S1 Lifetime Research Articles

Femtosecond Fluorescence Upconversion Studies of Light Harvesting by β-Carotene in Oxygenic Photosynthetic Core Proteins

Energy transfer from β-carotene to chlorophyll in the photosystem (PS) I core complex and the CP43, CP47, and reaction center (RC) proteins of PSII was studied by the femtosecond fluorescence upconversion technique. The carotenoid S2 lifetimes/transfer efficiencies, the latter obtained by comparison with β-carotene in solution, are ∼97 fs/33% (CP43), ∼100 fs/31% (CP47), ∼106 fs/26% (PSII−RC), and ∼62 fs/57% (PSI). By combining previous steady-state fluorescence excitation measurements with our ultrafast results on the carotenoid S2 lifetime and chlorophyll rise kinetics, we conclude that the β-carotene S1 state is insignificant for light harvesting in the PSII core proteins. Measurement of the steady-state fluorescence excitation spectrum of PSI at 77 K in this work yielded a β-carotene to chlorophyll energy transfer efficiency of 62 ± 3%, suggesting dominant transfer from the carotenoid S2 state. Interestingly, while both PSI and PSII almost exclusively utilize the carotenoid S2 state for light harvestin...

Ultrafast Formation of a Carotenoid Radical in LH2 Antenna Complexes of Purple Bacteria

The S1-mediated carotenoid to BChl a energy-transfer pathway and carotenoid radical formation have been studied using femtosecond time-resolved transient absorption spectroscopy. A series of LH2 complexes (LH2 = light-harvesting complex 2) from Rhodobacter sphaeroides incorporating carotenoids neurosporene, spheroidene, and spheroidenone were used to explore the effect of conjugation length. The S1 lifetimes of carotenoids in the LH2 complex are significantly shorter than in solution, resulting in efficiencies of the S1-mediated energy transfer of 94%, 82%, and 76%, respectively. In addition to the S1 decay, a low-amplitude component of ∼9 ps was revealed for all carotenoids in LH2 and assigned to the S* state. Besides energy transfer, excitation of the carotenoid S2 state in LH2 complexes also leads to the formation of a carotenoid radical, and this pathway is also conjugation length dependent. The carotenoid radical is formed as a result of electron transfer between the carotenoid and B800 BChl a molecules. The yield of the radical formation is highest for neurosporene (10−15%), while a lower efficiency was observed for spheroidene (5−8%). For spheroidenone, no distinct radical signal was found. The precursor of the carotenoid radical is a charge-transfer state that is populated directly from the S2 state. This charge-transfer state, which represents an excited state of the carotenoid−B800 complex, decays in 300 fs (neurosporene) or 400 fs (spheroidene) to form ground-state carotenoid cation and BChl anion. The charge-separated state has a lifetime of 2 ps (neurosporene) and 8 ps (spheroidene).

Nucleobase Pair Analogues 2-Pyridone·Uracil, 2-Pyridone·Thymine, and 2-Pyridone·5-Fluorouracil: Hydrogen-Bond Strengths and Intermolecular Vibrations

Due to the ultrafast nonradiative decay of the uracil (U), thymine (T), and 5-fluorouracil (5-FU) nucleobases, resolved vibronic spectra of nucleobase dimers with U, T, or 5-FU chromophores have not yet been observed in the gas phase. 2-Pyridone (2PY) is a fluorescent hydrogen-bonding analogue of U with a single N−H donor and CO acceptor group. We report the mass-selected S1 ← S0 vibronic spectra of 2PY·U, 2PY·T, and 2PY·5-FU, revealing properties of their H-bonds free of stacking interactions and backbone constraints. The in-plane H-bond vibrations, ω‘, χ‘, σ‘, are observed for all three dimers; the out-of-plane vibrations β‘, τ‘ and the overtone of the stagger mode 2δ‘ are measured for 2PY·T and 2PY·U. The experimental frequencies are in good agreement with the ab initio calculated frequencies of the most stable H-bonded isomer, which involves the N1−H and C2O groups of U, T, or 5-FU as H donor/acceptor. The H-bond dissociation energies follow the sequence T ≈ U < 5-FU. However, the experimental S1 state H-bond stretching force constants increase in the sequence U < T < 5-FU. The absence of fluorescence from these dimers implies that the S1 lifetimes of 2PY in these complexes are >20 times shorter than those in the other 2PY·X complexes measured so far. Energy transfer from 2PY to U (T, 5-FU) is suggested as the cause, which implies that the nπ* S1 states of U, T, and 5-FU, which have never been directly observed, lie below the S1 ππ* state of 2PY.

