be used for inactivation of pathogens on food surface, eradication of biofilms on equipment surfaces (e.g. conveyor belts, pipes) and packaging materials (e.g. glass, polystyrene). The invention thus also encompasses use of the compound or composition according to the invention in non-therapeutic applications of antimicrobial photodynamic therapy. The invention also encompasses a non-therapeutic use of a compound or composition according to the invention for water depollution, preferably wastewater of drinkable water, more preferably for the destruction of fungi, bacteria, viruses, parasites or chemical compounds.

The compound or composition according to the invention may also be for use in in vivo antimicrobial application. In vivo antimicrobial application may be eradication of biofilm or bacteria, preferably present in the oral cavity or in superficial wound, or therapeutic application of antimicrobial photodynamic therapy, preferably in the treatment of periodontal diseases, treatment of wound or skin infections, treatment of keratisis, or treatment of endophthalmitis.

Brief description of the figures:

Figure 1 represents the UV/vis absorption spectra of 1a-c measured in water solutions.

Figure 2 represents the emission spectra of compounds 1a (A), 1b (B) and 1c (C) in different solvents (PS 1a-c were excited at 400 nm).

Figure 3 represents the multi-step synthetic sequence towards hydrophilic phenalenones 1a-e.

EXAMPLES

Example 1 : Preparation of 1a, 1b, 1c, 1d and 1e according to the invention

General information: ¹ H NMR and ¹³ C spectra were recorded at room temperature on samples dissolved in deuterated solvents. Chemical shifts (5) are given in parts per million and coupling constants are given as absolute values expressed in Hertz. Structural assignments were made with additional information from COSY, HSQC and HMBC experiments. Electrospray (ES)-time of flight (TOF) mass spectrometry (MS) measurements were performed on a Xevo G2-XS QTOF spectrometer (Waters, USA) for ES+, ES-. An Atmospheric Solid Analysis Probe (ASAP) was used for the direct analysis of samples by atmospheric pressure ionization (ASAP+ or ASAP-). Calculated masses for the molecules are the sum of the atomic masses and do not consider the gain or loss of one electron. FTIR spectra were obtained from neat samples using the attenuated total reflection (ATR) technique. Thin-layer chromatography (TLC) was carried out on aluminum sheets precoated with silica gel. Column chromatography separations were performed using silica gel. All reagents were used without purification. All solvents were of HPLC grade or were distilled using standard drying agents prior to use. Starting chemical substrates and reagents were used as commercially provided unless otherwise indicated. SAPYR was prepared according to literature data [27]. Compound 2 was prepared according to literature data and similar procedures were used to prepare the compound 6 [31],

Experimental procedures:

8-(hydroxymethyl)-6-methoxy-7-phenyl-1 H-phenalen-1 -one 3

6-methoxy-1 -oxo-7-phenyl-1 /-/-phenalene-8-carbaldehyde 2 (100 mg, 0.32 mmol, 1 equiv.) was introduced in absolute ethanol (3.5 mL) and sodium borohydride (13.23 mg, 0.36 mmol, 1.1 equiv.) was then added. The reaction was left to stir for 2 hours 30 minutes. The reaction mixture was quenched with water and then extracted with ethyl acetate. The organic layer was dried over anhydrous MgSC and evaporated under reduced pressure. The crude compound was purified by column chromatography on silica gel (CH ₂ CI ₂ /Et ₂ O, 6/4) to afford the corresponding compound 3 as an orange solid (90.3 mg, 90%).

¹ H (300 MHz, CDCI ₃ ) 6 8.94 ppm (s, 1 H), 7.70-7.64 (m, 2H), 7.45-7.37 (m, 3H), 7.20-7.17 (m, 2H), 6.75 (d, J = 8.1 Hz, 1 H), 6.63 (d, J = 9.6 Hz, 1 H), 4.53 (s, 2H), 3.45 (s, 3H). All the physical and spectroscopic data were in complete agreement with the reported ones [42], 8-((dimethylamino)methyl)-6-methoxy-7-phenyl-1 /-/-phenalen-1 -one 4

