Photophysical properties of the ITI photoswitches substituted in the R₃ and R₄ position^a.

	R3	R4	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	H	429	515	6.2	4300	13.3 (t)	15.0	18.5
1l	OMe	H	438	546	6.1	3500	13.3 (p)	14.9	15.4
1m	Me	H	432	551	12.2	1600	13.2 (p)	14.1	3.7
1n	Me	Me	417	549	2.1	2500	13.4 (d)	13.2	0.8
1o	Et	H	433	543	6.2	3000	13.8 (p)	14.0	3.1
1p	iPr	H	432	542	5.7	3000	13.4 (d)	14.0	3.0
1q	Br	H	425	520	4.6	2100	13.7 (t)	14.7	9.8
1r	Br	Br	423	523	3.6	1500	13.4 (t)	14.4	5.9
1s	Cl	H	426	520	8.4	1400	13.6 (t)	14.7	10.5
1t	Cl	Cl	421	521	2.6	2700	14.2 (t)	14.5	7.6
1u	F	H	428	517	7.5	2000	13.8 (p)	15.4	36.5
1v	F	F	421	515	4.8	2100	14.8 (d)	15.9	83.3
1w	CF3	H	425	519	7.9	1400	13.1 (t)	14.2	4.2

^aPosition of the absorption band (λ_max), extinction coefficients for the Z- and E-isomers and quantum yields of forward switching of ITIs 1l–w 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/d/p stands for twisted/distorted/planar structure. Experimental activation energies were calculated with the Eyring equation (with κ = 1). Thermal relaxation rates of ITIs 1l–w 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