Anti-stokes Raman study of vibrational cooling dynamics in the primary photochemistry of rhodopsin

Picosecond Stokes and anti-Stokes Raman spectra are used to probe the structural dynamics and reactive energy flow in the primary cis-to-trans isomerization reaction of rhodopsin. The appearance of characteristic ethylenic, hydrogen out-of-plane (HOOP), and low-wavenumber photoproduct bands in the Raman spectra is instrument-response-limited, consistent with a subpicosecond product appearance time. Intense high and low-frequency anti-Stokes peaks demonstrate that the all-trans photoproduct is produced vibrationally hot on the ground-state surface. Specifically, the low-frequency modes at 282, 350, and 477 cm(-1) are highly vibrationally excited (T > 2000 K) immediately following isomerization, revealing that these low-frequency motions directly participate in the reactive curve-crossing process. The anti-Stokes modes are characterized by a approximately 2.5 ps temporal decay that coincides with the conversion of photorhodopsin to bathorhodopsin. This correspondence shows that the photo-to-batho transition is a ground-state cooling process and that energy storage in the primary visual photoproduct is complete on the picosecond time scale. Finally, unique Stokes vibrations at 290, 992, 1254, 1290, and 1569 cm(-1) arising from the excited state of rhodopsin are observed only at 0 ps delay.     

Time-resolved resonance Raman analysis of chromophore structural changes in the formation and decay of rhodopsin's BSI intermediate

Time-resolved resonance Raman microchip flow experiments are performed to obtain the vibrational spectrum of the chromophore in rhodopsin's BSI intermediate and to probe structural changes in the bathorhodopsin-to-BSI and BSI-to-lumirhodopsin transitions. Kinetic Raman spectra from 250 ns to 3 micros identify the key vibrational features of BSI. BSI exhibits relatively intense HOOP modes at 886 and 945 cm(-1) that are assigned to C(14)H and C(11)H=C(12)H A(u) wags, respectively. This result suggests that in the bathorhodopsin-to-BSI transition the highly strained all-trans chromophore has relaxed in the C(10)-C(11)=C(12)-C(13) region, but is still distorted near C(14). The low frequency of the 11,12 A(u) HOOP mode in BSI compared with that of lumirhodopsin and metarhodopsin I indicates weaker coupling between the 11H and 12H wags due to residual distortion of the BSI chromophore near C(11)=C(12). The C=NH(+) stretching mode in BSI at 1653 cm(-1) exhibits a normal deuteriation induced downshift of 23 cm(-1), implying that there is no significant structural rearrangement of the Schiff base counterion region in the transition of bathorhodopsin to BSI. However, a dramatic Schiff base environment change occurs in the BSI-to-lumirhodopsin transition, because the 1638 cm(-1) C=NH(+) stretching mode in lumirhodopsin is unusually low and shifts only 7 cm(-1) in D(2)O, suggesting that it has essentially no H-bonding acceptor. With these data we can for the first time compare and discuss the room temperature resonance Raman vibrational structure of all the key intermediates in visual excitation.     

Subpicosecond protein backbone changes detected during the green-absorbing proteorhodopsin primary photoreaction

Recent studies demonstrate that photoactive proteins can react within several picoseconds to photon absorption by their chromophores. Faster subpicosecond protein responses have been suggested to occur in rhodopsin-like proteins where retinal photoisomerization may impulsively drive structural changes in nearby protein groups. Here, we test this possibility by investigating the earliest protein structural changes occurring in proteorhodopsin (PR) using ultrafast transient infrared (TIR) spectroscopy with approximately 200 fs time resolution combined with nonperturbing isotope labeling. PR is a recently discovered microbial rhodopsin similar to bacteriorhodopsin (BR) found in marine proteobacteria and functions as a proton pump. Vibrational bands in the retinal fingerprint (1175-1215 cm(-1)) and ethylenic stretching (1500-1570 cm(-1)) regions characteristic of all-trans to 13-cis chromophore isomerization and formation of a red-shifted photointermediate appear with a 500-700 fs time constant after photoexcitation. Bands characteristic of partial return to the ground state evolve with a 2.0-3.5 ps time constant. In addition, a negative band appears at 1548 cm(-1) with a time constant of 500-700 fs, which on the basis of total-15N and retinal C15D (retinal with a deuterium on carbon 15) isotope labeling is assigned to an amide II peptide backbone mode that shifts to near 1538 cm(-1) concomitantly with chromophore isomerization. Our results demonstrate that one or more peptide backbone groups in PR respond with a time constant of 500-700 fs, almost coincident with the light-driven retinylidene chromophore isomerization. The protein changes we observe on a subpicosecond time scale may be involved in storage of the absorbed photon energy subsequently utilized for proton transport.     