Under argon atmosphere, 8-(hydroxymethyl)-6-methoxy-7-phenyl- 1 /-/-phenalen-1-one 3 (100 mg, 0.32 mmol, 1 equiv.) was introduced in anhydrous dichloromethane (2.4 mL). Triethylamine (89 pmol, 0.64 mmol, 2 equiv.) and methanesulfonyl chloride (29 pmol, 0.38 mmol, 1.2 equiv.) were added at 0°C. The mixture was stirred 30 minutes and dimethylamine (2 M in THF, 0.7 mL, 1.4 mmol, 4.4 equiv.) was then added. The reaction mixture was allowed to warm to room temperature and was left to stir for 1 hour, at which point water was added. The mixture was then extracted with ethyl acetate. The organic layer was dried over anhydrous Na2SO4 and evaporated under reduced pressure to afford the corresponding product 4 as a yellow solid (72.3 mg, 67%). mp = 201 -204°C. ¹ H NMR (400 MHz, CDCI ₃ ) 6 8.94 ppm (s, 1 H), 7.70 (d, J = 9.7 Hz, 1 H), 7.67 (d, J = 8.0 Hz, 1 H), 7.44-7.36 (m, 3H), 7.20-7.18 (m, 2H), 6.77 (d, J = 8.0 Hz, 1 H), 6.65 (d, J = 9.6 Hz, 1 H), 3.44 (s, 5H), 2.24 (s, 6H). ¹³ C NMR (100 MHz, CDCI3) 6 185.6 ppm, 161.6, 145.8, 142.4, 141.7, 133.6, 132.8, 129.2, 128.8, 128.3 (2C), 127.5 (3C), 126.7, 126.1 , 123.9, 121.1 , 106.5, 60.3, 55.7, 45.1 (2C). IR (ATR, cm’ ¹ ) 2821 , 1635, 1562, 1508, 848, 703. HRMS (ES+) m/z calculated for C23H22NO2 [M+H] ⁺ 344.1651 ; found 344.1638.

8-(chloromethyl)-6-methoxy-7-phenyl-1 /-/-phenalen-1 -one 5

Under an argon atmosphere, 8-(hydroxymethyl)-6-methoxy-7- phenyl-1 /-/-phenalen-1 -one 3 (100.9 mg, 0.32 mmol, 1 equiv.), anhydrous triethylamine (88 pL, 0.64 mmol, 2 equiv.) and then p-toluenesulfonyl chloride (91 mg, 0.48 mmol, 1.5 equiv.) were introduced in anhydrous dichloromethane (1 .5 mL). The reaction mixture was left to stir for 24 hours at room temperature, at which point a brine solution was added. The mixture was then extracted with dichloromethane. The organic layer was dried over anhydrous MgSC and evaporated under reduced pressure. The crude compound was purified by column chromatography on silica gel (Cyclohexane/EtOAc, 6/4) to afford the corresponding product 5 as a yellow solid (94.4 mg, 88%).

¹ H (400 MHz, CDCI ₃ ) 6 8.84 ppm (s, 1 H), 7.72 (d, J = 9.7 Hz, 1 H), 7.69 (d, J = 8.1 Hz, 1 H), 7.49-7.35 (m, 3H), 7.25-7.21 (m, 2H), 6.78 (d, J = 8.1 Hz, 1 H), 6.65 (d, J = 9.7 Hz, 1 H), 4.47 (s, 2H), 3.46 (s, 3H). All the physical and spectroscopic data were in complete agreement with the reported ones [42],

7-(5,8-dimethylnaphthalen-2-yl)-8-(hydroxymethyl)-6-metho xy-1 H- phenalen-1 -one 7

7-(5,8-dimethylnaphthalen-2-yl)-6-methoxy-1 -oxo-1 /-/-phenalene-8- carbaldehyde 6 (122.6 mg, 0.312 mmol, 1 equiv.) was introduced in absolute ethanol (4 mL) and sodium borohydride (12.98 mg, 0.343 mmol, 1.1 equiv.) was then added. The reaction was left to stir for 2 hours. The reaction mixture was quenched with water (20 mL). The aqueous layer was extracted with ethyl acetate (25 mL) then with dichloromethane (2 x 25 mL). The combined organic layer was dried over anhydrous MgSC and evaporated under reduced pressure to afford the pure compound 7 as an orange solid (114.2 mg, 93%). ¹ H NMR (300 MHz, DMSO-de) 5 8.87 ppm (s, 1 H), 8.07 (d, J = 8.6 Hz, 1 H), 7.98 (d, J = 9.7 Hz, 1 H), 7.96 (d, J = 8.1 Hz, 1 H), 7.74 (d, J = 1.3 Hz, 1 H), 7.42 (dd, J = 8.6, 1.3 Hz, 1 H), 7.27 (app t, J = 9.0 Hz, 2H), 7.04 (d, J = 8.1 Hz, 1 H), 6.59 (d, J = 9.7 Hz, 1 H), 5.40 (t, J = 5.4 Hz, 1 H), 4.32 (dd, J = 14.5, 5.4 Hz, 1 H), 4.19 (dd, J = 14.5, 5.3 Hz, 1 H), 3.30 (s, 3H), 2.69 (s, 3H), 2.54 (s, 3H). ¹³ C NMR (75 MHz, DMSO-de) 5 184.1 ppm, 161.1 , 142.9, 142.8, 140.2, 138.0, 134.4, 132.0, 131.8, 131.7, 130.9, 128.9, 128.4, 127.6, 126.5 (2C), 126.1 , 125.1 , 123.6, 122.7, 122.4, 120.3, 107.4, 60.9, 55.8, 19.0 (2C). HRMS (ASAP-) m/z calculated for C27H22O3 [M]’ 394.1569; found 394.1561. IR (ATR, cm’ ¹ ) 3445, 3048, 2959, 2856, 1634, 1558, 1511 , 1396, 1253, 1221 , 846. 8-(chloromethyl)-7-(5,8-dimethylnaphthalen-2-yl)-6-methoxy-1 H-phenalen-

