TWISTED INTRAMOLECULAR CHARGE-TRANSFER STATE OF A DONOR-ACCEPTOR MOLECULAR-SYSTEM WITH A BETA-DIKETONE BRIDGE - TUNING OF EMISSION THROUGH STRUCTURAL RESTRICTION BY METAL CATION COORDINATION

A novel donor-acceptor molecular system with a beta-diketone bridge, ) -3-(4-(N,N-dimethylamino)phenyl)-1,3-propanedione (DMA >$($) over bar CB) was synthesized and investigated in tuning of emission by metal ca tion coordination. From the solvent polarity dependence of Stokes shif t, it was revealed that the excited state has a considerable charge-tr ansfer (CT) character. The time-resolved fluorescence study in the pic osecond region indicates that the photophysical behavior can be interp reted with the typical two-state model (LE and TICT states) similar to the normal TICT compounds. Complexation of DMA >$($) over bar CB with Mg2+ resulted in a blue-shift of the emission maxima and a change of emission intensity, whose ratio was sensitively dependent on the solve nt polarity. The nonradiative decay rate constants (k(nr)) of DMA >$($ ) over bar CB and its Mg2+ complex (DMA(Mg)CB) increased exponentially with decreasing of the emission energy maxima (E(em)). However the sl opes of the in k(nr)-E(em) plots were quite different in each case. Fr om this relation it becomes obvious that the dominant vibrational acce pting mode to k(rn) is affected by the structural restriction around t he beta-diketone moiety by the Mg2+ coordination. This indicates the p robability of controlling the emission properties from TICT excited st ate with metal cation coordination.