Dual fluorescence and intramolecular charge transfer with crystalline 4-(diisopropylamino)benzonitrile

Dual fluorescence from a locally excited (LE) and an intramolecular charge transfer (ICT) state is observed with 4-(diisopropylamino)benzonitrile (DIABN) crystals, from 80 down to −110 °C. With crystalline 4-(methylamino)benzonitrile (MABN) only LE emission is observed over this temperature range. Crystals of 4-(dimethylamino)benzonitrile (DMABN) at room temperature mainly show LE fluorescence, whereas the red-shifted structured phosphorescence increases in importance upon cooling. Its spectral shape resembles that of DMABN phosphorescence in low-temperature glassy media, red-shifted by around 2800 cm−1. The ICT fluorescence of DIABN crystals at −110 °C has a risetime of 55 ps, which becomes shorter with increasing temperature.