Photodecomposition of peroxides containing a 1,4-bis(phenylethynyl)benzene chromophore

The photodecomposition dynamics of 1,4-bis(2-[4-tert-butylperoxycarbonylphenyl]ethynyl)benzene (1) have been compared with those of model compounds in the picosecond and nanosecond time domains by various photophysical techniques. Ultrafast visible transient absorption spectrometry revealed the singlet excited state of 1,4-bis(4-phenylethynyl)benzene (BPB) depopulates radiatively with a rate of 1.75 x 10(9) s(-1) and 95% efficiency. Phenyl ester moieties attached to the BPB core accelerate intersystem crossing (k = 2.8 x 10(8) s(-1)) and reduce the fluorescence quantum yield (phi(FL) = 0.82). The peroxide oxygen-oxygen bond of 1 cleaves (k = 3.6 x 10(11) s(-1)) directly from the singlet excited state (60% efficiency) causing a highly reduced fluorescence yield and leading to formation of aroyloxyl radicals. The next reaction step involves decarboxylation of the aroyloxyl radicals. Transient absorption signals in the MID IR region correspond to CO2 with the formation rate (2.5 x 10(6) s(-1)) as measured by nanosecond transient IR experiments. The transient IR spectra of the excited state of BPB, as well as of the aroyloxyl radical, evidenced a red shift in the acetylene triple bond absorption indicative of a decrease in the bond order. This clearly shows that delocalization of excitation energy over the BPB chromophore induces significant structural changes. The proposed mechanism is based on the rates of photophysical and photochemical channels and involves an additional population channel of the BPB triplet excited state from the upper singlet states.