Spectroscopic and Photophysical Characterization of Acetylenic Fluorophores: The Role of the Proximity Effect on Increasing Internal Conversion.

The spectroscopic and photophysical behavior of a series of six extended arylacetylenes and one 1,3-diyne derivative was studied in solvents of different polarity to gain insight into the relationships between molecular architecture and optical/photophysical properties. The radiative decay channel was revealed to be the most important one for all the compounds, particularly in nonpolar solvents. A notable fluoro-solvatochromism was observed for the 1,3-diyne derivative in line with the large increase of its dipole moment under excitation. A peculiar behavior was observed for nitro-substituted aryl acetylenes on increasing solvent polarity, which was explained by the presence of an upper forbidden singlet state (S ) that is more polar than the allowed S state; this was confirmed by quantum mechanical calculations. The large decrease of the S -S energy gap in a moderately polar solvent causes a strong increase of S -S internal conversion to the detriment of fluorescence, which is in agreement with Lim's proximity effect model.