1 -one 8

Under argon atmosphere, 7-(5,8-dimethylnaphthalen-2-yl)-8- (hydroxymethyl)-6-methoxy-1 H-phenalen-1 -one 7 (114.2 mg, 0.290 mmol, 1 equiv.), anhydrous triethylamine (81 pL, 0.58 mmol, 2 equiv.) and then p- toluenesulfonyl chloride (83 mg, 0.435 mmol, 1.5 equiv.) were introduced in anhydrous dichloromethane (3.8 mL). The reaction mixture was left to stir for 22 hours at room temperature, at which point 15 mL of dichloromethane and 20 mL of brine were added. The mixture was then extracted with dichloromethane (3 x 20 mL). The organic layer was dried over anhydrous MgSO4 and evaporated under reduced pressure. The crude compound was purified by column chromatography on silica gel (Cyclohexane/EtOAc, 70/30) to afford the corresponding product 8 as a yellow solid (88.7 mg, 74%). ¹ H NMR (300 MHz, CDCI ₃ ) 6 8.88 ppm (s, 1 H), 8.07 (dd, J = 8.6, 0.6 Hz, 1 H), 7.89 (dd, J = 1 .8, 0.6 Hz, 1 H), 7.74 (d, J = 9.7 Hz, 1 H), 7.72 (d, J = 8.0 Hz, 1 H), 7.43 (dd, J = 8.6, 1 .8 Hz, 1 H), 7.28 (app t, J = 9 Hz, 2H), 6.79 (d, J = 8.1 Hz, 1 H), 6.68 (d, J = 9.7 Hz, 1 H), 4.51 (d, J = 11.1 Hz, 1 H), 4.35 (d, J = 11.1 Hz, 1 H), 3.35 (s, 3H), 2.75 (s, 3H), 2.63 (s, 3H). ¹³ C NMR (75 MHz, CDCI3) 6 185.3 ppm, 161.7, 145.7, 142.8, 137.7, 136.0, 134.3, 132.9, 132.8, 132.4, 132.2, 131.7, 129.8, 129.2, 126.8, 126.5, 126.4, 126.1 , 124.0, 123.7, 123.3, 121.1 , 106.8, 55.7, 44.5, 19.6 (2C). HRMS (ASAP+) m/z calculated for C27H22O2CI [M+H] ⁺ 413.1308; found 413.1302.

A/-benzyl-1 -(6-methoxy-1 -oxo-7-phenyl-1 /-/-phenalen-8-yl)-A/, A/- dimethylmethanaminium chloride 1a

Under argon atmosphere, 8-((dimethylamino)methyl)-6-methoxy-7- phenyl-1 /-/-phenalen-1-one 4 (152.6 mg, 0.44 mmol, 1 equiv.), and (chloromethyl)benzene (581 pL, 4.44 mmol, 10 equiv.) were introduced in anhydrous dichloromethane (2.2 mL). The reaction mixture was refluxed for 12 hours. After cooling, the solvent was removed under reduced pressure. The solid was suspended in toluene and introduced into an Eppendorf tube, sonicated for 30 seconds, and centrifuged to remove the supernatant. This washing step was repeated 4 times. The solid was then dried under reduced pressure to afford the corresponding product 1a as an orange solid (204.6 mg, 98%).

