Ps In Methanol Research Articles

Thermalization of the Kerr index of refraction in acetone and methanol using femtosecond pump-probe scattering spectroscopy

Femtosecond time-resolved pump-probe scattering is used to temporally investigate the index thermalization Δn(t) of the molecular motions associated with the Kerr nonlinear index n2. This is done using an intense 300 fs 1034 nm pump pulse and temporally probed by a 517 nm and supercontinuum pulse in acetone and methanol. The optically pumped molecular states change the index of refraction and through the processes of Raman and Rayleigh scattering are shown to engage in a 4-step energy relaxation process. Here, the pump first initiates the Kerr effect as well as populate resonant vibrational molecular motions, which act as a “mother” energy state. The electronic component of the Kerr effect is shown to couple with these vibrational modes through the process of Born-Oppenheimer coupling, which can be measured through the Raman scattering of the probe. The “mother” coupled state then decays into “daughter” anharmonic non-resonant states, which then further decay into “granddaughter” and thermal bath states. The population of these states alter the index of refraction in time, Δn(t), and can be measured through the changing in the probe beam’s Rayleigh scattering. This thermalization process takes ∼5.5 ps in acetone and ∼3.7 ps in methanol. The scattering signals from each Δn(t) thermalization stage in the localized region, (“mother”, to “daughter”, to “granddaughter”, to the bath states, to the ground state), are fitted to theoretical decay equations that show the rise and fall times of the energy decay routes.

Influence of the Solvent Environment on the Ultrafast Relaxation Pathways of a Sunscreen Molecule Diethylamino Hydroxybenzoyl Hexyl Benzoate.

The excited-state dynamics of photoexcited diethylamino hydroxybenzoyl hexyl benzoate (DHHB), a UVA absorber widely used in sunscreen formulations, are studied with transient electronic and vibrational absorption spectroscopy methods in four different solvents. In the polar solvents methanol, dimethyl sulfoxide (DMSO), and acetonitrile, strong stimulated emission (SE) is observed at early time delays after photoexcitation at a near-UV wavelength of λex = 360 nm, and decays with time constants of 420 fs in methanol and 770 fs in DMSO. The majority (∼95%) of photoexcited DHHB returns to the ground state with time constants of 15 ps in methanol and 25 ps in DMSO. In the nonpolar solvent cyclohexane, ∼ 98% of DHHB photoexcited at λex = 345 nm relaxes to the ground state with a ∼ 10 ps time constant, and the SE is weak. DHHB preferentially adopts an enol form in its ground S0 state, but excited state absorption (ESA) bands seen in TEAS are assigned to both the S1-keto and S1-enol forms, indicating a role for ultrafast intramolecular excited state hydrogen transfer (ESHT). This ESHT is inhibited by polar solvents. The two S1 tautomers decay with similar time scales to the observed recovery of ground state population. For molecules that avoid ESHT, torsion around a central C-C bond minimizes the S1-enol energy, quenches the SE, and is proposed to lead to a conical intersection with the S0 state that mediates the ground state recovery. A competing trans-enol isomeric photoproduct is observed as a minor competitor to parent recovery in polar solvents. Evidence is presented for triplet (T1) enol production in polar solvents, and for T1 quenching by octocrylene, a common UVB absorber sunscreen additive. The T1 keto form is observed in cyclohexane solution.

New Insight into the Origin of the Red/Near-Infrared Intense Fluorescence of a Crystalline 2-Hydroxychalcone Derivative: A Comprehensive Picture from the Excited-State Femtosecond Dynamics.

Fluorescence upconversion and transient absorption techniques are used to explain the source of the intense red/near-infrared emission of crystalline 4-dimethylamino-2'-hydroxychalcone. We found that the initially excited enol form undergoes tautomerization in 3 ps to form the keto tautomer. The latter is stable in the ground state as a consequence of J-type aggregation in the crystal packing and is manifested in an absorption peak at 550 nm that spectrally overlaps with the short-lived enol emission, leading to self-reabsorption and adding a factor to the complete depletion of the enol emission. Relaxation of the keto tautomer takes place in the form of intense fluorescence (600-750 nm) with 1.7 ns lifetime. The different spectroscopy in solution is due to vibrational cooling (300 fs), followed by solvation dynamics (5 ps in methanol) and twisting of the hydroxyphenyl ring (16 ps), before relaxation of the enol tautomer in the form of weak green fluorescence with 350 ps lifetime.

Ultrafast Excited-State Deactivation of the Bacterial Pigment Violacein.

