Excited state interactions in the doubly hydrogen-bonded jet-cooled dimers of 4-aminophthalimide and 4-amino-N-methylphthalimide

The jet-cooled dimer of 4-amino-N-methylphthalimide (4ANMP) has four NH bonds, but the ground-state infrared spectrum shows just two strong N–H-stretch resonances in the range 3100–3800 cm−1. This confirms a symmetric structure (C2). In the excited state, there are six strong resonances in the same region, falling into two groups of three. A similar pattern is observed for the dimer of 4-aminophthalimide (4AP). These observations suggest that double proton transfer may be occurring across the two N–H⋯O hydrogen bonds.