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Register a new userDiversity of coherences and origin of electronic transitions of supermolecular nanoring
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Quantum coherence is highly involved in photochemical functioning of complex molecular systems. Co-existence and intermixing of electronic and/or vibrational coherences, while never unambiguously identified experimentally, has been proposed to be responsible for this phenomenon. Analysis of multidimensional spectra of a synthetic belt-shaped molecular six-porphyrin nanoring with an inner template clearly shows a great diversity of separable electronic, vibrational and mixed coherences and their cooperation shaping the optical response. The results yield clear assignment of electronic and vibronic states, estimation of excitation transfer rates, and decoherence times. Theoretical considerations prove that the complexity of excitation dynamics and spectral features of the nanoring excitation spectrum is due to combined effect of cyclic symmetry, small geometrical deformations, and vibronic coupling.
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Coherent dynamics of coupled molecules are effectively characterized by the two-dimensional (2D) electronic coherent spectroscopy. Depending on the coupling between electronic and vibrational states, oscillating signals of purely electronic, purely vibrational or mixed origin can be observed. Even in the mixed molecular systems two types of coherent beats having either electronic or vibrational character can be distinguished by analyzing oscillation Fourier maps, constructed from time-resolved 2D spectra. The amplitude of the beatings with the electronic character is heavily affected by the energetic disorder and consequently electronic coherences are quickly dephased. Beatings with the vibrational character depend weakly on the disorder, assuring their long-time survival. We show that detailed modeling of 2D spectroscopy signals of molecular aggregates providesdirect information on the origin of the coherent beatings.
The effect of nuclear dynamics and conical intersections on electronic coherences is investigated employing a two-state, two-mode linear vibronic coupling model. Exact quantum dynamical calculations are performed using the multi-configuration time-dependent Hartree method (MCTDH). It is found that the presence of a non-adiabatic coupling close to the Franck-Condon point can preserve electronic coherence to some extent. Additionally, the possibility of steering the nuclear wavepackets by imprinting a relative phase between the electronic states during the photoionization process is discussed. It is found that the steering of nuclear wavepackets is possible given that a coherent electronic wavepacket embodying the phase difference passes through a conical intersection. A conical intersection close to the Franck-Condon point is thus a necessary prerequisite for control, providing a clear path towards attochemistry.
The two-dimensional spectroscopy has recently revealed oscillatory behavior of excitation dynamics in molecular systems. However, in the majority of cases it is strongly debated if excitonic or vibrational wavepackets, or evidences of quantum transport have been observed. In this letter, the method for distinguishing between vibrational and excitonic wavepacket motion is presented, based on the phase and amplitude relationships of oscillations of distinct peaks, which has been revealed using fundamental analysis of two-dimensional spectrum of two representative systems.
Electronic spectra of C$_6$H are measured in the $18,950-21,100$ cm$^{-1}$ domain using cavity ring-down spectroscopy of a supersonically expanding hydrocarbon plasma. In total, 19 (sub)bands of C$_6$H are presented, all probing the vibrational manifold of the B$^2Pi$ electronically excited state. The assignments are guided by electronic spectra available from matrix isolation work, isotopic substitution experiments (yielding also spectra for $^{13}$C$_6$H and C$_6$D), predictions from ab initio calculations as well as rotational fitting and vibrational contour simulations using the available ground state parameters as obtained from microwave experiments. Besides the $0_0^0$ origin band, three non-degenerate stretching vibrations along the linear backbone of the C$_6$H molecule are assigned: the $ u_6$ mode associated with the C-C bond vibration and the $ u_4$ and $ u_3$ modes associated with C$equiv$C triple bonds. For the two lowest $ u_{11}$ and $ u_{10}$ bending modes, a Renner-Teller analysis is performed identifying the $mu^2Sigma$($ u_{11}$) and both $mu^2Sigma$($ u_{10}$) and $kappa^2Sigma$($ u_{10}$) components. In addition, two higher lying bending modes are observed, which are tentatively assigned as $mu^2Sigma$($ u_9$) and $mu^2Sigma$($ u_8$) levels. In the excitation region below the first non-degenerate vibration ($ u_6$), some $^2Pi-^{2}Pi$ transitions are observed that are assigned as even combination modes of low-lying bending vibrations. The same holds for a $^2Pi-^{2}Pi$ transition found above the $ u_6$ level. From these spectroscopic data and the vibronic analysis a comprehensive energy level diagram for the B$^2Pi$ state of C$_6$H is derived and presented.
We demonstrate that the coupling of excitonic and vibrational motion in biological complexes can provide mechanisms to explain the long-lived oscillations that have been obtained in non linear spectroscopic signals of different photosynthetic pigment protein complexes and we discuss the contributions of excitonic versus purely vibrational components to these oscillatory features. Considering a dimer model coupled to a structured spectral density we exemplify the fundamental aspects of the electron-phonon dynamics, and by analyzing separately the different contributions to the non linear signal, we show that for realistic parameter regimes purely electronic coherence is of the same order as purely vibrational coherence in the electronic ground state. Moreover, we demonstrate how the latter relies upon the excitonic interaction to manifest. These results link recently proposed microscopic, non-equilibrium mechanisms to support long lived coherence at ambient temperatures with actual experimental observations of oscillatory behaviour using 2D photon echo techniques to corroborate the fundamental importance of the interplay of electronic and vibrational degrees of freedom in the dynamics of light harvesting aggregates.
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