The photophysical properties of the natural pigment violacein extracted from an Antarctic organism adapted to high exposure levels of UV radiation were measured in a combined steady-state and time-resolved spectroscopic study for the first time. In the low-viscosity solvents methanol and acetone, violacein exhibits low fluorescence quantum yields on the order of 1 × 10-4, and femtosecond transient absorption measurements reveal excited-state lifetimes of 3.2 ± 0.2 and 4.5 ± 0.2 ps in methanol and acetone, respectively. As solvent viscosity is increased, both the fluorescence quantum yield and excited-state lifetime of this intensely colored pigment increase dramatically, and stimulated emission decays 30-fold more slowly in glycerol than in methanol at room temperature. Excited-state deactivation is suggested to occur via a molecular-rotor mechanism in which torsion about an interring bond leads to a conical intersection with the ground state.

Tracking Ultrafast Vibrational Cooling during Excited-State Proton Transfer Reaction with Anti-Stokes and Stokes Femtosecond Stimulated Raman Spectroscopy.

Energy dissipation following photoexcitation is foundational to photophysics and chemistry. Consequently, understanding such processes on molecular time scales holds paramount importance. Femtosecond stimulated Raman spectroscopy (FSRS) has been used to study the molecular structure-function relationships but usually on the Stokes side. Here, we perform both Stokes and anti-Stokes FSRS to track energy dissipation and excited-state proton transfer (ESPT) for the photoacid pyranine in aqueous solution. We reveal biphasic vibrational cooling on fs-ps time scales during ESPT. Characteristic low-frequency motions (<800 cm-1) exhibit initial energy dissipation (∼2 ps) that correlates with functional events of forming contact ion pairs via H-bonds between photoacid and water, which lengthens to ∼9 ps in methanol where ESPT is inhibited. The interplay between photoinduced dissipative and reactive channels is implied. Thermal cooling to bulk solvent occurs on the ∼50 ps time scale. These results demonstrate the combined Stokes and anti-Stokes FSRS as a powerful toolset to elucidate structural dynamics.

Charge-transfer-to-solvent reactions from I(-) to water, methanol, and ethanol studied by time-resolved photoelectron spectroscopy of liquids.

The charge-transfer-to-solvent (CTTS) reactions from iodide (I(-)) to H2O, D2O, methanol, and ethanol were studied by time-resolved photoelectron spectroscopy of liquid microjets using a magnetic bottle time-of-flight spectrometer with variable pass energy. Photoexcited iodide dissociates into a weak complex (a contact pair) of a solvated electron and an iodine atom in similar reaction times, 0.3 ps in H2O and D2O and 0.5 ps in methanol and ethanol, which are much shorter than their dielectric relaxation times. The results indicate that solvated electrons are formed with minimal solvent reorganization in the long-range solvent polarization field created for I(-). The photoelectron spectra for CTTS in H2O and D2O-measured with higher accuracy than in our previous study [Y. I. Suzuki et al., Chem. Sci. 2, 1094 (2011)]-indicate that internal conversion yields from the photoexcited I(-*) (CTTS) state are less than 10%, while alcohols provide 2-3 times greater yields of internal conversion from I(-*). The overall geminate recombination yields are found to be in the order of H2O > D2O > methanol > ethanol, which is opposite to the order of the mutual diffusion rates of an iodine atom and a solvated electron. This result is consistent with the transition state theory for an adiabatic outer-sphere electron transfer process, which predicts that the recombination reaction rate has a pre-exponential factor inversely proportional to a longitudinal solvent relaxation time.

Remarkable effects of solvent and substitution on the photo-dynamics of cytosine: a femtosecond broadband time-resolved fluorescence and transient absorption study.

