The photochemistry of 3,4,5-trideuteriopyridine

Irradiation of 3,4,5-trideuteriopyridine at 254 nm in the vapor phase results in the formation of a mixture of 2,3,4-trideuteriopyridine and 2,3,6-trideuteriopyridine. The formation of these products is consistent with a photoisomerization mechanism involving equilibrating azaprefulvene intermediates. This is the first direct evidence that pyridine vapor undergoes photoisomerization resulting in transposition of the pyridine ring atoms.     

Synthesis and spectroscopic properties of isomeric trideuterio‐ and tetradeuterio pyridines

The syntheses of the six possible isomeric trideuteriopyridines and the three possible isomeric tetradeuteriopyridines are described. These deuteriopyridines were characterized by their mass and NMR spectra.     

Synthesis and spectroscopic properties of some dideuterated cyanopyridines

4‐Cyanopyridine‐2,6‐d₂ ( 5a‐2,6‐d₂ ), 3‐cyanopyridine‐2,6‐d₂ ( 5b‐2,6‐d₂ ), and 2‐cyanopyridine‐4,6‐d₂ ( 5c‐4,6‐d₂ ) were synthesized from the corresponding 2‐, 3‐ or 4‐pyridinecarboxylic acid N‐oxides. These dideuterated products were characterized by their mass and NMR spectra.     

Medium effects on the direct cis-trans photoisomerization of 1,4-diphenyl-1,3-butadiene in solution

Quantum yields for the photoisomerization of trans,trans-1,4-diphenyl-1,3-butadiene (tt-DPB), determined in benzene, cyclohexane, methylcyclohexane, hexane, and perfluorohexane, confirm the low values reported earlier for benzene and cyclohexane and reveal even lower values in the last two solvents. In contrast to trans-stilbene (t-St), fluorescence and torsional relaxation leading to photoisomerization do not account exclusively for S(1)tt-DPB decay. Competing radiationless singlet excited-state decay pathways exist in tt-DPB, which do not lead to photoisomerization and may not involve large-amplitude torsional motions. Our results invalidate analyses of tt-DPB fluorescence quantum yields and lifetimes that assign all radiationless decay to the isomerization channel. Gas-phase chromatography analysis of tt-DPB photoisomerization in hexane shows the reaction to be concentration-independent and reveals, for the first time, a significant, two-bond photoisomerization pathway, φ(tt→tc) = 0.092 and φ(tt→cc) = 0.020. The dominant one-bond-twist (OBT) process is accompanied by a bicycle pedal (BP) process that accounts for almost 20% of tt-DPB photoisomerization. The OBT tt-DPB photoisomerization quantum yield is largest in benzene (Bz) and smallest in perfluorohexane (PFH). Contrary to expectations, reduction in medium friction in PFH is accompanied by a decrease in φ(tt→tc). The 1(1)B(u)/2(1)A(g) order and energy gap appear to control the contribution of torsional relaxation to radiationless decay. Lowering the 1(1)B(u) energy as in Bz favors photoisomerization. Reversal of the 1(1)B(u)/2(1)A(g) order in PFH is accompanied by short τ(f) and small φ(f) and φ(tt→tc) values that suggest the presence of competing 2(1)A(g) → 1(1)Ag relaxation paths that are unproductive with respect to photoisomerization. We conclude that the Birks extension to diphenylpolyenes of the Orlandi-Siebrand cis-trans photoisomerization mechanism is not valid. Photoisomerization appears to occur in the 1(I)B(u) state, and we argue that this applies to t-St as well.     

Vapor-phase photochemistry of methyl- and cyanopyridines: deuterium labeling studies

The three isomeric methylpyridines and the three isomeric cyanopyridines each constitute a photochemical triad. Irradition of each methylpyridine or each cyanopyridine in the vapor phase at 254 nm results in the formation of the other two isomers as primary photoproducts. Dideuteration of the 2-substituted or 3-substituted methyl or cyanopyridines expanded each triad to a pentad. Due to symmetry, 2,6-dideuteration of 4-methyl-or 4-cyanopyridine did not expand the triad. Trideuteration of 4-methylpyridine removed this symmetry and resulted in a photochemical pentad. These results are consistent with a mechanism involving 2,6-photocylization, migration of nitrogen around the five sides of the cyclopentenyl ring, and rearomatization. This mechanism exactly predicts the observed distribution of deuterium in the photoproducts.     

The vapor-phase phototransposition chemistry of pyridine: deuterium labeling studies

The six isomeric trideuteriopyridines and the three isomeric tetradeuteriopyridines undergo phototransposition upon S0 --> S2 (pi, pi*) excitation in the vapor phase at 254 nm. On the basis of the products formed, the six trideuteriopyridine isomers can be divided into two triads. Similarly, the three isomeric tetradeuteriopyridines also constitute a triad. Irradiation of any one member of each triad results in the formation of the other two members of that triad. These isomerizations are consistent with a mechanism involving photocyclization, nitrogen migration around the five sides of the cyclopentenyl ring, and rearomatization.     

Photoisomerization of cis,cis- to trans,trans-1,4-diaryl-1,3-butadienes in the solid state: the bicycle-pedal mechanism

The photoisomerizations of crystalline or powdered cis,cis-1,4-diphenyl- and 1,4-di(o-tolyl)-1,3-butadienes (cc-DPB and cc-DTB) to the trans,trans isomers were studied at room temperature. The progress of the reactions was monitored by fluorescence spectroscopy, powder X-ray diffraction, (1)H NMR, and high-performance liquid chromatography. Conversions to the trans,trans isomers were as high as 90% for cc-DPB and 20% for cc-DTB. Formation of the cis,trans isomers, the sole products obtained in solution and in very viscous glassy media at 77 K, is completely suppressed in the solid state. The observed two-bond photoisomerizations are explained by the bicycle-pedal (BP) photoisomerization mechanism. X-ray structure determinations show that o-methyl substitution causes a widening of the phenyl/diene dihedral angles from 40 degrees to 56 degrees and decreases the number of conformers in the crystal from two in cc-DPB to one in cc-DTB. The two conformers of cc-DPB molecules exist in crystals in edge-to-face alternating arrays, one of which has the two phenyls in parallel planes and the other in roughly perpendicular planes. The edge-to-face relationship is maintained in cc-DTB, but only the conformer with the o-tolyl groups in parallel planes is present. The time evolutions of fluorescence spectra measured in the course of the photoreaction show remarkable similarities, despite the different molecular conformations and crystal packing arrangements. Principal component analyses of the spectral matrices indicate the formation of discrete components, suggesting that the two-bond photoisomerizations proceed in stages involving molecules in different microcrystal environments. The structureless appearances of the initial fluorescence spectra show that the reactions are in part diabatic. The BP mechanism can account for the observations if the bicycle-pedal motion began in the excited state, S(1), and were completed in the ground state, S(0). Analysis of void spaces in the crystal lattice reveals much less compact packing of cc-DPB than of cc-DTB molecules, possibly explaining the much higher conversions to photoproduct from cc-DPB.