Ultrafast Excited State Dynamics of Tri- and Hexaporphyrin Arrays

An investigation of the ultrafast excited state dynamics of triporphyrin and hexaporphyrin arrays consisting of covalently linked Zn tetraphenylporphine (ZnTPP) and free base tetraphenylporphine (FbTPP) units is reported. The interchromophoric distance in the hexamers is of the order of 13 Å, while it varies from 26 to 70 Å in the trimers. These arrays exhibit several features that differ substantially from those of the monomeric units: a broadening of the Soret band, a shortening of the S2 lifetime of the ZnTPP chromophores, and additional ultrafast decay components of the S1 fluorescence. In the hexaporphyrin arrays, most of these features are attributed to the presence of excitonic states that result from the strong coupling between the Bx,y transition dipoles. The time constants for S1 energy transfer between ZnTPP chromophores as well as between ZnTPP and FbTPP moieties, deduced from anisotropic and isotropic time-resolved fluorescence measurements, were found to be of the order of a few tens of picoseconds. Moreover, back energy transfer from the FbTPP to ZnTPP units is also observed. At high to moderate excitation intensity, S1−S1 annihilation becomes an important decay mechanism of the excited state population of the hexaporphyrins. In the triporphyrins, the differences relative to the monomer are ascribed to the interaction with the phenylacetylene linkers, which lifts the degeneracy of the S2 states. S2 and S1 energy transfer were found to take place in the triporphyrin with the shortest linker only. In the other triporphyrins, an efficient energy transfer from the linker to the porphyrin units was observed.

Excited state direct dynamics of benzene with reparameterized multi-reference semiempirical configuration interaction methods

Abstract We investigate the feasibility of reparameterized multi-reference semiempirical methods in the context of direct wavepacket dynamics, where the electronic and nuclear Schrodinger equations are solved simultaneously. Using a very limited set of ab initio and experimental data, we refine the semiempirical parameters for benzene. The resulting reparameterized semiempirical method is compared to ab initio results including dynamic electron correlation over selected cuts of the ground and excited state PESs. We demonstrate that reparameterization with a limited data set can provide globally correct PESs. We also present the results of minimal energy conical intersection optimizations for benzene using correlated wavefunctions (CASPT2). Finally, we simulate the dynamics of benzene after excitation to S2 using both the standard and newly optimized parameter sets. We show that the excited state lifetime using the standard parameter set is inconsistent with experiment. In contrast, the newly optimized parameter set predicts an S2 lifetime of less than 100 fs, in agreement with experiment. The reparameterization strategy is a promising way to extend direct dynamics simulations of photochemistry to multi-picosecond time scales.

Observation of Proton-Coupled Electron Transfer by Transient Absorption Spectroscopy in a Hydrogen-Bonded, Porphyrin Donor−Acceptor Assembly

Proton-coupled electron transfer (PCET) kinetics of a Zn(II) porphyrin donor noncovalently bound to a naphthalene-diimide acceptor through an amidinium-carboxylate interface have been investigated by time-resolved spectroscopy. The S1 singlet excited-state of a Zn(II) 2-amidinium-5,10,15,20-tetramesitylporphyrin chloride (ZnP-beta-AmH+) donor is sufficiently energetic (2.04 eV) to reduce a carboxylate-diimide acceptor (DeltaG degrees = -460 mV, THF). Static quenching of the porphyrin fluorescence is observed and time-resolved measurements reveal more than a 3-fold reduction in the S1 lifetime of the porphyrin upon amidinium-carboxylate formation (THF, 298 K). Picosecond transient absorption spectra of the free ZnP-beta-AmH+ in THF reveal the existence of an excited-state isosbestic point between the S1 and T1 states at lambdaprobe = 650 nm, providing an effective 'zero-kinetics' background on which to observe the formation of PCET photoproducts. Distinct rise and decay kinetics are attributed to the build-up and subsequent loss of intermediates resulting from a forward and reverse PCET reaction, respectively (kPCET(fwd) = 9 x 108 s-1 and kPCET(rev) = 14 x 108 s-1). The forward rate constant is nearly 2 orders of magnitude slower than that measured for covalently linked Zn(II) porphyrin-acceptor dyads of comparable driving force and D-A distance, establishing the importance of a proximal proton network in controlling charge transport.

