Morrison Adrian F, Herbert John M
Department of Chemistry and Biochemistry, The Ohio State University , Columbus, Ohio 43210, United States.
J Phys Chem Lett. 2017 Apr 6;8(7):1442-1448. doi: 10.1021/acs.jpclett.7b00230. Epub 2017 Mar 17.
Singlet fission proceeds rapidly and with high quantum efficiency in both crystalline tetracene and pentacene, which poses a conundrum given that the process in tetracene is disfavored by the electronic energetics. Here, we use an ab initio exciton model to compute nonadiabatic couplings in the unit cell of tetracene in order to identify the modes that promote this process. Four intramolecular modes in the range of 1400-1600 cm, which are nearly resonant with the single-exciton/multiexciton energy gap, appear to play a key role. Ab initio calculations of the electron/phonon coupling constants for these modes reveal that they are almost entirely of "Holstein" type, modulating the site energies rather than the intersite couplings. The constants are used to parametrize a vibronic Hamiltonian, simulations with which suggest a vibronically coherent singlet fission mechanism that proceeds spontaneously despite unfavorable electronic energetics. In the absence of vibronic coupling, there is no significant fission according to our model.
在晶体并四苯和并五苯中，单线态裂变过程迅速且具有高量子效率，鉴于并四苯中的该过程在电子能量学方面并不有利，这就构成了一个难题。在此，我们使用从头算激子模型来计算并四苯晶胞中的非绝热耦合，以确定促进该过程的模式。在1400 - 1600厘米范围内的四种分子内模式，它们与单激子/多激子能隙几乎共振，似乎起着关键作用。对这些模式的电子/声子耦合常数的从头算计算表明，它们几乎完全是“霍尔斯坦”类型，调节的是位点能量而非位点间耦合。这些常数被用于参数化一个振动电子哈密顿量，基于此的模拟表明存在一种振动电子相干单线态裂变机制，尽管电子能量学不利，但该机制仍能自发进行。根据我们的模型，在没有振动电子耦合的情况下，不会发生显著的裂变。
