Spectroscopic Study of Thiophene–Pyrrole-Containing S,N-Heteroheptacenes Compared to Acenes and Phenacenes

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Rudnick, Alexander ; Wetzel, Christoph ; Tscheuschner, Steffen ; Schmalz, Holger ; Vogt, Astrid ; Greiner, Andreas ; Bässler, Heinz ; Mena-Osteritz, Elena ; Bäuerle, Peter ; Köhler, Anna:
Spectroscopic Study of Thiophene–Pyrrole-Containing S,N-Heteroheptacenes Compared to Acenes and Phenacenes.
In: The Journal of Physical Chemistry B. Bd. 121 (2017) Heft 31 . - S. 7492-7501.
ISSN 1520-5207
DOI: https://doi.org/10.1021/acs.jpcb.7b02935

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Abstract

In this study, we report a detailed spectroscopic study concerning the energy levels and vibrational structure of thiophene–pyrrole-containing S,N-heteroacenes. The aim of the study is first, to understand the differences in the photoluminescence (PL) efficiencies in this structurally similar series and second, to compare the electronic structure of S,N-heteroacenes to that of linear acenes and phenacenes, with a view to derive guidelines for the design of singlet fission materials. For S,N-heteroacenes comprising seven fused heterocyclic rings, we observe a higher PL quantum yield for derivatives with terminal thienothiophene units than for thienopyrrole-capped ones. This is assigned to a stronger tendency of the thienopyrrole-capped derivatives to form nonemissive associates in dilute solution, producing emissive excimers at higher concentration. By conducting time-resolved PL studies at 77 K, we further determine the lowest singlet and triplet energies for the S,N-heteroacenes with three, five, and seven fused rings. We show that their energies evolve with oligomer length analogously to those of phenacenes, yet in a fundamentally different way from that of linear acenes. This difference in evolution is attributed to the increasingly biradical character in acenes with increasing chain length in contrast to the S,N-heteroacenes and phenacenes.