Probing electronic coupling in excitonically coupled heterodimer complexes by two-color three-pulse photon echoes

Following the earlier work of Yang et al. [J. Chem. Phys. 110 (1999) 2983] analytical expressions for the downhill and uphill resonant two-color three-pulse photon echo peak shift (2C-3PEPS) of a heterodimer system are derived in the impulsive limit. It is shown how to obtain information about coupling between the components of the dimer from the combined one- and two-color peak shift measurements. Further analytical relations are derived which enable site specific information about the environment of the components, including the relative difference of the inhomogeneity and the difference between the energy-gap correlation functions on the heterodimer sites to be obtained. The simulations show only a very small influence of the laser pulse length on the measured values of coupling coefficient and other relevant quantities suggesting that current 2C-3PEPS measurements can find practical application in directly measuring couplings in excitonically coupled heterodimer complexes.

Femtosecond Pump-Probe Measurements of Solvation by Hydrogen-Bonding Interactions

An additional ultrafast blue shift in the transient absorption spectra of hydrogen-bonding complexes of a strong photoacid, 8-hydroxypyrene 1,3,6-trisdimethylsulfonamide (HPTA), over the solvation response of the uncomplexed HPTA and also over that of the methoxy derivative of the photoacid (MPTA) in the presence of the hydrogen-bonding base was observed on optical excitation of the photoacid. The additional 55 +/- 10 fs solvation response was found to be about 35 % and 19% of the total C(t) of HPTA in dichloromethane (DCM) when it was hydrogen-bonded to dimethylsulfoxide (DMSO) and dioxane, respectively, and about 29% of the total C(t) of HPTA in dichloroethane (DCE) when it was hydrogen-bonded to DMSO. We have assigned this additional dynamic spectral shift to a transient change in the hydrogen bond (O-H...O) that links HPTA to the complexing base, after the electronic excitation of the photoacid.

Toward an understanding of the mechanism of nonphotochemical quenching in green plants

Oxygenic photosynthesis in plants involves highly reactive intermediates and byproducts that can damage the photosynthetic apparatus and other chloroplast constituents. The potential for damage is exacerbated when the amount of absorbed light exceeds the capacity for light energy utilization in photosynthesis, a condition that can lead to decreases in photosynthetic efficiency. A feedback de-excitation mechanism (qE), measured as a component of nonphotochemical quenching of chlorophyll fluorescence, regulates photosynthetic light harvesting in excess light in response to a change in thylakoid lumen pH. qE involves de-excitation of the singlet excited state of chlorophyll in the light-harvesting antenna of photosystem II, thereby minimizing the deleterious effects of high light via thermal dissipation of excess excitation energy. While the physiological importance of qE has been recognized for many years, a description of its physical mechanism remains elusive. We summarize recent biochemical and spectroscopic results that have brought us closer to the goal of a mechanistic understanding of this fundamental photosynthetic regulatory process.

Phase-stabilized two-dimensional electronic spectroscopy

Two-dimensional (2D) spectroscopy is a powerful technique to study nuclear and electronic correlations between different transitions or initial and final states. Here we describe in detail our development of inherently phase-stabilized 2D Fourier-transform spectroscopy for electronic transitions. A diffractive-optic setup is used to realize heterodyne-detected femtosecond four-wave mixing in a phase-matched box geometry. Wavelength tunability in the visible range is accomplished by means of a 3 kHz repetition-rate laser system and optical parametric amplification. Nonlinear signals are fully characterized by spectral interferometry. Starting from fundamental principles, we discuss the origin of phase stability and the precise calibration of excitation-pulse time delays using movable glass wedges. Automated subtraction of undesired scattering terms removes experimental artifacts. On the theoretical side, the response-function formalism is extended to describe molecules with three electronic levels, and the shape of 2D spectral features is discussed. As an example for this technique, experimental 2D spectra are shown for the dye molecule Nile Blue in acetonitrile at 595 nm, recorded for a series of population times. Simulations explore the influence of different model parameters and qualitatively reproduce the experimental results. We show that correlations between different electronically excited states can be determined from the spectra. The technique described here can be used to measure the third-order response function of complex systems covering several electronic transitions.

Tunable two-dimensional femtosecond spectroscopy

We have developed a two-dimensional (2D) Fourier-transform femtosecond spectroscopy technique for the visible spectral region. Three-pulse photon echo signals are generated in a phase-matched noncollinear four-wave mixing box geometry that employs a 3-kHz repetition-rate laser system and optical parametric amplification. Nonlinear signals are fully characterized in amplitude and phase by spectral interferometry. Unlike for previous setups, we achieve long-term phase stability by employing diffractive optics and interferometric accuracy of excitation-pulse time delays by using movable glass wedges. As an example of this technique, 2D correlation and relaxation spectra at 600 nm are shown for a solution of Nile Blue dye in acetonitrile.

