I. Technical aspects of transient grating experimentsFemtosecond spectroscopy experiments are based on a Quantronix Q-lite seeded Integra C Titanium Sapphire amplifier generating 130 fs, 800 nm, 2.0 mJ laser pulses at 1 kHz. The laser system pumps two home-built noncollinear optical parametric amplifiers (NOPA). 1,2 The NOPA used for pump pulse generation has a spectral bandwidth corresponding to transform limited 15 fs laser pulses, whereas the NOPA from which probe pulses are derived generates spectra spanning the full 500-750 nm wavelength range. Portions of the probe spectrum are filtered in a fused silica prism compressor for use in experiments. Pump and probe pulses are compressed to 15-20 fs with a time-bandwidth product of 0.5-0.6 where residual third-order dispersion prevents compression to the Fourier transform limit.Transient grating (TG) experiments are performed with the interferometer shown in Figure S1. The interferometer generates a trapezoidal laser beam geometry with diffractive-optics (DO) for passively phase-stabilized interferometric signal detection. [3][4][5][6][7][8][9] The TG interferometer uses a DO (Holoeye) producing an angle of 4.65 degrees between the +/-1 diffraction orders at 590 nm. The pump (pulses 1 and 2) and probe (pulses 3) beams are crossed at angle of approximately 4.6 degrees in the DO. Pulses 1 and 2 arrive at the sample at the same time, and are delayed with respect to pulses 3 and 4 with a motorized translation stage. The signal is phase-matched so that it is automatically collinear with the reference pulse after the sample.

Transport processes and spectroscopic phenomena in light harvesting proteins depend sensitively on the characteristics of electron-phonon couplings. Decoherence imposed by low-frequency nuclear motion generally suppresses the delocalization of electronic states, whereas the Franck-Condon progressions of high-frequency intramolecular modes underpin a hierarchy of vibronic Coulombic interactions between pigments. This Article investigates the impact of vibronic couplings on the electronic structures and relaxation mechanisms of two cyanobacterial light-harvesting proteins, allophycocyanin (APC) and C-phycocyanin (CPC). Both APC and CPC possess three pairs of pigments (i.e., dimers) that undergo electronic relaxation on the subpicosecond time scale. Electronic relaxation is ~10 times faster in APC than in CPC despite the nearly identical structures of their pigment dimers. We suggest that the distinct behaviors of these closely related proteins are understood on the same footing only in a basis of joint electronic-nuclear states (i.e., vibronic excitons). A vibronic exciton model predicts well-defined rate enhancements in APC at realistic values of the site reorganization energies, whereas a purely electronic exciton model points to faster dynamics in CPC. Calculated exciton sizes (i.e., participation ratios) show that wave function delocalization underlies the rate enhancement predicted by the vibronic exciton model. Strong vibronic coupling and heterogeneity in the pigment sites are the key ingredients of the vibronic delocalization mechanism. In contrast, commonly employed purely electronic exciton models see heterogeneity as only a localizing influence. This work raises the possibility that similar vibronic effects, which are often neglected, may generally have a significant influence on energy transport in molecular aggregates and photosynthetic complexes.

