Higher Order Vibronic Sidebands of Chlorophyll a and Bacteriochlorophyll a for Enhanced Excitation Energy Transfer and Light Harvesting

Abstract

Optical absorption and fluorescence spectra of molecules in condensed phases often show extensive sidebands. Originating from electron−vibrational and electron−phonon couplings, these spectral tails bear important information on the dynamics of electronic states and processes the molecules are involved in. The vibronic sidebands observed in conjugate Q y absorption and fluorescence spectra of chlorophyll a and bacteriochlorophyll a are relatively weak, characterized by the total Huang−Rhys factor which is less than one. Therefore, it is widely considered that only fundamental intramolecular modes are responsible for their formation. Here, we provide evidence for extra-long and structured fluorescence tails of chlorophyll a and bacteriochlorophyll a as far as 4000 cm −1 from respective spectral origins, far beyond the frequency range of fundamental modes. According to quantum chemical simulations, these sidebands extending to ∼960 nm in chlorophyll a and ∼1140 nm in bacteriochlorophyll a into the infrared part of the optical spectrum are mainly contributed to by vibrational overtones of the fundamental modes. Because energy transfer and relaxation processes generally depend on vibronic overlap integrals, these findings potentially contribute to better understanding of many vital photo-induced phenomena, including photosynthetic light harvesting.

Establishment of the Qy Absorption Spectrum of Chlorophyll a Extending to Near-Infrared

“…As to the observed difference between the “hunches” in the spectra of Chl a and BChl a , it is much more difficult to evaluate theoretically. This is because according to our previous study [ 33 ], the PM7 method tends to underestimate vibrational intensities in the spectra of Chl a compared with those of BChl a .…”

“…Two sets of calculations were performed: one involving all fundamental modes plus up to 20 overtones of each vibrational mode and another, where additionally, the υ i + υ j -type combinations of the modes were included. Energy distributions of the vibrational modes accounted for in the calculations can be inspected in [ 33 ]. The respective spectra are in Figure 3 A, drawn by dashed and solid lines.…”

“…Given the low absorbance value, the light harvesting functionality in this spectral region is not very likely, except for some very special circumstances. It may, however, facilitate energy transfer through a network of antennas, as the energy transfer efficiency depends on spectral overlap between adjacent antenna complexes [ 33 ], (see [ 24 ] for a review of the possible roles of low-energy excited states in photosynthesis).…”

Asymmetry in the Qy Fluorescence and Absorption Spectra of Chlorophyll a Pertaining to Exciton Dynamics

“…Absorption and fluorescence measurements were made using an experimental setup that has been previously described (Rätsep et al, 2009a(Rätsep et al, , 2019a. It consists of a Andor Shamrock 0.3 m spectrograph equipped with a thermo-electrically cooled CCD camera, a highly stabilized tungsten light source, and a He bath cryostat.…”

“…For excited-state properties, hole-burning techniques are commonly applied, but there are still ambiguities in the baseline limit quantitative analysis, with fluorescence-excitation being a more robust technique provided that the contribution from the zero-phonon line (ZPL) can be determined by other means (Reimers et al, 2013;Pieper et al, 2018). For groundstate vibrations, only relatively recently has the differential fluorescence line-narrowing techniques ( FLN) Freiberg, 2003, 2007;Rätsep et al, 2009aRätsep et al, , 2019a been developed to deliver similar results. These techniques provide critical vibrational data required for studies of, e.g., coherent energy transport in photosystems (Renger et al, 1996;Huo and Coker, 2010;Rivera et al, 2013;Kreisbeck et al, 2014;Müh et al, 2014;Romero et al, 2014;Malý et al, 2016;Duan et al, 2017;Ren et al, 2018;Cao et al, 2020;Tomasi and Kassal, 2020).…”