Cytosine (Cyt) among all the nucleic acid bases features the most complex and least understood nonradiative deactivation, a process that is crucially important for its photostability. Herein, the excited state dynamics of Cyt and a series of its N1- and C5-derivatives, including the full set of Cyt nucleosides and nucleotides in DNA and RNA and the nucleosides of 5-methyl cytosine, 5-methylcytidine and 2'-deoxy-5-methylcytidine, have been investigated in water and in methanol employing femtosecond broadband time-resolved fluorescence coupled with fs transient absorption spectroscopy. The results reveal remarkable state-specific effects of the substitution and solvent in tuning distinctively the timescales and pathways of the nonradiative decays. For Cyt and the N1-derivatives, the nonradiative deactivations occur in a common two-state process through three channels, two from the light-absorbing ππ* state with respectively the sub-picosecond (∼0.2 ps) and the picosecond (∼1.5 ps) time constant, and the third is due to an optically dark nπ* state with the lifetime ranging from several to hundreds of picoseconds depending on solvents and substitutions. Compared to Cyt, the presence of the ribose or deoxyribose moiety at the N1 position of N1-derivatives facilitates the formation of the nπ* at the sub-picosecond timescale and at the same time increases its lifetime by ∼4-6 times in both water and methanol. In sharp contrast, the existence of the methyl group at the C5 position of the C5-derivatives eliminates completely the sub-picosecond ππ* channel and the channel due to the nπ*, but on the other hand slows down the decay of the ππ* state which after relaxation exhibits a single time constant of ∼4.1 to ∼7.6 ps depending on solvents. Varying the solvent from water to methanol accelerates only slightly the decay of the ππ* state in all the compounds; while for Cyt and its N1-derivatives, this change of solvent also retards strongly the nπ* channel, prolongs its lifetime from such as ∼7.7 ps in water to ∼52 ps in methanol for Cyt and from ∼30 ps in water to ∼186 ps in methanol for deoxycytidine. The spectral signatures we obtained for the ππ* and the nπ* states allow unambiguous evidence for clarifying uncertainties in the excited states of Cyt and the derivatives. The results provide a unifying experimental characterization at an improved level of detail about the photophysics of Cyt and its analogues under biologically relevant conditions and may help in understanding the photostability as well as photo-damages of the bases and related DNAs.

Chimeric behavior of excited thioxanthone in protic solvents: II. Theory.

The chimeric behavior of thioxanthone in protic solvents has been investigated employing computational chemistry methods. In particular, methanol and 2,2,2-trifluoroethanol have been chosen in this study. The solvent environment has been modeled using microsolvation in combination with a conductor-like screening model. The vertical excitation spectrum within the same solvent is seen to depend on the number of specific bonds formed between the chromophore and the solvent molecules. Two different models have been discussed in this work, namely, one and two H-bond models. In particular, the formation of the second H-bond causes the energy gap between the πHπL* and nOπL* states to increase further. Excited-state absorption spectra for the photophysically relevant electronic states have been theoretically determined for comparison with the time-resolved spectra recorded experimentally [Villnow, T.; Ryseck, G.; Rai-Constapel, V.; Marian, C. M.; Gilch, P. J. Phys. Chem. A 2014]. The equilibration of the 1(πHπL*) and 3(nOπL*) states holds responsible for the chimeric behavior. This equilibrium sets in with a calculated time constant of 23 ps in methanol and 14 ps in TFE (5 and 10 ps in experiment, respectively). The radiative decay from the optically bright 1(πHπL*) state is computed to occur with a time constant of 25 ns in both solvents (14–25 ns in experiment).

Palladium(II)-iodo-{1-alkyl-2-(arylazo)imidazole} complexes: Synthesis, structure, dynamics of photochromism and DFT computation

[Pd(Raai-CnH2n+1)2I)]2[Pd2I6] (Raai-CnH2n+1=1-alkyl-2-(arylazo)imidazole) has been characterized by the spectral data (UV–Vis, FTIR, Mass, 1H NMR). The single crystal X-ray structure of [Pd(Meaai-C2H5)2I]2[Pd2I6] shows that one Meaai-C2H5 acts as monodentate N(imidazolyl) donor, while the other one is bidentate N(imidazolyl), N(azo) chelator; the charge is neutralized with [Pd2I6]2− anion. UV light irradiation in DMF solution of the complexes show trans-to-cis isomerisation of Raai-CnH2n+1 about –NN– bond. Quantum yields (ϕt→c) of trans-to-cis isomerisation of the complexes are lower than that of the free ligand data. This observation is consistent with femtosecond transient absorption results of [Pd(Haai-C10H21)2][Pd2I6], which suggest that the trans→cis isomerization occurs in the monodentate azo-imidazole group and the bidentate azo-imidazole, because of chelation with Pd(II), does not exhibit photochromism. The isomerization proceeds with a time constant of ∼1.0ps in acetonitrile, 0.6ps in methanol, and 1.7ps in ethylene glycol, which are comparable to those the free azo-imidazole ligand. In contrast, the reverse transformation, i.e. cis-to-trans, is carried out by thermal process and the activation energy (Ea) of cis-to-trans isomerisation of the complexes is lower than that of free ligand. The spectral property and photochromic efficiency have been explained by DFT computation of optimized geometry of the complex.