Pump−Deplete−Probe Spectroscopy and the Puzzle of Carotenoid Dark States

Pump−deplete−probe and transient absorption spectroscopy are applied to carotenoids with N = 11 conjugated double bonds in solution to study the origin of recently observed transient features that have been previously assigned to new electronic states. The depletion pulse pumps the transient near-IR band, whose lifetime coincides with the fluorescence lifetime, and is hence attributed to the S2 state. The subsequent signal of any lower-lying dark excited-state populated by internal conversion from S2 should be affected by the depletion pulse. Correspondingly, the signal in the S1 deactivation channel is diminished by the depleted excited population. In contrast, the Ssol* signal, purportedly reflecting an intermediate state on a competing deactivation pathway, is not affected by the depletion pulse. When comparing our results with literature data for other carotenoids, we find that the Ssol* lifetime is constant at 6.2 ± 0.4 ps for any N ≥ 11 carotenoid; for shorter chain lengths, it is equal to the S1 lifetime. To explain this puzzle, Ssol* is identified as a vibrationally excited ground state (Ssol* = hot S0), populated by a combination of impulsive Raman scattering of the pump pulse and internal conversion (S1−S0), and decaying by vibrational relaxation. The Ssol* state is not identical to the ST* state, which appears in the same spectral region when the carotenoid is embedded in light-harvesting complexes.

Alanyl Side Chain Folding in Phenylalanine: Conformational Assignments through Ultraviolet Rotational Band Contour Analysis

Partially resolved ultraviolet rotational band contours associated with the S1 ← S0 origin bands of the six most populated conformers of jet-cooled phenylalanine have been recorded via resonant two-photon ionization. The strong dependence of their transition moment orientation on the conformation of the alanyl side chain has facilitated their structural assignment through simulations based upon ab initio computation. The S1 lifetimes of all six conformers, measured through pump−probe delayed ionization, reveal an efficient nonradiative decay pathway in the most stable conformer, which is stabilized through a chain of intramolecular hydrogen bonds linking the side chain to the benzene ring.

Femtosecond photoelectron imaging of pyridazine: S1 lifetime and (3s(n−1),3p(n−1)) Rydberg state energetics

The first real-time study on pyridazine in the S1(n,π*) state is presented. The S1 state is found to dephase with a time constant of 323±17 ps at its origin, and the electronic dephasing mechanism is attributed to the S1–S0 internal conversion. The S1 lifetime is found to decrease rather sharply as the internal energy increases. The 3s (n−1) and 3p (n−1) Rydberg states of pyridazine are clearly identified in angle- and energy-resolved photoelectron images obtained in the (1+2′) photoionization scheme, providing their respective term values of 5.68±0.03 and 6.28±0.04 eV.

Protonation effect on the excited state behaviour of EE-1-(n-pyridyl)-4-phenylbutadienes (n = 2, 3 and 4).

The acid-base equilibria of three aza-derivatives of EE-1,4-diphenylbutadiene and the excited state properties of their neutral and protonated forms have been studied in aqueous solutions. The prevalent relaxation of the neutral molecules is photoisomerization by a diabatic singlet mechanism accompanied by substantial internal conversion while the quantum yield of the radiative pathway is very small, particularly for the ortho- and para-substituted pyridines. Protonation affects only slightly the isomerization yield of these two compounds while drastically decreasing that of the meta-substituted analogue. Molecular orbital calculations and comparison with the corresponding azastilbenes indicate that the first excited singlet state of the protonated meta compound has a net charge transfer character, which leads to an increase of the torsional barrier in S1. Application of the Forster cycle to the three compounds showed that they become much stronger bases in S1. The 3-pyridyl derivative proved to be peculiar also in this respect since its equilibration with the proton occurs during the S1 lifetime thus allowing the acid-base equilibration in the excited state to be directly monitored by fluorimetric titration.

Singlet excited-state lifetimes of cytosine derivatives measured by femtosecond transient absorption.

