Excited state absorption spectra and intersystem crossing kinetics in diazanaphthalenes

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Abstract
Picosecond time‐resolved, excited state absorption spectra in the visible following excitation at 355 nm are discussed for room temperature solutions of four diazanaphthalenes (DN)—quinoxaline (1,4‐DN), quinazoline (1,3‐DN), cinnoline (1,2‐DN), and phthalazine (2,3‐DN). Kinetics of singlet state decay are obtained by monitoring the decay of Sn←S1 bands. The intersystem crossing rate constant (kisc) is found to vary as kisc(1,4‐DN)≳kisc(1,3‐DN)≳kisc(1,2‐DN). The kisc in phthalazine could not be determined from the weak, visible Sn←S1 absorption. Assuming rapid singlet vibrational relaxation and only minor effects due to energy gap variations, these experimental results agree with statistical limit predictions for the relative nonradiative rate. Calculations of the spin–orbit coupling matrix element βel= 〈S1(1nπ*)‖ Hso ‖ T1(3ππ*)〉, using INDO wave functions, give the ordering βel(1,4‐DN)≳βel(2,3‐DN)≳βel(1,3‐DN) ≳βel(1,2‐DN).