Photoexcited structural dynamics of an azobenzene analog 4-nitro-4'-dimethylamino-azobenzene from femtosecond stimulated Raman

Azobenzenes are used in many applications because of their robust and reversible light induced trans?cis isomerization about the N=N bond, but the mechanism of this ultrafast reaction has not been conclusively defined. Addressing this problem we have used Femtosecond Stimulated Raman Spectroscopy (FSRS) to determine the structural transients in the trans→cis photoisomerization of the azobenzene derivative, 4-nitro-4'-dimethylamino-azobenzene (NDAB). Key marker modes, such as the 1570/1590 cm(-1) NO(2) stretch and the 1630 cm(-1) C-N(Me)(2) stretch, enable the separation and analysis of distinct trans and cis photoproduct dynamics revealing the 400 fs Frank-Condon relaxation, the 800 fs timescale of the cis product formation and the 2 ps emergence and 8 ps relaxation of the unsuccessful ground state trans species. Based on these observations, we propose a reaction mechanism, including initial dilation of the CNN bend later joined by quick movement along the CCNN, CNNC and NNCC torsional coordinates that constitutes a mixed inversion-rotation mechanism.     

Calculation of vibrational spectra of linear tetrapyrroles. 4. Methine bridge C-H out-of-plane modes

Quantum chemical force fields obtained by density functional theory (DFT) calculations systematically overestimate the frequencies of normal modes including ethylenic C-H out-of-plane (HOOP) coordinates. Compensation of this deviation requires a specific scaling factor for this type of coordinate that is distinctly lower than those applicable to out-of-plane coordinates in general. Such a specific scaling factor (0.900) has been optimized for the DFT(B3LYP) level of theory on the basis of vibrational analyses of training molecules including the HOOP coordinate. Thus, the root-mean-square deviation for the calculated frequencies of these modes is reduced from 16 to 8 cm(-1). Although Raman intensities are yet not reproduced in a satisfactory manner, implementation of the HOOP scaling factor into the set of global scaling factors determined previously (Magdo et al. J. Phys. Chem. A 1999, 103, 289-303) allows for a substantially improved reproduction of the experimental (resonance) Raman spectra of test molecules including linear methine-bridged tetrapyrroles. A very good agreement between calculated and experimental spectra is noted for the phycocyanobilin dimethylester dimer as well as for the protein-bound phycocyanobilin in the antenna pigment alpha-CPC. However, for the phycocyanobilin chromophore in the P(r) state of the plant photoreceptor phytochrome phyA, considerable deviations remain in the spectral range between 800 and 500 cm(-1), which are attributed to the effect of specific protein-chromophore interactions. The influence of the protein environment is not considered in the present calculations that refer to the molecule in vacuo.     

A molecular spring for vision

Light absorption by the visual pigment rhodopsin leads to vision via a complex signal transduction pathway that is initiated by the ultrafast and highly efficient photoreaction of its chromophore, the retinal protonated Schiff base (RPSB). Here, we investigate this reaction in real time by means of unrestrained molecular dynamics simulations of the protein in a membrane mimetic environment, treating the chromophore at the density functional theory level. We demonstrate that a highly strained all-trans RPSB is formed starting from the 11-cis configuration (dark state) within approximately 100 fs by a minor rearrangement of the nuclei under preservation of the saltbridge with Glu113 and virtually no deformation of the binding pocket. Hence, the initial step of vision can be understood as the compression of a molecular spring by a minor change of the nuclear coordinates. This spring can then release its strain by altering the protein environment.     

