Photochemical synthesis and photophysical features of ethynylphenanthrenes studied by emission and transient absorption measurements.

Phenanthrenes substituted with trimethylsilylethynyl and phenylethynyl groups were photochemically prepared, and their photophysical properties were systematically investigated based on measurements of fluorescence quantum yields, lifetimes, and transient absorption. Introducing ethynyl groups into the phenanthrene skeleton caused an increase in the fluorescence quantum yields compared to that of phenanthrene. The quantum yields and rates of fluorescence were dependent on the substituting position(s) and the terminating group for the C-C triple bond. The observation of the triplet-triplet absorption of the substituted phenanthrenes was evident for the nonradiative process being intersystem crossing competitive with the fluorescence process. The mechanism of increasing the fluorescence abilities by substituting with the ethynyl group(s) was discussed with the aid of TD-DFT calculations.