Positional dependence of the heavy atom effect in the cis-trans photoisomerization of bromostilbenes

Abstract

Positional Dependence of the Heavy Atom Effect in the Cis-Trans Photoisomerization of Bromostilbenes Sir:Comparison of the temperature dependence of fluorescence, &, and isomerization, +t+c and quantum yields for the stilbenes and the p-halostilbenes has revealed a substantial bromine substituent effect. 3-5 Increasing the temperature in the parent hydrocarbon from -180 to 25" increases C$t-c while decreasing &, these two processes being coupled throughout the temperature range.

Pyrene-based metal–organic framework NU-1000 photocatalysed atom-transfer radical addition for iodoperfluoroalkylation and (Z)-selective perfluoroalkylation of olefins by visible-light irradiation

“…The heavy-atom effect of iodo group of product 6b was believed to facilitate the conversion from (E)-to (Z)-geometry. [74][75][76][77][78] Simultaneously, the EnT-induced intersystem crossing behaviour of conjugated arylalkene also imposed an competitive driving force to pull the peruoroalkyl group to the '(Z)'-side of aryl moiety (caution: here, the (Z)-geometry relative to aryl is equivalent to the (E)-geometry relative to iodine atom). The ratio of (Z)-/(E)-isomers should represent the balance status of 'tugof-war' between iodo and aryl group, and it was interesting to examine the (Z)-/(E)-ratio if iodine atom of 6b was absent, since it might provide a pathway for the facile preparation of (Z)enriched peruoroalkyl styrenes (caution: the (Z)-geometry here refers to the case of aryl moiety) which was of potential interest in pharmaceutical eld and generally needed multi-step synthesis.…”

Heavy-atom effect in photoisomerization of 4-pyrones and 4-pyridones

“…3.6-Bis(p-chlorphenyl)-4,5-dimethyl-2-pyrone (IVd): UV (CH3CN) Xmax 242 (c 12700), 325 nm (16700); IR (KBr) 1700, 1630, 1585,1530,1445,1390,1375,1330,1180,1080,1000,945,930, 870, 835, 825 cm"1; NMR (CDC13) 2.05 (s, 3 ), 2.10 (s, 3 ), 7.0-7.6 (m, 8 H); mass spectrum (m/e) 348 (5), 347 (6), 346 (28), 345 (9), 344 (M+, 48), 320 (12), 319 (13), 318 (65), 317 (20), 316 (100), 179 (13), 177 (40), 159 (8), 158 (12), 141 (25), 139 (25), 115 (9), 111 (15).…”

“…A solution of 0.3 g of l,2,6-trimethyl-3,5-bis(p-fluorophenyl)-4pyridone in 250 mL of acetonitrile was irradiated under nitrogen with a 500-W medium-pressure mercury lamp using Vycor filter at 29 °C for 2 h. The solvent was removed under reduced pressure and the residual solid was chromatographed on silica gel with chloroformether (8:2) as eluent. The first 600-mL collection afforded colorless crystals, which, after recrystallization from benzene-ligroin, gave 0.009 g (3.5%) of l,4,5-trimethyl-3,6-bis(p-fluorophenyl)-2-pyridone (Vic): mp 215-217 °C; IR (KBr) 1630, 1605, 1580, 1540, 1505, 1420, 1390, 1280, 1220, 1210, 1155, 1090, 1010, 910, 880, 845, 825 cm"1; NMR (CDClg) 1.80 (s, 3 ), 2.03 (s, 3 H), 3.23 (s, 3 ), 7.0-7.5 (m, 8 H); mass spectrum (m/e) 326 (18), 325 (M+, 86), 324 (100), 298 (5), 297 (23), 296 (24), 162 (8), 136 (13), 133 (9), 95 (12). Anal.…”

“…,805, 735, 715 cm"1; NMR (CC14) 1.78 (s, 3 ), 2.03 (s, 3 ), 2.40 (s, 3 H), 2.45 (s, 3 H), 3.22 (s, 3 H), 7.0-7.4 (m, 8 H); mass spectrum (m/e) 318 (21), 317 (M+, 87), 316 (100), 290 (7), 289 (74), 288 (17), 136 (7), 132 (9), 119 (14), 115 (8), 105 (6), 91 (19), 78 (28), 77 (10), 57 (14), 56 (11). 1.4.6-Trimethyl-3,5-bis(p-chlorophenyl)-2-pyridone (Vd): UV (MeOH) Xmax 246 («6400), 318 nm (6700); IR (KBr) 1615,1550,1490, 1390,1300,1105,1085,1015,990, 960,850,815,790,695 cm"1; NMR (CDC13) 1.66 (s, 3 ), 2.10 (s, 3 ), 3.60 (s, 3 )', 7.0-7.4 (m, 8 H); mass spectrum (m/e) 361 (12), 360 (22), 359 (63), 358 (M+, 81), 357 (100), 356 (95), 331 (19), 330 (22), 329 (28), 328 (22), 324 (14), 323 (22), 322 (26), 295 (10), 278 (10), 202 (11), 115 (11), 101 (11), 56 (40). 1.4.6-Trimethyl-3,5-bis(p-bromophenyl)-2-pyridone (Ve): UV (MeOH) Xmax 255 (e 10200), 318 nm (10400); NMR (CC14) 1.67 (s, 3 ), 2.12 (s, 3 ), 3.60 (s, 3 ), 7.0-7.6 (m, 8 H); mass spectrum (m/e) 449 (47), 448 (52), 447 (M+, 100), 446 (80), 445 (62), 444 (36), 419 (16), 418 (13), 369 (39), 368 (42), 367 (46), 366 (37), 339 (10), 202 (15), 144 (54), 143 (60), 137 (16), 135 (21), 122 (10), 105 (14), 56 (8).…”

Laser Flash Photolysis and Quantum Yields of the Direct Photochemical Cis‐Trans Isomerization of 4‐Nitro‐4′‐methoxystilbene in Cyclohexane Solutions in Presence of Quenchers. Model for the Photoisomerization of Nitrostilbenes

“…For unsubstituted stilbene [2] and also for 4-cyano-4-methoxystilbene [8 -111 it is assumed that the direct photoisomerization occurs by rotation in the singlet state. In these cases the triplet states are not formed by direct excitation at room temperature [5,6,8,9].…”