Dynamics of photoinduced processes in adenine and thymine base pairs

The excited-state dynamics of adenine and thymine dimers and the adenine-thymine base pair were investigated by femtosecond pump-probe ionization spectroscopy with excitation wavelengths of 250-272 nm. The base pairs showed a characteristic ultrafast decay of the initially excited pi pi* state to an n pi* state (lifetime tau(pi pi*) approximately 100 fs) followed by a slower decay of the latter with tau(n pi*) approximately 0.9 ps for (adenine)2, tau(n pi*) = 6-9 ps for (thymine)2, and tau(n pi*) approximately 2.4 ps for the adenine-thymine base pair. In the adenine dimer, a competing decay of the pi pi* state via the pi sigma* state greatly suppressed the n pi* state signals. Similarities of the excited-state decay parameters in the isolated bases and the base pairs suggest an intramonomer relaxation mechanism in the base pairs.     

STUDIES ON CHROMOPHORE COUPLING IN ISOLATED PHYCOBILIPROTEINS. IV. FEMTOSECOND TRANSIENT ABSORPTION STUDY OF ULTRAFAST EXCITED STATE DYNAMICS IN TRIMERIC PHYCOERYTHROCYANIN COMPLEXES*

The excited state dynamics of trimeric phycoerythrocyanin has been studied by two-color femtosecond transient absorption spectroscopy with a time-resolution better than 200 fs. Upon selective excitation of the short-wavelength phycobiliviolin chromophore at 575 nm absorption bleachings are observed. An isotropic ultrafast decay of the initial bleaching of 585 ± 40 fs has been resolved at short detection wavelength (578 nm). Upon stepwise increase of the detection wavelength up to 617 nm, the bleaching showed a delayed rise above 593 nm with r_rice=380–580 fs. All other isotropic kinetic components in this wavelength range were longer than 100 ps. The ultrafast component is discussed in terms of an energy transfer process from the α-84 phycobiliviolin chromophore to the β-84 chromophore in adjacent monomer subunits of the trimer. It is concluded that the β-155 chromophore is the longest-wavelength chromophore. Exciton relaxation between closely spaced chromophore pairs is discussed as an alternative interpretation for the ultrafast component.     

Ultrafast pump-probe study of the primary photoreaction process in pharaonis halorhodopsin: halide ion dependence and isomerization dynamics

Halorhodopsin is a retinal protein that acts as a light-driven chloride pump in the Haloarchaeal cell membrane. A chloride ion is bound near the retinal chromophore, and light-induced all- trans --> 13- cis isomerization triggers the unidirectional chloride ion pump. We investigated the primary ultrafast dynamics of Natronomonas pharaonis halorhodopsin that contains Cl (-), Br (-), or I (-) ( pHR-Cl (-), pHR-Br (-), or pHR-I (-)) using ultrafast pump-probe spectroscopy with approximately 30 fs time resolution. All of the temporal behaviors of the S n <-- S 1 absorption, ground-state bleaching, K intermediate (13- cis form) absorption, and stimulated emission were observed. In agreement with previous reports, the primary process exhibited three dynamics. The first dynamics corresponds to the population branching process from the Franck-Condon (FC) region to the reactive (S 1 (r)) and nonreactive (S 1 (nr)) S 1 states. With the improved time resolution, it was revealed that the time constant of this branching process (tau 1) is as short as 50 fs. The second dynamics was the isomerization process of the S 1 (r) state to generate the ground-state 13- cis form, and the time constant (tau 2) exhibited significant halide ion dependence (1.4, 1.6, and 2.2 ps for pHR-Cl (-), pHR-Br (-), and pHR-I (-), respectively). The relative quantum yield of the isomerization, which was evaluated from the pump-probe signal after 20 ps, also showed halide ion dependence (1.00, 1.14, and 1.35 for pHR-Cl (-), pHR-Br (-), and pHR-I (-), respectively). It was revealed that the halide ion that accelerates isomerization dynamics provides the lower isomerization yield. This finding suggests that there is an activation barrier along the isomerization coordinate on the S 1 potential energy surface, meaning that the three-state model, which is now accepted for bacteriorhodopsin, is more relevant than the two-state model for the isomerization process of halorhodopsin. We concluded that, with the three-state model, the isomerization rate is controlled by the height of the activation barrier on the S 1 potential energy surface while the overall isomerization yield is determined by the branching ratios at the FC region and the conical intersection. The third dynamics attributable to the internal conversion of the S 1 (nr) state also showed notable halide ion dependence (tau 3 = 4.5, 4.6, and 6.3 ps for pHR-Cl (-), pHR-Br (-), and pHR-I (-)). This suggests that some geometrical change may be involved in the relaxation process of the S 1 (nr) state.     

