Reaction dynamics of a molecular switch unveiled by coherent two-dimensional electronic spectroscopy

Coherent two-dimensional electronic spectroscopy is usually employed on molecular species with fixed geometric configuration. Here we present two-dimensional Fourier-transform electronic spectra of dissolved 6,8-dinitro-1',3',3'-trimethylspiro[2H-1-benzopyran-2,2'-indoline] (6,8-dinitro-BIPS), a photochromic system present in two ring-open forms differing in the cis/trans configuration of a double bond, which both undergo a photoinduced ring closure. The two-dimensional spectra, recorded with 20 fs pump pulses centered at 605 nm and a supercontinuum probe covering the complete visible spectral range, allow for a detailed analysis of the photophysics and photochemistry of the two isomers and directly reveal that cis/trans isomerization among them does not play a major role. This experiment demonstrates the potential of two-dimensional electronic spectroscopy for reactive processes.     

Coherent control of cross-peaks in chirality-induced two-dimensional optical signals of excitons

Polarization tuning of the interference of chirality-induced tensor components is used to simulate the suppression of diagonal peaks and amplification of cross peaks in femtosecond two-dimensional photon echo signals of excitons in a chiral porphyrin dimer. Superpositions of various tensor components which generate the optimized signals are constructed using a genetic learning algorithm. Exciton couplings and bath correlations may be extracted from these highly resolved signals.     

J-aggregates in matrix stabilized two-dimensional azobenzene derivatives

A two-component film technique at the air-water interface has been used for fabricating matrix stabilized azobenzene J-aggregates. Langmuir monolayers of (E)-1-(3-chloro-4-(alkyloxy)phenyl)-2-phenyldiazene (CnCD, n=8,10,12) have been prepared with stearic acid (STA) as the two-dimensional matrix. Miscibility studies at a molecular level, explored from the monolayer pressure-area isotherms revealed a phase separation of the CnCD from the stearic acid matrix at a compression pressure of 10 mN/m. A 43-nm strong red shift in the 350 nm pi-pi * absorption feature implied formation of highly ordered J-aggregates of CnCDs in conformity with atomic force microscopy and micro-Raman spectral characteristics. While a one-component CnCD failed to form a 2D monolayer, the STA supported CnCD binary system crossed a mixed monolayer phase followed by compression, leading to the formation of matrix stabilized CnCD J-aggregates.     

Probing electron correlations in molecules by two-dimensional coherent optical spectroscopy

The nonlinear optical signal generated in phenol by three femtosecond pulses with wavevectors k1, k2, and k3 in the phase-matching direction k1 + k2 - k3 is simulated. This two-dimensional coherent spectroscopy (2DCS) signal has a rich pattern containing information on double-excitation states. The signal vanishes for uncorrelated electrons due to interference among quantum pathways and, thus, provides direct signatures of correlated many-electron wavefunctions. This is illustrated by the very different 2DCS signals predicted by two levels of electronic structure calculations: state-averaged complete active space self-consistent field (SA-CASSCF) and multistate multiconfigurational second-order perturbation theory (MS-CASPT2).     

Enhanced intra-aggregate charge separation from binary excitons in mixed J-aggregates of cyanine dyes

Well-ordered, two-dimensional, amalgamation-type J-aggregates consisting of two different kinds of structurally analogous anionic cyanine dyes were self-assembled on an amino-functionalized Au(111) electrode surface. Similar binary J-aggregates, though considerably less ordered in either one- or three-dimensional structure, could also be grown easily in a concentrated ( approximately 1.5 M) aqueous solution of NaCl. These binary J-aggregates are characterized by a singly peaked J-band associated with a unique molecular exciton shared by the two different dye components coherently. This cooperative binary excitonic state can live with substantial (more than 0.1 eV) energy gaps between the HOMO (highest occupied molecular orbital) and LUMO (lowest unoccupied molecular orbital) levels of the two dyes mixed together. These energy gaps allow the binary exciton to relax very rapidly to an intra-aggregate charge-separated state in good competition with the metal-induced quenching of the excited-state energy. The enhanced charge separation leads to a high-efficiency photocurrent generation with a simple binary J-aggregate-coated Au(111) electrode in a reversible redox solution and results in strong quenching of the fluorescence from the corresponding aggregate grown in solution free from substrate-induced quenching.     

