Environment-dependent ultrafast photoisomerization dynamics in azo dye

Azoaromatic dyes have been extensively investigated over the past decade due to their potential use in a variety of optical devices that exploit their ultrafast photoisomerization processes. Among the azoaromatic dyes, Disperse Red 19 is a commercially available azobenzene nonlinear optical chromophore with a relatively high ground-state dipole moment. In the present study, we used ultrafast time-resolved spectroscopy to clarify the dynamics of a push-pull substituted azobenzene dye. Solution and film samples exhibited different ultrafast dynamics, indicating that the molecular environment affects the photoisomerization dynamics of the dye.     

Ab initio nonadiabatic quantum dynamics of cyclohexadiene/hexatriene ultrafast photoisomerization

Reaction mechanisms of the ultrafast photoisomerization between cyclohexadiene and hexatriene have been elucidated by the quantum dynamics on the ab initio potential energy surfaces calculated by multireference configuration interaction method. In addition to the quantum wave-packet dynamics along the two-dimensional reaction coordinates, the semiclassical analyses have also been carried out to correctly estimate the nonadiabatic transition probabilities around conical intersections in the full-dimensional space. The reaction time durations of radiationless decays in the wave-packet dynamics are found to be generally consistent with the femtosecond time-resolution experimental observations. The nonadiabatic transition probabilities among the ground (S0), first (S1), and second (S2) excited states have been estimated by using the semiclassical Zhu-Nakamura formula considering the full-dimensional wave-packet density distributions in the vicinity of conical intersections under the harmonic normal mode approximation. The cyclohexadiene (CHD) ring-opening process proceeds descending on the S1(1 1B) potential after the photoexcitation. The major part of the wave-packet decays from S1(1 1B) to S1(2 1A) by the first seam line crossing along the C2-symmetry-breaking directions. The experimentally observed ultrafast S1-S0 decay can be explained by the dynamics through the S1-S0 conical intersection along the direction toward the five-membered ring. The CHD: hexatriene (HT) branching ratio is estimated to be approximately 5:5, which is in accordance with the experiment in solution. This branching ratio is found to be mainly governed by the location of the five-membered ring S1-S0 conical intersection along the ground state potential ridge between CHD and HT.     

Effect of metal cation on photoisomerization of azobenzenes in zeolite nanocavities

In the photostationary state, the cis/trans isomer ratio of azobenzene and 3,3′-dimethylazobenzene adsorbed in zeolite NaY increases significantly to ca. 90:10, in contrast to the reaction in cyclohexane. However, for azobenzene-4,4′-dicarboxylic acid diethyl ester the formation of the cis isomer is remarkably suppressed to ca. 30%. On the basis of ab initio MO calculations, it is suggested that electrostatic interaction between these azobenzenes and the metal ions in zeolite nanocavities regulates the cis-trans photoisomerization process. In addition, it is found that the photoisomerization behavior of azobenzenes adsorbed on silica gel is similar to that in NaY.     

Reversible trans/cis photoisomerization in Langmuir-Blodgett multilayers from polyfunctional azobenzenes

Four novel, amphiphilic, N-acylaminoazobenzene derivatives containing a sulfonyl group and a β-alanine moiety were synthesized and their mono- and multilayers were prepared by the Langmuir-Blodgett (LB) technique. The opto-physical properties of the LB multilayers were investigated. Some relationship between the chemical structure of the azobenzenes and photosensitivity of their LB multilayers has been found. Reversible trans/cis photoisomerization was observed on alternate irradiation with ultraviolet and visible light of LB multilayers from azobenzenes modified by the introduction of a second aliphatic chain in the N-acylamino fragment as well as by introduction of the bulky N,N′-dicyclohexyl urea moiety.     

Mechanism and dynamics of azobenzene photoisomerization

The excited-state dynamics of trans-azobenzene were investigated by femtosecond time-resolved photoelectron spectroscopy and ab initio molecular dynamics. Two near-degenerate pipi* excited states, S2 and S3,4, were identified in a region hitherto associated with only one excited state. These results help to explain contradictory reports about the photoisomerization mechanism and the wavelength dependence of the quantum yield. A new model for the isomerization mechanism is proposed.     

