Nuclear dynamics of singlet exciton fission in pentacene single crystals

Abstract

Singlet exciton fission (SEF) is a key process for developing efficient optoelectronic devices. An aspect rarely probed directly, yet with tremendous impact on SEF properties, is the nuclear structure and dynamics involved in this process. Here, we directly observe the nuclear dynamics accompanying the SEF process in single crystal pentacene using femtosecond electron diffraction. The data reveal coherent atomic motions at 1 THz, incoherent motions, and an anisotropic lattice distortion representing the polaronic character of the triplet excitons. Combining molecular dynamics simulations, time-dependent density-functional theory, and experimental structure factor analysis, the coherent motions are identified as collective sliding motions of the pentacene molecules along their long axis. Such motions modify the excitonic coupling between adjacent molecules. Our findings reveal that long-range motions play a decisive part in the electronic decoupling of the electronically correlated triplet pairs and shed light on why SEF occurs on ultrafast time scales.

Orbital-resolved Observation of Singlet Fission

“…The orbital resolution allowed us to infer a large charge-transfer character of the bright singlet exciton and rule out the coherent mechanism with an instantaneous population of the bitriplet exciton. While optical excitation of the singlet state triggers coherent 12 as well as incoherent 42 nuclear motions, our results suggest that these are not dictating the general dynamics of the primary step of ultrafast fission in pentacene. Thus a conical intersection mechanism is not the governing process.…”

Spectroscopic analysis of vibrational coupling in multi-molecular excited states

“…In the case of excimer formation, exciton resonance [36,37] as well as a CT state [12] have been proposed as driving forces behind geometrical relaxations but, meanwhile, the consensus prevailed that only an adiabatic mixing of Frenkel and CT excitons can fully explain the observed phenomena [11,15,25,[38][39][40][41]. For example, in crystalline pentacene, the first excited state is known to have a significant admixture of CT character [42,43] and a considerable geometric reorganization accompanies its photo-excitation [44]. In fact, Tvingstedt et al successfully used a displaced harmonic oscillator model using inter-and intra-molecular vibrational modes to describe the emission of CT states formed at donoracceptor hetero-junctions [24].…”

Machine-learning assisted design principle search for singlet fission: an example study of cibalackrot

“…The currently widely accepted mechanism proceeds from a twosite singly excited state (S 1 ,S 0 ), to a coupled singlet of two triplets 1 (T 1 , T 1 ) and finally to two independent spin-dephased triplet excitons (T 1 + T 1 ) after decoupling [14][15][16] . However, recent reports suggest that there may be an additional spatially separated exciton state 1 (T 1 …T 1 ) between formation and fission 17 , and that quintet spin states may also contribute to the final spin-dephasing mechanism 18,19 . Ultimately, when the energy of the decoupled T 1 states are high enough, both triplet excitons can be used to drive a photovoltaic current 20 .…”