¹ H NMR (300 MHz, D ₂ O) 5 8.26 ppm (s, 1 H), 7.65 (d, J = 8.2 Hz, 1 H), 7.62 (d, J = 9.6 Hz, 1 H), 7.58-7.55 (m, 1 H), 7.45 (app. t, J = 7.6 Hz, 2H), 7.34- 7.23 (m, 3H), 7.19 (app. t, J= 7.6 Hz, 2H), 6.79 (d, J= 8.2 Hz, 1 H), 6.69-6.56 (m, 2H), 6.38 (d, J= 9.6 Hz, 1 H), 4.36 (s, 2H), 4.35 (s, 2H), 3.32 (s, 3H), 2.58 (s, 6H). ¹³ C NMR (75 MHz, D ₂ O) 5 186.0 ppm, 162.5, 150.1 , 145.4, 138.4, 138.3, 134.6, 132.7 (2C), 131.1 , 129.4 (2C), 128.5 (2C), 128.4, 128.0, 127.9 (2C), 127.4, 126.5, 124.1 , 123.7, 123.2, 119.5, 108.6, 70.3, 61.5, 56.0, 49.2 (2C). IR (ATR, cm’ ¹ ) 3348, 3023, 2360, 1635, 1571 , 1458, 1104, 839, 702. HRMS (ES+) m/z calculated for C ₃ oH ₂₈ N0 ₂ [M+] 434.2120; found 434.2110.

1 -((6-methoxy-1 -oxo-7-phenyl-1 H-phenalen-8-yl)methyl)pyridin-1 -ium chloride 1b

Under argon atmosphere, 8-(chloromethyl)-6-methoxy-7-phenyl- 1 /-/-phenalen-1-one 5 (150 mg, 0.448 mmol, 1 equiv.) and anhydrous pyridine (361 pL, 4.48 mmol, 10 equiv.) were added in anhydrous dichloromethane (2.5 mL). The reaction mixture was refluxed for 16 hours. After cooling, the solvent was removed under reduced pressure. The solid was suspended in diethyl ether and introduced into an Eppendorf tube, sonicated for 30 seconds, and centrifuged to remove the supernatant. This washing step was repeated 4 times. The solid was then dried under reduced pressure to afford the corresponding product 1b as an orange solid (161.9 mg, 88%).

¹ H NMR (400 MHz, D ₂ O) 5 8.48 ppm (tt, J = 7.8, 1 .3 Hz, 1 H), 8.47 (s, 1 H), 8.12-8.10 (m, 2H), 7.88-7.84 (m, 2H), 7.54 (tt, J = 7.8, 1.3 Hz, 1 H), 7.54 (d, J= 8.3 Hz, 1 H), 7.49 (d, J = 9.7 Hz, 1 H), 7.43 (app. t, J = 7.3 Hz, 2H), 6.67- 6.73 (m, 3H), 6.30 (d, J = 9.7 Hz, 1 H), 5.78 (s, 2H), 3.40 (s, 3H). ¹³ C NMR (100 MHz, D ₂ O) 5 185.8 ppm, 162.4, 148.2, 145.8, 144.8, 143.9 (2C), 139.2, 138.1 , 133.1 , 129.6, 128.8 (2C), 128.3, 128.0 (3C), 127.8, 126.6 (2C), 123.6,

123.1 , 119.3, 108.2, 63.0, 56.0. IR (ATR, cm’ ¹ ) 3393, 3057, 1634, 1575, 1486, 835, 701. HRMS (ES+) m/z calculated for C26H20NO2 [M+] 378.1493; found 378.1494.

8-((bis(2-hvdroxyethyl)amino)methyl)-6-methoxy-7-phenyl-1 /-/-phenalen-1- one chloride 1c

Under argon atmosphere, 8-(chloromethyl)-6-methoxy-7-phenyl- 1 /-/-phenalen-1-one 5 (70 mg, 0.21 mmol, 1 equiv.), and A/- methyldiethanolamine (121 pL, 1.05 mmol, 5 equiv.) were introduced in anhydrous tetrahydrofuran (1.1 mL). The reaction mixture was refluxed for 16 hours. After cooling, the solvent was removed under reduced pressure. The solid was suspended in tetrahydrofuran and introduced into an Eppendorf tube, sonicated for 30 seconds, and centrifuged to remove the supernatant. This washing step was repeated 3 times. This purification step was repeated with toluene. The solid was then dried under reduced pressure to afford the corresponding product 1c as an orange solid (83.8 mg, 89%). ¹ H NMR (400 MHz, D ₂ O) 5 8.4-8.2 ppm (m, 1 H), 7.7-7.4 (m, 5H), 7.02-6.93 (m, 2H), 6.9-6.7 (m, 1 H), 6.3-6.2 (m, 1 H), 4.77 (s, 2H), 3.77 (app. t, J = 4.8 Hz, 4H), 3.39 (s, 3H), 3.29 (qd, J = 13.8, 6.6 Hz, 4H), 2.67 (s, 3H). ¹³ C NMR (100 MHz, D ₂ O) 5 185.8 ppm, 162.4, 150.1 , 145.1 , 138.8, 138.2, 134.4, 129.0 (2C), 128.2 (3C), 128.1 , 127.2, 124.2, 123.5, 123.1 , 119.2, 108.4, 63.5 (3C), 55.9, 55.1 (2C), 48.3. HRMS (ES+) m/z calculated for C26H28NO4 [M+] 418.2018; found 418.2011 .

