Quantum Beats in Crystalline Tetracene Delayed Fluorescence Due to Triplet Pair Coherences Produced by Direct Singlet Fission

Abstract

A detailed analysis of the oscillations seen in the delayed fluorescence of crystalline tetracene is presented in order to study the mechanism of singlet fission. Three quantum beat frequencies of 1.06 ± 0.05, 1.82 ± 0.05, and 2.92 ± 0.06 GHz are resolved, which are damped on a time scale of 20 ns. The effects of sample morphology, excitation wavelength, and temperature are examined. A density matrix model for singlet fission is developed that quantitatively describes the frequencies, amplitudes, and damping of the oscillations. The model assumes a direct coupling of the initially excited singlet exciton to the triplet pair manifold. There is no electronic coherence between the singlet and triplet pair states, but the rapid singlet decay time of ∼200 ps in solution-grown single crystals provides the impulsive population transfer necessary to create a coherent superposition of three zero-field triplet pair states |xx>, |yy>, and |zz> with overall singlet character. This superposition of the three states gives rise to the three quantum beat frequencies seen in the experiment. Damping of the quantum beats results from both population exchange between triplet and singlet manifolds and pure dephasing between the triplet pair states. By lowering the temperature and slowing the SF rate, the visibility of the oscillations decreases. There is no evidence of magnetic dipole-dipole coupling between the product triplets. Our model provides good overall agreement with the data, supporting the conclusion that singlet fission in tetracene proceeds through the "direct" mechanism without strong electronic coupling between the singlet and triplet pair states.

Visualization of exciton transport in ordered and disordered molecular solids

“…As has been shown by Burdett et al 28 , correlations between pairs of triplets can persist up to B10 ns after fission. Such correlations modify the rate equation at early times and give a linear relationship between triplet and singlet exciton density.…”

Singlet fission in linear chains of molecules

“…1, 2 Decades after its first observation in anthracene crystals, SF has been recently experimented in a wide variety of crystalline organic materials, [3][4][5][6][7][8][9][10] polymers, 11 quantum dots, 12 and in solution. 13 One of the reasons behind a renewed interest on this physical phenomenon is given by the opportunity to exploit it, in order to overcome the Shockley-Queisser theoretical limit of conversion efficiency for single junction solar cells, 14 a possibility that has been verified in few instances 4,15 but it is still under debate.…”

“…Burdett and Bardeen studied the temperature-dependence for the decay of singlet exciton in tetracene crystals, in particular at low temperatures (200 K), where it is affected by a possible change of polymorphism. 5,6 However, Friend's group suggests that the generation of triplet pairs, in absence of the driving force given by exothermicity, is due to an equilibrium-like process between S1 and 2T1 populations, thus explaining both lower rates and high quantum yield. 3 It is known that the mechanism of SF features an intermediate state which is typically described by a doubly excited configuration where two triplets with opposite spins are accommodated on two different excitation sites.…”

“…22 A main controversy in SF mechanism arises from the coupling between ME and S1 states: some studies suggest that the main interaction driving the ME←S1 transition is the direct coupling between these two states; 5,17,18,31 another school of thought indicates that direct coupling is not sufficient to explain the rates of SF and suggests that indirect coupling via charge transfer (CT)…”

Magnetic field dependence of singlet fission in solutions of diphenyl tetracene

“…In thin films, fission has been previously characterized by transient [8,15] and continuous wave [10] optical probes. Both show a strong magnetic field dependence in the fluorescence.…”