Novel coherent two-dimensional optical spectroscopy probes of chirality exchange and fluctuations in molecules

We demonstrate how stochastic transitions between molecular configurations with opposite senses of chirality may be probed by 2D optical signals with specific pulse polarization configurations. The third-order optical response of molecular dimers (such as biphenyls) with dynamical axial chirality is calculated to order of k(2) in the wavevector of light. Spectroscopic signatures of equilibrium chirality fluctuations are predicted for three dynamical models (Ornstein-Uhlenbeck, two-state jump, and diffusion in double well) of the dihedral angle that controls the chirality.     

Controlled formation of the two-dimensional TTBC J-aggregates in an aqueous solution

Strong experimental and theoretical evidence was provided on the controlled formation of the two-dimensional J-aggregates that were assembled in the herringbone morphology. The exciton-band structure formation of 1,1',3,3'-tetraethyl-5,5',6,6'-tetrachlorobenzimidazolocarbocyanine (TTBC) J-aggregates was investigated in ionic (NaOH) aqueous solution at room temperature. The control was achieved by changing the [TTBC] at a given [NaOH], or vice versa, and was monitored through the changes in the absorption, fluorescence excitation, and emission spectra. Specific attention was paid to expose the excited-state structure and dynamics through simulations of the excitonic properties, which included diagonal energetic disorder and phonon-assisted exciton relaxation. Aggregates were characterized by an asymmetrically split Davydov pair, an H-band (approximately 500 nm, 1300 cm(-1) wide, Lorentzian-like) and a J-band (approximately 590 nm, 235 cm(-1) wide, with a band shape typical of a one-dimensional J-aggregate), whose relative intensities showed a strong dependence on the [TTBC]/[NaOH]. The H-band is favored by high [TTBC] or high [NaOH]. An explanation of the control on the aggregate formation was given by correlating the changes in the absorption with the structural modifications and the subsequent changes in the dynamics, which were induced by variations in the dye and NaOH concentrations. The J-band shape/width was attributed to disorder and disorder-induced intraband phonon-assisted exciton relaxation. The intraband processes in both bands were estimated to occur in the same time scale (about a picosecond). It has been suggested that the wide energetic gap between the Davydov split bands (3000 cm(-1)) could get bridged by the excitonic states of the loosely coupled chains, in addition to the monomeric species at low [TTBC]. Phonon-assisted interband relaxation, through the band gap states and/or directly from the H- to the J-band, are suggested for accounting the difference between the bandwidths and shapes of the two bands. Energy transfer between the H-band and the monomeric species is suggested as crucial for tuning the relative strengths of the two bands.     

Unusual optical properties of porphyrin fractal J-aggregates

A simple mix and shake procedure in aqueous solution from water-soluble tetrakis(4-sulfonatophenyl)porphyrin and spermine leads to fractal non-covalent supramolecular systems which exhibit unusual large enhancement of resonant light scattering, behaving as metal particles.     

Chirality-induced signals in coherent multidimensional spectroscopy of excitons

The nonlocal second- and third-order susceptibilities of an isotropic ensemble of aggregates are calculated by solving the nonlinear exciton equations which map the system into coupled anharmonic oscillators. Both electric and magnetic contributions are included using the minimal-coupling Hamiltonian. The various tensor components are evaluated to first order in the optical wave vector k. Additional structural information about the interchromophore distances, which is not accessible through zeroth-order contributions (the dipole approximation), is contained to the first order in k. New resonant second- and third-order signals predicted for chiral molecules provide multidimensional extensions of circular dichroism spectroscopy. Numerical simulations demonstrate the sensitivity of third-order signals to the secondary structural motiffs of peptides.     

Mesoscopic organization of two-dimensional J-aggregates of thiacyanine in Langmuir-Schaefer films

When dioctadecyl dimethylammonium bromide (DODAB) is compressed on a subphase containing 3,3'-disulfopropyl-5,5'-dichlorothiacyanine (THIAMS), adsorption of the dye to the DODAB monolayer results in the formation of J-aggregates which spontaneously organize into polygonal domains of micron size. The features of the domains depend on the surface pressure. The fluorescence of the individual domains is polarized. The shapes of the domains determined by fluorescence microscopy and atomic force microscopy (AFM) are identical. The domains differ however significantly from those observed after injection of a 3,3'-disulfopropyl-5,5'-dichloro-9-ethylthiacarbocyanine (THIATS) or THIAMS solution below a precompressed DODAB film, as well as from the domains observed upon compression of a DODAB monolayer on a subphase containing 10(-6) M THIATS.     