1 -((7-(5,8-dimethylnaphthalen-2-yl)-6-methoxy-1 -oxo-1 H-phenalen-8- yl)methyl)pyridin-1 -ium chloride 1d

Under argon atmosphere, 8-(chloromethyl)-7-(5,8-dimethylnaphthalen-2-yl)- 6-methoxy-1 H-phenalen-1-one 8 (43.2 mg, 0.105 mmol, 1 equiv.), and pyridine (82 pL, 1.05 mmol, 10 equiv.) were introduced in anhydrous dichloromethane (2.15 mL). The reaction mixture was heated under reflux for 4 days. After cooling, the solvent was removed under reduced pressure. The solid was suspended in diethyl ether and introduced into an Eppendorf tube, sonicated for 30 seconds, and centrifuged to remove the supernatant. This washing step is repeated 5 times. The solid was then dried under vacuum to afford the corresponding product 1d as an orange solid (41 .5 mg, 80%). ¹ H NMR (300 MHz, D ₂ O) 5 8.58 ppm (s, 1 H), 8.17 (t, J = 7.8 Hz, 1 H), 8.03 (d, J = 8.6 Hz, 1 H), 7.84 (d, J = 6.1 Hz, 2H), 7.67 (dd, J = 9.0, 5.8 Hz, 2H), 7.40 (dd, J = 12.4, 6.0 Hz, 3H), 7.28 (d, J = 7.8 Hz, 2H), 7.13-7.04 (m, 1 H), 6.82 (d, J = 8.3 Hz, 1 H), 6.44 (d, J = 9.5 Hz, 1 H), 5.85 (d, J = 14.7 Hz, 1 H), 5.68 (d, J = 14.8 Hz, 1 H), 3.27 (s, 3H), 2.69 (s, 3H), 2.17 (s, 3H). HRMS (ES+) m/z calculated for C32H26NO2 [M] ⁺ 456.1964 found 456.1961.

N-((7-(5,8-dimethylnaphthalen-2-yl)-6-methoxy-1 -oxo-1 H-phenalen-8- yl)methyl)-2-hvdroxy-N-(2-hvdroxyethyl)-N-methylethan-1 -aminium chloride 1e

Under argon atmosphere, 8-(chloromethyl)-7-(5,8-dimethylnaphthalen-2-yl)- 6-methoxy-1 H-phenalen-1-one 8 (87.7 mg, 0.212 mmol, 1 equiv.), and N- methyl diethanolamine (0.12 mL, 1.06 mmol, 5 equiv.) were introduced in anhydrous tetrahydrofuran (4.4 mL). The reaction mixture was heated under reflux for 4 days. After cooling, the solvent was removed under reduced pressure. The solid was suspended in tetrahydrofuran and introduced into an Eppendorf tube, sonicated for 30 seconds, and centrifuged to remove the supernatant. This washing step is repeated 3 times. This purification step was repeated with toluene. The solid was then dried under vacuum to afford the corresponding product 1e as an orange solid (51.5 mg, 46%). ¹ H NMR (300 MHz, D ₂ O) 5 8.40 ppm (s, 1 H), 8.12 (d, J = 8.7 Hz, 1 H), 7.88-7.72 (m, 3H), 7.41 (d, J = 7.2 Hz, 1 H), 7.29 (d, J = 7.2 Hz, 1 H), 7.21 (dd, J = 8.7, 1 .7 Hz, 1 H), 6.96 (d, J = 8.3 Hz, 1 H), 6.52 (d, J = 9.5 Hz, 1 H), 5.16 (d, J = 13.4 Hz, 1 H), 4.65 (d, J = 13.7 Hz, 2H), 3.72-3.52 (m, 4H), 3.38 (s, 3H), 3.32-2.93 (m, 3H), 2.66 (d, J = 16.0 Hz, 6H), 2.22 (s, 3H). HRMS (ES+) m/z calculated for C32H34NO4 [M] ⁺ 496.2488 found 496.2488. Example 2: Results and discussion