Tracking of the nuclear wavepacket motion in cyanine photoisomerization by ultrafast pump-dump-probe spectroscopy

Understanding ultrafast reactions, which proceed on a time scale of nuclear motions, requires a quantitative characterization of the structural dynamics. To track such structural changes with time, we studied a nuclear wavepacket motion in photoisomerization of a prototype cyanine dye, 1,1'-diethyl-4,4'-cyanine, by ultrafast pump-dump-probe measurements in solution. The temporal evolution of wavepacket motion was examined by monitoring the efficiency of stimulated emission dumping, which was obtained from the recovery of a ground-state bleaching signal. The dump efficiency versus pump-dump delay exhibited a finite rise time, and it became longer (97 fs → 330 fs → 390 fs) as the dump pulse was tuned to longer wavelengths (690 nm → 950 nm → 1200 nm). This result demonstrates a continuous migration of the leading edge of the wavepacket on the excited-state potential from the Franck-Condon region toward the potential minimum. A slowly decaying feature of the dump efficiency indicated a considerable broadening of the wavepacket over a wide range of the potential, which results in the spread of a population distribution on the flat S(1) potential energy surface. The rapid migration as well as broadening of the wavepacket manifests a continuous nature of the structural dynamics and provides an intuitive visualization of this ultrafast reaction. We also discussed experimental strategies to evaluate reliable dump efficiencies separately from other ultrafast processes and showed a high capability and possibility of the pump-dump-probe method for spectroscopic investigation of unexplored potential regions such as conical intersections.     

Photochemical tuning of the helical structure of cholesteric liquid crystals by photoisomerization of chiral azobenzenes, and their structural effects

Several chiral azobenzene compounds having different chiral substituents were synthesized. A cholesteric phase was induced by mixing each chiral azobenzene compound with a host non-chiral nematic liquid crystal (E44). The helical twisting power (HTP) as well as the change in HTP by trans-cis photoisomerization of the chiral azobenzene compound was dependent on the structure of the chiral substituents. A compensated nematic phase was induced by combination of E44, a chiral azobenzene compound and a non-photochromic chiral compound. Reversible switching between the compensated nematic phase and cholesteric phase was brought about by trans-cis photoisomerization of the chiral azobenzene compound in the liquid crystalline systems. An azobenzene compound substituted with a menthyl group showed the highest efficiency as the trigger for the switching; this efficiency was related to the compactness of the chiral group substituted within the azobenzene core moiety.     

Quantum dynamics of the femtosecond photoisomerization of retinal in bacteriorhodopsin

The membrane protein bacteriorhodopsin contains all-trans-retinal in a binding site lined by amino acid side groups and water molecules that guide the photodynamics of retinal. Upon absorption of light, retinal undergoes a subpicosecond all-trans-->13-cis phototransformation involving torsion around a double bond. The main reaction product triggers later events in the protein that induce pumping of a proton through bacteriorhodopsin. Quantum-chemical calculations suggest that three coupled electronic states, the ground state and two closely lying excited states, are involved in the motion along the torsional reaction coordinate phi. The evolution of the protein-retinal system on these three electronic surfaces has been modelled using the multiple spawning method for non-adiabatic dynamics. We find that, although most of the population transfer occurs on a timescale of 300 fs, some population transfer occurs on a longer timescale, occasionally extending well beyond 1 ps.     

Optimal control of ultrafast cis-trans photoisomerization of retinal in rhodopsin via a conical intersection

Optimal control simulation is applied to the cis-trans photoisomerization of retinal in rhodopsin within a two-dimensional, two-electronic-state model with a conical intersection [S. Hahn and G. Stock, J. Phys. Chem. B 104, 1146 (2000)]. For this case study, we investigate coherent control mechanisms, in which laser pulses work cooperatively with a conical intersection that acts as a "wave-packet cannon." Optimally designed pulses largely consist of shaping subpulses that prepare a wave packet, which is localized along a reaction coordinate and has little energy in the coupling mode, through multiple electronic transitions. This shaping process is shown to be essential for achieving a high target yield although the envelopes of the calculated pulses depend on the local topography of the potential-energy surfaces around the conical intersection and the choice of target. The control mechanisms are analyzed by considering the motion of reduced wave packets in a nuclear configuration space as well as by snapshots of probability current-density maps.     

