Exciton Fission and Fusion in Bis(tetracene) Molecules with Different Covalent Linker Structures

Abstract

Bichromophoric molecules can support two spatially separated excited states simultaneously and thus provide novel pathways for electronic state relaxation. Exciton fission, where absorption of a single photon leads to two triplet states, is a potentially useful example of such a pathway. In this paper, a detailed study of exciton fission in three novel phenylene-linked bis(tetracene) molecules is presented. Their spectroscopy is analyzed in terms of a three-state kinetic model in which the singlet excited state can fission into a triplet pair state, which in turn undergoes recombination on a time scale longer than the molecule's radiative lifetime. This model allows us to fit both the prompt and delayed fluorescence decay data quantitatively. The para-phenylene linked bis(tetracene) molecules 1,4-bis(tetracen-5-yl)benzene (1) and 4,4'-bis(tetracen-5-yl)biphenylene (2) show intramolecular exciton fission with yields of approximately 3%, whereas no delayed fluorescence is observed for tetracene or the meta-linked molecule 1,3-bis(tetracen-5-yl)benzene 3. Analysis of the temperature-dependent fluorescence dynamics yields activation energies for fission of (10.0 +/- 0.6) kJ/mol for 1 and (4.1 +/- 0.5) kJ/mol for 2, with Arrhenius prefactors of (1.48 +/- 0.04) x 10(8) s(-1) for 1 and (1.72 +/- 0.02) x 10(7) s(-1) for 2. The observed trends in activation energies are reproduced by ab initio calculations of the independently optimized singlet and triplet energies. The calculations indicate that electronic coupling between the two tetracene units is primarily through-bond, allowing differences in fission rates to be qualitatively explained in terms of the linker structure as well. Our results show that it is important to consider the effects of the linker structure on both energy relaxation and electronic coupling in bichromophoric molecules. This study provides insight into the structural and energetic factors that should be taken into account in the design of exciton fission molecules for possible solar cell applications.

Singlet fission in pentacene through multi-exciton quantum states

“…A detailed analysis of the electronic configurations comprising the wavefunctions of D shows that they are best described as multiple charge carrier pairs or, more specifically, two electron-hole pairs. Because D has the character of multiple excitons, this specific type of state is likely to be important in many systems that exhibit multiple-carrier generation from single photons [12][13][14]16 . Dark states, which are difficult to observe experimentally, are therefore central to multiple charge carrier generation mechanisms.…”

“…Although earlier studies outlined the energetic criteria for singlet fission 15,16 , the lack of a proper description of the ME wavefunction has prevented elucidation of a detailed mechanism. Some have postulated that the singlet fission process involves interconversion of the optically allowed excited state S 1 to a nearby state that has the character of two triplets [11][12][13][14] .…”

Solution-based intramolecular singlet fission in cross-conjugated pentacene dimers

“…[26][27][28] Solution-state SF has also been demonstrated in tetracene dimers, but with a SF yield of only 3%. 25 It is logical to hypothesize that the relative proximity between neighboring acenes is critical to the process of SF, and the molecular geometry as well as the π-π overlap would be dominant factors for efficient SF. [32][33][34][35] One can envision a plethora of acenes that might be synthesized to explore specific aspects of SF in dimeric structures.…”

Singlet fission in pentacene dimers

“…As such, the potential of coupled chromophores to afford two triplet excited states via SF has been elucidated in, for example, a tetracene dimer with an SF yield of around 3% (3,7). Additionally, past experiments in single-crystal, polycrystalline, and amorphous solids of pentacene have documented that the efficiency of SF relates to the electronic coupling between these two chromophores (8,9).…”