Photophysical properties of indicaxanthin in aqueous and alcoholic solutions

Photophysical properties of indicaxanthin in solution were studied at room temperature using stationary and time-resolved spectroscopies. The results obtained suggest the photoprotective role of indicaxanthin in its natural environment due to the efficient light to heat conversion, and the absence of triplet excited state formation. Transient absorption spectra were obtained for the first time for indicaxanthin in solutions. Indicaxanthin in its S1 first singlet excited state appears instantaneously upon photoexcitation at λexc = 483 nm (S0 → S1 transition). Its band shows maximum at about λmax = 395 nm in both aqueous and alcoholic solutions. The S1 population decay can be described by a double-exponential function; the two characteristic time-constants were tentatively assigned to the presence of two stereoisomeric indicaxanthin forms. The S1 lifetimes extend in viscous solvents, from 6.5 and 31 ps in methanol to 14.1 and 161 ps in ethylene glycol.

The solvent effect and identification of a weakly emissive state in nonradiative dynamics of guanine nucleosides and nucleotides — a combined femtosecond broadband time-resolved fluorescence and transient absorption study

A combined method of femtosecond broadband time-resolved fluorescence (fs-TRF) and transient absorption (fs-TA) was employed to investigate the excited state dynamics of 2'-deoxyguanosine (dG) and 2'-deoxyguanosine 5'-monophosphate (dGMP). Comparative fs-TRF and fs-TA measurements were conducted on dG and dGMP in neutral water, deuterated water, and methanol with excitation wavelengths of 245, 267 and 285 nm. Very similar results were observed with dG and dGMP. The data provide compelling evidence for the co-existence of two nonradiative pathways. One is the generally recognized Laππ* mediated channel, the other involves an unprecedented weakly emissive state which plays a significant role in the overall deactivation processes. The Laππ* channel features biphasic dynamics with time constants (τ1/τ2) of ~0.2/0.8 ps in water and ~0.25/1.0 ps in methanol. The biphasic decay arises due to a partial transfer with τ1 of the Laππ* population to the newly identified state followed by conversion in τ2 of the remaining Laππ* molecules into the electronic ground state. The channel mediated by the weakly emissive species shows solvent-dependent dynamics with time constants (τ3) of ~2.0 ps in water, ~2.3 ps in deuterated water, and ~4.1 ps in methanol. The species features absorption at UV wavelengths (~300-400 nm) and exhibits deeply red-shifted fluorescence (λmax ~ 520 nm) with polarization direction varied markedly from that of the Laππ* but close to the Lbππ*. This species acts as an effective quenching state to the radiative decay of the brightly emissive Laππ* and Lbππ*. It sets in promptly (<~50 fs) after the photoexcitation and is further populated through nonadiabatic coupling with the Laππ*. The overall involvement of this state is facilitated with excitation at high energy and is favoured in methanol over water. According to the spectral character and the solvent effect in particular the kinetic isotope effect, the species is tentatively associated to the πσ* state with charge transfer (CT) character which is considered to be preferentially stabilized by hydrogen-bonding between the guanine amino and surrounding solvent molecules. The result of this study leads to a dramatically different picture of guanine deactivation. It demonstrates a crucial role of the solvent in shaping the nonradiative dynamics of guanine nucleosides and nucleotides. The data presented are important for understanding the detailed photophysics of not only the monomeric guanine but also DNA assemblies that contain guanine in base pairs or have a guanine tetrad as the structural motif.

Excited-State Dynamics of Monomeric and Aggregated Carotenoid 8′-Apo-β-carotenal

Excited-state properties of monomeric and aggregated carbonyl carotenoid 8'-apo-β-carotenal were studied by means of femtosecond transient absorption spectroscopy. For monomers, the polarity-dependent behavior characteristic of carotenoids with conjugated carbonyl group was observed. In n-hexane the S(1) lifetime is 25 ps, but it is shortened to 8 ps in methanol. This shortening is accompanied by the appearance of new spectral bands in transient absorption spectrum. On the basis of analysis of the transient absorption spectra of monomeric 8'-apo-β-carotenal in n-hexane and methanol, we propose that the polarity-induced spectral bands are due to the S(1)(A(g)(-))-S(3)(A(g)(+)) transition, which is enhanced upon breaking the symmetry of the molecule. This symmetry breaking is caused by the conjugated carbonyl group; it is much stronger in polar solvents where the S(1) state gains significant charge-transfer character. Upon addition of water to methanol solution of 8'-apo-β-carotenal we observed formation of aggregates characterized by either blue-shifted (H-aggregate) or red-shifted (J-aggregate) absorption spectrum. Both aggregate types exhibit excited-state dynamics significantly different from those of monomeric 8'-apo-β-carotenal. The lifetime of the relaxed S(1) state is 20 and 40 ps for the H- and J-aggregates, respectively. In contrast to monomers, aggregation promotes formation of triplet state, most likely by homofission occurring between tightly packed molecules within the aggregate.

