Photoionization versus photoheterolysis of all-trans-retinol. The effects of solvent and laser radiation intensity

The time-resolved formation of the retinyl carbocation from all-trans-retinol and all-trans-retinol acetate was studied by use of picosecond flash photolysis. From both precursors, the retinyl cation is produced by heterolytic C-O bond cleavage in solvents of medium polarity (acetonitrile, tetrahydrofuran, propanol with Reichardt polarity parameter ET(N) approximately 0.5) and high polarity (EtOH, MeOH, TFE, HFIP, ET(N) > 0.6) during the laser pulse (< or =5 ps) where its lifetime is >10 ns. The absorption maximum of the cation at early times (t < 100 ps) is at lambda = 590-600 nm; it shifts to shorter wavelengths (Deltalambda = 5-10 nm) within 1-10 ns. This spectral shift is suggested to be due to contact ion pair --> solvent-separated ion pair --> free-ion transformation. The quantum yield of cation formation phi(cat) is independent of excitation wavelength (213, 266 or 355 nm). Photoheterolysis proceeds via a one-quantum process. In chlorinated solvents, i.e. n-BuCl, 1,2-dichloroethane, chloroform or CCl(4), formation of the retinol radical cation (which is characterized by a peak at 610 nm and further absorption maxima at approximately 840 and approximately 940 nm) by intermolecular electron transfer to the solvent molecules was detected. The radical cation lifetime in all these solvents is 1.5-2 ns, except for CCl(4) where it is 0.25 ns. The formation of the radical cation or cation was not detected in the low polarity solvents: cyclohexane, hexane, dioxane and p-xylene. However, in solvents of medium and high polarity, at high radiation intensities the radical cation may form in addition to the cation (as a result of two-quantum ionization). DFT calculations confirm our experimental results. The rate of retinol S(1) depopulation (k = 0.3-1 x 10(9) s(-1)) is almost independent of the solvent polarity in the range from cyclohexane to methanol. In highly polar solvents (ET(N) > 0.9) the rate increases to (0.5-5) x 10(10) 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.     

Simulation of the photodeactivation of formamide in the nO-pi* and pi-pi* states: an ab initio on-the-fly surface-hopping dynamics study

The short-time photodynamics (1 ps) of formamide in its low-lying singlet excited n(O)-pi(*) and pi-pi(*) states have been investigated by the direct trajectory surface-hopping method based on multiconfigurational ab initio calculations. The simulations showed that in both states, the primary deactivation process is C-N bond dissociation. In the ground state, the energy is transferred to (a) translational motion of the HCO and NH(2) fragments, (b) additional C-H dissociation from the vibrationally hot HCO fragment, or (c) formation of NH(3) and CO. In addition to the C-N dissociation pathway, C-O bond fission is found to be an additional primary deactivation path in the pi-pi(*) dynamics. From fractional occupations of trajectories, lifetimes of formamide were estimated: tau(S(1))=441 fs and tau(S(2))=66 fs.     

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.     

Ultrafast study of 9-diazofluorene: direct observation of the first two singlet states of fluorenylidene

Ultrafast photolysis of 9-diazofluorene (DAF) produces a broadly absorbing transient within the instrument time resolution (300 fs), which is assigned to an excited state of the diazo compound. The diazo excited state fragments to form fluorenylidene (Fl) in both its lowest energy singlet state (1Fl, 405-430 nm, depending on the solvent) and a higher energy singlet state (370 nm, 1Fl*). The excited singlet carbene has a lifetime of 20.9 ps in acetonitrile and decays to the lower energy singlet state (1Fl), which relaxes to the triplet ground state (3Fl) in acetonitrile, cyclohexane, benzene, and hexafluorobenzene. The equilibrium mixture of singlet and triplet fluorenylidene reacts with these solvents. Singlet fluorenylidene reacts with methanol and cyclohexene in competition with relaxation to 3Fl. One of the reaction products in methanol is the 9-fluorenyl cation. The rate of intersystem crossing (ISC) in hexafluorobenzene and other halogenated solvents is remarkably slow given that carbene ISC rates are generally fastest in nonpolar solvents. An explanation of this effect is advanced.     

