Photophysical properties of the ITI photoswitches substituted in the R₁ 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
1c	NO2	0.78	419	527	10.9	2900	12.7 (t)	14.7	10.3
1d	F	0.06	435	537	6.8	4600	13.1 (t)	14.9	14.6
1e	Me	−0.17	437	536	6.5	4200	13.6 (t)	15.0	18.0
1f	OMe	−0.27	460	540	3.9	1400	14.0 (t)	15.1	20.0
1g	NMe2	−0.83	545	504 (calc)	N.A.	1000	13.6 (t)	N.A.	N.A.

^aPosition of the absorption band (λ_max), extinction coefficients for the Z- and E-isomers and quantum yields of forward switching of ITIs 1c–g in MeOH. Experimental λ_max,E values are obtained from TA. 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 1c–g 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