Photoreduction of Oxoisoaporphine Dyes by Amines:  Transient-Absorption and Semiempirical Quantum-Chemical Studies

Abstract

Photoreduction by amines of oxoisoaporphine dyes occurs via a stepwise mechanism of electron-proton-electron transfer that leads to the metastable N-hydrogen oxoisoaporphine anion. During photoreduction that occurs from the triplet manifold of the oxoisoaporphine, a radical ion A(-)(*), a neutral-hydrogenated radical A-NH(*), and the metastable ion A-NH(-) of the oxoisoaporphine are formed. We present time-resolved spectroscopic data and quantum mechanical semiempirical PM3 and ZINDO/S results for the transient species formed during the flash photolysis of oxoisoaporphines in the presence of amines. These calculations reproduce adequately the experimental spectra of the triplet-triplet absorption near 450 nm, and that of neutral hydrogenated radical of the studied oxoisoaporphines centered at 390 nm. A transient absorption observed near 490 nm, for all of the studied systems, was explained by considering the formation of radical ion pair between the radical anion of the oxoisoaporphine, A(-)(*), and the radical cation of the amine, whose ZINDO/S calculated spectra generate the strongest transition near the experimentally observed absorption maximum at 490 nm, supporting the formation of a radical ion pair complex as the first step of the photoreduction.

Unexpected Imidazoquinoxalinone Annulation Products in the Photoinitiated Reaction of Substituted‐3‐Methyl‐Quinoxalin‐2‐Ones with N‐Phenylglycine

“…This photoreduction mechanism was further supported by laser flash photolysis experiments which allowed the identification of the photoreduction intermediate absorptions (20). The stepwise electron-proton-electron transfer mechanism observed for quinoxalin-2-one derivatives is similar to the one we reported for the oxoisoaporphines derivatives studied in the last years (21)(22)(23)(24)(25). Both kinds of compounds, quinoxalin-2-ones and oxoisoaporphines, contain conjugated C=N and C=O moieties in their structures, which are the groups inducing their photoreactivity toward amines, showing the unusual thermal reversion of metastable semireduced photoproducts.…”

Photoreduction of Azaoxoisoaporphines by Amines: Laser Flash and Steady‐State Photolysis and Pulse Radiolysis Studies

“…The 1 H-NMR study of the photoreduction of AOIA and OIA has provided the identity of the metastable photoproducts hydrogenated at the carbonyl O-atom (AOH À ). Their formation rationalizes the observed isotopic H/D exchange and supports the stepwise photoreduction mechanism elaborated earlier for 2,3-dihydro-oxoisoaporphines (2,3-dh-OIA) (21)(22)(23) and OIA (24).…”

“…This stabilization can be attributed to the presence of the second electronegative N-atom in the AOIA structure. The presence of the second N-atom does not affect the triplet quantum yield, Φ T , as compared with the OIA, (24) but these newly measured triplet quantum yields are significantly larger than those measured for 2,3-dihydro-oxoisoaporphines derivatives, which were 0.38; 0.42 and 0.55 for the 5,6-dimethoxy; 5-methoxy and the unsubstituted one respectively (22,23). However, this second N-atom, substantially affects the photoconsumption quantum yields, Φ pc .…”

“…Laser flash photolysis experiments were performed with a Q-switched Nd:YAG laser, Quantel Brilliant with the excitation at 355 nm. The flash photolysis setup was described previously (22,28,29).…”

“…On the other hand, permanent products, for example aldehydes, secondary amines and diethylaminobutadiene, are formed from the iminium cation (Eq. 7) (21,22).…”