The photophysical properties of menthyl anthranilate: a UV-A sunscreen

The results of a comprehensive investigation of the photophysical properties of the sunscreen agent menthyl anthranilate in various solvent systems are reported. Luminescence studies reveal that this ester is highly fluorescent (phi f = 0.64 +/- 0.06 in ethanol) and has a solvent-dependent emission maximum in the range of 390-405 nm. Phosphorescence has also been detected in low-temperature glasses with an emission maximum at 445 nm and a lifetime of 2.5 s. Kinetic UV-visible absorption measurements revealed a transient species with absorption maxima at 480 nm and solvent-dependent lifetimes of 26-200 microseconds that are attributed to the triplet state. The triplet state is efficiently quenched by oxygen, leading to the formation of singlet oxygen in all of the solvent systems studied. The singlet-oxygen quantum yields (phi delta) determined by time-resolved near-infrared luminescence measurements were determined to be in the range 0.09-0.12 for all systems.     

Characterization of the triplet state of tris(8-hydroxyquinoline)aluminium(III) in benzene solution

The lowest triplet state of tris(8-hydroxyquinoline)aluminium(III) (Alq3) has been prepared by pulse radiolysis/energy transfer from appropriate donors in benzene solutions and has an absorption maximum around 510 nm with a lifetime of about 50 mus. It is quenched by molecular oxygen, leading to singlet oxygen formation. From flash photolysis and singlet oxygen formation measurements, a quantum yield of triplet formation of 0.24 was determined for direct photolysis of the complex. A value of 2.10 +/- 0.10 eV was determined for the energy of the lowest triplet state by energy transfer studies and was confirmed by phosphorescence measurements on Alq3, either in the heavy atom solvent ethyl iodide or photosensitized by benzophenone in benzene. Dexter (exchange) energy transfer was observed from triplet Alq3 to platinum(II) octaethylporphyrin.     

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.     

Laser Flash Photolysis Studies of the UVA Sunscreen Mexoryl SX

Abstract The results of a nanosecond laser flash photolysis investigation of the UVA sunscreen Mexoryl* SX in various solvent environments and within a commercial sunscreen formulation are reported. To the best of our knowledge this is the first laser flash photolysis study of a commercial suncare formulation. In each of these environments kinetic UV-visible absorption measurements following nanosecond 355 nm laser excitation reveals a short-lived species with a solvent-dependent absorption maximum around 470–500 nm and a solvent-dependent lifetime of 50–120 ns. This transient absorption is attributed to the triplet state of Mexoryl* SX on the basis that it is quenched by molecular oxygen leading to the formation of singlet oxygen in acetonitrile. The singlet oxygen quantum yield (φ_Δ), determined by comparative time-resolved near-infrared luminescence measurements and extrapolated to the limit of complete triplet state quenching, is estimated as 0.09 ± 0.03 in acetonitrile. In aqueous solution the shorter triplet state lifetime combined with lower ambient oxygen concentrations precludes significant triplet state quenching. For the commercial sunscreen formulation there was no observable difference in the measured triplet lifetime between samples exposed to oxygen or argon, suggesting that the singlet oxygen quantum yield in such environments is likely to be orders of magnitude lower than that measured in acetonitrile.     

Tetra-2,3-pyrazinoporphyrazines with externally appended pyridine rings. 6. Chemical and redox properties and highly effective photosensitizing activity for singlet oxygen production of penta- and monopalladated complexes in dimethylformamide solution

