Correlation of exciplex formation with ground state conformations in flexible bichromophoric esters: 2-(1-pyrenyl)ethyl p-cyanobenzoate and its modelcompounds

Two kinds of model compounds for a bichromophoric ester, 2-(1-pyrenyl)ethyl p-cyanobenzoate (P2CN), were synthesized: 1-substituted 2-( 1-pyrenyl)ethy l p-cyanobenzoates (RP2CN, R = Me, Et, and Ph) as flexible model compounds and trans-2-(1-pyrenyl)cyclohexyl p-cyanobenzoate (cyclo-P2CN) as a rigid m odel compound. P2CN and all the model compounds had an intramolecular excip lex emission (phi(EX) approximate to 0.010) and efficient quenching of fluo rescence from the locally excited pyrene part (phi(LE) approximate to 0.001 ). The decay lifetimes of the locally excited pyrene were 365 ps for P2CN a nd 66 ps for cyclo-P2CN, which were respectively identical with the risetim e for the exciplex formation. The H-1 NMR of both P2CN and RP2CN showed the shifts of H-2" and H-3" of the p- cyanobenzoyl to the higher and H-10' of 1-pyrenyl signals to the lower magnetic field compared with those of the re ference compounds, approaching towards those of cyclo-P2CN. NOE was observe d between H-10' and H-2" of cyclo-P2CN, and, although very weak, between H- 2' and H-3" of MeP2CN. In the solid state, P2CN adopts a folded conformatio n, which is roughly close to the gauche conformation, with the dihedral ang le of 57.5 degrees between p-cyanophenyl ring and pyrenyl ring based on X-r ay crystallographic analysis on a leaflet monoclinic crystal, suggesting th e exciplex formation from this conformation in P2CN.