Computational Study of Photoexcited Dynamics in Bichromophoric Cross-Shaped Oligofluorene

Abstract

The non-adiabatic excited state molecular dynamics (NA-ESMD) approach is applied to investigate photoexcited dynamics and relaxation pathways in a spiro-linked conjugated polyfluorene at room (T = 300 K) and low (T = 10 K) temperatures. This dimeric aggregate consists of two perpendicularly oriented weakly interacting α-polyfluorene oligomers. The negligible coupling between the monomer chains results in an initial absorption band composed of equal contributions of the two lowest excited electronic states, each localized on one of the two chains. After photoexcitation, an efficient ultrafast localization of the entire electronic population to the lowest excited state is observed on the time scale of about 100 fs. Both internal conversion between excited electronic states and vibronic energy relaxation on a single electronic state contribute to this process. Thus, photoexcited dynamics of the polyfluorene dimer follows two distinct pathways with substantial temperature dependence on their efficiency. One relaxation channel involves resonance electronic energy transfer between the monomer chains, whereas the second pathway concerns the relaxation of the electronic energy on the same chain that has been initially excited due to electron-phonon coupling. Despite the slower vibrational relaxation, a more efficient ultrafast electronic relaxation is observed at low temperature. Our numerical simulations analyze the effects of molecular geometry distortion during the electronic energy redistribution and suggest spectroscopic signatures reflecting complex electron-vibrational dynamics.

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

“…In this way, electronic state populations can be obtained by replacing N in eqn (24) and (23) by the adiabatic population operator P K = |f (n) K ihf (n) K | to obtain:…”

“…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.…”

Ultrafast Electronic Energy Transfer in an Orthogonal Molecular Dyad

“…18 For large sized systems such dynamics require semiempirical methods like OM2/MRCI and DFTB for the quantum chemical calculations 19,20 that are combined with the classical propagation of the nuclei in the excited states including nonadiabatic couplings allowing switches between different electronic states. [21][22][23] For the system in our study, nonadiabatic ab initio molecular dynamics (NA-AIMD) simulations are nowadays feasible. [24][25][26] In our case the description of electronic states and excitations is firmly based on (time-dependent) density functional theory (TDDFT),…”

Photoinduced Intra- and Intermolecular Energy Transfer in Chlorophyll a Dimer

“…The ratio of simulated relaxation constants in Chla and Chlb monomers agreed well with that from ultrafast transient absorption spectroscopy. This approach has also been successfully applied to study ultrafast intramolecular exciton redistri-bution and energy relaxation after excitation in large-scale organic conjugated molecules [22][23][24][25][26] . In photosynthesis, intermolecular excitation energy relaxation is critical to energy transfer in photosynthetic systems since many pigments can be excited simultaneously and are participating in the global energy flux.…”