Tetrakis-2,3-[5,6-di-(2-pyridyl)pyrazino]porphyrazinatopalladium(II) [Py 8TPyzPzPd] ( 1) and the corresponding pentapalladated species [(PdCl 2) 4Py 8TPyzPzPd] ( 2), dissolved (c approximately 10 (-5)-10 (-6) M) in preacidified dimethylformamide ([HCl] approximately 10 (-4) M), behave as potent photosensitizing agents for the production of singlet oxygen, (1)O 2, with Phi Delta values of 0.89 +/- 0.04 and 0.78 +/- 0.05, respectively. The related octacation [(2-Mepy) 8TPyzPzPd] (8+) ( 3), examined under similar experimental conditions, exhibits lower Phi Delta values, that is, 0.29 +/- 0.02 (as an iodide salt) and 0.32 +/- 0.02 (as a chloride salt). In view of the very high values of Phi Delta, the photophysics of complexes 1 and 2 has been studied by means of pump and probe experiments using ns laser pulses at 532 nm as excitation source. Both complexes behave like reverse saturable absorbers at 440 nm because of triplet excited-state absorption. The lifetimes of the triplet excited states are 65 and 96 ns for the penta- and mononuclear species, respectively. Fluorescence quantum yields (Phi f) are approximately 0.1% for both 1 and 2. Such low Phi f values for the two complexes are consistent with the high efficiency of triplet excited-state formation and the measured high yields of (1)O 2. Time-dependent density-functional theory (TDDFT) calculations of the lowest singlet and triplet excited states of the mono- and pentapalladated species help to rationalize the photophysical behavior and the relevant activity of the complexes as photosensitizers for the (1)O 2 ( (1)Delta g) generation.     

4'-(5'-R-pyrimidyl)-2,2':6',2'-terpyridyl (R = H, OEt, Ph, Cl, CN) platinum(II) phenylacetylide complexes: synthesis and photophysics

A series of new square-planar 4'-(5'-R-pyrimidyl)-2,2':6',2'-terpyridyl platinum(II) phenylacetylide complexes ( 1a- 5a) bearing different substituents (R = H, OEt, Ph, Cl, CN) on the pyrimidyl ring have been synthesized and characterized. The electronic absorption, photoluminescence, and triplet transient difference absorption spectra were investigated. All of the complexes exhibit broad, moderately strong absorption between 400 and 500 nm that can be tentatively assigned to the metal-to-ligand charge transfer ( (1)MLCT) transition, possibly mixed with some ligand-to-ligand charge transfer ( (1)LLCT) character. Photoluminescence arising from the (3)MLCT state was observed both in fluid solutions at room temperature and in a rigid matrix at 77 K. The (1)MLCT/ (1)LLCT absorption bands and the (3)MLCT emission bands for 1a- 5a red-shift in comparison to those of the corresponding 4'-toly-2,2':6',2'-terpyridyl platinum(II) phenylacetylide complex. In addition, the energies of the (1)MLCT/ (1)LLCT absorption and the (3)MLCT emission bands exhibit a linear correlation with the Hammett constant (sigma p) of the 5'-substituent on the pyrimidyl ring. The lifetime of the (3)MLCT emission at room temperature is governed by the energy gap law. The triplet transient difference absorption spectra of 1a- 5a exhibit a broad absorption band from 500 to 800 nm, and a bleaching band between 420 and 500 nm. Complex 5a, which contains the -CN substituent, exhibits a lower-energy triplet absorption band at 785 nm and a shorter lifetime (130 ns) in CH 3CN than 2a, which has the -OEt substituent, does (lambda T1-Tn (max) = 720 nm, tau T = 660 ns). The triplet excited-state absorption coefficients at the band maxima for 1a- 5a vary from 36 600 L.mol (-1).cm (-1) to 115 090 L.mol (-1).cm (-1), and the quantum yields of the triplet excited-state formation range from 0.19 to 0.66. All complexes exhibit a moderate nonlinear transmission for nanosecond laser pulses at 532 nm. Moreover, these complexes can generate singlet oxygen efficiently in air-saturated CH 3CN solutions, with the singlet oxygen generation quantum yield (Phi Delta) varying from 0.24 to 0.46.     

Triplet formation involving a polar transition state in a well-defined intramolecular perylenediimide dimeric aggregate