■ Photophysical characterizations

UV/Visible and fluorescence spectroscopy: UV-Vis spectra were recorded on an Agilent 8453 Diode-Array spectrophotometer in the range of 250-700 nm. Emission spectra were measured in an ISS PC1 spectrofluorometer at room temperature. Fluorescence quantum yields were determined using equation (1 ) and 6-ethoxy-phenalenone in methanol as reference [35] >F is the fluorescence quantum yields. A: is the absorbance in the wavelength of excitation, I correspond to area of the emission spectra and q is the refractive index of the solvent. The excitation wavelength was 400 nm for all steady state fluorescence experiments.

Luminescence lifetime measurements were carried out using a PicoQuant FluoTime 200 fluorescence lifetime spectrometer with a multichannel scaler (PicoQuant’s Timeharp 250) with the time correlated single photon counting (TCSPC) method. As excitation resource laser 405 nm was employed.

Singlet oxygen quantum yield measurements: Lifetime decays of singlet oxygen, O2( ¹ A _g ), were acquired with a FluoTime 200 equipped with multichannel scaler Nanoharp 200. Excitation at 355 nm was achieved with a laser FTSS355-Q3, (Crystal Laser, Berlin, Germany) working at 1 kHz repetition rate. Excitation at 473nm was achieved with a CNI laser AO-V-473 (Changchun New Industries Optoelectronics Technology Co. Ltd., China), working at 1 .5 kHz repetition rate. For the detection at 1270 nm a NIR PMT H10330A (Hamamatsu) was employed. The O2( ¹ A _g ) quantum yields ( >A) were determined by comparing the intensity at zero time of the 1270 nm signals to those of optically-matched solutions of SAPYR or Eosin Y as references. Chemical trapping experiments: Quantum yields of singlet oxygen produced by 1a-c was evaluated by monitoring the UV/Visible absorption change in an Agilent 8453 Diode-Array spectrophotometer of 3,3'- anthracene-9,10-diyldipropanoic acid (ADPA) which is a well-known singlet oxygen trap. Excitation was achieved at 473nm with a CNI laser AO-V-473 (Changchun New Industries Optoelectronics Technology Co. Ltd., China), working at 1.5 kHz repetition rate. A solution of 1a-c in deuterated water (A473nm = 0.06) was prepared and an aliquot of ADPA in deuterated water was added to reach A378nm = 0.74. The photoabsorption spectral change of ADPA solution with photosensitizers 1a-c was monitored following the decrease of the band at 382 nm. Plots of Ln(A/Ao) vs. time (s) give linear graphs for which the slope is the experimental kinetic rate constant (k, s ^-1 ) (Ao is the initial absorption intensity and A is the absorption intensity at the time t). From comparison of experimental rate constants between phenalenone derivatives 1a-c and two selected actinometers (Eosin Y and Riboflavin, see supporting information for results using Riboflavin) using optically-matched solutions, singlet oxygen quantum yields of the photosensitizers A can be determined from the following equation:

Photochemical stability: Solutions of the photosensitizers SAPYR or 1a-c in ultrapure water were prepared to reach an absorbance close to 1 for their longest-wavelength absorption maximum. These solutions were then irradiated under white light (solar simulator, 100 mw.cm’ ² ) in a quartz cell at room temperature. To determine photostability, UV-Visible spectra were periodically recorded and the different UV-Visible spectra were compared over time.

■ Antibacterial photodynamic inactivation studies

To evaluate the biological activity of the molecules, the minimal inhibitory concentration (MIC) and the minimal bactericidal concentration (MBC) were determined on bacterial cultures (S.a. : Staphylococcus aureus CIP76.25, E.f. : Enteroccocus faecalis, E.c. : Echerichia coli CIP53.126, et P.a. : Pseudomonas aeruginosa CIP76.110) by micromethod in 96-well plates. Briefly, bacterial cultures in log-phase were diluted in sterile PBS 1X at a concentration equivalent to 4x10 ^A 6 UFC/mL. Serial dilutions of different molecules were performed in PBS 1X to a final concentration from 200 to 1 .5 pM and 50 pl of each dilution were added in the microplate wells. Then, 50 pl of diluted bacterial culture were added in each well. Two different plates for each strain were prepared: one plate for the dark condition and a second plate for light irradiation.