Unraveling ultrafast dynamics in photoexcited aniline

A combination of ultrafast time-resolved velocity map imaging (TR-VMI) methods and complete active space self-consistent field (CASSCF) ab initio calculations are implemented to investigate the electronic excited-state dynamics in aniline (aminobenzene), with a perspective for modeling (1)πσ* mediated dynamics along the amino moiety in the purine derived DNA bases. This synergy between experiment and theory has enabled a comprehensive picture of the photochemical pathways/conical intersections (CIs), which govern the dynamics in aniline, to be established over a wide range of excitation wavelengths. TR-VMI studies following excitation to the lowest-lying (1)ππ* state (1(1)ππ*) with a broadband femtosecond laser pulse, centered at wavelengths longer than 250 nm (4.97 eV), do not generate any measurable signature for (1)πσ* driven N-H bond fission on the amino group. Between wavelengths of 250 and >240 nm (<5.17 eV), coupling from 1(1)ππ* onto the (1)πσ* state at a 1(1)ππ*/(1)πσ* CI facilitates ultrafast nonadiabatic N-H bond fission through a (1)πσ*/S(0) CI in <1 ps, a notion supported by CASSCF results. For excitation to the higher lying 2(1)ππ* state, calculations reveal a near barrierless pathway for CI coupling between the 2(1)ππ* and 1(1)ππ* states, enabling the excited-state population to evolve through a rapid sequential 2(1)ππ* → 1(1)ππ* → (1)πσ* → N-H fission mechanism, which we observe to take place in 155 ± 30 fs at 240 nm. We also postulate that an analogous cascade of CI couplings facilitates N-H bond scission along the (1)πσ* state in 170 ± 20 fs, following 200 nm (6.21 eV) excitation to the 3(1)ππ* surface. Particularly illuminating is the fact that a number of the CASSCF calculated CI geometries in aniline bear an exceptional resemblance with previously calculated CIs and potential energy profiles along the amino moiety in guanine, strongly suggesting that the results here may act as an excellent grounding for better understanding (1)πσ* driven dynamics in this ubiquitous genetic building block.     

Enhanced diastereoselectivity via confinement: photoisomerization of 2,3-diphenylcyclopropane-1-carboxylic acid derivatives within zeolites

From the perspective of asymmetric induction, the photochemistry of 24 chiral esters and amides of cis-2,3-diphenylcyclopropane-1-carboxylic acid from excited singlet and triplet states has been investigated within zeolites. The chiral auxiliaries placed at a remote location from the isomerization site functioned far better within a zeolite than in solution. Generally, chiral auxiliaries with an aromatic or a carbonyl substituent performed better than the ones containing only alkyl substituents. A model based on cation-binding-dependent flexibility of the chiral auxiliary accounts for the observed variation in de between aryl (and carbonyl) and alkyl chiral auxiliaries within zeolites. Cation-dependent diastereomer switch was also observed in select examples.     

Significant impact of deprotonated status on the photoisomerization dynamics of bacteriophytochrome chromophore

We report that the photoinduced dynamics of the phytochrome chromophore is strongly dependent on the protonation/deprotonation states of the pyrrole ring. The on-the-fly surface hopping dynamics simulations were performed to study the photoisomerization of different protonation/deprotonation phytochrome chromophore models. The simulation results indicate that the deprotonations at the pyrrole rings significantly modify the photoinduced nonadiabatic dynamics, leading to distinctive population decay dynamics and different reaction channels. Such feature can be well explained by the formation of the different hydrogen bond network patterns. Therefore, the proper understanding of the photoisomerization mechanism of phytochrome chromophore must take the hydrogen bond network into account. This work provides the new insights into the photobiological functions of phytochrome chromophore and suggests the possible ideas to control of its photoconversion processes for further rational engineering in optical applications.     

