Photoinduced Intra- and Intermolecular Energy Transfer in Chlorophyll a Dimer

Abstract

Applying nonadiabatic excited-state molecular dynamics, we investigate excitation energy transfer and exciton localization dynamics in a chlorophyll a (Chla) dimer system at the interface of two monomers of light-harvesting complex II trimer. After its optical excitation at the red edge of the Soret (B) band, the Chla dimer experiences an ultrafast intra- and intermolecular nonradiative relaxation process to the lowest band (Q). The energy relaxation is found to run faster in the Chla dimer than in the Chla monomer. Once the molecular system reaches the lowest Q band composed of two lowest excited states S and S, the concluding relaxation step involves the S → S population transfer, resulting in a relatively slower relaxation rate. The strength of thermal fluctuations exceeds intraband electronic coupling between the states belonging to a certain band (B, Q, and Q), producing localized states on individual chromophores. Therefore, time evolution of spatial electronic localization during internal conversion reveals transient trapping on one of the Chla monomers participating in the events of intermonomeric energy exchange. In the phase space domains where electronic states are strongly coupled, these states become nearly degenerate promoting Frenkel-exciton-like delocalization and interchromophore energy transfer. As energy relaxation occurs, redistribution of the transition density on two Chla monomers leads to nearly equal distribution of the exciton among the molecules. For a single Chla, our analysis of excitonic dynamics reveals wave function amplitude transfer from nitrogen and outer carbon atoms to inner carbon atoms during nonradiative relaxation.

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.…”

Relation between Vibrational Dephasing Time and Energy Gap Fluctuations

“…36 Nonadiabatic excited-state MD has been applied to chlorophyll dimers only very recently. 37 In the present study, the LH2 and FMO complexes contain bacteriochlorophylls (BChls) as their pigments, whereas the two cryptophyte complexes, PE545 as well as PE555, contain bilin molecules for capturing the sun light. The CHARMM as well as AMBER force field sets have been employed for the 300 K MD simulations, as listed in Table 1.…”

“…Treating the dynamics of the system at least partially quantum mechanically to be more consistent with the computations of the vertical excitation energies is computationally much more expensive and so far has only been performed for a relatively small light-harvesting complex, that is, the water-soluble chlorophyll-binding protein . Nonadiabatic excited-state MD has been applied to chlorophyll dimers only very recently . In the present study, the LH2 and FMO complexes contain bacteriochlorophylls (BChls) as their pigments, whereas the two cryptophyte complexes, PE545 as well as PE555, contain bilin molecules for capturing the sun light.…”

Back-and-Forth Energy Transfer during Electronic Relaxation in a Chlorin–Perylene Dyad

“…NEXMD Overview . The nonadiabatic excited state molecular dynamics (NEXMD) computational package combines the fewest switching surface hopping (FSSH) algorithm , with “on the fly” calculation of excited state energies, gradients, and nonadiabatic couplings at the configuration interaction singles (CIS) level of theory using the semiempirical Austin model 1 Hamiltonian using the Collective Electronic Oscillator (CEO) approach. − NEXMD has been previously used in a large variety of processes in multichromophore organic conjugated molecular systems like light harvesting in dendrimers, − energy transfer in donor–acceptor systems, , intramolecular energy redistribution in chorophylls, , etc …”