Modulation of the 4-aminophthalimide spectral properties by hydrogen bonds in water.

TD-DFT and DFT calculations have been performed to examine the relationship between the spectral shifts of 4-aminophthalimide (4AP) and the formation of hydrogen bonds in water solution. The computations of the S0 state are at the IEFPCM-B3LYP/6-311++G(d, p) level and the S1 state at the TD-IEFPCM-B3LYP/6-311++G(d, p) level. The eleven structures of the hydrogen-bonded 4AP clusters formed with different number water molecules in both S0 and S1 states were optimized. The absorption, fluorescence and infrared spectra were calculated. The results of the hydrogen bond energy and length reveals that the hydrogen bonds formed by the nitrogen atom of the amine group with water molecule (A type) are significantly weakened from states S0 to S1. In contrast, the hydrogen bonds formed by the oxygen atoms of the two carbonyl groups (B type) with water molecules and those formed by the two hydrogen atoms of the amine group (C type) with water molecules are remarkably strengthened. Comparing with the 4AP monomer spectra, the weakening for the hydrogen bond of A type could be responsible for the blueshifts of the electronic absorption spectra and the stretching vibrational spectra of the two NH groups in 4AP from states S0 to S1. The significant redshifts of the electronic spectra and the S0-S1 downshifts of the stretching vibrational modes of the two NH groups and the two carbonyl groups in 4AP could be attributed to the strengthening of hydrogen bonds for B and C types.