Triplet excimers of fluoroquinolones in aqueous media

Generation of triplet eximers of 6-fluoro-7-piperazinyl-quinolone-3-carboxylic acids (FQs) have been detected in aqueous media using laser flash photolysis (LFP). These transient species (SS) are generated by self-quenching reactions of FQ triplet excited states such as pefloxacin (PFX), norfloxacin (NFX), the N-acetylated form of NFX (ANFX), and its methyl ester (EANFX) with their ground states. In this context, self-quenching rate constants in the range of (1-7) × 10(8) M(-1) s(-1) were determined. The triplet excimers show transient absorption spectra with λ(max) ca. 710 nm for SS(NFX), 740 nm for SS(PFX), and 620 nm for SS(ANFX) and E(ANFX), which are red-shifted with respect to their predecessors triplet excited states. These excimers can be also observed in the presence of phosphate buffer (PB). Experiments performed with NFX and ANFX at different PB concentrations showed that deprotonation processes are not involved in the generation of SS. The triplet multiplicity of the FQ excimers was confirmed by energy transfer reactions with naproxen. The correlation between fluorescence, intersystem crossing, excimer and photodegradation quantum yields of (A)NFX indicated that FQ self-quenching reactions are mainly a deactivation pathway. On the other hand, generation of FQ radical anions absorbing at λ(max) ca. 620 nm has been observed by an efficient electron transfer reaction from Trp to NFX, PFX, and ANFX (rate constants ca. 1 × 10(9) M(-1) s(-1)).     

Ultrafast study of p-biphenylyldiazomethane and p-biphenylylcarbene

p-Biphenylyldiazomethane was excited by femtosecond pulses of UV light in acetonitrile, in cyclohexane, and in methanol. Ultrafast photolysis produces a singlet excited state of p-biphenylyldiazomethane with lambdamax = 490 nm, and lifetimes of less than 300 fs in acetonitrile, in cyclohexane, and in methanol. The decay of the excited state is accompanied by the growth of transient absorption with lambdamax = 360 nm. The carrier of this transient absorption is attributed to singlet p-biphenylylcarbene, a result that is consistent with the predictions of TD-DFT calculations. The singlet carbene lifetimes are 200 and 77 ps in acetonitrile and cyclohexane, respectively, and are controlled by intersystem crossing to the lower energy triplet state. The transient absorption does not decay to baseline in acetonitrile, because of the formation of nitrile ylide. The equilibrium mixture of singlet and triplet p-biphenylylcarbene reacts with acetonitrile to form a nitrile ylide (lambdamax = 370 nm), and with cyclohexane by C-H insertion 1-20 ns after the laser pulse. The singlet carbene lifetime is only 7.9 ps in methanol, owing to a rapid reaction with the solvent. Reaction with the solvent gives rise, in part, to a p-biphenylylbenzyl cation (lambdamax = 450 nm, tau = 6.3 ps) in methanol.     

Competition between alpha-cleavage and energy transfer in alpha-azidoacetophenones

Molecular modeling demonstrates that the first excited state of the triplet ketone (T1K) in azide 1b has a (pi,pi*) configuration with an energy that is 66 kcal/mol above its ground state and its second excited state (T2K) is 10 kcal/mol higher in energy and has a (n,pi*) configuration. In comparison, T1K and T2K of azide 1a are almost degenerate at 74 and 77 kcal/mol above the ground state with a (n,pi*) and (pi,pi*) configuration, respectively. Laser flash photolysis (308 nm) of azide 1b in methanol yields a transient absorption (lambdamax=450 nm) due to formation of T1K, which decays with a rate of 2.1 x 105 s-1 to form triplet alkylnitrene 2b (lambdamax=320 nm). The lifetime of nitrene 2b was measured to be 16 ms. In contrast, laser flash photolysis (308 nm) of azide 1a produced transient absorption spectra due to formation of nitrene 2a (lambdamax=320 nm) and benzoyl radical 3a (lambdamax=370 nm). The decay of 3a is 2 x 105 s-1 in methanol, whereas nitrene 2a decays with a rate of approximately 91 s-1. Thus, T1K (pi,pi*) in azide 1b leads to energy transfer to form nitrene 2b; however, alpha-cleavage is not observed since the energy of T2K (n,pi*) is 10 kcal/mol higher in energy than T1K, and therefore, T2K is not populated. In azide 1a both alpha-cleavage and energy transfer are observed from T1K (n,pi*) and T2K (pi,pi*), respectively, since these triplet states are almost degenerate. Photolysis of azide 1a yields mainly product 4, which must arise from recombination of benzoyl radicals 3a with nitrenes 2a. However, products studies for azide 1b also yield 4b as the major product, even though laser flash photolysis of azide 1b does not indicate formation of benzoyl radical 3b. Thus, we hypothesize that benzoyl radicals 3 can also be formed from nitrenes 2. More specifically, nitrene 2 does undergo alpha-photocleavage to form benzoyl radicals and iminyl radicals. The secondary photolysis of nitrenes 2 is further supported with molecular modeling and product studies.     