Excited-state symmetry breaking in cofacial and linear dimers of a green perylenediimide chlorophyll analogue leading to ultrafast charge separation

Photoexcitation of chromophoric dimers constrained to a symmetric pi-stacked geometry by their molecular structure usually produces excimers independent of solvent polarity, while dimers with edge-to-edge perpendicular pi systems undergo excited-state symmetry breaking in highly polar solvents leading to intradimer charge separation. We present direct evidence for symmetry breaking in the lowest excited singlet state of a symmetric cofacial dimer of 1,7-bis(pyrrolidin-1'-yl)-perylene-3,4:9,10-bis(dicarboximide) (5PDI) in the low polarity solvent toluene to produce a radical ion pair quantitatively. This dimer, cof-5PDI2, was synthesized by attaching two 5PDI chromophores via imide groups to a xanthene spacer. For comparison, a linear symmetric dimer, lin-5PDI2, was prepared in which the 5PDI chromophores are linked end-to-end via a N-N single bond between their imides. The edge-to-edge pi systems of the 5PDI chromophores within lin-5PDI2 are perpendicular to one another. Ground-state absorption spectra of both 5PDI dimers show exciton coupling, which is consistent with the orientation of the 5PDI chromophores relative to one another. Ultrafast transient absorption spectroscopy following excitation of the dimers with 700 nm, 100 fs laser pulses shows that quantitative intradimer electron transfer occurs in cof-5PDI2 in toluene with tau = 0.17 ps followed by charge recombination to the ground state with tau = 222 ps. Similar measurements on lin-5PDI2 reveal that photoinduced electron transfer does not occur in toluene, but occurs in more polar solvents such as 2-methyltetrahydrofuran, wherein tau = 55 ps for charge separation and tau = 99 ps for charge recombination. Excited-state symmetry breaking in 5PDI dimers provides new routes to biomimetic charge separation and storage assemblies that can be more easily prepared and modified than those based on multiple tetrapyrrole macrocycles.     

Evidence for an intramolecular charge transfer state in 12'-apo-beta-caroten-12'-al and 8'-apo-beta-caroten-8'-al: influence of solvent polarity and temperature

The ultrafast excited-state dynamics of two carbonyl-containing carotenoids, 12'-apo-beta-caroten-12'-al and 8'-apo-beta-caroten-8'-al, have been investigated by transient absorption spectroscopy in a systematic variation of solvent polarity and temperature. In most of the experiments, 12'-apo-beta-caroten-12'-al was excited at 430 nm and 8'-apo-beta-caroten-8'-al at 445 or 450 nm via the S0 --> S2 (11Ag- --> 11Bu+) transition. The excited-state dynamics were then probed at 860 nm for 12'-apo-beta-caroten-12'-al and at 890 or 900 nm for 8'-apo-beta-caroten-8'-al. The temporal evolution of all transient signals measured in this work can be characterized by an ultrafast decay of the S2 --> SN absorption at early times followed by the formation of a stimulated emission (SE) signal, which subsequently decays on a much slower time scale. We assign the SE signal to a low-lying electronic state of the apocarotenals with intramolecular charge-transfer character (ICT --> S0). This is the first time that the involvement of an ICT state has been detected in the excited-state dynamics of a carbonyl carotenoid in nonpolar solvents such as n-hexane or i-octane. The amplitude ratio of ICT-stimulated emission to S2 absorption was weaker in nonpolar solvents than in polar solvents. We interpret the results in terms of a kinetic model, where the S1 and ICT states are populated from S2 through an ultrafast excited-state branching reaction (tau2 < 120 fs). Delayed formation of a part of the stimulated emission is due to the transition S1 --> ICT (tau3 = 0.5-4.1 ps, depending on the solvent), which possibly involves a slower backward reaction ICT --> S1. Determinations of tau1 were carried out for a large set of solvents. Especially in 12'-apo-beta-caroten-12'-al, the final SE decay, assigned to the nonradiative relaxation ICT --> S0, was strongly dependent on solvent polarity, varying from tau1 = 200 ps in n-hexane to 6.6 ps in methanol. In the case of 8'-apo-beta-caroten-8'-al, corresponding values were 24.8 and 7.6 ps, respectively. This indicates an increasing stabilization of the ICT state with increasing solvent polarity, resulting in a decreasing ICT-S0 energy gap. Tuning the pump wavelength from the blue wing to the maximum of the S0 --> S2 absorption band resulted in no change of tau1 in acetone and methanol. Additional measurements in methanol after excitation in the red edge of the S0 --> S2 band (480-525 nm) also show an almost constant tau1 with only a 10% reduction at the largest probe wavelengths. The temperature dependence of the tau1 value of 12'-apo-beta-caroten-12'-al was well described by Arrhenius-type behavior. The extracted apparent activation energies for the ICT --> S0 transitions were in general small (on the order of a few times RT), which is in the range expected for a radiationless process.     