Heterogeneous exciton dynamics revealed by two-dimensional optical spectroscopy

We show that optical two-dimensional (2D) spectroscopy can recover ultrafast heterogeneous dynamics of closely spaced delocalized exciton states from a molecular exciton manifold characterized by a single absorption band. The complete experimental third-order nonlinear optical response from room-temperature J-aggregates in liquid phase is reproduced for the first time with self-consistent Frenkel exciton theory combined with modified Redfield theory. We show that exciton relaxation between the exciton states and nuclear-motion-induced exchange-narrowed energy fluctuations of individual delocalized exciton states can be distinguished because these two processes lead to a distinctively different evolution of the absolute 2D spectrum. Our technique also allows recovery of the variation of the exciton relaxation rates as well as the degree of exciton delocalization across the absorption band.     

The relationship between microwave spectroscopy,coherent triplet excitons,and density of states functions in molecular crystals

A theory of microwave band-to-band transitions in triplet Frenkel excitons in the coherent migration limit for a one-dimensional exciton associated with translationally equivalent molecules is developed. Because of selective spin-orbit coupling to the magnetic sublevels, an anisotropy in the zero field splitting across the band (k = 0 to k = ±π/a) occurs which, on a reduced energy scale, “mirrors” the energy dispersion of triplet exciton states. As a result, if the coherence time of a k state of the triplet band is longer than the intrinsic time associated with a microwave field connecting the magnetic sublevels of the band, one can measure both the bandwidth and density of states function with a zero field electron spin resonance experiment.     

Energy-transfer and charge-separation pathways in the reaction center of photosystem II revealed by coherent two-dimensional optical spectroscopy

The excited state dynamics and relaxation of electrons and holes in the photosynthetic reaction center of photosystem II are simulated using a two-band tight-binding model. The dissipative exciton and charge carrier motions are calculated using a transport theory, which includes a strong coupling to a harmonic bath with experimentally determined spectral density, and reduces to the Redfield, the F?rster, and the Marcus expressions in the proper parameter regimes. The simulated third order two-dimensional signals, generated in the directions -k(1)+k(2)+k(3), k(1)-k(2)+k(3), and k(1)+k(2)-k(3), clearly reveal the exciton migration and the charge-separation processes.     

High efficiency photocurrent generation by two-dimensional mixed J-aggregates of cyanine dyes

Two-dimensional mixed J-aggregates of structurally and spectrally analogous anionic cyanine dyes, coadsorbed on a self-assembled monolayer of aminoalkanethiolate on Au(111), generated a high-efficiency (20-30% quantum efficiency) cathodic photocurrent and a significant photovoltaic effect in reversible Fe2+/Fe3+ redox solution.     

Backbone dynamics of proteins studied by two-dimensional heteronuclear NMR spectroscopy and molecular dynamics simulations

To investigate the backbone dynamics of proteins ¹⁵N longitudinal and transverse relaxation experiments combined with {¹H, ¹⁵N{ NOE measurements together with molecular dynamics simulations were carried out using ribonuclease T₁ and the complex of ribonuclease T₁ with 2′GMP as a model protein. The intensity decay of individual amide cross peaks in a series of (¹H, ¹⁵N)HSQC spectra with appropriate relaxation periods was fitted to a single exponential by using a simplex algorithm in order to obtain ¹⁵N T₁ and T₂ relaxation times. The relaxation times were analyzed in terms of the “model-free” approach introduced by Lipari and Szabo. In addition, a nanosecond molecular dynamics (MD ) simulation of ribonuclease T₁ and its 2′GMP complex in water was carried out. The angular reorientations of the backbone amide groups were classified with several coordinate frames following a transformation of NH vector trajectories. In this study, NH librations and backbone dihedral angle fluctuations were distinguished. The NH bond librations were found to be similar for all amides as characterized by correlation times of librational motions in a subpicosecond scale. The angular amplitudes of these motions were found to be about 10°–12° for out-of-plane displacements and 3°–5° for the in-plane displacement. The contributions from the much slower backbone dihedral angle fluctuations strongly depend on the secondary structure. The dependence of the amplitude of local motion on the residue location in the backbone is in good agreement with the results of NMR relaxation measurements and the X-ray data. The protein dynamics is characterized by a highly restricted local motion of those parts of the backbone with defined secondary structure as well as by a high flexibility in loop regions. Comparison of the MD and NMR data of the free liganded enzyme ribonuclease T₁ clearly indicates a restriction of the mobility within certain regions of the backbone upon inhibitor binding. © 1996 John Wiley & Sons, Inc.     

