Tuning from π,π* to charge-transfer excited states in styryl-substituted terthiophenes:: An ultrafast and steady-state emission study

The steady-state and transient emission properties of unsubstituted terthiophene and a series of 3'-[E-2-(4-R- phenyl) ethenyl]-2,2':5',2''-terthiophenes (where R) H, MeO, NH2, CN, NMe2, NO2) have been examined. The R=NO2 compound is nonemissive at room temperature in all solvents but cyclohexane. All of the other compounds show measurable steady-state emission in a variety of solvents. The behavior of these spectra may be split into two groups. The first group, those substituted compounds with R=CN, NH2 and NMe2, show solvatochromic behavior, where their Lippert-Mataga plots suggest changes in dipole upon photo-excitation ranging from 12.5 to 16.0 D. For the second group, where R=H and MeO (and unsubstituted terthiophene as well), the Lippert-Mataga plots indicate dipole moment changes ranging from 0 to 7.9 D. The difference in behavior between the two groups of emissive compounds can be attributed to a charge-transfer character of the emitting state in the first group. This conclusion is supported by density functional theory calculations, which show that the frontier MOs in the group one compounds are spatially separated whereas those of group two have frontier MOs that are delocalized over both the styryl and terthiophene moieties. Picosecond time-resolved fluorescence spectroscopy reveals that unsubstituted terthiophene has the shortest emission lifetime of 140 ps in acetonitrile. For the styryl substituted terthiophenes, the lifetimes are much longer and range from 320 to 670 ps for R=CN and NMe2 respectively, a result that can be explained in terms of a smaller rate of intersystem crossing in these compounds.