Methoxy-substituted stilbenes, styrenes, and 1-arylpropenes: photophysical properties and photoadditions of alcohols

The photochemistry of trans-stilbene and four methoxy-substituted stilbene derivatives has been investigated in a variety of solvents. The fluorescence of all five trans isomers was quenched by 2,2,2-trifluoroethanol (TFE). Upon irradiation of the five substrates in TFE, the products derived from photoaddition of the solvent were detected. Nuclear magnetic resonance spectroscopy of the products formed by irradiation in TFE-OD indicated that the proton and nucleophile are attached to two adjacent atoms of the original alkene double bond. Irradiation of the corresponding methoxy-substituted styrenes and trans-1-arylpropenes in TFE produced the analogous solvent adducts. The photoaddition of TFE proceeded with the general order of reactivity: styrenes > trans-1-arylpropenes > trans-stilbenes. Transient carbocation intermediates were observed following laser flash photolysis of the stilbenes in 1,1,1,3,3,3-hexafluoro-2-propanol (HFIP). The results are consistent with a mechanism that involves photoprotonation of the substrates by TFE or HFIP, followed by nucleophilic trapping of short-lived carbocation intermediates. Compared to the other stilbene derivatives, trans-3,5-dimethoxystilbene displayed a large quantum yield of fluorescence and a low quantum yield of trans-cis isomerization in polar organic solvents. The unique photophysical properties of trans-3,5-dimethoxystilbene are attributed to formation of a highly polarized charge-transfer excited state (mu(e) = 13.2 D).     

Ultrafast transient lens spectroscopy of various C40 carotenoids: lycopene, beta-carotene, (3R,3'R)-zeaxanthin, (3R,3'R,6'R)-lutein, echinenone, canthaxanthin, and astaxanthin

The ultrafast internal conversion (IC) dynamics of seven C(40) carotenoids have been investigated at room temperature in a variety of solvents using two-color transient lens (TL) pump-probe spectroscopy. We provide comprehensive data sets for the carbonyl carotenoids canthaxanthin, astaxanthin, and-for the first time-echinenone, as well as new data for lycopene, beta-carotene, (3R,3'R)-zeaxanthin and (3R,3'R,6'R)-lutein in solvents which have not yet been investigated in the literature. Measurements were carried out to determine, how the IC processes are influenced by the conjugation length of the carotenoids, additional substituents and the polarity of the solvent. TL signals were recorded at 800 nm following excitation into the high energy edge of the carotenoid S2 band at 400 nm. For the S2 lifetime solvent-independent upper limits on the order of 100-200 fs are estimated for all carotenoids studied. The S1 lifetimes are in the picosecond range and increase systematically with decreasing conjugation length. For instance, in the sequence canthaxanthin/echinenone/beta-carotene (13/12/11 double bonds) one finds tau1 approximately 5, 7.7 and 9 ps for the S1-->S0 IC process, respectively. Hydroxyl groups not attached to the conjugated system have no apparent influence on tau1, as observed for canthaxanthin/astaxanthin (tau1 approximately 5 ps in both cases). For all carotenoids studied, tau1 is found to be insensitive to the solvent polarity. This is particularly interesting in the case of echinenone, canthaxanthin and astaxanthin, because earlier measurements for other carbonyl carotenoids like, e.g., peridinin partly showed dramatic differences. The likely presence of an intramolecular charge transfer state in the excited state manifold of C40 carbonyl carotenoids, which is stabilized in polar solvents, has obviously no influence on the measured tau1.     