Enhanced diastereoselectivity via confinement: diastereoselective photoisomerization of 2,3-diphenyl-1-benzoylcyclopropane derivatives within zeolites

Photochemistry of optically pure trans-2,3-diphenyl-1-benzoylcyclopropane has been examined in isotropic solution and within zeolites. Results suggest that it isomerizes by cleavage of either the C1-C2 or C1-C3 bond. From the perspective of chiral induction, photoisomerization of cis-2,3-diphenyl-1-benzoylcyclopropane derivatives with chiral auxiliaries placed at the meta and para positions of the benzoyl group have been examined both in isotropic solution and within zeolites. Whereas in isotropic solution the chiral auxiliaries placed at the meta position exhibit very little influence during the conversion of triplet cis-2,3-diphenyl-1-benzoylcyclopropane derivatives, they have significant influence within zeolites. For example, alpha-methyl benzylamine placed at the meta position of the benzoyl group (via an amide bond) yields the trans isomer with a diastereoselectivity (de) of 71% within NaY zeolite, whereas in solution no de is obtained. The chiral induction process within zeolites depends on the nature of the alkali ion and on the presence of water. Results suggest that the chiral auxiliary is able to control the bond being cleaved (C1-C2 vs. C1-C3 bond) within a zeolite, but it is unable to do so in an isotropic solution.     

Role of monovalent counterions in the ultrafast dynamics of DNA

This paper examines the contribution of counterion motion to the electric-field dynamics in the interior of DNA. The electric field is measured by a coumarin fluorophore that is synthetically incorporated into an oligonucleotide, where it replaces a native base pair. The DNA is a 17-base-pair oligomer with no A- or G-tracts. Time-resolved Stokes-shift measurements on the coumarin are made from 40 ps to 40 ns with each of the alkali ions and or one of several tetraalkylammonium ions as the DNA counterion. With the possible exception of rubidium, there are no indications of site-specific binding of the counterions. For sodium and other ions with a smaller hydrodynamic radius, the dynamics are identical and are fit to a power law. For larger ions, there is a progressive increase in the rate of shifting after 1 ns. This effect correlates with the hydrodynamic radius of the counterion. The lack of change in the spectral shape of the emission shows that neither the broadly distributed power-law relaxation nor the extra nanosecond dynamics are due to heterogeneity in the relaxation rates of different helices.     

Coherent control of the ultrafast dissociative ionization dynamics of bromochloroalkanes

We report on the coherent control of the ultrafast ionization and fragmentation dynamics of the bromochloroalkanes C(2)H(4)BrCl and C(3)H(6)BrCl using shaped femtosecond laser pulses. In closed-loop control experiments on bromochloropropane (C(3)H(6)BrCl) the fragment ion yields of CH(2)Cl(+), CH(2)Br(+), and C(3)H(3)(+) are optimized with respect to that of the parent cation C(3)H(6)BrCl(+). The fragment ion yields are recorded in additional experiments in order to reveal the energetics of cation fragmentation, where laser-produced plasma radiation is used as a tunable pulsed nanosecond vacuum ultraviolet radiation source along with photoionization mass spectrometry. The time structure of the optimized femtosecond laser pulses leads to a depletion of the parent ion and an enhancement of the fragment ions, where a characteristic sequence of pulses is required. Specifically, an intense pump pulse is followed by a less intense probe pulse where the delay is 0.5 ps. Similarly optimized pulse shapes are obtained from closed-loop control experiments on bromochloroethane (C(2)H(4)BrCl), where the fragment ion yield of CH(2)Br(+) is optimized with respect to that of C(2)H(4)BrCl(+) as well as the fragment ion ratios C(2)H(2)(+)/CH(2)Br(+) and C(2)H(3)(+)/C(2)H(4)Cl(+). The assignment of the underlying control mechanism is derived from one-color 804 nm pump-probe experiments, where the yields of the parent cation and several fragments show broad dynamic resonances with a maximum at Δt = 0.5 ps. The experimental findings are rationalized in terms of dynamic ionic resonances leading to an enhanced dissociation of the parent cation and some primary fragment ions.     

Disentangling intrinsic ultrafast excited-state dynamics of cytosine tautomers

Gas-phase ultrafast excited-state dynamics of cytosine, 1-methylcytosine, and 5-fluorocytosine were investigated in molecular beams using femtosecond pump-probe photoionization spectroscopy to identify the intrinsic dynamics of the major cytosine tautomers. The results indicate that, upon photoexcitation in the first absorption band, the cytosine enol tautomer exhibits a significantly longer excited-state lifetime than its keto and imino counterparts. The initially excited states of the cytosine keto and imino tautomers decay with sub-picosecond dynamics for excitation wavelengths shorter than 300 nm, whereas that of the cytosine enol tautomer decays with time constants ranging from 3 to 45 ps for excitation between 260 and 285 nm.     

