Excited State Behavior of Diarylethenes in the Subnanosecond Timescale: The Role of an Upper Singlet

Picosecond absorption measurements of the trans isomers of 9-styrylanthracene (9-StAn), n-styrylnaphthalene (n-StN, with n = 1 and 2), and n-styrylphenanthrene (n-StPh, with n = 1, 2, and 9) were carried out in a number of solvents at room temperature. In all cases, a short-lived transient (with tau(s) = 0.1 divided by 0.8 ns) was produced monophotonically within the laser pulse. This transient, which absorbs in a wide wavelength range (400-750 nm), was assigned to the S-2 state. At longer delay time, the S-1 --> S-n absorption (with lifetime tau(L)) replaced that of S-2, its assignment being strongly supported by the comparison between tau(L) and the fluorescence lifetime of these diarylethenes. The S-1 --> S-n absorption of two distinct rotamers was detected for 2-StN in solvents of different polarity, while for the other compounds only one longer-lived transient was detected. Complementary fluorimetric measurements by phase-modulation technique confirmed the presence of a second decay component in the picosecond region with a lifetime corresponding to the tau(s) values measured by laser flash photolysis. The effect of temperature studied in both picosecond and nanosecond regions suggests that the S-2 --> S-1 internal conversion is not the only decay pathway of S-2. In particular, for 9-StAn, it is shown that both S-1 and S-2 are involved in the singlet-triplet intersystem crossing, and, at least in the cases of 1-StN and 2-StPh, the S-2 State (or both S-i and S-2) is (are) involved in the trans --> perp decay process. An investigation of the effect of excitation wavelength on the fluorescence and trans --> cis photoisomerization quantum yields was found to support such hypothesis of the role played by upper excited states in the relaxation processes of some of the styrylarenes investigated.