Extremely strong solvent dependence of the S1--> S0 internal conversion lifetime of 12'-apo-beta-caroten-12'-al

The ultrafast internal conversion (IC) dynamics of the carbonyl carotenoid 12'-apo-beta-caroten-12'-al has been investigated in solvents of varying polarity using time-resolved femtosecond transient absorption spectroscopy. The molecules were excited to the S(2) state by a pump beam of either 390 or 470 nm. The subsequent intramolecular dynamics were detected at several probe wavelengths covering the S(0)--> S(2) and S(1)--> S(n) bands. For the S(1)--> S(0) internal conversion process, a remarkably strong acceleration with increasing polarity was found, e.g., lifetimes of tau(1) = 220 ps (n-hexane), 91 ps (tetrahydrofuran) and 8.0 ps (methanol) after excitation at 390 nm. The observation can be rationalized by the formation of a combined S(1)/ICT (intramolecular charge transfer) state in the more polar solvents. The effect is even stronger than the strongest one reported so far in the literature for peridinin. Addition of lithium salts to a solution of 12'-apo-beta-caroten-12'-al in ethanol leads only to small changes of the IC time constant tau(1). In addition, we estimate an upper limit for the time constant tau(2) of the S(2)--> S(1) internal conversion process of 300 fs in all solvents.     

Dynamics of carbocation formation in the photolysis of 1,2,2,3-tetramethyl-1,2-dihydroquinoline in alcohols

Dynamics of the formation of the carbocation in the ground state as a result of photoinduced proton transfer from a solvent to the excited state of 1,2,2,3-tetramethyl-1,2-dihydroquinoline (3MDHQ) in MeOH and 2,2,2-trifluoroethanol (TFE) was registered by pump-probe laser photolysis (λ_pump = 310 nm) with femtosecond time resolution. The lifetimes of the excited singlet state of 3MDHQ τ = 115 and 780 ps were determined in TFE and MeOH, respectively. The transient species with absorption spectrum corre-sponding to the spectrum of the carbocation from 3MDHQ (λ_max = 480 nm) is generated at time delays lower than 500 fs from the unrelaxed excited singlet state.     

Studies of 9-fluorenyl carbocations. intramolecular hydride migration in a substituted 9-fluorenyl carbocation

The substituted fluorenyl cation, 9-(diphenylmethyl)fluoren-9-yl cation (4), is formed under stable ion conditions (low temperature/strong acid) from its corresponding alcohol 3. This ion is transformed to a substituted diphenyl methyl cation 8 at ambient temperature via an apparent 1,2-hydrogen shift. Irradiation of 9-(diphenylmethyl)fluoren-9-ol in methanol gives products derived from the corresponding cation along with radical-derived products from C-C and C-O homolysis processes. The laser flash photolysis of this alcohol gave a transient corresponding to cation 4. All of the photoproducts are derived from cation 4 or radical pathways. High level MO calculations point to a high barrier (23.8 kcal x mol(-1)) for the 1,2-hydride shift. This barrier is the consequence of the minimum energy conformation of this fluorenyl cation which is less than ideal for the periplanar geometry necessary for this process.     

Exciplex and radical ion intermediates in the photochemical reaction of 9-cyanophenanthrene with 2,3-dimethyl-2-butene

The photochemical reaction of 9-cyanophenanthrene and 2,3-dimethyl-2-butene, first reported by Mizuno, Pac and Sakurai, has been reinvestigated. The formation of a [2+2]-cycloadduct via a singlet exciplex is the exclusive reaction in the nonpolar solvents benzene and ethyl acetate. Photochemical behavior in polar solvents is far more complicated than previously reported. Mechanisms consistent with the effects of solvent polarity, methanol concentration, methanol deuteration, and light intensity upon product yields are proposed. Formation of a 9-cyanophenthrene anion radical and 2,3-dimethyl-2-butene cation radical is the primary photoinitiated process in polar solvent. The cation radical can undergo deprotonation to yield an allyl radical or nucleophilic attack by methanol to yield a methoxyalkyl radical. Covalent bonding of these radicals and the 9-cyanophenanthrene anion radical gives rise to the acyclic adducts obtained in polar solvents. The anion radical can also be protonated, leading ultimately to the formation of 9,10-dihydro-9-cyanophenanthrene.     

