Does electronic coherence enhance anticorrelated pigment vibrations under realistic conditions?

Abstract

The light-harvesting efficiency of a photoactive molecular complex is largely determined by the properties of its electronic quantum states. Those, in turn, are influenced by molecular vibrational states of the nuclear degrees of freedom. Here, we reexamine two recently formulated concepts that a coherent vibronic coupling between molecular states would either extend the electronic coherence lifetime or enhance the amplitude of the anticorrelated vibrational mode at longer times. For this, we study a vibronically coupled dimer and calculate the nonlinear two-dimensional (2D) electronic spectra which directly reveal electronic coherence. The timescale of electronic coherence is initially extracted by measuring the antidiagonal bandwidth of the central peak in the 2D spectrum at zero waiting time. Based on 1 the residual analysis, we identify small-amplitude long-lived oscillations in the cross-peaks, which, however, are solely due to groundstate vibrational coherence, regardless of having resonant or off-resonant conditions. Our studies neither show an enhancement of the electronic quantum coherence nor an enhancement of the anticorrelated vibrational mode by the vibronic coupling under ambient conditions.In the initial steps of photosynthesis, photoactive molecular complexes capture the sunlight energy and transfer it to the reaction center on an ultrafast time scale and with unity quantum efficiency 1 . The performance is determined by the molecular electronic properties, in concert with the molecular vibrations and coupling to the environment given by a solvent and the surrounding pigments and proteins. To investigate the energy transfer, ultrafast 2D electronic spectroscopy 2-4 is able to resolve fs time scales. It is able to reveal the interactions between the energetically closeby lying molecular electronic states, for which the linear spectra are commonly highly congested and broadened by the strong static disorder 5 . Recent experimental studies of the Fenna-Matthews-Olson (FMO) complex reported long-lived oscillations of the cross-peaks both at low 6 and at room temperature 7 which have been assigned to enhanced electronic coherence. This has generated tremendous interest in this new field of quantum biology 8 , aiming to reveal a functional connection between photosynthetic energy transfer and long-lived quantum coherence. Moreover, also in photoactive marine cryptophyte algae 9 , the light-harvesting complex LHCII 10 and in the Photosystem II reaction center 11, 12 , long-lived oscillations have been experimentally reported at low and room temperature.

Quantum biology revisited

“…However, the fast electronic dephasing, which appears to be general for light-harvesting systems, destroys coherence in the electronic sector faster than the vibrational period. Consequently, the vibrational coherence in the sector of the antiresonant vibrational mode, under realistic conditions, remains unaffected in this limit of short-lived electronic coherences (102). In this respect, we believe that considerable care must be taken in interpreting the resulting spectral signatures of vibronic coherencesand, in particular, when attempting to place this physics in the context of biological function.…”

Equation-of-Motion Methods for the Calculation of Femtosecond Time-Resolved 4-Wave-Mixing and N-Wave-Mixing Signals

“…The 3-pulse DM EOM-PMA has been used to simulate signals for single chromophores, − dimers, − trimers, multichromophore light harvesting complexes, − organic semiconductors, and quantum dots. − In addition, the 3-pulse EOM-PMA has been applied to calculate TA PP signals for a model system under a scanning tunneling microscope, to obtain photocurrent-detected 2D spectra simulated with time-dependent density functional theory (TDDFT), and to evaluate TA PP signals for a model of a photoinduced condensed-phase electron-transfer reaction with the QCLE method. − …”

Cooperation between Excitation Energy Transfer and Antisynchronously Coupled Vibrations

“…In handling the vibration coupled to the donor and the acceptor, especially when the two are identical pigment units, the motion can be described with correlated and anticorrelated bases, which are respectively defined as the addition and the subtraction of the individual vibrational displacement vectors of the two units. It was suggested that the anticorrelated components play critical roles in vibronic mixing as their associated motions can produce excitation energy gap fluctuations between the donor and the acceptor, whereas correlated components are incapable of generating such fluctuations. , While the existence and the influence of such correlations between vibrations in separate pigment units in a noisy system are still under debate, some studies have further suggested that an anticorrelated component of the vibrations may be enhanced by vibronic coupling and that it may outlive electronic coherence once it is activated. ,− …”