Two-dimensional optical spectroscopy: Two-color photon echoes of electronically coupled phthalocyanine dimers

Two-color photon echo peak shift spectroscopy was used to study electronic coupling in a phthalocyanine homodimer. Two optical parametric amplifiers were used to produce pulses to excite the split lower states of LuPc2-. The existence of a two-color peak shift indicates the existence of correlation between these two dipole-allowed states. The nature of this correlation is discussed based on theoretical predictions of the interactions between exciton and charge resonance states.

Energy transfer in photosystem I of cyanobacteria Synechococcus elongatus: model study with structure-based semi-empirical Hamiltonian and experimental spectral density

We model the energy transfer and trapping kinetics in PSI. Rather than simply applying Förster theory, we develop a new approach to self-consistently describe energy transfer in a complex with heterogeneous couplings. Experimentally determined spectral densities are employed to calculate the energy transfer rates. The absorption spectrum and fluorescence decay time components of the complex at room temperature were reasonably reproduced. The roles of the special chlorophylls (red, linker, and reaction center, respectively) molecules are discussed. A formally exact expression for the trapping time is derived in terms of the intrinsic trapping time, mean first passage time to trap, and detrapping time. The energy transfer mechanism is discussed and the slowest steps of the arrival at the primary electron donor are found to contain two dominant steps: transfer-to-reaction-center, and transfer-to-trap-from-reaction-center. The intrinsic charge transfer time is estimated to be 0.8 approximately 1.7 ps. The optimality with respect to the trapping time of the calculated transition energies and the orientation of Chls is discussed.

Photochemistry of dianthrylsilanes: a study of sigma,pi-interaction

In this article, we demonstrated by the application of time-resolved spectroscopy, X-ray structural analysis and other spectroscopic techniques that 9-Anthrylsilanes exhibits sigma,pi-interaction between 9-anthryl group and the Si-Si linkage in anthryl-disilanes, ASi(2), ASi(2)A, and ASi(3)A which does not occur in the analogous alkyl derivatives as well as the pyrenylsilane derivatives, in spite of the fact that the 0,0-band of PSi(2) is about 12.8 KJ more energetic than that of ASi(2) (Figure 1). More interestingly, the X-ray structural studies reveal that ASi(3)A exists in a butterfly-like structure in agreement with other spectroscopic analyses that the two anthryl groups do not interact in their excited states, while those in ASi(2)A do. This is in contrast to the analogous pyrenylsilanes; the trisilanes exhibits a stronger excimer interaction than that of disilane.(10b) Our results show that the sigma,pi-interactions in ASi(3)A has imparted rigidity to the tri-silyl linkage. Potential applications of anthrylsilanes in material sciences will be explored.(5) This work provides evidence that sigma,pi-interaction between the 9-anthryl group and disilyl linkage does play an important role in the properties of disilanes. We attribute this enhanced sigma,pi-interaction to the nature of the lowest excited state (S(1) state) of anthracenes, the L(a) transition, which has a much higher oscillator strength than the S(1)L(b)-transition of pyrenes (Figure 1). We define the interaction in anthracene as a sigma,pi(S(1,)L(a)) interaction. This interaction lends a substantial barrier to the Si-Si bond with the excited anthryl nucleus in anthrylsilanes. The scope and potential applications of this phenomenon are discussed.

Evidence for direct carotenoid involvement in the regulation of photosynthetic light harvesting

Nonphotochemical quenching (NPQ) refers to a process that regulates photosynthetic light harvesting in plants as a response to changes in incident light intensity. By dissipating excess excitation energy of chlorophyll molecules as heat, NPQ balances the input and utilization of light energy in photosynthesis and protects the plant against photooxidative damage. To understand the physical mechanism of NPQ, we have performed femtosecond transient absorption experiments on intact thylakoid membranes isolated from spinach and transgenic Arabidopsis thaliana plants. These plants have well defined quenching capabilities and distinct contents of xanthophyll (Xan) cycle carotenoids. The kinetics probed in the spectral region of the S(1) --> S(n) transition of Xans (530-580 nm) were found to be significantly different under the quenched and unquenched conditions, corresponding to maximum and no NPQ, respectively. The lifetime and the spectral characteristics indicate that the kinetic difference originated from the involvement of the S(1) state of a specific Xan, zeaxanthin, in the quenched case.