Absorption Spectra and Photoreactivity of Para-aminobenzophenone by Time-dependent Density Functional Theory

The absorption spectral properties of para-aminobenzophenone (p-ABP) were investigated in gas phase and in solution by time-dependent density functional theory. Calculations suggest that the singlet states vary greatly with the solvent polarities. In various polar solvents, including acetonitrile, methanol, ethanol, dimethyl sulfoxide, and dimethyl formamide, the excited S1 states with charge transfer character result from π→π* transitions. However, in nonpolar solvents, cyclohexane, and benzene, the S1 states are the result of n→π* transitions related to local excitation in the carbonyl group. The excited T1 states were calculated to have ππ* character in various solvents. From the variation of the calculated excited states, the band due to π→π* transition undergoes a redshift with an increase in solvent polarity, while the band due to n→π* transition undergoes a blueshift with an increase in solvent polarity. In addition, the triplet yields and the photoreactivities of p-ABP in various solvents are discussed.     

Transient lens spectroscopy of ultrafast internal conversion processes in citranaxanthin

The ultrafast internal conversion (IC) dynamics of the apocarotenoid citranaxanthin have been studied for the first time by means of two-color transient lens (TL) pump-probe spectroscopy. After excitation into the high-energy edge of the S2 band by a pump pulse at 400 nm, the subsequent intramolecular processes were probed at 800 nm. Experiments were performed in a variety of solvents at room temperature. Upper limits for the S2 lifetime tau2 on the order of 100-200 fs are estimated. The S1 lifetime tau1 varies only slightly between solvents (10-12 ps), and the only clear decrease is observed for methanol (8.5 ps). The findings are consistent with earlier results from transient absorption studies of other apocarotenoids and carotenoid ketones and transient lens experiments of C40 carbonyl carotenoids. Possible reasons for the observed weak solvent dependence of tau1 for citranaxanthin are discussed.     

Toward efficient nanoporous catalysts: controlling site-isolation and concentration of grafted catalytic sites on nanoporous materials with solvents and colorimetric elucidation of their site-isolation

We report that the polarity and dielectric constants of solvents used for grafting organosilanes on mesoporous materials strongly affect the concentration of grafted organic groups, the degree of their site-isolation, and the catalytic properties of the resulting materials. Polar and nonpolar organosilanes as well as polar-protic, dipolar-aprotic, and nonpolar solvents were investigated. Polar-protic solvents, which have high dielectric constants, resulted in smaller concentrations ( approximately 1-2 mmol/g) of polar organic groups such as 3-aminopropyl groups, higher surface area materials, site-isolated organic groups, and more efficient catalytic properties toward the Henry reaction of p-hydroxybenzaldehyde with nitromethane. On the other hand, dipolar-aprotic and nonpolar solvents resulted in larger concentrations ( approximately 2-3 mmol/g) of grafted polar functional groups, lower-to-higher surface area materials, more densely populated catalytic groups, and poor-to-efficient catalytic properties toward the Henry reaction. Both the polar-protic and dipolar-aprotic solvents resulted in significantly lower concentration of grafted groups for nonpolar organosilanes such as (3-mercaptopropyl)trimethoxysilane compared to corresponding grafting of the polar amino-organosilanes. The relationship between the solvent properties and the percentage and degree of site-isolation of the grafted functional groups was attributed to differences in solvation of the organosilanes and silanols in various solvents and possible hydrogen-bonding between the organsilanes and the solvents. The degree of site-isolation of the amine groups, which affect the material's catalytic properties, was elucidated by a new colorimetric method involving probing of the absorption maxima (lambdamax) on the d-d electronic spectrum of Cu2+ complexes with the amine-functionalized materials and the colors of the samples. The absorption lambdamax and the colors of the materials were found to be uniquely dependent on the type of solvents used for grafting the organoamines. For instance, the monoamine- and diamine-functionalized samples grafted in methanol resulted in pale blue and light purple colors with lambdamax at approximately 720 and 650 nm, respectively. These correspond to CuNO5 and CuN2O4 structures, respectively, which are indicative of the presence of site-isolated organoamines in samples grafted in methanol. The monoamine and diamine samples grafted in toluene resulted in purple and deep purple colors with lambdamax at approximately 590 and 630 nm, respectively. These correspond to CuN2O4 and CuN4O2, which are indicative of the presence of closely spaced organoamines in samples grafted in toluene. The samples grafted in isopropanol gave colors and lambdamax intermediate between those of samples grafted in toluene and methanol.     