Spatial confinement effect on the atomic structure of solid argon

Molecules confined in nanopores show unusual behavior not seen in bulk systems. The present paper reports on molecular dynamics simulations of unusual freezing behavior in confined Ar. Similar to bulk Ar, liquid Ar confined in pores with a diameter D>15sigma (5.1 nm), where sigma is the diameter of the Ar atom, crystallizes when the cooling rate is lower than a critical value (Qc). We also find that the spatial confinement does not have significant influence on Qc when D>15sigma (5.1 nm). In the pore of 10sigma (3.4 nm) in diameter, on the other hand, the behavior is dramatically changed. Crystalline Ar does not appear inside the pore even when the system is cooled at a rate lower than the Qc in the bulk system by over two orders of magnitude. Instead, amorphous Ar characterized by local icosahedral configurations is formed in the pore. We further find that, even when crystalline Ar is formed outside the pore, it does not grow deeply into the pore. This supports that the amorphous Ar is actually the most stable phase in the pore. It is well known that Ar is a poor glass former. Our finding that even such an amorphous Ar is the most stable in the pore suggests that, in any system, it is possible to prepare amorphous structure selectively by using nano-molds.     

Spatial confinement of avidin domains in microwell arrays

Microwell arrays were chemically etched across the distal faces of coherent fiber-optic bundles. A typical 1.6 mm diameter array comprised approximately 3000 individual microwells that were approximately 1-14 microm deep and approximately 22 microm wide. A methodology involving organosilane functionalized microwell surfaces and site-selective photobiotin chemistry was developed to partially fill microwells with a thin avidin layer. Avidin microwell arrays were characterized using charge coupled device optical microscopy and scanning electron microscopy. The avidin microwell arrays had individual well volumes that were six orders of magnitude smaller and up to 30-fold more numerous than commercially available avidin-coated microtiter plates. Preliminary results indicated that individual avidin microwells were ideally suited to house single biological cells. Using standard epifluorescence microscope optics and a mercury-arc lamp, an individual 22 microm wide microwell could be optically addressed and selectively filled with avidin without the use of a photolithographic mask. The ability to control both the size and position of avidin domains on the microwell array surface demonstrates the utility of this methodology towards fabricating a single microwell array with multianalyte sensing capabilities.     

Carotenoid response to retinal excitation and photoisomerization dynamics in xanthorhodopsin

We present a comparative study of xanthorhodopsin, a proton pump with the carotenoid salinixanthin serving as an antenna, and the closely related bacteriorhodopsin. Upon excitation of retinal, xanthorhodopsin exhibits a wavy transient absorption pattern in the region between 470 and 540 nm. We interpret this signal as due to electrochromic effect of the transient electric field of excited retinal on salinixanthin. The spectral shift decreases during the retinal dynamics through the ultrafast part of the photocycle. Differences in dynamics of bacteriorhodopsin and xanthorhodopsin are discussed.     

Confined ultrafast torsional dynamics of Thioflavin-T in a nanocavity

The influence of confinement in the supramolecular β-cyclodextrin nanocavity on the excited state torsional dynamics of the amyloid fibril sensor, Thioflavin-T, is explored using subpicosecond fluorescence up-conversion spectroscopy. In the presence of β-cyclodextrin, the emission intensity and the fluorescence lifetime of Thioflavin-T significantly increases, indicating the confinement effect of the nanocage on the photophysical behaviour of the dye. Detailed time-resolved fluorescence studies show an appreciable dynamic Stokes' shift for the dye in the β-cyclodextrin nanocavity. Analysis of the time-resolved area normalized emission spectra (TRANES) indicates the formation of an emissive TICT state. The rate of formation of the TICT state, as calculated from the time dependent changes in the peak frequency and the width of the emission spectra, is found to be substantially slower in the β-cyclodextrin nanocavity compared to that in bulk water. Present results indicate that ultrafast torsional motion in Thioflavin-T is significantly retarded due to confinement by the β-cyclodextrin nanocavity.