Non-radiative relaxation of photoexcited chlorophylls: theoretical and experimental study

Abstract

Nonradiative relaxation of high-energy excited states to the lowest excited state in chlorophylls marks the first step in the process of photosynthesis. We perform ultrafast transient absorption spectroscopy measurements, that reveal this internal conversion dynamics to be slightly slower in chlorophyll B than in chlorophyll A. Modeling this process with non-adiabatic excited state molecular dynamics simulations uncovers a critical role played by the different side groups in the two molecules in governing the intramolecular redistribution of excited state wavefunction, leading, in turn, to different time-scales. Even given smaller electron-vibrational couplings compared to common organic conjugated chromophores, these molecules are able to efficiently dissipate about 1 eV of electronic energy into heat on the timescale of around 200 fs. This is achieved via selective participation of specific atomic groups and complex global migration of the wavefunction from the outer to inner ring, which may have important implications for biological light-harvesting function.

Photoinduced Intra- and Intermolecular Energy Transfer in Chlorophyll a Dimer

“…Extensive previous studies have already adopted AM1 and similar semiempirical models to investigate excited state properties of porphyrinbased molecules 23,42,[45][46][47][48][49][50][51][52][53][54] . The excited states calculated with the AM1/CIS method reproduce the measured energy gap between the Soret band and the Q y band, and has been successfully applied to calculate the excited states 2 of Chla and Chlb monomers 13,14 . A detailed discussion on the parameter testings, algorithm implementation, the advantages, and various applications of NA-ESMD can be found in the recent literature 21,22,55 .…”

“…The fitting is conducted through a least-squares regression analysis. The rate constants and the corresponding time constants (τ 1 = k −1 1 , τ 2 = k −1 2 ) are presented in Table 1 in comparison to the respective parameters obtained for the Chla monomer 13 . Shown in Fig.…”

“…This assumption is based on the fact that Table 1: Comparison of internal conversion rates obtained from a first-order irreversible population model for B → Q x , Q x → Q y , and B → Q y pathways for Chla dimer and monomer 13 systems. Populations of specific excited states are averaged from 500 trajectories for Chla dimer and 500 trajectories for Chla monomer 13 in the NA-ESMD simulations.…”

“…Upon photoexcitation, nonradiative relaxation occurs from high-lying energy excited states to the lowest excited state of pigments in photosynthetic pigment-protein complexes. As one of the key processes in the first step of photosynthesis, nonradiative relaxation in the single Chla and Chlb molecules 13,14 has been studied with the nonadiabatic excited-state molecular dynamics (NA-ESMD) 21,22 . The ratio of simulated relaxation constants in Chla and Chlb monomers agreed well with that from ultrafast transient absorption spectroscopy.…”

An ab initio multiple cloning approach for the simulation of photoinduced dynamics in conjugated molecules

“…9 An adequate theoretical treatment of such processes can be achieved by using direct or on-the-fly non-adiabatic molecular dynamics methods. [10][11][12] A sub-family of these approaches, based on trajectory surface hopping (SH) algorithms, [13][14][15][16] have been extensively used to study the photophysics and photochemistry of a wide variety of organic molecules: dendrimers, [17][18][19][20] nanohoops, [21][22][23] fluorenes, 24 fullerenes, 25 Ru(II)-based complexes, 26 chlorophylls, [27][28][29] retinal, 30 nucleotides [31][32][33][34][35][36][37] and so on. Different SH computational implementations are represented by NEWTON-X, 38,39 SHARC (Surface Hopping including ARbitrary Couplings), 40 PYXAID (PYthon eXtension for Ab Initio Dynamics) 41,42 and NEXMD (Non-adiabatic EXcited-states Molecular Dynamics), 12,43 among others.…”