The spectrophysics of warfarin: implications for protein binding

The photophysical behavior of the isomers of the anticoagulant drug warfarin in various solvents and solvent mixtures was investigated using absorption, 1H NMR, and steady-state and time-resolved fluorescence spectroscopies in conjunction with B3LYP-based theoretical treatments. Complex absorption patterns were observed, indicative of the presence of different isomers of warfarin in the various solvents studied. In alkaline aqueous solution, the deprotonated open side form of warfarin is highly dominant and only one S0-->S1 singlet transition could be observed in the absorption spectrum centered at 320 nm. These observations were supported by theoretical density functional calculations (B3LYP) in which the geometries of nine isomers of warfarin were optimized and their respective eight lowest singlet and three lowest triplet excitation energy levels were predicted. Examination of the fluorescence excitation and emission spectra of the isomers in nonpolar and polar organic solvents showed the presence of the deprotonated open side chain form of warfarin in 2-propanol, ethanol, and acetonitrile. Time-resolved fluorescence experiments revealed a short decay time constant, tau1, in all solvents studied while in more polar environments a second longer one, tau2, was evident varying between 0.5 and 1.6 ns depending on solvent polarity. The variation of number and length of fluorescence lifetimes as a function of solvent environment has provided a tool for examining warfarin protein binding. Studies on the binding of warfarin to human serum albumin (HSA) have been undertaken, and different modes of binding were observed which are indicative of binding to the anion-selective Sudlow I and, second, a lower affinity mode of interaction.     

Solvent-dependent ultrafast internal conversion dynamics of n'-apo-beta-carotenoic-n'-acids (n = 8, 10, 12)

The ultrafast internal conversion dynamics of 12'-apo-beta-carotenoic-12'-acid (12'CA), 10'-apo-beta-carotenoic-10'-acid (10'CA) and 8'-apo-beta-carotenoic-8'-acid (8'CA) have been investigated by femtosecond pump-probe spectroscopy. The three apocarotenoic acids were excited to the S(2) state with different excess energies. Time constants tau(1) for the IC process S(1)/ICT --> S(0) were measured by probing the dynamics at 390 nm (S(0) --> S(2)), 575 nm (S(1)/ICT --> S(n)), 850, 860 and 890 nm (S(2) --> S(n) and S(1)/ICT --> S(0)). In nonpolar solvents, the observed reduction of the tau(1) values with increasing conjugation length of the acids is consistent with a reduction of the energy gap between the S(1)/ICT and S(0) states. The values are in good agreement with those of the corresponding apocarotenals studied previously in our groups. In polar solvents, a pronounced reduction of tau(1) values was observed for 12'CA, however the behavior was different from that observed for the respective aldehyde 12'-apo-beta-caroten-12'-al studied previously: First, the degree of tau(1) reduction in methanol was milder for 12'CA (218 --> 55 ps) than for 12'-apo-beta-caroten-12'-al (220 --> 8 ps). Secondly, for 12'CA the plateau of solvent independent tau(1) values extended further into the mid-polar range (up to 0.5 on the Deltaf scale) than previously observed for the 12'-aldehyde. For 10'CA the polarity effect on the tau(1) values was weaker ( approximately 71 ps in n-hexane and 34 ps in methanol) and for 8'CA it disappeared completely ( approximately 24 ps averaged over all solvents). The polarity-induced reduction of tau(1) is likely due to the stabilization of an intramolecular charge transfer state in polar solvents. This S(1)/ICT state is also responsible for the stimulated emission in the near IR, which has been observed in this specific class of carotenoids with a terminal carboxyl group for the first time. The occurrence of stimulated emission in the near IR region is also consistent with the steady-state fluorescence spectra which are reported along with the absorption spectra of these species. Possible reasons for the different behavior of the apocarotenoic acids compared to the respective aldehydes are discussed.     