Selective formation of triplet alkyl nitrenes from photolysis of beta-azido-propiophenone and their reactivity

Photolysis of beta-azido propiophenone derivatives, 1, with built-in sensitizer units, leads to selective formation of triplet alkyl nitrenes 2 that were detected directly with laser flash photolysis (lambdamax = 325 nm, tau = 27 ms) and ESR spectroscopy (|D/hc| = 1.64 cm-1, |E/hc| = 0.004 cm-1). Nitrenes 2 were further characterized with argon matrix isolation, isotope labeling, and molecular modeling. The triplet alkyl nitrenes are persistent intermediates that do not abstract H-atoms from the solvent but do decay by dimerizing with another triplet nitrene to form azo products, rather than reacting with an azide precursor. The azo dimer tautomerizes and rearranges to form heterocyclic compound 3. Nitrene 2a, with an n,pi* configuration as the lowest triplet excited state of the its ketone sensitizer moiety, undergoes intramolecular 1,4-H-atom abstraction to form biradical 6, which was identified by argon matrix isolation, isotope labeling, and molecular modeling. beta-Azido-p-methoxy-propiophenone, with a pi,pi* lowest excited state of its triplet sensitizer moiety, does not undergo any secondary photoreactions but selectively yields only triplet alkyl nitrene intermediates that dimerize to form 3b.     

Kinetic model of energy transfer processes between low-coordinated ions on MgO by photoluminescence decay measurements.

Photoluminescence decay studies of emitting species on MgO nanocubes at room temperature provide evidence of three surface species characterized by an excitation and emission wavelength couple {lambda(exc);lambda(em)}. Species A corresponds to {lambda(exc)=240 nm; lambda(em)=380 nm}, whereas the couple {lambda(exc)=280 nm; lambda(em)=470 nm} is assigned to two species: B and B', the former is involved in energy transfer from excited state A* and the latter in direct emission from excited state B'*. A simple model for energy transfer from species A* to B is proposed. The numerical resolution of equations corresponding to this model is in good agreement with experimental data. This method quantifies the kinetics of intrinsic emission and energy transfer processes. Lifetime values indicate that phosphorescence is taking place, and species A, B and B' are identified as edge O(2-) (4 C), corner O(2-) (3 C) and kink O(2-) (3 C) oxide ions respectively.     

Spectrofluorimetric determination of fluoroquinolones in pharmaceutical preparations

