Higher excited-state photoisomerization and singlet to triplet intersystem-crossing in DODCI Original Research Article

An intensity dependent N-isomer fluorescence reduction was observed by picosecond laser pump and probe experiments for DODCI (3,3′-diethyloxadicarbocyanine iodide) in ethylene glycol and methanol. The S1–S0 fluorescence signal caused by time-delayed probe pulse excitation is reduced due to pump pulse induced enhanced N → P photoisomerisation, higher excited-state photodisintegration, and Sm–Tm′ intersystem-crossing. The transmission of a time-delayed picosecond probe light continuum is lowered by enlarged P-isomer absorption and triplet–triplet absorption. The excited-state absorption dynamics, photoisomerisation, photodisintegration and intersystem-crossing dynamics are simulated numerically. The quantum yields of P-isomer formation caused by higher excited-state singlet absorption are φNP′=0.06±0.02 and 0.1±0.02 for DODCI in ethylene glycol and methanol, respectively. Sm–Tm′ quantum yields of triplet formation of φSmTm′=0.05±0.02 for DODCI in ethylene glycol and φSmTm′=0.04±0.02 for DODCI in methanol are determined. Triplet–triplet absorption cross-section spectra are deduced from the picosecond light continuum transmission measurements.