Key role of chromophore-modified vibrational modes on thermal broadening of single-molecule spectra in disordered solids

For decades, the thermal broadening of zero-phonon lines of single organic molecules embedded in amorphous solids was associated with their weak electron-phonon coupling to quasilocal vibrational modes. In this paper we show that the generally accepted perturbative approach for the pure dephasing problem is not applicable to the studied chromophore-doped glass, just as there is no evidence of weak coupling to individual quasilocal modes. Experimental and theoretical analysis of the temperature-dependent fluorescence excitation spectra of single tetra-$tert$-butylterrylene molecules in a polymer matrix of polyisobutylene is used as an example. Analysis of wide-range temperature measurements (9--60 K) with the nonperturbative theory of electron-phonon coupling demonstrates that the observed zero-phonon line broadening is due to the appearance of a wide spectrum of resonant modes induced by the impurity molecules themselves. This spectrum and the associated thermal broadening are obtained in the long-wave approximation for propagating waves using the density of the vibrational states for the host matrix and two quadratic coupling constants describing the interaction between the host and guest molecules. We also test the model considering the resonant modes arising from the host normal modes of a quasilocal character and find it completely incompatible with the experimental results.