Femtosecond transient absorption spectroscopy of copper(ii)–curcumin complexes

Ligand-metal interaction between curcumin and Cu(II) in methanol and sodium dodecyl sulfate (SDS) micelles was investigated using fluorescence spectroscopy and transient absorption spectroscopy. The Cu(II) ion exhibits a high efficiency in quenching the fluorescence of curcumin. By quantifying fluorescence quenching as a function of Cu(II) concentration, the complexation constants, K(1) and K(2), for the formation of the 1 : 1 and 1 : 2 Cu(II)-curcumin complexes, [Cu(II)-Cur](+) and [Cu(II)-Cur(2)], have been determined. In methanol, K(1) and K(2) are (1.33 ± 0.47) × 10(8) M(-1) and (6.79 ± 1.77) × 10(5) M(-1), respectively, whereas those in SDS micelles are (9.90 ± 1.68) × 10(5) M(-1) and (1.70 ± 0.48) × 10(6) M(-1), respectively. The transient absorption spectra of curcumin and the Cu(II)-curcumin complexes from 520 nm to 700 nm show a combination of stimulated emission and excited state absorption (ESA). However, the transient absorption signal at 500 nm corresponds to ESA exclusively. For curcumin, the ESA kinetics exhibit two rising components with time constants of 0.9 ps and 8.2 ps in methanol, and 0.5 ps and 2.5 ps in SDS micelles, which are consistent with solvation dynamics of excited state curcumin in these media. In addition, the ESA kinetics show a decay component with a time constant of 125 ps in methanol and 64 ps in SDS micelles, reflecting the excited state intramolecular hydrogen atom transfer of curcumin in these media. The ESA kinetics of the Cu(II)-curcumin complexes exhibit a sharp rise and a fast decay with a time constant of approximately 1 ps in both media due to the strong interaction between Cu(II) and curcumin.

Size Tunable Nano Patterns Using Nanosphere Lithography with Ashing and Annealing Effect

This work presents a fabrication procedure to make large-area, size-tunable, periodically different shape metal arrays using nanosphere lithography (NSL) combined with ashing and annealing. A polystyrene (PS, 580 mu m) monolayer, which was used as a mask, was obtained with a mixed solution of PS in methanol by multi-step spin coating. The mask morphology was changed by oxygen RIE (Reactive Ion Etching) ashing and temperature processing by microwave heating. The Au or Pt deposition resulted in size tunable nano patterns with different morphologies such as hole and dots. These processes allow outstanding control of the size and morphology of the particles. Various sizes of hole patterns were obtained by reducing the size of the PS sphere through the ashing process, and by increasing the size of the PS sphere through annealing treatment, which resulted in tcontrolling the size of the metallic nanoparticles from 30 nm to 230 nm.

Solvent-Dependent Spectral Diffusion in a Hydrogen Bonded “Vibrational Aggregate”

Two-dimensional infrared spectroscopy (2DIR) is used to measure the viscosity-dependent spectral diffusion of a model vibrational probe, Mn(2)(CO)(10) (dimanganese decacarbonyl, DMDC), in a series of alcohols with time scales ranging from 2.67 ps in methanol to 5.33 ps in 1-hexanol. Alcohol-alkane solvent mixtures were found to produce indistinguishable linear IR spectra, while still demonstrating viscosity-dependent spectral diffusion. Using a vibrational exciton model to characterize the inhomogeneous energy landscape, several analogies emerge with multichromophoric electronic systems, such as J-aggregates and light-harvesting protein complexes. An excitonic, local vibrational mode Hamiltonian parametrized to reproduce the vibrational structure of DMDC serves as a starting point from which site energies (i.e., local carbonyl frequencies) are given Gaussian distributed disorder. The model gives excellent agreement with both the linear IR spectrum and the inhomogeneous widths extracted from 2DIR, indicating the system can be considered to be a "vibrational aggregate." This model naturally leads to exchange narrowing due to disorder-induced exciton localization, producing line widths consistent with our 1D and 2D measurements. Further, the diagonal disorder alone effectively reduces the molecular symmetry, leading to the appearance of Raman bands in the IR spectrum in accord with the measurements. Here, we show that the static inhomogeneity of the excitonic model with disorder successfully captures the essential details of the 1D spectrum while predicting the degree of IR activity of forbidden modes as well as the inhomogeneous widths and relative magnitudes of the transition moments.