Lifetimes of the lowest excited singlet (S1) electronic states of various derivatives of the pyrimidine nucleobase cytosine (Cyt) were measured by the femtosecond transient absorption technique. The bases were excited in room-temperature aqueous solution at 265 nm using approximately 200 fs pump pulses from a titanium-sapphire laser system. The decay of excited-state absorption (ESA) at visible probe wavelengths was used to determine the S1 lifetimes of a variety of modified Cyt compounds at different pH values by global fitting. Identical lifetimes were observed for Cyt and cytidine (Cyd) within experimental uncertainty, but ESA by the ribonucleoside was considerably stronger, suggesting that the ribose group increases the oscillator strength of the S1 --> SN transition. The S1 lifetime of the important minor base 5-methylcytosine (m5Cyt) is 7.2 +/- 0.4 ps at pH 6.8. The same lifetime was measured for the ribonucleoside 5-methylcytidine, but sugar substitution again increased the strength of the ESA signal. Protonation of Cyd and m5Cyt at low pH led to a modest decrease in their S1 lifetimes. On the other hand, deprotonation of Cyt and m5Cyt significantly increased the lifetime of their respective S1 states. These trends support the intermediacy of the n,pi* state localized on the carbonyl oxygen in the nonradiative decay mechanism of Cyt. Longer S1 lifetimes were observed for 5-fluorocytosine and N4-acetylcytosine. Collectively, these results illustrate the great potential of femtosecond laser spectroscopy for investigating excited-state dynamics in DNA and DNA components.

Photophysical properties of xanthophylls in carotenoproteins from human retinas.

The macula of the human retina contains high amounts of the xanthophyll carotenoids lutein and zeaxanthin [a mixture of (3R,3'R)-zeaxanthin and (3R,3'S-meso)-zeaxanthin]. Recently, it was shown that the uptake and the stabilization of zeaxanthin and lutein into the retina are likely to be mediated by specific xanthophyll-binding proteins (XBP). Here, we have used femtosecond pump-probe spectroscopy to study the dynamics of the S1 state of these xanthophylls in xanthophyll-enriched and native XBP. The results from the native XBP and the enriched XBP were then compared with those for carotenoids in organic solvents and in detergent micelles. Steady-state and transient absorption spectra show that the incorporation of xanthophylls into the protein causes a redshift of the spectra, which is stronger for lutein than for zeaxanthin. The transient absorption spectra further indicate that a part of the xanthophylls remains unbound in the xanthophyll-enriched XBP. The transient absorption spectra of the native XBP prove the presence of both xanthophylls in native XBP. Although the S1 lifetime of lutein does not exhibit any changes when measured in solution, micelles or XBP, we have observed the influence of the environment on the S1 lifetime of meso-zeaxanthin, which has a longer (12 ps) lifetime in XBP than in solution (9 ps). The most pronounced effect was found for vibrational relaxation in the S1 state, which is significantly slower for xanthophylls in XBP compared with micelles and solution. This effect is more pronounced for meso-zeaxanthin, suggesting a specific site of binding of this carotenoid to XBP.

The Carotenoid S1 State in LH2 Complexes from Purple Bacteria Rhodobacter sphaeroides and Rhodopseudomonas acidophila: S1 Energies, Dynamics, and Carotenoid Radical Formation

Using near-infrared femtosecond absorption spectroscopy, we have determined the S1 energies of the carotenoids spheroidene and rhodopin glucoside in LH2 complexes of purple bacteria. The S1 energies in the LH2 complexes yield values of 13400 ± 100 cm-1 for spheroidene and 12550 ± 150 cm-1 for rhodopin glucoside, which are very close to the S1 energies obtained for both carotenoids in solution. The 850 cm-1 difference between the S1 energies of these two carotenoids significantly affects the energy transfer pathways within the LH2 complexes. The S1 energy of spheroidene in the LH2 complex of Rhodobacter (Rb.) sphaeroides is high enough to allow efficient energy transfer from the S1 state to bacteriochlorophylls, resulting in a substantial shortening of the spheroidene S1 lifetime in the LH2 complex (1.7 ps) compared with the lifetime in solution (8.5 ps). Rhodopin glucoside, which occurs in Rhodopseudomonas (Rps.) acidophila, has an S1 energy in the LH2 complex too low for efficient S1-mediated energy tran...