Simple, rapid and highly sensitive spectrofluorimetric method is presented for the determination of four fluoroquinolone (FQ) drugs, ciprofloxacin, enoxacin, norfloxacin and moxifloxacin in pharmaceutical preparations. Proposed method is based on the derivatization of FQ with 4-chloro-7-nitrobenzofurazan (NBD-Cl) in borate buffer of pH 9.0 to yield a yellow product. The optimum experimental conditions have been studied carefully. Beer's law is obeyed over the concentration range of 23.5-500 ng mL(-1) for ciprofloxacin, 28.5-700 ng mL(-1) for enoxacin, 29.5-800 ng mL(-1) for norfloxacin and 33.5-1000 ng mL(-1) for moxifloxacin using NBD-Cl reagent, respectively. The detection limits were found to be 7.0 ng mL(-1) for ciprofloxacin, 8.5 ng mL(-1) for enoxacin, 9.2 ng mL(-1) for norfloxacin and 9.98 ng mL(-1) for moxifloxacin, respectively. Intra-day and inter-day relative standard deviation and relative mean error values at three different concentrations were determined. The low relative standard deviation values indicate good precision and high recovery values indicate accuracy of the proposed methods. The method is highly sensitive and specific. The results obtained are in good agreement with those obtained by the official and reference method. The results presented in this report show that the applied spectrofluorimetric method is acceptable for the determination of the four FQ in the pharmaceutical preparations. Common excipients used as additives in pharmaceutical preparations do not interfere with the proposed method.     

The discrimination between triplet state and radical cation in the pulse radiolysis of bithiophene in CCl4

The triplet state (³2T) and the radical cation (2T⁺√) of 2,2′-bithiophene (2T) are characterized by pulse radiolysis in CCl₄. Two main absorption bands at 360 and 420 nm are respectively attributed to ³2T* and to 2T⁺√. The triplet, induced in an excited state through a Förster mechanism, undergoes a conformational rearrangement (k₆=(6.8±0.9)×10⁶ s⁻¹). The radical cation is produced both through a resonance charge transfer and a second diffusional process; the two oxidizing species are respectively CCl₄⁺√ and (CCl⁺₃Cl⁻)_solv through the mediation of a singlet excited state, ¹2T*.     

Does bimolecular charge recombination in highly exergonic electron transfer afford the triplet excited state or the ground state of a photosensitizer?

The investigations were made on photoinduced electron transfer (ET) from the singlet excited state of rubrene (1RU*) to p-benzoquinone derivatives (duroquinone, 2,5-dimethyl-p-benzoquinone, p-benzoquinone, 2,5-dichloro-p-benzoquinone, and p-chloranil) in benzonitrile (PhCN) by using the steady state and time-resolved spectroscopies. The photoinduced ET produces solvent-separated type charge-separated (CS) species and the charge-recombination (CR) process between RU radical cation and semiquinone radical anions obeys second-order kinetics. Not only the CS species but also the triplet excited state of RU (3RU*) is seen in the transient absorption spectra upon laser excitation of a PhCN solution of RU and p-benzoquinone derivatives. The comparison of their time profiles clearly suggests that the CR process between RU radical cation and semiquinone radical anions to the ground state is independent from the deactivation of 3RU*. This indicates that the CR in a highly exergonic ET occurs at a longer distance with a large solvent reorganization energy, which results in faster ET to the ground state than to the triplet excited state that is lower in energy than the CS state. Photoinduced ET from 3RU* in addition from 1RU* also occurs when p-benzoquinone derivatives with electron-withdrawing substituents were employed as electron acceptors.     

Role of excited state intramolecular charge transfer in the photophysical properties of norfloxacin and its derivatives

The photophysical properties of 1-ethyl-6-fluoro-7-(1-piperazinyl)-1,4-dihydro-4-oxoquinoline-3-carboxylic acid (norfloxacin, NFX) and some of its derivatives have been studied to evaluate the role of the free carboxylic acid and the nonprotonated piperazinyl group in the behavior of the 1,4-dihydro-4-oxoquinoline ring. Steady state and time-resolved fluorescence measurements at different pHs provide clear evidence in favor of singlet excited-state deactivation of NFX and its N(4')-methyl derivative pefloxacin (PFX) via intramolecular electron transfer from the N(4') atom of the piperazinyl ring to the fluoroquinolone (FQ) main system. This is a very efficient, energy-wasting pathway, which becomes dramatically enhanced in basic media. Acetylation at N(4') (as in ANFX) decreases the availability of the lone pair, making observable its fluorescence and the transient absorption spectrum of its triplet excited state even at high pH. It also reveals that the geometry of FQs changes from an almost sp3 hybridization of the N(1') of the piperazinyl substituent in the ground state to nearly sp2 in the singlet excited state (rehybridization accompanied by intramolecular charge transfer, RICT); accordingly, the singlet energy of ANFX is significantly lower than that of NFX and PFX. The fluorescence measurements using acetonitrile as a polar nonprotic organic solvent further support deactivation of the singlet excited state of nonacetylated NFX derivatives via intramolecular electron transfer from the N(4') atom.     

