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Unravelling the Reaction Mechanism for the Fast Photocyclisation of 2‐Benzoylpyridine in Aqueous Solvent by Time‐Resolved Spectroscopy and Density Functional Theory Calculations
Author(s) -
Du Yong,
Xue Jiadan,
Li MingDe,
Guan Xiangguo,
McCamant David W.,
Phillips David Lee
Publication year - 2010
Publication title -
chemistry – a european journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.687
H-Index - 242
eISSN - 1521-3765
pISSN - 0947-6539
DOI - 10.1002/chem.200903073
Subject(s) - chemistry , intersystem crossing , photochemistry , aqueous solution , protonation , singlet state , ultrafast laser spectroscopy , acetonitrile , solvent , resonance raman spectroscopy , pyridine , triplet state , solvent effects , spectroscopy , excited state , raman spectroscopy , molecule , medicinal chemistry , ion , organic chemistry , physics , quantum mechanics , nuclear physics , optics
A combined femtosecond transient absorption (fs‐TA) and nanosecond time‐resolved resonance Raman (ns‐TR 3 ) spectroscopic investigation of the photoreaction of 2‐benzoylpyridine (2‐BPy) in acetonitrile and neutral, basic and acidic aqueous solvents is reported. fs‐TA results showed that the nπ* triplet 2‐BPy is the precursor of the photocyclisation reaction in neutral and basic aqueous solvents. The cis triplet biradical and the cis singlet zwitterionic species produced during the photocyclisation reaction were initially characterised by ns‐TR 3 spectroscopy. In addition, a new species was uniquely observed in basic aqueous solvent after the decay of the cis singlet zwitterionic species and this new species was tentatively assigned to the photocyclised radical anion. The ground‐state conformation of 2‐BPy in acidic aqueous solvent is the pyridine nitrogen‐protonated 2‐BPy cation (2‐BPy‐NH + ) rather than the neutral form of 2‐BPy. After laser photolysis, the singlet excited state (S 1 ) of 2‐BPy‐NH + is generated and evolves through excited‐state proton transfer (ESPT) and efficient intersystem crossing (ISC) processes to the triplet exited state (T 1 ) of the carbonyl oxygen‐protonated 2‐BPy cation (2‐BPy‐OH + ) and then photocyclises with the lone pair of the nitrogen atom in the heterocyclic ring. Cyclisation reactions take place both in neutral/basic and acidic aqueous solvents, but the photocyclisation mechanisms in these different aqueous solvents are very different. This is likely due to the different conformation of the precursor and the influence of hydrogen‐bonding of the solvent on the reactions.