Modelling the bacterial photosynthetic reaction center. V. Assignment of the electronic transition observed at 2200 cm−1 in the special-pair radical-cation as a second-highest occupied molecular orbital to highest occupied molecular orbital transition

Abstract

Modeling the bacterial photosynthetic reaction center. VII. Full simulation of the intervalence hole-transfer absorption spectrum of the special-pair radical cation Modelling the bacterial photosynthetic reaction center. VI. Use of density-functional theory to determine the nature of the vibronic coupling between the four lowest-energy electronic states of the special-pair radical cation Modelling the bacterial photosynthetic reaction center. V. Assignment of the electronic transition observed at 2200 cm À1 in the special-pair radical-cation as a second-highest occupied molecular orbital to highest occupied molecular orbital transition Primary charge separation in photoexcited photosynthetic reaction centers produces the radical cation P ϩ of a bacteriochlorophyll dimer known as the special-pair P. P ϩ has an intense electronic transition in the vicinity of 1800-5000 cm Ϫ1 which is usually assigned to the interchromophore hole-transfer excitation of the dimer radical cation; in principle, this spectrum can give much insight into key steps of the solar-to-electrical energy-conversion process. The extent to which this transition is localized on one-half of the dimer or delocalized over both is of utmost importance; an authoritative deduction of this quantity from purely spectroscopic arguments requires the detailed assignment of the observed high to medium resolution spectra. For reaction centers containing bacteriochlorophylls a or b, a shoulder is observed at 2200 cm Ϫ1 on the low-energy side of the main hole-transfer absorption band, a band whose maximum is near 2700 cm Ϫ1 . Before quantitative analysis of the hole-transfer absorption in these well-studied systems can be attempted, the nature of the processes leading to this shoulder must be determined. We interpret it as arising from an intrachromophore SHOMO to HOMO transition whose intensity arises wholly through vibronic coupling with the hole-transfer band. A range of ab initio and density-functional calculations are performed to estimate the energy of this transition both for monomeric cations and for P ϩ of Blastochloris viridis, Rhodobacter sphaeroides, Chlorobium limicola, Chlorobium tepidum, Chlamydomonas reinhardtii, Synochocystis S.6803, spinach photosystems I and II, Heliobacillus mobilis, and finally Heliobacterium modesticaldum, with the results found to qualitatively describe the available experimental data. Subsequent papers in this series provide quantitative analyses of the vibronic coupling and complete spectral simulations based on the model developed herein.

Examination of the Photophysical Processes of Chlorophyll d Leading to a Clarification of Proposed Uphill Energy Transfer Processes in Cells of Acaryochloris marina¶

“…This assignment gives a spacing of Q x (0,0) − Q y (0,0) = 2060 cm −1 , a value which is in good agreement with calculated spacings for Chl a from Symmetry Adapted Cluster‐Configuration Interaction (SAC‐CI) (30) (2900 cm −1 ) and Density Functional Theory and Multi‐reference Configuration Interaction (DFT/MRCI) (29) (2100 cm −1 ). Variations in this quantity based on the same assignment for Chl a , Chl b , bacteriochlorophyll (BChl) a , BChl b and BChl g follow a consistent and anticipated pattern based on calculated orbital spacings (36). TDDFT calculations (37) predict a much smaller spacing, however, of around only 130 cm −1 and predict a large number of transitions between the Soret and Q bands.…”

Organic Mixed‐Valence Compounds: A Playground for Electrons and Holes

“…In addition, the immense potential of (organic) MV compounds to mimic key steps in the even more complex ET reactions in nature (e.g. photosynthesis) [24,[53][54][55][56][57][58][59][60] was recognized, and the requirement for a detailed understanding of ET processes prompted chemists to synthesize various organic MV systems whose ET behavior was studied extensively (especially by spectroscopic methods). This process was supported additionally by continuous refinement of ET theories.…”

Diabatic models with transferrable parameters for generalized chemical reactions

“…The critical feature is the universality of the model parameters for the qualitative description of system properties. Simple models can also be extended to universal quantitative ones provided that full quantum solutions [26,50], sufficient interfering processes [43,[51][52], and motions orthogonal to the reaction coordinate are also included [27,[53][54]. The assignment [55] of the Q-band spectrum of chlorophyll-a, arguably the world's most important chromophore whose properties display a strong pseudo-Jahn-Teller effect, following 50 years of intense debate, provides another example of the power of this approach.…”