Multiple time scales in solvation dynamics of DNA in aqueous solution: the role of water, counterions, and cross-correlations

Recent time domain experiments have explored solvation dynamics of a probe located inside a DNA duplex, in an effort to gain information, e.g., on the dynamics of water molecules in the DNA major and minor grooves and their environment. Multiple time constants in the range of a few picoseconds to several nanoseconds were obtained. We have carried out 15 ns long atomistic molecular dynamics simulations to study the solvation dynamics of bases of a 38 base-pair long DNA duplex in an aqueous solution containing counterions. We have computed the energy-energy time correlation function (TCF) of the four individual bases (A, T, G, and C) to characterize the solvation dynamics. All the TCFs display highly nonexponential decay with time. When the trajectories are analyzed with 100 fs time resolution, the TCF of each base shows initial ultrafast decay (with tau1 approximately equal 60-80 fs) followed by two intermediate components (tau2 approximately equal 1 ps, tau3 approximately equal 20-30 ps), in near complete agreement with a recent time domain experiment on DNA solvation. Interestingly, the solvation dynamics of each of the four different nucleotide bases exhibit rather similar time scales. To explore the existence of slow relaxation at longer times reported recently in a series of experiments, we also analyzed the solvation TCFs calculated with longer time trajectories and with a larger time resolution of 1 ps. In this case, an additional slow component with a time constant of the order of 250 ps is observed. Through an analysis of partial solvation TCFs, we find that the slow decay originates mainly from the interaction of the nucleotides with the dipolar water molecules and the counterions. An interesting negative cross-correlation between water and counterions is observed, which makes an important contribution to relaxation at intermediate to longer times.     

State preparation and excited electronic and vibrational behavior in hemes

The temporally overlapping, ultrafast electronic and vibrational dynamics of a model five-coordinate, high-spin heme in a nominally isotropic solvent environment has been studied for the first time with three complementary ultrafast techniques: transient absorption, time-resolved resonance Raman Stokes, and time-resolved resonance Raman anti-Stokes spectroscopies. Vibrational dynamics associated with an evolving ground-state species dominate the observations. Excitation into the blue side of the Soret band led to very rapid S2 --> S1 decay (sub-100 fs), followed by somewhat slower (800 fs) S1 --> S0 nonradiative decay. The initial vibrationally excited, non-Boltzmann S0 state was modeled as shifted to lower energy by 300 cm(-1) and broadened by 20%. On a approximately 10 ps time scale, the S0 state evolved into its room-temperature, thermal distribution S0 profile largely through VER. Anti-Stokes signals disappear very rapidly, indicating that the vibrational energy redistributes internally in about 1-3 ps from the initial accepting modes associated with S1 --> S0 internal conversion to the rest of the macrocycle. Comparisons of anti-Stokes mode intensities and lifetimes from TRARRS studies in which the initial excited state was prepared by ligand photolysis [Mizutani, T.; Kitagawa, T. Science 1997, 278, 443, and Chem. Rec. 2001, 1, 258] suggest that, while transient absorption studies appear to be relatively insensitive to initial preparation of the electronic excited state, the subsequent vibrational dynamics are not. Direct, time-resolved evaluation of vibrational lifetimes provides insight into fast internal conversion in hemes and the pathways of subsequent vibrational energy flow in the ground state. The overall similarity of the model heme electronic dynamics to those of biological systems may be a sign that the protein's influence upon the dynamics of the heme active site is rather subtle.     

