Effects of Charge Transfer State and Exciton Migration on Singlet Fission Dynamics in Organic Aggregates

Abstract

A time-dependent wavepacket diffusion method is used to investigate the effects of charge transfer (CT) states, singlet exciton and multiexciton migrations on singlet fission (SF) dynamics in organic aggregates. The results reveal that the incorporation of CT states can result in a different SF dynamics from the direct interaction between singlet exciton and multiexciton, and an obvious SF interference is also observed between the direct channel and the indirect channel mediated by CT states. In the case of direct interaction, although the fast population transfer of singlet exciton in monomers, by the increase of exciton-exciton interaction, can accelerate the SF process, the spatial coherence alternatively has a counter-productive effect, and their competition leads to an optimal exciton-exciton interaction at which SF has a maximal rate. This trade-off relationship in SF dynamics is further analysed from different perspectives, specifically in spatial and energy representations, is also confirmed through the indication that static energy disorders can speed up SF process by destructing the coherence.Meanwhile, it is found that the couplings among multiexciton states decrease SF rates by the multiexciton coherence and backward conversion from multiexciton to singlet exciton states.

Quantum Master Equation Approach to Singlet Fission Dynamics in Pentacene Linear Aggregate Models: Size Dependences of Excitonic Coupling Effects

“…where H ex is the exciton Hamiltonian; H ph and H ex -ph represent the phonon Hamiltonian and vibronic (exciton-phonon) coupling Hamiltonian, respectively. In the exciton Hamiltonian H ex , we consider only interactions between neighboring monomers, [32] and ignore the direct couplings between FE and TT states, couplings between CT states and couplings between TT states because they are known to be mostly much smaller than other couplings. [5,28,32] These approximations are not predicted to affect the essential results in the present study since we focus on the FE coupling effects on the indirect SF pathways, which are usually primary processes, [5,6] in linear aggregates with different numbers of monomers (N).…”

“…Among them, as a research toward the understanding of SF process in crystals, SF dynamics of aggregate models beyond dimer models has recently started to be investigated. For example, a time‐dependent wavepacket diffusion (TDWPD) method has been applied to clarifying the electronic coupling effects among FE, CT, and TT states on the SF dynamics in one‐dimensional aggregate models . It has been found in the case of direct interaction between FE and TT that the SF rates increase with increasing the aggregate length, reaching a convergent value at sufficiently long length, and that there is an optimal FE coupling at which SF has a maximal rate due to the mutually opposite contribution to SF rates between population transfer among FE states (accelerating the SF) and spatial coherence of FE (suppressing the SF) although the mechanism is not revealed in detail.…”

“…Namely, in addition to the energy level matching conditions in step (i), design of intermolecular configuration (crystal packing) is found to be indispensable for describing the SF rate, which is proportional to transition probability from FE to TT state. [5,6] Another essential factor for determining the SF efficiency is involved in the exciton dynamics with vibronic couplings, [14][15][16][17][18][19][20][21][22][23][24][25][26][27][28][29][30][31][32] which is the main topic in step (iii). In step (iii), various results obtained in steps (i) and (ii) are combined and are mutually correlated with the exciton dynamics caused by vibronic couplings.…”

“…For example, a time-dependent wavepacket diffusion (TDWPD) method [33] has been applied to clarifying the electronic coupling effects among FE, CT, and TT states on the SF dynamics in one-dimensional aggregate models. [32] It has been found in the case of direct interaction between FE and TT that the SF rates increase with increasing the aggregate length, reaching a convergent value at sufficiently long length, and that there is an optimal FE coupling at which SF has a maximal rate due to the mutually opposite contribution to SF rates between population transfer among FE states (accelerating the SF) and spatial coherence of FE (suppressing the SF) although the mechanism is not revealed in detail. Conversely, such features of dynamics are also speculated to depend on the monomer energy levels and electronic couplings in aggregate models, that is, intermonomer configuration, aggregate architecture, and size.…”

Assessment of Ab Initio and Density Functional Theory Methods for the Excitations of Donor-Acceptor Complexes: The Case of the Benzene-Tetracyanoethylene Model

“…usually contain electron donor and acceptor materials constructing an active layer [ 2 , 3 ]. Charge transports in donor/acceptor materials and charge transfer (CT) at the interfaces between electron donors and acceptors have great importance in the improvement of device performance [ 4 , 5 , 6 , 7 , 8 , 9 , 10 , 11 , 12 ]. Usually, the charge transfer in organic optoelectronic devices involves molecular excitation behavior [ 13 , 14 ].…”