Analysis of fluoroquinolone-mediated photosensitization of 2'-deoxyguanosine, calf thymus and cellular DNA: determination of type-I, type-II and triplet-triplet energy transfer mechanism contribution

Fluoroquinolone (FQ) antibacterials are known to exhibit photosensitization properties leading to the formation of oxidative damage to DNA. In addition, photoexcited lomefloxacin (Lome) was recently shown to induce the formation of cyclobutane pyrimidine dimers via triplet-triplet energy transfer. The present study is aimed at gaining further insights into the photosensitization mechanisms of several FQ including enoxacin (Enox), Lome, norfloxacin (Norflo) and ofloxacin (Oflo). This was achieved by monitoring the formation of DNA base degradation products upon UVA-mediated photosensitization of 2'-deoxyguanosine, isolated and cellular DNA. Oflo and Norflo act mainly via a Type-II mechanism whereas Lome and, to a lesser extent, Enox behave more like Type-I photosensitizers. However, the extent of oxidative damage was found to be relatively low. In contrast, it was found that cyclobutane thymine dimers represent the major class of damage induced by Enox, Lome and Norflo within isolated and cellular DNA upon UVA irradiation. This striking observation confirms that FQ are able to promote efficient triplet energy transfer to DNA. The levels of photosensitized formation of strand breaks, alkali-labile sites and oxidative damage to cellular DNA, as measured by the comet assay, were confirmed to be rather low. Therefore, we propose that the phototoxic effects of FQ are mostly accounted for energy transfer mechanism rather than by Type-I or -II photosensitization processes.     

The direct detection of an aryl azide excited state: an ultrafast study of the photochemistry of para- and ortho-biphenyl azide

Ultrafast laser flash photolysis (266 nm) of para- and ortho-biphenyl azide in acetonitrile produces azide excited states that have broad absorption bands centered at 480 nm. The para-biphenyl azide excited singlet state has a lifetime of 100 fs. The excited-state lifetime of the ortho-azide isomer is 450 +/- 150 fs. Decay of the azide excited states is accompanied by the formation of the corresponding known singlet nitrenes (para, lambdamax = 350 nm, ortho, lambdamax = 400 nm). Singlet para-biphenylnitrene is born with excess energy and undergoes vibrational cooling with a time constant of 11 ps to form the long-lived (tau approximately 9 ns) relaxed singlet nitrene. Singlet ortho-biphenylnitrene decays with a lifetime of 16 ps in acetonitrile at ambient temperature.     

Study of acyl group migration by femtosecond transient absorption spectroscopy and computational chemistry

The primary photophysical and photochemical processes in the photochemistry of 1-acetoxy-2-methoxyanthraquinone (1a) were studied using femtosecond transient absorption spectroscopy. Excitation of 1a at 270 nm results in the population of a set of highly excited singlet states. Internal conversion to the lowest singlet npi* excited state, followed by an intramolecular vibrational energy redistribution (IVR) process, proceeds with a time constant of 150 +/- 90 fs. The 1npi* excited state undergoes very fast intersystem crossing (ISC, 11 +/- 1 ps) to form the lowest triplet pipi* excited state which contains excess vibrational energy. The vibrational cooling occurs somewhat faster (4 +/- 1 ps) than ISC. The primary photochemical process, migration of acetoxy group, proceeds on the triplet potential energy surface with a time constant of 220 +/- 30 ps. The transient absorption spectra of the lowest singlet and triplet excited states of 1a, as well as the triplet excited state of the product, 9-acetoxy-2-methoxy-1,10-anthraquinone (2a), were detected. The assignments of the transient absorption spectra were supported by time-dependent DFT calculations of the UV-vis spectra of the proposed intermediates. All of the stationary points for acyl group migration on the triplet and ground state singlet potential energy surfaces were localized, and the influence of the acyl group substitution on the rate constants of the photochemical and thermal processes was analyzed.     