Resolution of Benzophenone Delayed Fluorescence and Phosphorescence with Compensation for Thermal Broadening

Principal component analysis with self-modeling based on the van't Hoff equation constraint was applied to a set of benzophenone luminescence spectra from degassed carbon tetrachloride solutions in the 14.7−88.5 °C range. This analysis differs from an earlier treatment in that it was preceded by compensation of the spectra for differential broadening with increasing T. A set of Gaussians was derived as previously described [Saltiel, J.; Sears, D. F., Jr.; Turek, A. M. J. Phys. Chem. A 2001, 105, 7569−7578] that, on convolution with the spectrum for each corresponding T, creates a uniformly broadened spectral set. Significant improvements in the accuracy of the resolved delayed fluorescence and phosphorescence spectra lead to a reliable comparison between prompt and delayed fluorescence spectra that confirms the conclusion that the short S1 lifetime prevents full relaxation of emitting benzophenone singlets. The enthalpy difference for the T1/S1 equilibrium, 5.50 ± 0.06 kcal/mol, agrees with the energy dif...

Intra- and Intermolecular Photoinduced Energy and Electron Transfer between Oligothienylenevinylenes and N-Methylfulleropyrrolidine

The photophysical properties of a homologous series of oligothienylenevinylenes (nTVs) and the inter- and intramolecular photoinduced energy- and electron-transfer processes between an nTV as a donor and N-methylfulleropyrrolidine (MP−C60) as an acceptor are described as a function of oligomer length (n = 2, 3, 4, 6, 8, and 12) in apolar and polar solvents. Whereas the shorter oligomers fluoresce and have singlet excited-state [nTV(S1)] lifetimes of 280 ps (2TV) and 1360 ps (3TV), the S1 lifetimes of the longer oligomers (n > 3) are extremely short because of a fast thermal decay, resulting in negligible quantum yields for fluorescence and intersystem crossing to the triplet state [nTV(T1)]. Using photoinduced absorption (PIA) spectroscopy, we demonstrate, however, that the nTV(T1) state can be populated indirectly via intermolecular triplet energy transfer using photoexcited MP−C60 as a triplet sensitizer for n > 2. This enabled the spectral determination of the nTV (Tn ← T1) transition as a function of chain length (n = 3−12) in toluene (ε = 2.38). In a more polar solvent, o-dichlorobenzene (ODCB, ε = 9.93), the MP−C60(T1) state acts both as an oxidizing agent toward the nTVs, resulting in the formation of a metastable radical ion pair (nTV+• + MP−C60-•) for n > 2, and as a triplet sensitizer, to produce the nTV(T1) state. In addition to the intermolecular transfer reactions, we investigated the corresponding intramolecular photoinduced energy- and electron-transfer reactions in systems in which MP−C60 and nTV (n = 2−4) are covalently linked. The results indicate that, after photoexcitation of the nTV moiety, an ultrafast singlet energy transfer to MP−C60 occurs, followed by an intramolecular electron transfer. The intramolecularly charge-separated state is the lowest-energy excited state in polar media, e.g., ODCB. In apolar media, e.g., toluene, the formation of the intramolecularly charge-separated state occurs only for n > 2 and concurrent with fluorescence and intersystem crossing to the MP−C60(T1) state. The discrimination between energy and electron transfer is rationalized using a continuum model.

The relaxation of electronically excited molecules: Effects of alkyl substitution on the S2 and T1 states of benzopyranthione in solution

The effects of n-alkyl group substitution on the photophysics, photochemistry and spectroscopy of benzopyranthione (BPT), a rigid model aromatic thione, have been investigated. The absorption, emission and excitation spectra of BPT and its 2-substituted n-C4H9 (I) and n-C10H21 (II) derivatives, their S2 → S0 fluorescence and T1 → S0 phosphorescence quantum yields and their S2 and T1 lifetimes have been measured in several solvents, in the presence and absence of oxygen. Perfluorinated solvents have been used to minimize the effects of solute–solvent interactions on the excited state dynamics. The rate constants of radiative and non-radiative decay of the S2 and T1 states and of their bimolecular quenching by molecular oxygen and by ground state thione have been obtained. The spectra of I and II are nearly identical in every respect, except that the S2←S0 absorption spectrum of II exhibits a greater absorbance on its red edge, attributable to its larger density of low-frequency torsional states. The expected effect of intramolecular interaction of the longer alkyl chain with the strongly H-abstracting CS group was not observed in the S2 states of II compared with I, and this lack of effect was confirmed by showing that the excited state lifetimes were unaffected by a 40-fold change in the viscosity of the perfluoroalkane solvents. By contrast, a significantly shorter lifetime of the triplet state of II compared with I suggests that intramolecular interaction beween the hydrocarbon tail of II and the excited CS group may induce non-radiative relaxation in the longer-lived triplets.

