Charge Transfer-Induced Weakening of Vibronic Coupling for Single Terrylene Molecules Adsorbed onto Hexagonal Boron Nitride

Fluorescence spectra of single terrylene molecules adsorbed on hexagonal boron nitride flakes were recorded at cryogenic temperatures. The pure electronic transitions of terrylene molecules are spread over a broad energy scale from 570 to 610 nm. Surprisingly, peaks in the vibrationally resolved fluorescence spectrum show intensity variations of ≤20-fold between molecules. We find an extreme case in which the Debye-Waller-Franck-Condon factor of the zero-phonon line exceeds 0.8. The vibronic intensity correlates with both the spectral position of the electronic transition and the frequency of the longitudinal stretch mode, which varies between 243 and 257 cm^-1. Using density functional theory calculations, we show that these observations can be explained by terrylene chemisorption on charge-donating defect sites. The electronic states of molecules at such chemisorption sites would be very attractive for the efficient emission of single photons with narrow lines and for the generation of indistinguishable photons.