Ultrafast excited-state dynamics of tetraphenylethylene studied by semiclassical simulation

Detailed simulation study is reported for the excited-state dynamics of photoisomerization of cis-tetraphenylethylene (TPE) following excitation by a femtosecond laser pulse. The technique for this investigation is semiclassical dynamics simulation, which is described briefly in the paper. Upon photoexcitation by a femtosecond laser pulse, the stretching motion of the ethylenic bond of TPE is initially excited, leading to a significant lengthening of ethylenic bond in 300 fs. Twisting motion about the ethylenic bond is activated by the energy released from the relaxation of the stretching mode. The 90 degrees twisting about the ethylenic bond from an approximately planar geometry to nearly a perpendicular conformation in the electronically excited state is completed in 600 fs. The torsional dynamics of phenyl rings which is temporally lagging behind occurs at about 5 ps. Finally, the twisted TPE reverts to the initial conformation along the twisting coordinate through nonadiabatic transitions. The simulation results provide a basis for understanding several spectroscopic observations at molecular levels, including ultrafast dynamic Stokes shift, multicomponent fluorescence, viscosity dependence of the fluorescence lifetime, and radiationless decay from electronically excited state to the ground state along the isomerization coordinate.     

Ultrafast solvation dynamics of coumarin 153 in imidazolium-based ionic liquids

The dynamic Stokes shift of coumarin 153 has been measured in two room-temperature ionic liquids, 1-(3-cyanopropyl)-3-methylimidazolium bis(trifluoromethylsulfonyl)imide and 1-propyl-3-methylimidazolium tetrafluoroborate, using the fluorescence up-conversion technique with a 230 fs instrumental response function. A component of about 10-15% of the total solvation shift is found to take place on an ultrafast time scale < 10 ps. The amplitude of this component is substantially less than assumed previously by other authors. The origin of the difference in findings could be partly due to chromophore-internal conformational changes on the ultrafast time scale, superimposed to solvation-relaxation, or due to conformational changes of the chromophore ground state in polar and apolar environments. First three-pulse photon-echo peak-shift experiments on indocyanine green in room-temperature ionic liquids and in ethanol indicate a difference in the inertial component of the early solvent relaxation of <100 fs.     

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.     

Femtosecond electronic relaxation of excited metalloporphyrins in the gas phase

A systematic study of the ultrafast decay of metalloporphyrins containing various transition metals with partially filled 3d shells and zinc (3d filled) is reported here after excitation in the second excited state of the system (Soret band). Both time-of-flight mass spectrometry and velocity map imaging have been used for detection. A general biexponential decay with a short time constant tau1 approximately 100 fs is observed for the transition metal porphyrins, followed by a tau2 approximately 1 ps time decay. This evolution is interpreted as a porphyrin-to-metal charge transfer, tau1, followed by a back transfer, tau2, which leads to an excited state (d,d*) localized on the metal. These conclusions stem from the different behaviors of zinc and the transition metal porphyrins. A porphyrin-to-metal charge transfer model is chosen to describe the relaxation mechanism, based upon the fact that transition metalloporphyrins can accept electrons on the metal site, in contrast to zinc porphyrins.     

