Photophysical properties of the ITI photoswitches substituted in the X position^a.

	X	λ max,Z (nm)	λ max,E (nm)	Φ Z–E (%)	ε Z (M−1 cm−1)	ΔG°# (kcal mol−1) calc.	ΔG°# (kcal mol−1) exp.	t 1/2 (ms)
1a	S	429	515, 552	6.2	4300	13.3 (t)	15.0	18.5
1b	Se	446	546	16.2	1800	12.8 (t)	13.8	2.3

^aPosition of the absorption band (λ_max), extinction coefficients for the Z- and E-isomers and quantum yields of forward switching of ITIs 1a and 1b in MeOH. Where possible, experimental λ_max,E values are obtained from transient absorption (TA) spectra where two absorption maxima are identified; the most intense absorption is highlighted in bold. Theoretical activation barriers for thermal relaxation were obtained at the M06-2X/6-31+G(d)//6-311++G(2df, 2p) level. The nature of TS is specified in parentheses; t/p stands for twisted/planar structure. Experimental activation energies were calculated with the Eyring equation (with κ = 1). Thermal relaxation rates of ITIs 1a and 1b were recorded in MeOH. Experimental half-lives were obtained from ns TA spectroscopy. Data for 1a have been reported earlier and are reported for comparison.¹³