Two controls were included; a bacterial growth control with no molecule added and a negative control with PBS without bacterial inoculum and no molecules.

The microplate was incubated at 37°C for 1 h in dark condition. After that, bacteria were irradiated with LED white light (total fluence: 25J/cm ² ). During the time of irradiation, the second plate was maintained in the dark. Then, 100 pL of 2-fold concentrated TS broth were added in each well and the microplate was incubated at 37°C for 24h. The MIC values were determined as the lowest concentration without observed bacterial growth. For determination of the MBC, 100 pl from wells where no evident growth was observed were diluted in physiological water (NaCI 0.9%). All 10-fold serial dilutions were spread on TS agar plates using an automatic plater (easySPIRAL®, Interscience, Saint-Norn La Breteche, France). After incubation at 37 °C for 24 h, colonies were counted to determine total CFU per mL (CFU/mL). The MBC corresponds to the concentration of the active compound for which 99.99% of the bacteria have been killed (i.e. 4 log reduction compared to the untreated control). Three independent experiments were performed with each strain. ■ Results

Photophysical Characterization

Absorption and fluorescence spectra of photosensitizers 1a-c were performed in different polar solvents (Figures 1 , 2). The spectroscopic characterization of 1a-c displayed a broad band in the 400-520 nm region and a weak absorption band around 355 nm. The signatures of 1a-c are in line with previous studies about photophysical characterizations of 6- alkoxyphenalenones such as 2 [31,35]. Compared to phenalenone 2, the introduction of positive charged moieties not directly connected to the phenalenone framework has a low impact on the absorption profile (AAmax in MeOH < 10 nm). Switching from DMSO to methanol has a negligible influence on the longest-wavelength absorption maximum of 1a-c while the use of water induced a red shift of absorption wavelength (+18-19 nm, Table 1 ).

Fluorescence spectra recorded at room temperature were shown in Figure 2 and the data were collected in Table 1 . Dyes 1a-c are characterized by fluorescence emission in the yellow spectral range (A _e m = 533-551 nm) with large Stokes shifts (AA = 88-96 nm) regardless of the solvents. The development of fluorescent organic dyes with large Stokes shifts (AA > 80 nm) still attracts considerable interest for biological imaging owing to the reduction of the overlap between the absorption and emission spectra which lowers the interferences [36] A slight red shift of the emission was observed from DMSO to methanol and water and the emission intensities are higher in methanol and water with promising emission properties in the yellow spectral range. As expected, the fluorescence lifetimes determined at room temperature were short (5-8 ns) and consistent with previous studies dealing with photophysical characterizations of non-hydrophilic phenalenones [31], Table 1. Photophysical properties of the photosensitizers in different solvents

High photostability of photosensitizers is an important criterion for practical aPDT applications. The results were compared to SAPYR which is a potent and well-established photosensitizer eliciting phototoxic activities against bacteria [27-28], Upon white light irradiation (100 mW. cm ^-2 ), dyes

1a-c turned out to be stable with no change of the UV-visible absorption spectra while a slight decomposition of SAPYR was observed after one hour of irradiation suggesting a higher photostability of dyes 1a-c compared to

SAPYR dye: 28, 44]

While dyes 1a-c display good emission yields in methanol and water, phenalenone derivatives have generally low fluorescent quantum yields due to a very efficient intersystem crossing producing the corresponding triplet state involved in the production of singlet oxygen [37], Therefore, the ability of dyes 1a-c to produce singlet oxygen was then investigated upon light illumination. Two techniques are commonly used for determining singlet oxygen quantum yields. The first strategy for quantifying singlet oxygen involves the use of a specific chemical probe whose reaction with singlet oxygen can be monitored by spectroscopy or other analytical techniques. The second approach is based on the direct observation of ¹ O2 phosphorescence emission band at around 1270 nm. Both techniques have been used for determining singlet oxygen quantum yields of 1a-c in selected solvents and the data are collected in Table 2.