Excited-State Intramolecular Hydrogen Atom Transfer and Solvation Dynamics of the Medicinal Pigment Curcumin

The potential use of the naturally occurring yellow-orange pigment curcumin as a photodynamic therapy agent is one of the most exciting applications of this medicinal compound. Although subnanosecond spectroscopy has been used to investigate the photophysical processes of curcumin, the time resolution is insufficient to detect important and faster photoinduced processes, including solvation and excited-state intramolecular hydrogen atom transfer (ESIHT). In this study, the excited-state photophysics of curcumin is studied by means of ultrafast fluorescence upconversion spectroscopy. The results show two decay components in the excited-state kinetics with time scales of 12-20 ps and approximately 100 ps in methanol and ethylene glycol. The resulting prominent isotope effect in the long component upon deuteration indicates that curcumin undergoes ESIHT in these solvents. The short component (12-20 ps) is insensitive to deuteration, and multiwavelength fluorescence upconversion results show that this decay component is due to solvation of excited-state curcumin.

Femtosecond midinfrared study of the photoinduced Wolff rearrangement of diazonaphthoquinone

Time-resolved vibrational femtosecond spectroscopy is employed to investigate the photoinduced Wolff rearrangement reaction of diazonaphthoquinone (DNQ) dissolved in different solvents (methanol and water). DNQ is an important compound in commercial Novolak photoresists. Upon photoexcitation the ketene intermediate appears within 300 fs, indicating that the ketene is formed in a very fast concerted process involving N(2) loss and rearrangement. The strong shift of the vibrational band, assigned to the ketene by density functional theory calculations and experimental infrared spectra, toward higher wavenumbers is attributed to vibrational cooling. The relaxation time depends on the solvent (10 ps in methanol and 3 ps in water). However, the spectroscopic data show that the indirect ketene formation via a carbene intermediate might also be involved in the reaction process contributing to the ketene formation on the 10 ps time scale.

Effect of π-electron conjugation length on the solvent-dependent S 1 lifetime of peridinin

Peridinin exhibits an anomalous solvent dependence of its S 1 excited state lifetime attributed to the presence of an intramolecular charge transfer (ICT) state. The nature of this state has yet to be elucidated. Ultrafast time-resolved optical spectroscopy has been performed on a synthetic analog, C 35-peridinin, having one less conjugated double bond than peridinin. The data reveal the lifetime decreases from 1.5 ns in n-hexane to 9.2 ps in methanol, an order of magnitude larger than peridinin. This is the strongest solvent dependence on the lifetime of an S 1 state of a carotenoid yet reported. The data support the view that the S 1 and ICT states are strongly coupled.

Ultrafast studies of some diarylcarbenes

The photochemistry of 5-diazo-10,11-dihydrodibenzo[a,d]diazocycloheptene (DDBC) and 9-diazoanthrone (DAN) were studied by ultrafast time resolved techniques. The excited states of these diazo compounds were observed by UV-Vis spectroscopy and were found to decay in 300 fs. The diazo excited state decays led to the appearance and first direct observation of singlet 5-diazo-10,11-dihydrodibenzo[a,d]cycloheptenylidene (DBC) and 9-anthronylidene (AN). The dynamics of DBC and AN were studied in acetonitrile, cyclohexane and methanol. The lifetimes of (1)DBC are 83 ps and 72 ps in acetonitrile and cyclohexane, respectively. The lifetime of (1)DBC shortens to 9 ps in methanol due to rapid reaction with the solvent. The lifetime of (1)AN is 87 ps and 66 ps in acetonitrile and cyclohexane, respectively. In methanol, the lifetime of (1)AN cannot be determined due to spectral overlap of (1)AN and cation ANH+. The decays of (1)DBC and (1)AN are controlled by intersystem crossing (ISC) in acetonitrile and cyclohexane and the rates of ISC of (1)DBC or (1)AN are similar in these two solvents. This differs from the solvent dependence of other diarylcarbene intersystem crossing rates. This is attributed to the relatively large singlet-triplet (S-T) gaps of these carbenes and this factor dominates the influence of solvent.

Evidence for an Intramolecular Charge Transfer State in 12‘-Apo-β-caroten-12‘-al and 8‘-Apo-β-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.