Picosecond dynamics of nonthermalized excited states in tris(2,2-bipyridine)ruthenium(II) derivatives elucidated by high energy excitation

The picosecond excited-state dynamics of several derivatives have been investigated using high photon energy excitation combined with picosecond luminescence detection. Instrument response-limited fluorescence (tau(1) approximately equal to 3-5 ps) at 500 nm was observed for all of the complexes, while longer-lived emission (tau(2) > 50 ps), similar in energy, was observed for only some of the complexes. Interestingly, the presence of tau(2) required substitution at the 4,4-positions of the bipyridine ligands and D(3) symmetry for the complex; only the 4,4-substituted homoleptic complexes exhibited tau(2). On the basis of previous assignments of the ultrafast dynamics measured for Ru(bpy)(2+)3 and Ru(dmb)(2+)3, tau(2) has been tentatively ascribed to relaxation from higher electronic or vibrational levels in the triplet manifold having slightly more triplet character than the state responsible for tau(1). However, given that the kinetics for these transition metal complexes are highly dependent on both pump and probe wavelengths and that there is considerable interest in utilizing such complexes for electron transfer in the nonergodic limit, further characterization of the state giving rise to tau(2) is warranted.     

Electron and nuclear dynamics of molecular clusters in ultraintense laser fields. II. Electron dynamics and outer ionization of the nanoplasma

We explore electron dynamics in molecular (CD4)(1061) clusters and elemental Xen (n=249-2171) clusters, responding to ultraintense (intensity I=10(16)-10(19) W cm(-2)) laser fields. Molecular dynamics simulations (including magnetic field and relativistic effects) and analyses of high-energy electron dynamics and nuclear ion dynamics in a cluster interacting with a Gaussian shaped laser field (frequency 0.35 fs(-1), photon energy 1.44 eV, phase 0, temporal width 25 fs) elucidated the time dependence of inner ionization, the formation of a nanoplasma of unbound electrons within the cluster or its vicinity, and of outer ionization. We determined the cluster size and the laser intensity dependence of these three sequential-parallel electronic processes. The characteristic times for cluster inner ionization (tau(ii)) and for outer ionization (tau(oi)) fall in the femtosecond time domain, i.e., tau(ii)=2-9 fs and tau(oi)=4-15 fs for (CD4)(1061), tau(ii)=7-30 fs and tau(oi)=5-13 fs for Xe(n) (n=479,1061), with both tau(ii) and tau(oi) decreasing with increasing I, in accord with the barrier suppression ionization mechanism for inner ionization of the constituents and the cluster barrier suppression ionization mechanism for outer ionization. The positive delay times Deltatau(OI) between outer and inner ionization (e.g., Deltatau(OI)=6.5 fs for Xen at I=10(16) W cm(-2) and Deltatau(OI)=0.2 fs for (CD4)(1061) at I=10(19) W cm(-2)) demonstrate that the outer/inner ionization processes are sequential. For (CD4)(1061), tau(ii)tau(oi), reflecting on the energetic hierarchy in the ionization of the Xe atoms. Quasiresonance contributions to the outer ionization of the nanoplasma were established, as manifested in the temporal oscillations in the inner/outer ionization levels, and in the center of mass of the nanoplasma electrons. The formation characteristics, dynamics, and response of the nanoplasma in molecular or elemental clusters were addressed. The nanoplasma is positively charged, with a high-average electron density [rho(P)=(2-3)10(22) cm(-3)], being characterized by high-average electron energies epsilon(av) (e.g., in Xe(1061) clusters epsilon(av)=54 eV at I=10(16) W cm(-2) and epsilon(av)=0.56-0.37 keV at I=10(18) W cm(-2), with epsilon(av) proportional, variant I(1/2)). Beyond the cluster boundary the average electron energy markedly increases, reaching electron energies in the range of 1.2-40 keV for outer ionization of Xe(n) (n=249-2171) clusters. The nanoplasma exhibits spatial inhomogeneity and angular anisotropy induced by the laser field. Femtosecond time scales are predicted for the nanoplasma production (rise times 7-3 fs), for the decay (decay times approximately 5 fs), and for the persistence time (30-10 fs) of a transient nanoplasma at I=10(17)-10(18) W cm(-2). At lower intensities of I=10(16) W cm(-2) a persistent nanoplasma with a "long" lifetime of > 50 fs will prevail.     

