Covalently Linked Acceptor−Donor Systems Based on Isoquinoline N-oxide Acceptor:  Photoinduced Electron Transfer Produces Dual-Channel Luminescent Systems that Evolve Chemically to Photohydroxylation of the Aromatic Donor

Abstract

Acceptor-donor compounds containing the isoquinoline N-oxide acceptor and (methoxy)(n)benzene (n = 0, 1, 2, 3) electron donors were studied. The two chromophores are connected by a CH(2) bridging unit. All acceptor-donor compounds exhibit photoinduced electron transfer in acid medium that results in the formation of a charge-transfer (CT) state. Measurements of the corresponding electronic emission spectra revealed that these bichromophoric systems exhibit a dual fluorescence that is strongly dependent on the protonation of the N-oxide function and the donor ability. The CT state responsible for the red-shifted luminescence in the studied compounds is directly connected with the initial excited state S(1). On the basis of the spectroscopic and photochemical evidence, N[bond]O scission is the dominant primary photochemical process involving the CT state, the subsequent radical coupling resulting in efficient aromatic hydroxylation. The outcome of both quenching and sensitization experiments confirms this assertion. The results strongly suggest that the ensuing photohydroxylation reaction is not a concerted process, but rather a two-step N[bond]O scission followed by C[bond]O formation, which is regioselectively guided by the electronic distribution of the resulting donor cation-radical.

Heteroarene N‐Oxides as Oxygen Source in Organic Reactions

“…Another example of a photoinduced aromatic hydroxylation is the intramolecular reaction of 1‐benzylisoquinoline 2‐oxides to form the corresponding phenols in moderate to good yields (Scheme ) . In this system, two chromophores, the quinolone N ‐oxide ( e – ‐acceptor) and the arene ( e – ‐donor), are connected through a bridging methylene unit.…”

Fe(ii), Ru(ii) and Re(i) complexes of endotopic, sterically non-hindering, U-shaped 8,8′-disubstituted-3,3′-biisoquinoline ligands: syntheses and spectroscopic properties

“…1, and absorption data are listed in Table 2 are ascribed to allowed 1 pp* transitions, while for the region 300-400 nm transitions of different character overlap. 23,24 In this region, contributions from both weaker 1 np* and intraligand charge transfer ( 1 ILCT) transitions might be expected, the latter due also to the presence of electron-releasing (methoxy) and electron-accepting (chloride) subunits interacting with the biisoquinoline ligand frame. The spectra appear reasonably homogeneous within the series, except for a progressive extension of the absorption tail in the range 350-400 nm on passing from 1 to 3.…”

Six‐Membered Heterocycles: Quinoline and Isoquinoline

“…Reissert compounds are useful materials in preparing 1-alkyl-and 1-cyanoisoquinolines. Deprotonation of 377 with NaH followed by alkylation and further removal of the acyl and cyanide groups gives rise to 379 [498]. Furthermore, the N-sulfonyl analogues, such as 380, are prone to eliminate an arylsulfinate to yield 381 [499].…”