Femtosecond transient grating and photon echo spectroscopies with a sub-20 fs time resolution are applied to allophycocyanin (APC), a protein located at the base of the phycobilisome antenna of cyanobacteria. Coupling between pairs of phycocyanobilin pigments with nondegenerate energy levels gives rise to the four-level exciton electronic structure of APC. Spectroscopic signals obtained in multiple experiments (e.g., linear absorption, fluorescence, transient grating, 2D Fourier transform photon echo) are used to constrain the parameters of a Frenkel exciton Hamiltonian. Comparison between experiment and theory yields a robust microscopic understanding of the electronic and nuclear relaxation dynamics. In agreement with previous work, transient absorption anisotropy establishes that internal conversion between the exciton states of the dimer occurs with time constants of 35, 220, and 280 fs. The sub-100 fs dynamics are decomposed into three distinct relaxation processes: electronic population transfer, intramolecular vibrational energy redistribution, and the dephasing of electronic and nuclear coherences. Model calculations show that the sub-100 fs red-shift in the transient absorption signal spectrum reflects interference between stimulated emission (ESE) and excited state absorption (ESA) signal components. It is also established that the pigment fluctuations in the dimer are not well-correlated, although further experiments will be required to precisely quantify the amount of correlation. The findings of this paper suggest that the light harvesting function of APC is enhanced by nondegeneracy of the pigments comprising the dimer and strong vibronic coupling of intramolecular modes on the phycocyanobilins. We find that the exciton states are 96% localized to the individual molecular sites within a particular dimer. Localization of the transition densities, in turn, is suggested to promote significant vibronic coupling which serves to both broaden the absorption line shape and open channels for fast internal conversion. The dominant internal conversion channel is assigned to a promoting mode near 800 cm(-1) involving hydrogen out-of-plane (HOOP) wagging motion similar to that observed in phytochrome and retinal. This rate enhancement ensures that all photoexcitations quickly and efficiently relax to the electronic origin of the lower energy exciton state from which energy transfer to the reaction center occurs.

Heterodyne-detected transient grating (TG) and two-dimensional photon echo (2DPE) spectroscopies are extended to the mid-UV spectral range in this investigation of photoinduced relaxation processes of adenine in aqueous solution. These experiments are the first to combine a new method for generating 25 fs laser pulses (at 263 nm) with the passive phase stability afforded by diffractive optics-based interferometry. We establish a set of conditions (e.g., laser power density, solute concentration) appropriate for the study of dynamics involving the neutral solute. Undesired solute photoionization is shown to take hold at higher peak powers of the laser pulses. Signatures of internal conversion and vibrational cooling dynamics are examined using TG measurements with signal-to-noise ratios as high as 350 at short delay times. In addition, 2DPE line shapes reveal correlations between excitation and emission frequencies in adenine, which reflect electronic and nuclear relaxation processes associated with particular tautomers. Overall, this study demonstrates the feasibility of techniques that will hold many advantages for the study of biomolecules whose lowest-energy electronic resonances are found in the mid-UV (e.g., DNA bases, amino acids).

Femtosecond electronic relaxation dynamics of a cylindrical molecular aggregate are measured with transient grating (TG) and two-dimensional Fourier transform photon echo (PE) spectroscopies. The aggregates are double-walled cylindrical structures formed by self-assembly of amphiphilic cyanine dye molecules in water. The diameters of the inner and outer cylinders are approximately 6 and 10 nm. The linear absorption spectrum of the aggregate exhibits four spectrally resolved single exciton transitions corresponding to excited states localized on particular regions of the structure: (1) an excited state localized on the inner cylinder corresponds to the lowest energy transition at 16670 cm(-1); (2) a transition at 17150 cm(-1) represents a state localized on the outer cylinder, (3) whereas an overlapping peak found at 17330 cm(-1) is more closely associated with the inner cylinder; (4) an excited state delocalized between the inner and outer cylinder is assigned to a transition in the linear absorption spectrum at 17860 cm(-1). TG spectra show a series of resonances reflecting the electronic structure of both the single and double exciton manifolds. In addition, PE spectra reveal coherent modulation of both diagonal and cross-peak amplitudes persisting for 100 fs, where the coherence frequency matches the energy gap between transitions 1 and 4 in the linear absorption spectrum. PE line shapes suggest correlated energy level fluctuations for the exciton states associated with these two transitions, which is consistent with this fairly long-lasting coherence at room temperature in aqueous solution. The impact of these correlations on Forster energy transfer efficiency is discussed. The observations imply fairly long-range correlations between the molecular sites (>0.6 nm), which in turn reflects the length scale of the environmental motion inducing the fluctuations. We suggest that this environmental motion is most likely associated with water confined inside the cylinder and/or fluctuations of the dye's aliphatic functional groups.