Direct determination of four fluoroquinolones,enoxacin, norfloxacin,ofloxacin, and ciprofloxacin,in pharmaceuticals and blood serum by HPLC

A rapid, accurate and sensitive method has been developed for the quantitative determination of four fluoroquinolone antimicrobial agents, enoxacin, norfloxacin, ofloxacin and ciprofloxacin, with high in-vitro activity against a wide range of Gram-negative and Gram-positive organisms.A Kromasil 100 C(8) 250 mm x 4 mm, 5 microm analytical column was used with an eluting system consisting of a mixture of CH(3)CN-CH(3)OH-citric acid 0.4 mol L(-1) (7:15:78 %, v/v). Detection was performed with a variable wavelength UV-visible detector at 275 nm resulting in limits of detection: 0.02 ng per 20 microL injection for enoxacin and 0.01 ng for ofloxacin, norfloxacin and ciprofloxacin. Hydrochlorothiazide (HCT) was used as internal standard at a concentration of 2 ng microL(-1). A rectilinear relationship was observed up to 2 ng microL(-1) for enoxacin, 12 ng microL(-1) for ofloxacin, 3 ng microL(-1) for norfloxacin, and 5 ng microL(-1) for ciprofloxacin. Separation was achieved within 10 min. The statistical evaluation of the method was examined by performing intra-day (n=8) and inter-day precision assays (n=8) and was found to be satisfactory with high accuracy and precision. The method was applied to the direct determination of the four fluoroquinolones in human blood serum. Sample pretreatment involved only protein precipitation with acetonitrile. Recovery of analytes in spiked samples was 97+/-6% over the range 0.1-0.5 ng microL(-1).     

Photophysical and photochemical processes of riboflavin (vitamin B_2) by means of the transient absorption spectra in aqueous solution

Using time-resolved techniques of 337 and 248 nm laser flash photolysis, the photo-physical and photochemical processes of riboflavin (RF, vitamin B2) were studied in detail in aqueous solution. The excited triplet state of riboflavin (3RF*) was produced with 337 nm laser, while under 248 nm irradiation, both 3RF* and hydrated electron (eaq) formed from photoionizationcould be detected. Photobiological implications have been inferred on the basis of reactivity of 3RF* including energy transfer, electron transfer and hydrogen abstraction. The RF.+ was generated by oxidation of SO4.- radical with the aim of confirming the results of photolysis.     

Intramolecular NH/pi complexes of 2-allylaniline derivatives in the ground and excited states

The photochemical and photophysical properties of three 8-allyl-1,2,3,4-tetrahydroquinolines (1a-c) have been studied. These compounds exhibit a 2-allylaniline-like photochemical behavior, undergoing photocyclization to lilolidines (3a-c). The absorption, emission, and excitation spectra of 1a-c, employing convenient model compounds for comparison, demonstrate the formation of a NH/pi intramolecular ground-state complex (AB). This species can absorb light at long wavelengths (330-340 nm), giving rise to the corresponding excited complex AB*. Emission from AB* is red-shifted (420 nm) with respect to that observed when the monomer A is excited (lambda(exc) = 300 nm). These experimental results have been rationalized by means of density-functional theory calculations.     