Exploring 12'-apo-beta-carotenoic-12'-acid as an ultrafast polarity probe for ionic liquids

The ultrafast excited-state dynamics of the carbonyl-containing carotenoid 12'-apo-beta-carotenoic-12'-acid (12'CA) have been used for probing the microscopic environment in various ionic liquids (ILs). The following IL cations were investigated: 1,3-di-n-alkyl-imidazolium featuring different n-alkyl chain lengths and also additional methylation at the C2 position, triethylsulfonium, as well as two tetraalkylammonium ions. These were combined with different anions: [BF4]-, [PF6]-, ethyl sulfate ([EtOSO3]-), and bis(trifluoromethylsulfonyl)amide ([Tf2N]-). The probe molecule was excited via the S0 --> S2 transition at 425 or 430 nm, and the characteristic stimulated emission decay of the low-lying excited electronic S1/ICT (intramolecular charge transfer) state of 12'CA was monitored in the near IR (850 or 860 nm). Its lifetime tau1 is sensitive to the micropolarity-induced stabilization of S1/ICT relative to S0. The lifetime tau1 of the S1/ICT state varies only moderately in all ionic liquids studied here ( approximately 40-110 ps), which lies in the range between ethanol (109 ps) and methanol (49 ps). While organic solvents show an excellent correlation of tau1 with the solvent polarity function Deltaf = (epsilon - 1)/(epsilon + 2) - (n2 - 1)/(n2 + 2), where epsilon and n are the static dielectric constant and the refractive index of the solvent, respectively, this is not the case for ILs. This is due to dominant local electrostatic probe-cation interactions which cannot be easily quantified by macroscopic quantities. Methylation at the C2 position of 1,3-di-n-alkyl-imidazolium reduces the accessibility of the cation and therefore the electrostatic stabilization of the probe, resulting in an increase of tau1. A similar increase is observed upon extension of one of the n-alkyl chains from ethyl to n-decyl. Tetraalkylammonium ILs show an increased tau1 probably due to their more delocalized positive charge which cannot interact so favorably with the probe, in contrast to trialkylsulfonium ILs where the charge is more localized on the sulfur atom. The dependence of tau1 on the IL anion is much weaker, the only notable exception being [EtOSO3]-, where 12'CA experiences a less polar local environment than expected on the basis of extrapolated static dielectric constants. This is possibly due to the competition of the anion and probe for the cation interaction sites. Considerable electrostatic probe-cation interactions can be also introduced by addition of large amounts of LiClO4 salt to ethanol and diethyl ether. In this case, tau1 also strongly decreases, indicating an efficient coordination of Li+ cation(s) with the carbonyl oxygen at the negative end of the probe molecule. The S1/ICT --> S0 internal conversion of the 12'CA probe in ILs accelerates with increasing temperature, which can be characterized by an apparent activation energy of a few kJ mol-1, which is expected for energy-dependent nonradiative processes.     

Solvent and solvent isotope effects on the vibrational cooling dynamics of a DNA base derivative

Vibrational cooling by 9-methyladenine was studied in a series of solvents by femtosecond transient absorption spectroscopy. Signals at UV and near-UV probe wavelengths were assigned to hot ground state population created by ultrafast internal conversion following electronic excitation by a 267 nm pump pulse. A characteristic time for vibrational cooling was determined from bleach recovery signals at 250 nm. This time increases progressively in H2O (2.4 ps), D2O (4.2 ps), methanol (4.5 ps), and acetonitrile (13.1 ps), revealing a pronounced solvent effect on the dissipation of excess vibrational energy. The trend also indicates that the rate of cooling is enhanced in solvents with a dense network of hydrogen bonds. The faster rate of cooling seen in H2O vs D2O is noteworthy in view of the similar hydrogen bonding and macroscopic thermal properties of both liquids. We propose that the solvent isotope effect arises from differences in the rates of solute-solvent vibrational energy transfer. Given the similarities of the vibrational friction spectra of H2O and D2O at low frequencies, the solvent isotope effect may indicate that a considerable portion of the excess energy decays by exciting relatively high frequency (>/=700 cm-1) solvent modes.