Photophysical properties of the ITI photoswitches substituted in the R₂ position^a.

	R2	Hammett (R)	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	H	0	429	515	6.2	4300	13.3 (t)	15.0	18.5
1h	CF3	0.54	424	500	4.9	2100	13.1 (t)	14.5	7.5
1i	F	0.06	425	512, 542	0.4	4000	13.8 (p)	15.2	22.9
1j	OMe	−0.27	448	516, 553	4.5	11 000	13.2 (p)	14.3	5.3
1k	NMe2	−0.83	516	620	6.2 × 10−3	27 000	10.3 (p)	13.8	2.0

^aPosition of the absorption band (λ_max), extinction coefficients for the Z- and E-isomers and quantum yields of forward switching of ITIs 1h–k in MeOH. Experimental λ_max,E values are obtained from TA spectra where two absorption maxima are identified; the most intense absorption is highlighted in bold. Theoretical activation barriers for thermal relaxation were derived at the M06-2X/6-31+G(d)//6-311++G(2df, 2p) level. The nature of TS is specified in the 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 1h–k were recorded in MeOH. Experimental half-lives were obtained from ns TA spectroscopy. Data for 1a, 1i, and 1j have been reported earlier and are reported for comparison.¹³