Laser-induced fluorescence spectroscopy of N,N'-dimethyl 3,4,9,10-perylene tetracarboxylic diimide monomers and oligomers attached to helium nanodroplets.

Laser-induced fluorescence spectroscopy has been applied to study the molecule N,N'-dimethyl 3,4,9,10-perylene tetracarboxylic diimide (MePTCDI) and its oligomers embedded in helium nanodroplets. The 0(0)0-transition of the monomer absorbs at 20571.6 cm(-1) as the dominant line. The vibrational structure in the S1 state of the molecule is clearly resolved at higher frequencies. The measured vibrational spectrum is compared to Raman spectra of thin films and calculated frequencies of the isolated molecule. When doping the helium droplets with MePTCDI oligomers, narrow (< 1 cm(-1)) as well as broad (approximately 500 cm(-1)) excitation lines are observed. These are attributed to electronic molecular dimer transitions and excitonic transitions of sandwich-like MePTCDI oligomers, respectively. The findings are compared with the corresponding absorptions of 3,4,9,10-perylene tetracarboxylic dianhydrid (PTCDA) in helium droplets and with those of thin solid films.