Unusual solvent effect of molecular charge transfer complexes: Stacking/non-stacking interaction revealed by characterization of structure and photophysical aspects

Abstract

The molecular charge transfer complexes using 9-methyl anthracene (9MA) and pyrene as donors and pyromellitic diimide (PMDI) as acceptor have been characterized by crystal structure analysis and photophysical studies. A π-π stacking interaction is observed in case of the 1:1 PMDI-Pyrene CT complex whereas the 1:1 PMDI-9MA CT complex exhibits selective stacking structure of the acceptor and donor in an interlock manner. The optimized structures from DFT calculations of PMDI-9MA and PMDI-Pyrene CT complexes in gas phase support the results. The photophysical studies of these complexes have been compared with the CT complexes of PMDA with four donors Naphthalene, Anthracene, Methyl Anthracene and Pyrene and the CT complexes of NTCDA with Methyl Anthracene and Pyrene. All the PMDA CT complexes exhibit blue shift of the charge transfer absorption and the corresponding emission band with increasing polarity of the solvents. This unusual solvent effect has been ascribed to specific interaction of the acceptor with the solvent. The red shift of the CT band in the solid state for the complexes compared to that observed in solvents support the contention of specific interaction. The binding constants for all the CT complexes are found to be of the order of 103 and the interplanar spacing of the stacked complexes is found to be around 3.3Å. Time resolved emissions of all the complexes reveal two different conformations in solution. The average lifetime of fluorescence in transparent KBr pellet is observed to be longer than that in solution. The structural properties, the spectral data and the geometry of the complexes in the excited states evaluated by TDDFT and DFT methods match well with the experimental results. These studies will help understand π-π interaction in supramolecular systems and in various biological interactions.