Discrimination and interpretation of spectral phenomena by room-temperature single-molecule spectroscopy

The single-molecule fluorescence properties of the two perylenediimide dyes, 9-amino-N-(2,6-diisopropylphenyl)perylene-3,4-dicarboximide (API), and the derivative NN-di(tert-butoxycarbonyl)-API (DAPI), are investigated in the amorphous host polystyrene at room temperature. For API, three spectroscopically different types of spectra can be distinguished, two of them can be assigned to the off-resonance and the in-resonance conformer of the amino group and the chromophore's pi -electron system, repectively. The bathochromic third spectrum is suggested to originate also from an off-resonance conformer. In accordance with the sterically demanding substituents only off-resonance spectra are found for DAPI. Not being limited by the inhomogeneous spectral distribution, many molecules show clearly resolved vibronic structure and can be characterized by their spectral means, the vibronic maxima positions, and the vibronic band gaps. Sequences of consecutive spectra reveal different forms of dynamic behavior such as diffusion and abrupt jumps, both in the spectral domain and in the intensity trajectories. These can be summarized in a rough classification of the observed dynamic phenomena.