One-electron oxidation of alcohols by the 1,3,5-trimethoxybenzene radical cation in the excited state during two-color two-laser flash photolysis

One-electron oxidation of alcohols such as methanol, ethanol, and 2-propanol by 1,3,5-trimethoxybenzene radical cation (TMB*+) in the excited state (TMB*+*) was observed during the two-color two-laser flash photolysis. TMB*+ was formed by the photoinduced bimolecular electron-transfer reaction from TMB to 2,3,5,6-tetrachlorobenzoquinone (TCQ) in the triplet excited-state during the first 355-nm laser flash photolysis. Then, TMB*+* was generated from the selective excitation of TMB*+ during the second 532 nm laser flash photolysis. Hole transfer rate constants from TMB*+* to methanol, ethanol, and 2-propanol were calculated to be (5.2 +/- 0.5) x 10(10), (1.4 +/- 0.3) x 10(11), and (3.2 +/- 0.6) x 10(11) M-1 s-1, respectively. The order of the hole transfer rate constants is consistent with oxidation potentials of alcohol. Formation of TCQH radical (TCQH*) with a characteristic absorption peak at 435 nm was observed in the microsecond time scale, suggesting that deprotonation of the alcohol radical cation occurs after the hole transfer and that TCQ radical anion (TCQ*-), generated together with TMB*+ by the photoinduced electron-transfer reaction, reacts with H+ to give TCQH*.     

Ligand-Localized Triplet-State Photophysics in a Platinum(II) Terpyridyl Perylenediimideacetylide

The synthesis, electrochemistry, and photophysical behavior of a Pt(II) terpyridyl perylenediimide (PDI) acetylide (1) charge-transfer complex is reported. The title compound exhibits strong (ε ≈ 5 × 10(4) M(-1)cm(-1)) low-energy PDI acetylide-based π-π* absorption bands in the visible range extending to 600 nm, producing highly quenched singlet fluorescence (Φ = 0.014 ± 0.001, τ = 109 ps) with respect to a nonmetalated PDI model chromophore. Nanosecond transient absorption spectroscopy revealed the presence of a long excited-state lifetime (372 ns in 2-methyltetrahydrofuran) with transient features consistent with the PDI-acetylide triplet state, ascertained by direct comparison to a model Pt(II) PDI-acetylide complex lacking low-energy charge-transfer transitions. For the first time, time-resolved step-scan FT-IR spectroscopy was used to characterize the triplet excited state of the PDI-acetylide sensitized in the title compound and its associated model complex. The observed red shifts (～30-50 cm(-1)) in the C═O and C≡C vibrations of the two Pt(II) complexes in the long-lived excited state are consistent with formation of the (3)PDI acetylide state and found to be in excellent agreement with the expected change in the relevant DFT-calculated IR frequencies in the nonmetalated PDI model chromophore (ground singlet state and lowest triplet excited state). Formation of the PDI triplet excited state in the title chromophore was also supported by sensitization of the singlet oxygen photoluminescence centered at ～1275 nm in air-saturated acetonitrile solution, Φ((1)O(2)) = 0.52. In terms of light emission, only residual PDI-based red fluorescence could be detected and no corresponding PDI-based phosphorescence was observed in the visible or NIR region at 298 or 77 K in the Pt(II) terpyridyl perylenediimideacetylide.     

Primary photophysical properties of ofloxacin

Steady-state fluorescence has been used to study the excited singlet state of ofloxacin (OFLX) in aqueous solutions. Fluorescence emission was found to be pH dependent, with a maximum quantum yield of 0.17 at pH 7. Two pKa*s of around 2 and 8.5 were obtained for the excited singlet state. Laser flash photolysis and pulse radiolysis have been used to study the excited states and free radicals of OFLX in aqueous solutions. OFLX undergoes monophotonic photoionization from the excited singlet state with a quantum yield of 0.2. The cation radical so produced absorbs maximally at 770 nm with an extinction coefficient of 5000 +/- 500 dm3 mol-1 cm-1. This is confirmed by one-electron oxidation in the pulse radiolysis experiments. The hydrated electron produced in the photoionization process reacts with ground state OFLX with a rate constant of 2.0 +/- 0.2 x 10(10) dm3 mol-1 s-1, and the anion thus produced has two absorption bands at 410 nm (extinction coefficient = 3000 +/- 300 dm3 mol-1 cm-1) and at 530 nm. Triplet-triplet absorption has a maximum at 610 nm with an extinction coefficient of 11,000 +/- 1500 dm3 mol-1 cm-1. The quantum yield of triplet formation has been determined to be 0.33 +/- 0.05. In the presence of oxygen, the triplet reacts to form both excited singlet oxygen and superoxide anion with quantum yields of 0.13 and < or = 0.2, respectively. Moreover, superoxide anion is also formed by the reaction of oxygen with the hydrated electron from photoionization. Hence the photosensitivity due to OFLX could be initiated by the oxygen radicals and/or by OFLX radicals acting as haptens.     

