Both electronic and vibrational coherences are involved in primary electron transfer in bacterial reaction center

Abstract

Understanding the mechanism behind the near-unity efficiency of primary electron transfer in reaction centers is essential for designing performance-enhanced artificial solar conversion systems to fulfill mankind’s growing demands for energy. One of the most important challenges is distinguishing electronic and vibrational coherence and establishing their respective roles during charge separation. In this work we apply two-dimensional electronic spectroscopy to three structurally-modified reaction centers from the purple bacterium Rhodobacter sphaeroides with different primary electron transfer rates. By comparing dynamics and quantum beats, we reveal that an electronic coherence with dephasing lifetime of ~190 fs connects the initial excited state, P*, and the charge-transfer intermediate ; this step is associated with a long-lived quasi-resonant vibrational coherence; and another vibrational coherence is associated with stabilizing the primary photoproduct, . The results show that both electronic and vibrational coherences are involved in primary electron transfer process and they correlate with the super-high efficiency.

Coherent intradimer dynamics in reaction centers of photosynthetic green bacterium Chloroflexus aurantiacus

“…Our observations and interpretations on RCs from the green bacterium Cfx. aurantiacus are in line with the work of van Grondelle and co-workers 50 . Based on their results, we can say more definitely that the P 2 * state in our model can be the P*(P A…”

“…Very recently, two dimentional electronic spectroscopy technique with 17-fs pulses was applied to mutant RCs from the purple bacterium Rba. sphaeroides at 77 K 50 . This work strongly supported a well-known sequential scheme of charge separation, in which the very first step is a formation of the P A δ+ P B δ− state.…”

Vibronic coherence contributes to photocurrent generation in organic semiconductor heterojunction diodes

“…Our transient results demonstrate that the ultrafast process from EX to CS is temperature independent ( Supplementary Fig. 5), and the corresponding oscillation signal dephases quickly (in <200 fs, consistent with the recently resolved electronic coherence lifetime 28 ). Furthermore, the energy gap between the initial EX (emission peak~800 nm) and the CS states (emission peak~780 nm) ( Fig.…”

“…Coherence phenomena have been proposed to contribute in some biological photosynthetic systems 29,[45][46][47] , and some reviews have also proposed that coherence might enhance charge transfer 46,48 . Very recently, coherence was found to play a key role for the near-unity CS efficiency in a natural reaction center complex 28 . To explore the next-generation of photovoltaic materials, new design strategies based on understanding the working mechanism are needed.…”

Theory of delocalised charge transfer​​​​​​​

“…The coupling of charge transfer states to nuclear modes was suggested to play a functional role in the efficiency of charge transfer since the 1990s 105,106 , and recent work with 2DES has detected quantum beats at a number of frequencies that correlate with charge transfer, exciton, and vibrational states at room temperature [106][107][108] . As such, vibronic coherences have been suggested to play an active role in charge separation at physiological temperatures in both bacterial and oxygenic photosynthesis 43,98,[106][107][108][109] . Although the mere presence of quantum beating doesn't mean that coherence is playing a functional role in charge separation, this is an active field study that is currently seeing much activity.…”