Identifying the Pathways of Energy Transfer between Carotenoids and Chlorophylls in LHCII and CP29. A Multicolor, Femtosecond Pump−Probe Study

Spectral and kinetic information on energy transfer from carotenoids (Cars) to chlorophylls (Chls) within light-harvesting complex II (LHCII) and CP29 was obtained from femtosecond transient absorption study by using selective Car excitation (489 and 506 nm) and detecting the induced changes over a wide spectral interval (460−720 nm). By examining the evolution of entire spectral bands rather than looking at a few single traces, we were able to identify the species (pigments and/or electronic states) which participate in the energy flow, as well as the lifetimes and quantum yields of individual processes. Hence, it was found that the initially excited Car S2 state decays very fast, with lifetimes of 70−90 fs in CP29 and 100 ± 20 fs in LHCII, via two competing channels: energy transfer to Chls (60−65%) and internal conversion to the lower, optically forbidden S1 state (35−40%). In CP29, the energy acceptors are exclusively Chls a, while in LHCII, this is only valid for lutein and violaxanthin. In the latter case, neoxanthin transfers energy mostly to Chls b. In both complexes, ca. 15−20% of the initial Car excitations are transferred to Chls a via the S1 level, with a time constant of around 1 ps, thus bringing the total Car−Chl transfer efficiency to ca. 80%. Given the yield of this process and the large difference between the transfer time and the intrinsic S1 lifetime (∼20 ps), it seems that lutein is the only species active on this pathway. From the measured transfer rates, we estimated that a coupling of 280−330 cm-1 drives the transfer via the S2 route, while a coupling value of around 100 cm-1 was estimated for the S1 transfer. The Car S2 state is coupled to both Qx and Qy states of the Chl through a Coulombic mechanism; from the available structural information, we estimated the dipole−dipole contribution to be 450−500 cm-1. The S1 state is coupled to the Chl a Qy transition via an exchange and/or a Coulombic mechanism.

Effect of Terminal Groups, Polyene Chain Length, and Solvent on the First Excited Singlet States of Carotenoids

The effect of terminal groups, polyene chain length, and solvent on the first excited singlet states (S1) of carotenoids was studied by steady-state and transient optical absorption spectroscopy, and AM1 semiempirical molecular orbital calculations. The carotenoids studied were ethyl 8‘-apo-β-caroten-8‘-oate (I), ethyl 6‘-apo-β-caroten-6‘-oate (II), ethyl 4‘-apo-β-caroten-4‘-oate (III), 8‘-apo-β-caroten-8‘-nitrile (IV), 6‘-apo-β-caroten-6‘-nitrile (V), 4‘-apo-β-caroten-4‘-nitrile (VI), 8‘-apo-β-caroten-8‘-al (VII), and 6‘-apo-β-caroten-6‘-al (VIII). Solvents were 3-methylpentane (3-MP) and MeCN. The effect of solvent on the S1 absorption maxima is similar to that on the ground state (S0) absorption maxima, which suggests that both effects stem from the same type of interaction, i.e., the dispersive interaction between carotenoids and solvents. Carotenoids with terminal CHO groups have S1 absorption maxima at longer wavelenths than those with terminal CN or CO2Et groups. The S1 absorption maxima are red-shifted with increasing polyene chain length. In the nonpolar solvent 3-MP, the S1 lifetimes of carotenoids depend mainly on the polyene chain length. With a one CC bond increase, the S1 lifetime decreases by a factor of ca. 2 (ca. 24 ps for I, IV, and VII; 12 ps for II, V, and VIII; and 7 ps for III and VI). Terminal groups have little effect on the S1 lifetimes in 3-MP. However, in the polar solvent MeCN, carotenoids with terminal CHO groups have decreased S1 lifetimes (ca. 8 ps for VII and 6 ps for VIII), while carotenoids with terminal CN and CO2Et groups have essentially unchanged S1 lifetimes. This observation, along with the data of β-carotene and 7‘-apo-7‘,7‘-dicyano-β-carotene, suggests that polar solvents could decrease the S1 lifetimes of carotenoids, when, and only when, there is considerable charge transfer character in their excited states.