Ultrafast aggregate-to-aggregate energy transfer within self-assembled light-harvesting columns of zinc phthalocyanine tetrakis(perylenediimide)

Light harvesting in photosynthetic antenna proteins involves a series of highly efficient ultrafast energy transfers between spectroscopically different populations of chlorophylls. Several strategies have recently been employed to mimic this natural energy transfer process, including polymers, dendrimers, and oligomeric porphyrin arrays linked by covalent bonds or by self-assembly. In all of these systems, excitation energy transfer occurs from one molecule to another, while very few of them involve energy transfer from one very strongly interacting chromophore aggregate to another such aggregate. Here we report the synthesis and characterization of a covalent zinc phthalocyanine-2,3,9,10,16,17,23,24-octacarboxytetraimide in which all four imide nitrogen atoms are substituted with N-octyl-N'-(4-aminophenyl)-1,7(3',5'di-tert-butylphenoxy)perylene-3,4:9,10-bis(dicarboximide) (ZnPcIm4-PDI4). The individual molecules self-assemble into stacked heptamers in solution as evidenced by small-angle X-ray scattering and form long fibrous structures in the solid as evidenced by TEM. The ZnPcIm4 and PDI molecules both stack in register with the same components in an adjacent covalent building block. Ultrafast energy transfer occurs with tau = 1.3 ps from the aggregated peripheral PDI chromophores to the core ZnPcIm4 chromophore aggregate. Exciton hopping between the ZnPcIm4 chromophores occurs with tau = 160 fs.     

Femtosecond Time-Resolved Stimulated Raman Spectroscopy of the S(2) (1B(u)) Excited State of beta-Carotene

The electronic and vibrational structure of beta-carotene's early excited states are examined using femtosecond time-resolved stimulated Raman spectroscopy. The vibrational spectrum of the short-lived ( approximately 160 fs) second excited singlet state (S(2),1B(u) (+))of beta-carotene is obtained. Broad, resonantly enhanced vibrational features are observed at approximately 1100, 1300, and 1650 cm(-1) that decay with a time constant corresponding to the electronic lifetime of S(2). The temporal evolution of the vibrational spectra are consistent with significant population of only two low-lying excited electronic states (1B(u) (+) and 2A(g) (-)) in the ultrafast relaxation pathway of beta-carotene.     

Development of a tunable femtosecond stimulated raman apparatus and its application to beta-carotene

We have developed a tunable femtosecond stimulated Raman spectroscopy (FSRS) apparatus and used it to perform time-resolved resonance Raman experiments with <100 fs temporal and <35 cm(-1) spectral resolution. The key technical change that facilitates this advance is the use of a tunable narrow-bandwidth optical parametric amplifier (NB-OPA) presented recently by Shim et al. (Shim, S.; Mathies, R. A. Appl. Phys. Lett. 2006, 89, 121124). The practicality of tunable FSRS is demonstrated by examining the photophysical dynamics of beta-carotene. Using 560 nm Raman excitation, the resonant S1 state modes are enhanced by a factor of approximately 200 compared with 800 nm FSRS experiments. The improved signal-to-noise ratios facilitate the measurement of definitive time constants for beta-carotene dynamics including the 180 fs appearance of the S1 vibrational features due to direct internal conversion from S2 and their characteristic 9 ps decay to S0. By tuning the FSRS system to 590 nm Raman excitation, we are able to selectively enhance vibrational features of the hot ground state S hot 0 and monitor its approximately 5 ps cooling dynamics. This tunable FSRS system is valuable because it facilitates the direct observation of structural changes of selected resonantly enhanced states and intermediates during photochemical and photobiological reactions.