Substituent Effects on Singlet Exciton Fission in Polycrystalline Thin Films of Cyano-Substituted Diaryltetracenes

Cyano-substituted
tetracenes (5,11-dicyano-6,12-diphenyltetracene, Tet)
undergo exoergic singlet fission (SF), a spin-allowed
photophysical process that generates a pair of triplet excitons from
one singlet exciton. To elucidate substituent effects on SF, we have
measured the SF dynamics and triplet yields of thin films, formed
by Tet bearing hydrogen (H), methyl (Me), fluoro (F), and trimethylsilyl (TMS) substituents on the p-phenyl positions and on
the 3 and 9 positions of the tetracene core, by time-resolved spectroscopy
in the vis-NIR and IR regions. The H-, Me-, and F-Tet display strong intramolecular electronic
coupling (π–π distances <4 Å), and SF gives
high triplet exciton yields up to 200% (quantitative). In addition,
a charge-transfer state mediates SF in F-Tet films, while H-Tet and Me-Tet show no evidence for such a
state. Correlations between the SF yields and the crystal structure
show that chromophore slippage along both their short and long axes
allows efficient SF and that a large degree of π contact between
the chromophores is not necessary for rapid and efficient SF in the
solid state. As expected, the large interchromophore distance in TMS-Tet (>4 Å) reduces its SF triplet yield to about
60%.