Anisotropic Singlet Fission in Organic Single Crystals
American Chemical Society (ACS) National Meeting 2016, Philadelphia, PA, USA
Singlet fission (SF) is one of solutions to the breaking of the Shockley-Queisser theoretical limit for higher photovoltaic conversion efficiency. Despite recent efforts, the mechanisms by how singlet fission occurs remain still unclear. In this work, we explore anisotropic singlet fission mechanisms of hexacene single crystal by employing polarized static and transient absorption microscopy. Polarized static absorption spectra of hexacene single crystal show a large Davydov splitting of 1160 cm-1, originating from a strong charge transfer character for the crystal sites. Polarized transient absorption spectra display a clear triplet Davydov splitting of 240 cm-1 (30 meV), indicating that the adiabatic T1 states inherit the charge transfer characteristic of the singlet states after the photoexcitation. Transient kinetics analyses show anisotropic singlet fission along the long a- axis and short b- axis of hexacene single crystal. We found from our kinetics data that the singlet fission of hexacene single crystal is a two-step process, instead of the one-dominant-step process as reported in the literature. The rates for both the a- and b-axis in the first step are very close, while the rate in the second step along the b-axis is faster than that along the a-axis. The anisotropic rates depend merely on the probe polarization instead of polarization of the excitation, which is attributed to the delocalized excitation over a few molecules. The first step from the mixed Frenkel and charge transfer to the [ME] state is in the adabatic regime of strong electronic coupling, while the second step from the [ME] state to the triplet state is nonadiabatic from nonlocal electron-vibration couplings (Peierls interactions) of the intermolecular interactions. Our work offers direct evidence that the singlet fission in hexacene single crystal is an anisotropic and charge-transfer mediated many-molecule process. We believe that this work gives new insights into the singlet fission mechanism for both theory and applications.