A cofacially stacked perylenediimide (PDI) dimer with a xanthene linker was studied under a variety of conditions (solvent, temperature) and serves as a model for the molecular interactions occurring in solid films. Intrinsically, the PDI units have a fluorescence quantum yield (Phi F) close to unity, but Phi F is lowered by a factor of 6-50 at room temperature when two PDI moieties are held in a cofacial arrangement, while the decay time of the most emissive state is increased significantly (tau F = 27 ns in toluene) compared to a monomeric PDI molecule (tau F = 4 ns). Fluorescence measurements show a strong solvent and temperature dependence of the characteristics of the emissive excited state. In a glassy matrix of toluene (TOL) or 2-methyltetrahydrofuran (2-MeTHF), Phi F is high, and the decay time is long (tau F = approximately 50 ns). At higher temperature, both Phi F and tau F are reduced. Interestingly, at room temperature, Phi F and tau F are also reduced with increasing solvent polarity, revealing the presence of a polar transition state. Photoinduced absorption of the stacked molecules from the picosecond to the microsecond time scale shows that after photoexcitation reorganization occurs in the first nanoseconds, followed by intersystem crossing (ISC), producing the triplet excited state. Using singlet oxygen ( (1)Delta g) luminescence as a probe, a triplet quantum yield (Phi T) greater than 50% was obtained in air-saturated 2-Me-THF. Triplet formation is exceptional for PDI chromophores, and the enhanced ISC is explained by a decay involving a highly polar transition state.     

A clean, well-defined solid system for photosensitized 1O2 production measurements

Generation of singlet molecular oxygen ((1)O(2)) by photosensitization with methylene blue (MB) supported in Nafion-Na films has been quantified by integration of the (1)O(2) emission decay at 1270 nm. The quantum yield of (1)O(2) production (Phi(Delta)) in the air-equilibrated solid phase is 0.24 +/- 0.03. Information on the (1)O(2) generation environment has been gained from complementary techniques such as UV-Vis absorption and emission spectroscopy, as well as MB fluorescence and triplet-triplet absorption decay. Results are compared with the (1)O(2) generation by MB in methanol solution (Phi(Delta) = 0.51) and in methanol-swollen Nafion films (Phi(Delta) = 0.49 +/- 0.06). Differences and similarities are discussed in terms of the factors that influence Phi(Delta) in solution and in the solid media. The optical and mechanical features of Nafion, ease of dye loading, compatibility with most solvents, homogeneity, reproducibility and stability of the photosensitizing material makes it a convenient reference for (1)O(2) generation quantum yield measurements in transparent (micro)heterogeneous and homogeneous media.     

Axial halogen ligand effect on photophysics and optical power limiting of some indium naphthalocyanines

Three axially substituted complexes, 2,3-octa(3,5-di-tert-butylphenoxy)-2,3-naphthalocyaninato indium chloride (1a), 2,3-octa(3,5-di-tert-butylphenoxy)-2,3-naphthalocyaninato indium bromide (1b), and 2,3-octa(3,5-di-tert-butylphenoxy)-2,3-naphthalocyaninato indium iodide (1c) have been synthesized and their photophysical properties have been investigated. Optical power limiting of nanosecond (ns) and picosecond (ps) laser pulses at 532 nm using these complexes has been demonstrated. All complexes display strong Q(0,0) absorption and measurable emission in the near-infrared region and exhibit strong excited-state absorption in the range of 470-700 nm upon ns laser excitation. The different axial ligands show negligible effect on the linear absorption, emission, and transient difference absorption spectra. However, the excited-state lifetime, triplet excited-state quantum yield, and efficiency to generate singlet oxygen are affected significantly by the heavier axial ligand. Brominated and iodinated complexes 1b and 1c show higher triplet excited-state quantum yield, while chlorinated complex 1a has longer excited-state lifetime and is more efficient in generating singlet oxygen. The iodinated complex 1c displayed the best optical limiting due to the higher ratio of excited-state absorption cross section to ground state absorption cross section (sigma(eff)/sigma(0)).     

Enhanced intersystem crossing via a high energy charge transfer state in a perylenediimide-perylenemonoimide dyad