Coherence in electronically excited dimers. The observation of coherent dimers and its relationship to coherent excitons

The coherent nature of molecular excited dimers in their triplet states is unequivocally established by a series of low temperature optically detected magnetic resonance experiments in zero field. The results clearly establish that a minimum coherence time for the dimer ψ(+) state in 1,2,4,5-tetrachlorobenzene (≈ 1.7 K) is longer than 5 × 10^?7 s while the exchange time in these systems is approximately 16 × 10^?12 s. This implies that the coherence persists for a time approximately 10⁵ times that associated with the stochastic limit. The coherence time could, however, be even longer. Moreover, these new results clearly demonstrate that there is a direct and quantitative relationship between coherent properties of dimers and the coherent properties of triplet Frenkel excitons.     

Stochastic Liouville equations for coherent multidimensional spectroscopy of excitons

Signatures of chemical exchange and spectral diffusion in 2D photon-echo line shapes of molecular aggregates are studied using model calculations for a dimer whose Hamiltonian parameters are stochastically modulated. Cross peaks induced by chemical exchange and by exciton transport have different dynamics and distinguish two models which have the same absorption spectrum (a two-state jump bath modulation model of a dimer and a four-state jump bath model of a single chromophore). Slow Gaussian-Markovian spectral diffusion of a symmetric dimer induces new peaks which are damped as the dipole moment is equilibrated. These effects require an explicit treatment of the bath and may not be described by lower-level theories such as the Redfield equations, which eliminate the bath.     

Orientational correlations in two-dimensional liquid crystals studied by molecular dynamics simulation

Orientational correlations in Langmuir monolayers of nematic and smectic-C liquid crystal (LC) phases are investigated by molecular dynamics simulation. In both phases, the orientational correlation functions decay algebraically yet with the different exponents of 1.9 and 0.2 for the nematic and the smectic-C monolayers, respectively. The power law decay, i.e., the absence of long-range orientational order, means the both monolayers should be the ideal 2D system with a continuous symmetry, whereas the large difference in the exponents of power law gives rise to the crucial difference in their optical properties; the nematic monolayer is optically isotropic while the smectic-C monolayer exhibits an anisotropy on the length scale of visible light. Since the exponent is inversely proportional to the molecular exchange energy, the averaged molecular interaction in the nematic monolayer should be an order of magnitude smaller than that in the smectic-C monolayer, which is ascribed to the low molecular density and the weak molecular dipole due to the water molecule. The relation between the molecular interaction and the orientational correlation calculated for the 2D LC system offers much information not only about the 2D LCs but also on the bulk system.     

Nonlinear optical properties of confined excitons in clusters

Size effects in femtosecond photon echo spectroscopy of neat clusters are calculated using a quasiparticle representation of the nonlinear response. We extend our previous study of cooperative effects on the nonlinear response of assemblies of two level molecules [J. A. Leegwater and S. Mukamel, Phys. Rev. A46, 452 (1992)] to allow for nuclear motion and to have an s-p model of polarizable atoms. Photon echos in Benzene/Argon clusters are calculated using a semiclassical phase averaging procedure [L. E. Fried and S. Mukamel, Adv. Chem. Phys. (in Press)].     

Internal dynamics features in the free jet rotational spectrum of the acetaldehyde-Kr molecular complex

The structure and the dynamics of internal motions in the complex formed between acetaldehyde and Kr are studied by free jet absorption microwave spectroscopy performed in the range 60-78 GHz. The fourfold structure of each rotational line is evidence of the vibration-rotation coupling between the overall rotation of the complex, a tunneling motion of the Kr atom between two equivalent positions and the internal rotation of the methyl group in the acetaldehyde moiety. The four sets of transitions could be fitted with a coupled Hamiltonian which allows for the Coriolis interaction obtaining the energy separation between the vibrational energy levels related to the tunneling motion, while the observed splittings due to the methyl group internal rotation were analyzed independently with an appropriate model. The potential energy barriers for the tunneling motion and the internal rotation of the methyl group have been calculated and the interaction of the rare gas atom with the acetaldehyde moiety is reflected in the change of the V(3) barrier to internal rotation in going from the molecule to the weakly bound complex.