Ultrafast Electronic Relaxation and Hydrogen-Bond-Formation/Dissociation Dynamics of Photoexcited All-trans Retinal in Protic Solvents

The ultrafast electronic relaxation and the hydrogen-bond-formation/dissociation dynamics of photoexcited all-trans retinal in 1-butanol/cyclohexane mixed solvents have been studied by femtosecond time-resolved visible absorption spectroscopy. Four transient absorption bands, which can be assigned to the S3, S2, S1, and T1 states, were observed in neat cyclohexane. The shapes and the dynamics of these absorption bands agree very well with those reported previously for all-trans retinal in hexane. In contrast, only three transient absorption bands, which can be assigned to the S3, S2, and T1 states, were identified in the mixed solvents. The band assigned to the S2 state showed a time-dependent peak shift, which is attributed to solvent reorganization on a picosecond time scale. A kinetic analysis of the three transient absorption bands has led to the conclusion that no state-ordering change of the (n, π*) and (π, π*) states takes place in the excited singlet manifold upon hydrogen-bond formation. The 1-butanol concentration dependence of the absorption spectra shows that the free and hydrogen-bonded species coexist in the S3 and T1 states, but that all of the retinal molecules are hydrogen-bonded in the S2 state. These observations indicate that an ultrafast hydrogen-bond-formation reaction takes place during or just after the S3 → S2 internal conversion and is complete within a time scale much shorter than the S2 lifetime. Dissociation of the hydrogen bonding is most likely to take place during or after the S1 → T1 intersystem crossing and is complete within a time scale much shorter than the T1 lifetime. The observed longer lifetime of the hydrogen-bonded S2 state is consistent with the higher isomerization quantum yield in protic solvents than in aprotic nonpolar solvents.

Effect of solvent density and species on static and dynamic fluorescence Stokes shifts of coumarin 153

Solvent density dependence of the absorption and fluorescence spectra of coumarin 153 in argon (Ar), ethane (C2H6), carbon dioxide (CO2), and trifluoromethane (CF3H) has been studied at 323.2 K over a wide density region up to ρr∼3, where ρr is the reduced density by the critical density (ρc) of the solvent (ρr≡ρ/ρc). By analyzing the line shape functions of the absorption and fluorescence spectra, the first moment and the full width of the half maximum (FWHM) have been determined. Both the free energy difference (ΔG) between the ground and the S1 states and the fluorescence Stokes shift (Δν), evaluated by the first moments of the absorption and fluorescence line shape functions, show larger shift to the values in vapor in the order of Ar, C2H6, CO2, and CF3H if compared at the same ρr. There are three characteristic density regions on the density dependence of ΔG and Δν; these values show larger dependence in the lower-density region (ρr&amp;lt;0.5) and the higher-density region (ρr&amp;gt;2), than that in the medium-density region. The solvent density and species dependence of the FWHM of the absorption spectrum is quite similar to that of Δν. The density dependence of Δν and FWHM in Ar and C2H6 indicates that the intramolecular reorganization energy decreases with increasing the solvent density, while the effect is small. We have estimated the solvent reorganization energy (λS) in CO2 and CF3H by assuming no contribution of λS in C2H6 and Ar. We have demonstrated that the dielectric continuum model makes a good prediction on the relation between ΔG and λS, while it is poor on their density dependence. The invariance of ΔG and λS in the medium-density region can be interpreted as the density saturation around the solute molecule. In the lower-density region (ρr&amp;lt;∼0.5), the way of the solvation is suggested to be different from those in the medium- and high-density regions. The lifetime of S1 shows a large dependence on the solvent density and species in the very low-density region (ρr∼0.05). We have also found that slow solvation dynamics exists in the lower-density region of CF3H. The origin of the dynamics is discussed in relation with the static Stokes shift.