TWISTED INTERNAL CHARGE-TRANSFER (TICT) EFFECT IN BIS(4-CYANOPHENYL)PIPERAZINE, A PSEUDO-DIMER OF DIMETHYLAMINOBENZONITRILE - A COMPARATIVE-STUDY

Dimethylaminobenzonitrile (DMABN) and N,N'-bis(4-cyanophenyl)piperazine (BCPPZ) have been comparatively studied by X-ray diffraction, fluorescence and time-resolved emission and absorption. BCPPZ can be considered as a pseudo-dimer made up of two weakly interacting DMABN moieties. The crystal structure of DMABN is reported for the first time: it shows an almost perfect planarity. By contrast, BCPPZ is appreciably twisted around the C-N bonds connecting the piperazine part and the cyanophenyl group. The twist is assigned to intramolecular H ... H repulsions and thus should also be present in solution. The photophysical properties of BCPPZ and DMABN are very similar and, in particular, the dual luminescence typical of the formation of a twisted internal charge transfer (TICT) state is also observed in BCPPZ. The only difference lies in the values of the kinetic and thermodynamic parameters describing the interconversion between the planar excited state and the TICT state: this latter state is thermodynamically more stable for BCPPZ than for DMABN. The general analogy in the photophysical behaviour implies that, while the ground state of BCPPZ is centrosymmetric, the excited states exhibit a broken symmetry which can be qualitatively attributed to the weak orbital interaction between the constituent parts.