Table 2. Singlet oxygen quantum yields of photosensitizers 1a-c

Singlet oxygen quantum yields measured in DMSO revealed the high propensity of photosensitizers 1a-c to produce singlet oxygen. The best singlet oxygen quantum yield was found for 1c ( >A = 0.91 in DMSO) with respect to SAPYR ( >A = 1.02) [27] under excitation at 355 nm. Switching from DMSO to methanol has a dramatic impact on singlet oxygen production with quantum yields between 0.10 and 0.15 for 1a-c. The increase of the fluorescence observed in protic solvent (MeOH, H2O - Table 1 ) could explain the low production of singlet oxygen. Due to the short lifetime of singlet oxygen in water which makes difficult the detection of the phosphorescence signal at 1270 nm (TA = 3.45 ps at 25°C) [38], the phosphorescence experiments have been carried out in D2O (T = 67.9 ps at 25°C) for which the non-radiative decay rate towards ground state oxygen is slower. The singlet oxygen quantum yields in D2O are comparable with those obtained in methanol. Dyes 1a-c does not deviate from the Kasha- Vavilov rule because excitation wavelengths has no influence on singlet oxygen quantum yields with similar values obtained under excitation at 355 nm and 470 nm in D2O [39], The singlet oxygen quantum yields obtained by phosphorescence were compared with the chemical probe method using 3,3'-anthracene-9,10-diyldipropanoic acid (ADPA) as the trapping agent [40], Upon excitation at 470 nm, the change of absorbance of ADPA due to singlet oxygen trapping was monitored by UV/visible spectroscopy and the results gave similar singlet oxygen quantum yield ( >A = 0.08-0.15 using Eosin Y as reference) than those obtained by phosphorescence method.

Antibacterial photodynamic inactivation studies

The photodynamic activities of 1a-c was investigated against Grampositive (Staphylococcus aureus, Enterococcus faecalis) and Gramnegative bacteria (Escherichia coli, Pseudomonas aeruginosa) belonging to the ESKAPE bacteria group and for which new therapeutic solutions must be found due to their ability to counter the antibacterial action of common medicines [41]. The chemical structure of the photosensitizers plays a crucial role in their bactericidal ability and therefore, the results were compared to the pyridinium-based phenalenone SAPYR known to exhibit a strong phototoxicity against diverse bacterial strains (Table 3 and Table 4).

Table 3. Minimum Inhibitory Concentration (MIC, values in pM) and Minimum Bactericidal Concentration (MBC, values in pM) of SAPYR and 1a- c tested against 2 bacterial strains (S.a.: Staphylococcus aureus CIP76.25, P.a. Pseudomonas aeruginosa CIP76.110) under dark (D) and light (L) conditions.

Table 4. Minimum Inhibitory Concentration (MIC, values in jjM) and Minimum Bactericidal Concentration (MBC, values in M) of SAPYR and 1a- c tested against 2 bacterial strains (E.f. Enteroccocus faecalis, Echerichia coli CIP53. 126) under dark (D) and light (L) conditions.

Minimum Inhibitory Concentration (MIC) defined as the lowest concentration of photosensitizer that prevents the visible bacterial growth of a bacteria and Minimum Bactericidal Concentration (MBC) which is the lowest concentration that kills bacteria are collected in Tables 3 and 4. The antibacterial tests have been carried out under dark conditions and upon illumination with white light LED (total fluence: 25 J/cm ² ) for 6 hours. Under dark conditions, no significant activity was observed for the photosensitizers against the strains, except for 1a with a MIC value of 167 pM (Table 3). Under light irradiation, bactericidal activities at the micromolar range were observed with different values depending on the structures of the photosensitizers. Therefore, light is essential to the photoinactivation of the bacteria which demonstrates the absence of dark toxicity. Interestingly, the MIC and MBC values showed photosensitizers 1a-c were more effective against Staphylococcus aureus than the reference compound SAPYR. Comparison of the antibacterial activity of SAPYR and 1b possessing both a pyridinium moiety showed that not only quaternarization but also the structure of the phenalenone framework and the position of the cationic group plays an important role in their ability to eradicate bacteria. Compared to SAPYR, photosensitizer 1b was more effective against Staphylococcus aureus, Enterococcus faecalis and Escherichia coH. and the best results were obtained with photosensitizer 1c. Upon illumination under white light, SAPYR and dyes 1a-b did not exhibit any photodynamic effects against Pseudomonas aeruginosa while 1c was effective against Staphylococcus aureus (MIC 41 .7 pM) and Pseudomonas aeruginosa (MIC 33 pM).

Conclusion

In summary, herein is reported the synthesis of new hydrophilic phenalenone-based photosensitizers containing a quaternary ammonium or a pyridinium unit. All derivatives produce singlet oxygen and exhibit yellow fluorescence upon blue light irradiation. Antibacterial photodynamic inactivation studies revealed promising phototoxicity activities of the photosensitizers against Gram-positive (Staphylococcus aureus, Enterococcus faecalis) and Gram-negative bacteria (Pseudomonas aeruginosa, Escherichia coli). List of references

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