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Abstract
Due to their substantial fluorescence quantum yields in the crystalline phase, propeller-shaped molecules have recently gained significant attention as potential emissive materials for optoelectronic applications. For the family of cyclopentadiene derivatives, light-emission is highly dependent on the nature of heteroatomic substitutions. In this paper, we investigate excited state relaxation pathways in the tetraphenyl-furan molecule (TPF), which in contrast with other molecules in the family, shows emission quenching in the solid-state. For the singlet manifold, our calculations show nonradiative pathways associated with C-O elongation are blocked in both vacuum and the solid state. A fraction of the population can be transferred to the triplet manifold and, subsequently, to the ground state in both phases. This process is expected to be relatively slow due to the small spin-orbit couplings between the relevant singlet-triplet states. Emission quenching in crystalline TPF seems to be in line with more efficient exciton hopping rates. Our simulations help clarify the role of conical intersections, population of the triplet states and crystalline structure in the emissive response of propeller-shaped molecules.
Highlights
The optimisation of highly emissive organic molecules has become a milestone in the technology of optoelectronic materials
The experimental absorption spectrum of tetraphenyl-furan molecule (TPF) in THF solution features an intense band at 327 nm and a low-intensity band at 270 nm, whereas fluorescence peaked at 383 nm [11]
We tested the performance of single-reference (TD-DFT and RI-CC2) and multi-reference methods (CASPT2/CASSCF) for the description of TPF absorption (Franck-Condon point, FC) and emission spectra (S1 minimum) in vacuum, solution and crystal phase (Table 1)
Summary
The optimisation of highly emissive organic molecules has become a milestone in the technology of optoelectronic materials. According to the RACI model, the conical intersections, which act as funnels for internal conversion (IC) to the ground state, are destabilised in the crystal environment due to the steric hindrance, decreasing the internal conversion rate, and increasing the fluorescence yield [2,3,4,5,6]. This model is appropriate when a molecule possesses enough energy to explore regions of excited-state surfaces with strong nonadiabatic couplings between the ground and excited states. In contrast with related systems, the intermolecular processes seem to play an important role in excited state relaxation in the TPF crystal
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