The electronic relaxation processes of a photoexcited linear perylenediimide-perylenemonoimide (PDI-PMI) acceptor-donor dyad were studied. PDI-PMI serves as a model compound for donor-acceptor systems in photovoltaic devices and has been designed to have a high-energy PDI (-*)-PMI (+*) charge transfer (CT) state. Our study focuses on the minimal Gibbs free energy (Delta G ET) required to achieve quantitative CT and on establishing the role of charge recombination to a triplet state. We used time-resolved photoluminescence and picosecond photoinduced absorption (PIA) to investigate excited singlet (S 1) and CT states and complemented these experiments with singlet oxygen ( (1)Delta g) luminescence and PIA measurements on longer timescales to study the population of triplet excited states (T 1). In an apolar solvent like cyclohexene (CHX), photoinduced electron transfer does not occur, but in more polar solvents such as toluene (TOL) and chlorobenzene (CB), photoexcitation is followed by a fast electron transfer, populating the PDI (-*)-PMI (+*) CT state. We extract rate constants for electron transfer (ET; S 1-->CT), back electron transfer (BET; S 1<--CT), and charge recombination (CR) to lower-energy states (CT-->S 0 and CT-->T 1). Temperature-dependent measurements yield the barriers for the transfer reactions. For ET and BET, these correspond to predictions from Marcus-Jortner theory and show that efficient, near quantitative electron transfer ( k ET/ k BET >or= 100) can be obtained when Delta G ET approximately -120 meV. With respect to triplet state formation, we find a relatively low triplet quantum yield (Phi T < 25%) in CHX but much higher values (Phi T = 30-98%) in TOL and CB. We identify the PDI (-*)-PMI (+*) state as a precursor to the T 1 state. Recombination to T 1, rather than to the ground-state S 0, is required to rationalize the experimental barrier for CR. Finally, we discuss the relevance of these results for electron donor-acceptor films in photovoltaic devices.     

Femtosecond time-resolved photophysics of 1,4,5,8-naphthalene diimides

The photophysical properties of a tetrahedral molecule with naphthalene diimide (NDI) moieties and of two model compounds were investigated. The absorption and fluorescence spectra of dialkyl-substituted NDI are in agreement with literature. While the absorption spectra of phenyl-substituted molecules are similar to all other NDIs, their fluorescence showed a broad band between 500 and 650 nm. This band is sensitive to the polarity of the solvent and is attributed to a CT state. The absorption spectra and lifetime (10+/-2 ps) of the electronically excited singlet state of a dialkyl-substituted NDI was determined by femtosecond transient absorption spectroscopy, and the latter was confirmed by picosecond fluorescence spectroscopy. Nanosecond flash photolysis showed the subsequent formation of the triplet state. The presence of a phenyl substituent on the imide nitrogen of NDI resulted in faster deactivation of the singlet state (lifetime 0.5-1 ps). This is attributed to the formation of a short-lived CT state, which decays to the local triplet state. The faster deactivation was confirmed by fluorescence lifetime measurements in solution and in a low-temperature methyl-tetrahydrofuran glass.     

Effect of aza substitution on the photophysical and electrochemical properties of porphycenes: Characterization of the near-IR-absorbing photosensitizers 2,7,12,17-tetrakis(p-substituted phenyl)-3,6,13,16-tetraazaporphycenes

The need for new photodynamic-therapy photosensitizers has stimulated the search of new families of compounds absorbing strongly in the 700-900 nm range, the region where tissue is most transparent to radiation capable to induce the photodynamic effect. Using computational chemistry techniques, 3,6,13,16-tetraazaporphycenes were previously identified as interesting target candidates. This work reports on the photophysical and electrochemical properties of selected members of this new family of macrocycles. Compared to porphycenes, the tetra-aza counterparts show stronger absorption in the near-infrared, lower-lying singlet and triplet excited states, and substantially larger internal conversion quantum yield (Phi(IC) = 0.93). Energy transfer to oxygen is observed, which results in the formation of the cytotoxic species singlet oxygen. The process is found to be reversible, consistent with a triplet-energy value close to that of singlet oxygen.     

Monomeric azaheterofullerene derivatives RC59N: influence of the R moiety on spectroscopic and photophysical properties

We have synthesised nine monomeric azaheterofullerene (AZA) derivatives, RC(59)N, with a wide variety of different side chains R and investigated their spectroscopic and photophysical properties in toluene and o-dichlorobenzene (ODCB). Measurements include their ground-state absorption spectra, molar absorption coefficient (epsilon(G)), fluorescence spectra, fluorescence quantum yields (Phi(F)), singlet-state lifetimes (tau(F)), triplet-state absorption spectra, triplet molar absorption coefficients (epsilon(T)), singlet oxygen (Phi(Delta)), and triplet state (Phi(T)) quantum yields. The replacement of a carbon by a nitrogen atom in the C(60) sphere strongly affects most of the spectroscopic and photophysical properties. The chemical nature of the R moiety has definite effects on these properties in contrast with minor effects on the chemical nature of the addends in [6,6]-ring bridged monoadduct methano[60]fullerene derivatives. These effects concern properties of the ground state, singlet excited state, and triplet states of our nine RC(59)N derivatives and in particular the values of photophysical parameters epsilon(G), epsilon(T), Phi(Delta), and Phi(T), which are significantly lower than those of analogous monoadduct [6,6]-ring bridged methano[60]fullerene derivatives.     