An intramolecular charge transfer state of carbonyl carotenoids: implications for excited state dynamics of apo-carotenals and retinal

Excited state dynamics of two apo-carotenals, retinal and 12'-apo-β-carotenal, were studied by femtosecond transient absorption spectroscopy. We make use of previous knowledge gathered from studies of various carbonyl carotenoids and suggest that to consistently explain the excited-state dynamics of retinal in polar solvents, it is necessary to include an intermolecular charge transfer (ICT) state in the excited state manifold. Coupling of the ICT state to the A(g)(-) state, which occurs in polar solvents, shortens lifetime of the lowest excited state of 12'-apo-β-carotenal from 180 ps in n-hexane to 7.1 ps in methanol. Comparison with a reference molecule lacking the conjugated carbonyl group, 12'-apo-β-carotene, demonstrates the importance of the carbonyl group; no polarity-induced lifetime change is observed and 12'-apo-β-carotene decays to the ground state in 220 ps regardless of solvent polarity. For retinal, we have confirmed the well-known three-state relaxation scheme in n-hexane. Population of the B(u)(+) state decays in <100 fs to the A(g)(-) state, which is quenched in 440 fs by a low-lying nπ* state that decays with a 33 ps time constant to form the retinal triplet state. In methanol, however, the A(g)(-) state is coupled to the ICT state. This coupling prevents population of the nπ* state, which explains the absence of retinal triplet formation in polar solvents. Instead, the coupled A(g)(-)/ICT state decays in 1.6 ps to the ground state. The A(g)(-)/ICT coupling is also evidenced by stimulated emission, which is a characteristic marker of the ICT state in carbonyl carotenoids.     

Photophysical and photochemical studies of 2-phenylbenzimidazole and UVB sunscreen 2-phenylbenzimidazole-5-sulfonic acid

The sunscreen agent 2-phenylbenzimidazole-5-sulfonic acid (PBSA) and its parent 2-phenylbenzimidazole (PBI) cause DNA photodamage via both Type-I and Type-II mechanisms when UVB irradiated. We have studied the photophysical and photochemical properties of these compounds and their ability to photogenerate reactive oxygen species including free radicals. PBI and PBSA exhibit both oxidizing and reducing properties in their excited state. The absorption and fluorescence properties of PBSA depend strongly upon pH, and hence the photochemistry of PBSA was studied in both neutral and alkaline solutions. PBSA showed strong oxidizing properties when UV irradiated in neutral aqueous solution (pH 7.4) in the presence of cysteine, glutathione and azide, as evidenced by the detection of the corresponding S-cysteinyl, glutathiyl and azidyl radicals with the aid of the spin trap, 5,5-dimethyl-1-pyrroline N-oxide (DMPO). However, when an aqueous anaerobic solution (pH 10) of PBSA and either nitromethane (NM) or 4-nitrobenzoic acid (4-NBA) were irradiated, the corresponding nitro anion radicals were observed. This finding suggests that both NM and 4-NBA are reduced by direct electron transfer from the excited state PBSA. During UV irradiation of an aerobic solution of PBSA, O2*- and *OH radical were generated and trapped by DMPO. Further, PBI (in ethanol) and PBSA (in ethylene glycol : water 2: 1 mixture) showed low temperature (77 K) phosphorescence (lambdamax = 443, 476 and 509 nm) and also an electron paramagnetic resonance half-field transition (deltaMs = +/-2), which is evidence for a triplet state. This triplet produced singlet oxygen (1O2) with quantum yields 0.07 and 0.04 in MeCN for PBI and PBSA, respectively. These studies demonstrate that UV irradiation of PBSA and PBI generates a variety of free radicals and active oxygen species that may be involved in the photodamage of DNA.