Hydrogen-bonded dimers, excitation transfer

Mukherjee, Bandyopadhyay and Chakraborty " investigated the photophysics of 7-azaindole homodimer (Fig. 52) in Ar and N2 matrices and the souble-proton transfer reaction taken place upon UV excitation. The IR spectra revealed that a doubly hydrogen bonded dimer is predominantly produced upon annealing the seeded matrices at 28 K. The authors found that, unlike in hydrocarbon solutions or in a supersonic jet expansion, the matrix-isolated doubly hydrogen-bonded dimer emits exclusively in ultraviolet from the locally excited state, and the main photophysical process (excited state double proton transfer ESDTP) was slowed down dramatically. They interpreted these observations in terms of matrix effect on the double proton transfer potential, specifically the increase of the effective ESDPT barriers in the rigid inert gas matrices compared to those in the gas phase or hydrocarbon solution. 

Figure 3.19 Resolution of the concerted versus stepwise mechanism of excited-state double proton transfer in doubly hydrogen-bonded dimer of 7-azaindole [90, 91]. Schematic presentations of (a) the concerted, one-step, synchronous double proton transfer in 7-azaindole dimer and (b) the stepwise proton transfer following formation...

The mode of asymmetric induction can be rationalized from the mechanism of the photopinacolization in the presence of aliphatic amines. The electron transfer from the amine to the excited triplet ketone furnishes charge transfer complex 5, from which a radical pair is formed by protoirtransfer. The weakly coordinated chiral amine seems to favor the dimerization of radical 6 from the si face leading to the (/ , ft)-enantiomer 3. The much lower selectivities observed with methanol as the cosolvent (3% ee at 27°C) indicate dipolar or hydrogen bonding interactions between the chiral diamine and the prochiral radical (Scheme 4). 

RRKM calculations on triplet species in diazirine photolysis,647 BEBO calculations on the activation energies for hydrogen-transfer reactions,648 stereochemistry as a probe for photochemical reaction mechanisms,649 photochemistry with polarized light,550 dimer formation,651 the formation of molecular complexes,552 hydrogen bonding in electronically excited states,553 and the interactions between excited-state aromatic molecules and 02654 have been the subjects of recent theoretical treatments. 

The main produets formed after irradiation were the trans-cis dimers [tc-3 and tc-4), whieh are also stabilized by N-H 0=C interactions. In contrast to the photoehemistry of the monomers, no dissociative products were observed after 248 nm irradiation of the dimers. The absence of dissociative produets was explained by a proton-transfer mechanism in the excited state that is faster than the photodissociative mechanism. The fact that hydrogen bonding has such a significant effect on the photochemical stability of JV-methylformamide has important implications to understand the stability of peptide-bonded systems to UV irradiation. 

In summary, the triplet (do po) excited states of the d -d metal dimers [Ir(p-pz)(C0D)]2 and Pt2(pop)4 " undergo a variety of photochemical reactions. Electron transfer to one-electron quenchers such as pyridinium cations or halocarbons readily occurs with acceptors that have reduction potentials as negative as -2.0 V. With the latter reagents, net two-electron, photoinduced electron transfer yields d -d oxidative addition products. Additionally, the triplet (da pa) excited state of Pt2(pop)4 apparently is able to react by extracting a hydrogen atom from a C-H bond of an organic substrate. 

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