Triplet formation of 4-thiothymidine and its photosensitization to oxygen studied by time-resolved thermal lensing technique

Excited-state dynamics of 4-thiothymidine (S4-TdR) and its photosensitization to molecular oxygen in solution with UVA irradiation were investigated. Absorption and emission spectra measurements revealed that UVA photolysis of S4-TdR gives rise to a population of T1(pipi*), following S2(pipi*) --> S1(npi*) internal conversion. In transient absorption measurement, the 355 nm laser photolysis gave broad absorption (380-600 nm) bands of triplet S4-TdR. The time-resolved thermal lensing (TRTL) signal of S4-TdR containing the thermal component due to decay of triplet S4-TdR was clearly observed by the 355 nm laser excitation. The quantum yield for S1 --> T1 intersystem crossing was estimated to be unity by a triplet quenching experiment with potassium iodide. In the presence of molecular oxygen, the photosensitization from triplet S4-TdR gave rise to singlet oxygen O2 (1Deltag) with a quantum yield of 0.50. Therapeutic implications of such singlet oxygen formation are discussed.     

The photophysics of monomeric bacteriochlorophylls c and d and their derivatives: properties of the triplet state and singlet oxygen photogeneration and quenching

Measurements of pigment triplet-triplet absorption, pigment phosphorescence and photosensitized singlet oxygen luminescence were carried out on solutions containing monomeric bacteriochlorophylls (Bchl) c and d, isolated from green photosynthetic bacteria, and their magnesium-free and farnesyl-free analogs. The energies of the pigment triplet states fell in the range 1.29-1.34 eV. The triplet lifetimes in aerobic solutions were 200-250 ns; they increased to 280 +/- 70 microseconds after nitrogen purging in liquid solutions and to 0.7-2.1 ms in a solid matrix at ambient or liquid nitrogen temperatures. Rate constants for quenching of the pigment triplet state by oxygen were (2.0-2.5) x 10(9) M-1 s-1, which is close to 1/9 of the rate constant for diffusion-controlled reactions. This quenching was accompanied by singlet oxygen formation. The quantum yields for the triplet state formation and singlet oxygen production were 55-75% in air-saturated solutions. Singlet oxygen quenching by ground-state pigment molecules was observed. Quenching was the most efficient for magnesium-containing pigments, kq = (0.31-1.2) x 10(9) M-1 s-1. It is caused mainly by a physical process of singlet oxygen (1O2) deactivation. Thus, Bchl c and d and their derivatives, as well as chlorophyll and Bchl a, combine a high efficiency of singlet oxygen production with the ability to protect photochemical and photobiological systems against damage by singlet oxygen.     

Primary Photoprocesses in a Fluoroquinolone Antibiotic Sarafloxacin†

The photophysical properties of the fluoroquinolone antibiotic sarafloxacin (SFX) were investigated in aqueous media. SFX in water, at pH 7.4, shows intense absorption with peaks at 272, 322 and 335 nm, (? = 36800 and 17000 dm³ mol^?1 cm^?1, respectively). Both the absorption and emission properties of SFX are pH‐dependent; pK_a values for the protonation equilibria of both the ground (5.8 and 9.1) and excited singlet states (5.7 and 9.0) of SFX were determined spectroscopically. SFX fluoresces weakly, the quantum yield for fluorescence emission being maximum (0.07) at pH 8. Laser flash photolysis and pulse radiolysis studies have been carried out in order to characterize the transient species of SFX in aqueous solution. Triplet–triplet absorption has a maximum at 610 nm with a molar absorption coefficient of 17,000 ± 1000 dm³ mol^?1 cm^?1. The quantum yield of triplet formation has been determined to be 0.35 ± 0.05. In the presence of oxygen, the triplet reacts to form excited singlet oxygen with quantum yield of 0.10. The initial triplet (³A*) was found to react with phosphate buffer to form triplet ³B* with lower energy and longer lifetime and having an absorption band centered at 700 nm. SFX triplet was also found to oxidize tryptophan to its radical with concomitant formation of the anion radical of SFX. Hence the photosensitivity of SFX could be initiated by the oxygen radicals and/or by SFX radicals acting as haptens.     

