Limitations of generic chromophore concept for femtosecond vibrational coherences

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The spectrum of vibrational coherences modulating transient triplet–triplet absorption of bis-triisopropylsilylethynylpentacene that propagate in the course of the fission process from the singlet state of this molecule to the triplet manifold has been lately reproduced by ab initio calculations. This has led to the conclusion that, contrary to a recent hypothesis, spontaneous generation of coherent vibrational wave packets is not a constitutive feature of singlet exciton fission. Here we demonstrate by calculations that the corresponding spectrum of unsubstituted pentacene is significantly different, so that the two systems cannot be viewed as different models of the same generic chromophore. This shows that femtosecond pump–probe spectra of vibrational coherences are sensitive to subtle features of molecular structure, which we identify here, thereby rationalizing the registered spectral differences. The crucial factor is the substituent-induced coupling between some pentacene normal modes that in other contexts may be safely ignored. Overall, our study indicates that this kind of spectroscopy requires great caution in using the generic chromophore concept for interpretative purposes and experimental design.