Table 2.
Computational studies on substituent effects on ITI photo-isomerization
| Z-isomer | |||||||
|---|---|---|---|---|---|---|---|
| R (Hammet constant σ) | λmax,Z (nm) | Transition | |||||
| exp. | calc. | ππ*/nπ* | θ1(C1-C2-N4-C5)/θ2(C2-N4-C5-C6) | ΔμES-GS,Z (D) | ε (mol−1cm−1) | ||
| 1a | H (0.00) | 429 | 398 |
S0→S1 0.58/0.38 |
−179.8/54.1 | 1.55 | 4300 |
| 1b | MeO (−0.27) | 448 | 413 |
S0→S1 0.62/0.29 |
−179.1/38.6 | 4.43 | 11000 |
| 1c | Me (−0.17) | 434 | 406 |
S0→S1 0.61/0.32 |
−179.9/50.3 | 2.66 | 5700 |
| 1d | COOMe (0.45) | 427 | 399 |
S0→S1 0.56/−0.15 |
−179.1/60.8 | 0.85 | 2300 |
| 1e | CF3 (0.54) | 424 | 391 |
S0→S1 0.54/0.37 |
−179.1/62.0 | −0.62 | 2100 |
| 1 f | NO2 (0.78) | – | 390 |
S0→S1 0.53/−0.33 |
−178.9/67.3 | −0.26 | 2600 |
| E-isomer | ||||||
|---|---|---|---|---|---|---|
| R (Hammet constant σ) | λmax,E (nm) | Transition | ||||
| exp. | calc. | ππ*/nπ* | θ1(C1-C2-N4-C5)/ θ2(C2-N4-C5-C6) | ΔμES-GS,E (D) | ||
| 1a | H (0.00) | 515, 552 | 505 |
S0→S1 0.54/0.30 |
9.3/66.0 | −4.16 |
| 1b | MeO (−0.27) | 553 | 533 |
S0→S1 0.58/−0.32 |
12.4/52.9 | 2.37 |
| 1c | Me (−0.17) | 511, 548, | 519 |
S0→S1 0.58/−0.31 |
9.9/63.2 | −1.01 |
| 1d | COOMe (0.45) | 503 | 484 |
S0→S1 0.52/−0.31 |
0.26/93.0 | −4.45 |
| 1e | CF3 (0.54) | 500 | 482 |
S0→S1 0.52/−0.31 |
2.23/86.1 | −5.81 |
| 1 f | NO2 (0.78) | 501 | 470 |
S0→S1 0.54/0.30 |
0.00/−92.9 | −5.85 |
| Transition state | |||||||||
|---|---|---|---|---|---|---|---|---|---|
| R (Hammet constant σ) | Δλmax (nm) | t1/2 (ms) | Ea,E-Z (kcal/mol) | α(C2-N4-C5)/ | ΔμGS-TS,Z | ||||
| exp. | calc. | exp. | exp. | calc. | θ2(C2-N4-C5-C6) | (D) | φZ-E (%) | ||
| 1a | H (0.00) | 86, 123 | 107 | 18.5 ± 1.4 | 14.4 | 14.4 | 177.2/87.5 | −0.02 | 6.2 |
| 1b | MeO (−0.27) | 105 | 120 | 12.7 ± 0.5 | 14.2 | 13.0 | 177.7/0.0 | −2.45 | 4.5 |
| 1c | Me (−0.17) | 77, 114 | 113 | 21.1 ± 1.2 | 14.5 | 14.0 | 177.7/0.0 | −2.68 | 5.4 |
| 1d | COOMe (0.45) | 76 | 85 | 4.0 ± 0.3 | 13.6 | 13.1 | 177.6/92.0 | 0.38 | 6.3 |
| 1e | CF3 (0.54) | 76 | 91 | 9.9 ± 1.0 | 14.1 | 13.6 | 177.6/90.1 | 0.94 | 4.9 |
| 1 f | NO2 (0.78) | – | 80 | 2.8 ± 0.5 | 13.4 | 12.0 | 177.8/90.1 | 2.48 | 4.1 |
Shifts of λmax for the Z (Top) and E (Middle) isomers of ITIs 1a–f in MeOH. Experimental λmax,E values are obtained from TA that show two absorption maxima which are both reported with the maximum with the highest absorption highlighted in bold. Theoretical λmax values and the difference of GS and ES dipole moments (ΔμES-GS) were obtained at the SMD-TD-M06-2X/6-311++G(2df,2p) level using the SMD-M06-2X /6-31+G(d) geometries, from which also twisting angles (θ1 and θ2, Fig. 2a) were derived. Bottom: Thermal relaxation of ITIs 1a–f in MeOH. Experimental half-lives were calculated from ms TA spectroscopy. The differences of dipole moment of the transition state and that of the Z-form in their GS (ΔμGS-TS,Z) were obtained at the SMD-M06-2X/6-31+G(d) level, at which also the transition state twisting angles (θ1 and θ2) as well as the activation barriers for thermal relaxation were derived