Laser induced photochromic mechanism of 3-phenyl-3-[1,2-dimethylindol-3-YL]-3H-naphtho[2,1-B]pyran

The photochromic process of 3-phenyl-3-[1,2-dimethylindol-3-yl]-3H-naphtho[2,1-b]pyran [I] has been examined with nanosecond laser flash photolysis techniques in cyclohexane and acetonitrile respectively. Both excited singlet state and triplet state are involved in the photocoloration process. The decay kinetics of photoproducts are also studied. The maximum absorption wavelength and lifetime of the transient species are solvent dependent.     

Femtosecond ground state recovery: measuring the intersystem crossing yield of polyspirobifluorene

Measurement of the quantum yield of triplet formation has been made for the prototypical conjugated polymer polyspirobifluorene in solution and solid state. An updated method has been described based on femtosecond time resolved ground state recovery following photoexcitation of the polymer. The two components to the recovery of the ground state due to the decay of the singlet and triplet excited states are clearly visible and from these it is possible to calculate Phi(T)=0.05+/-0.01 in solution, this gives k(isc)=5.4 x 10(7) s(-1) which compares favorably with other conjugated polymers. In polymer films an increased triplet yield of Phi(T)=0.12+/-0.02 is found to be independent of temperature, the increased yield is attributed to triplet recombination from charged states.     

Photochemical elimination of leaving groups from zwitterionic intermediates generated via electrocyclic ring closure of alpha,beta-unsaturated anilides

Methacrylanilides, ArN(CH3)COC(CH2LG)=CH2, with allylic leaving groups (LG(-) = BocAla, PhCO2(-), PhCH2CO2(-), PhO(-)) undergo photochemical electrocyclic ring closure to produce a zwitterionic intermediate. Further reaction of the intermediate results in expulsion of the leaving group to give an alpha-methylene lactam as the major product. In addition, a lactam product that retains the leaving group is formed via a 1,5-H shift in the intermediate. Elimination of the leaving group is generally preferred, even for LG(-) = PhO(-), although in benzene as the solvent the lactam retaining the phenolate group becomes the sole photoproduct. The electrocyclic ring closure occurs in the singlet excited-state for the para-COPh-substituted anilide derivative and is not quenched by 0.15 M piperylene or 0.01 M sodium 2-naphthalenesulfonate (2-NPS) as triplet quenchers. Comparable concentrations of 2-NPS strongly quench the transient absorption of the triplet excited state observed at 450-700 nm according to laser flash photolysis experiments. In aqueous media, quantum yields for total products are insensitive to leaving group ability, and Phi(tot)(para-CO2CH3) = 0.04-0.06 at 310 nm and Phi(tot)(para-COPh) = 0.08-0.1 at 365 nm, for which Phi(isc) = 0.15.     

How the pi conjugation length affects the fluorescence emission efficiency

How the pi conjugation length affects the fluorescence emission efficiency is elucidated by examination of the theoretical and experimental relationship between absolute quantum yield (Phi(f)) and magnitude (Api) of the pi conjugation length in the excited singlet state, which provides a novel concept for molecular design for highly fluorescent organic compounds. As a tool to predict Phi(f) from a structural model, (nu(a) - nu(f))1/2 x a3/2 (nu(a): wavenumber of absorption maximum, nu(f): wavenumber of emission maximum, a: molecular radius) could be used instead of Api. The concept should be valuable for potential applications to (1) examination of an excited singlet state structure (for example, coplanarity of excited-state molecules) and (2) molecular design of novel materials, in which the excited singlet state plays an important role, such as highly efficient fluorophores, electroluminescent materials, photoconducting materials, and nonlinear optical materials. A remarkably intense green fluorophore (Phi(f) 0.88, log epsilon 4.72, lambda(em) 527 nm) is created based on this concept, which is of great interest in relation to a green fluorescent protein (Topaz, T203Y type, Phi(f) 0.60, log epsilon 4.98, lambda(em) 527 nm).