Signatures of Quantum–Effects in Complex Molecules

Two-Dimensional Coherent Electronic Spectroscopy (2D-ES) has been increasingly used, over the past few years, in the study of biological & artificial light harvesters and has clarified numerous subtleties of energy-transfer dynamics. The phase-sensitive Four-Wave-Mixing technique, measuring the time-domain signal S(3) (k s ,t 1 ,t 2 ,t 3 ) in vector-space ks = k 1 k2 +k 3 resolves cross-peaks in 2D frequency-frequency correlation maps S(3) (1 , t 2 , 3 ) and visualizes macroscopic quantum-interference effects such as electronic wave-packets and inter-band electronic coherence. 2D-ES traces the physics of molecular many-body correlations, it highlights the early, non-perturbative exciton and its phase relaxation, thereby probing electronic coupling and electron-phonon interaction as the fundamental underpinnings that dictate the elements of coherent/incoherent dynamics in light-harvesting energy transfer and excitonic transport. In this follow-up program of the successfully completed P 18233, auxiliary instrumentations and novel variants of 2D-ES are proposed, varying sequences of phase-locked sub-20 fs excitation pulses via rotation of polarization and wave-vector geometry. In Sec. II.2 A - 2D-ES made easy and upgraded by polarization pulse shaping - a new experimental 2D configuration based upon double-pulse pump/probe and polarization shaping is proposed. Systematic experiments are intended to disentangle and simplify the landscape of cross-peaks. New studies on the bi-tubular aggregates C8O3 and its thio-analog C8S3 are planned, with the objective to suppress diagonal peaks and amplify & unveil off-diagonal signal patterns of single- and double excitons. Varying polarization target areas and identifying Liouville pathways may enable to raise the selective manipulation of energy transfer through optimum control experiments. The second big enterprise in this proposal, Two-Quantum 2D Electronic Correlation Spectroscopy, 2Q-2D-ECS (Sec.II.2 B) concerns experiments that specifically probe bi-exciton-/molecular double excitation states and their correlation energies. These measurements detect the correlation signal in vector-space kS = +k 1 +k 2 -k3 and realize the bi-exciton correlation via double-quantum excitation, while its 2D Fourier projection gives rise to the double- quantum correlation spectrum S(3) (t1 , 2 , 3 ). The double-quantum correlation technique opens new avenues for probing higher correlation effects and separating bi-exciton from single-exciton pathways. 2Q-2D-ECS may sense the specific role of bi-exciton band-to-band couplings as powerful dissipative channels of excess energy caused by intensive light-flux in native biological and synthetic aggregate environments. Furthermore, since the signal-pulse is generated only when the electrons are correlated, experimental realization of the double-quantum optical response in carotenoids would give a clear decision about mystery and reality of the two-quantum resonance hypothesis. 2D electronic correlation experiments in tandem with theory (cooperation S. Mukamel, UCI) are, therefore, conjectured to be powerful fs-instrumentations to probe the fundamental doctrine of quantum-dynamics and dissipation in complex molecules: coupling makes motion and motion causes function.

With the advancement of Two-Dimensional Fourier Transform Electronic Spectroscopy (2D-FT ES) in the last decade, femto-chemical physics of bio- & synthetic molecular/material systems has experienced ground-breaking experiments in time- and frequency optical cycle space, with stabilized relative phase in the hundreds of THz regime. The method goes beyond established photon echo or transient grating multi-wave mixing by correlating coherent dynamics during two time periods and subsequent double FT in tandem with spectral heterodyning. The ability to probe molecular couplings between states is a quite particular strength of 2D-FT spectroscopy dominating the topics in this project. Pushing 2D-FT-ECS into the ultra-broadband energy regime with sub-10 fs resolution prepares additional nuclear degrees-of-freedom in the excited-state(s) of complex molecular systems which established (single-quantum, 1Q) 2D-Vibron-Electronic FT spectroscopy (wave-vector geometry kI = - k1 + k2 k3) and new molecular correlations, in the last funding period. One central key in these investigations was to study the role of additive and non-additive vibrations convolved in electronic/excitonic molecular systems and in exciton - charge-transfer dynamics. In a second endeavour of equal impact, we employed Two-Quantum 2D- spectroscopy (2Q-2D-FT-ECS) in wave-vector geometry kIII = k2 + k3 k1 and measured the 2Q -2D coherent FT signal SIII(3)(?2, ?3 ; t1 =0) of bi-exciton and second LUMO states, with the objective to unveil the local molecular orbital aspects of (bound vs. unbound) electron-electron correlation (vide infra) Several achievements made by 1Q 2D-FT studies have some transformative relevance. Our simulation studies on the photosynthetic Fenna-Matthews-Olson complex (FMO) in the non-adiabatic coupling regime illuminated in combination with real-time experiments vibration-exciton mixing, vibron-exciton carriers and, implicitly, features of conical funnel dynamics. The results show that coupling bet-ween pigments in the simple Frenkel exciton picture is critical in biological Light Harvesting (LH) and, rather, favor the vibron-exciton as a more realistic concept to understand natures response to light. Due to its nuclear origin, its oscillation is long-lasting enough to drive ET and reach the Reaction Center (RC). By using the Lu-Phthalocyanine-anion dimer as a bio-mimetic analog of the photosynthetic special pair, we could recover the 2D (vibronic)-excitonic CT dynamics in unprecedented detail. We look into electron- hole-transfer and charge separation in a molecular movie on a 50 fs time base, hence illuminating the incipience of elementary chemical change, similar to the events in the bio-LH machinery. Finally, most impressive biological LH surrogate systems disentangled by 2Q-2D-FT-ECS and analyzed in this program by atomistic many-body theory are cylindrical molecular aggregates. They show well-resolved two-exciton resonances of weakly scattering exciton pairs, where two excitons (unbound) scatter elastically and conserve their population. They can be treated as weakly coupled bosons stemming from the extended exciton delocalization made possible by the nano-tube geometry. Nanotubes are, there-fore, attractive geometric templates for excitation energy accumulation as a way of protecting the LH architecture from damage via intense photons and/or, after weak dephasing (quasi-repulsive pair!), as a way of increasing the C-T efficiency in RC, successfully, realized by nature in photosynthetic chlorosomes.