Evidence for Singlet Fission Driven by Vibronic Coherence in Crystalline Tetracene

Abstract

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.

Vibronic exciton theory of singlet fission. III. How vibronic coupling and thermodynamics promote rapid triplet generation in pentacene crystals

“…We note that this procedure has in essence been followed by studies on singlet fission using few-state models as proxies for a extended crystals. 8,11,13,14,51,52 Lastly, a truncation of the Redfield tensor (see Sec. II B) is performed such that adiabatic states having energies higher than 1.92 eV (15 500 cm −1 ) are excluded.…”

“…Both Holstein and Peierls coupling have been theoretically demonstrated to benefit the intramolecular fission rate in chemically linked pentacenes. 16 Nevertheless, non-perturbative treatments of these mechanisms in a crystalline setting have remained limited to phenomenological models based on few-state systems, 8,11,[13][14][15] in which the potentially extended character of the involved excited states are not accounted for. As such, the significance of such modes for capturing the realistic, microscopic behavior representative of extended crystals remains to be verified.…”

Singlet fission in a hexacene dimer: energetics dictate dynamics

“…[31][32][33][34] It has been established that molecular vibrations play a key role in mediating singlet ssion in both oligoacene molecular crystals (intermolecular singlet ssion, xSF) and in acene intramolecular singlet ssion (iSF) compounds. 17,[35][36][37][38] In crystals, it is now commonly accepted that both coherent and incoherent formation of triplet pairs is possible, even within the same system. 35,39 Experimental signatures of vibrational coherences have been detected using ultrafast vibrational and 2D electronic spectroscopy, where both inter-and intramolecular vibrational modes have been found to be important.…”

Analytic derivative couplings and first-principles exciton/phonon coupling constants for an ab initio Frenkel-Davydov exciton model: Theory, implementation, and application to compute triplet exciton mobility parameters for crystalline tetracene

“…The exciton-site basis also includes a ground-state configuration and can be expanded with more states to fit the problem at hand, e.g., by inclusion of charge-separated states 43 Coefficients K In specify the contribution to eigenstate I of the supersystem that arises from the excitation of monomer n. These coefficients are determined by solving the generalized eigenvalue problem,…”

“…Notably, the resulting expressions for the derivative couplings H [x] JK involve intermediate quantities that are equivalent to the so-called "Holstein" and "Peierls" coupling constants that are routinely discussed in the literature on organic photovoltaics. Indeed, we have recently used the AIF-DEM formalism to compute these exciton/phonon couplings from first principles for the process of singlet fission in crystalline tetracene, 43 but the details of the implementation are provided here for the first time. Finally, we use this formalism to compute the exciton/phonon coupling constants that modulate triplet exciton mobility in crystalline tetracene, quantities that are also relevant in the context of singlet fission.…”