Calixarene building block bis(2-hydroxyphenyl)methane (2HDPM) and hydrogen-bonded 2HDPM-H2O complex in electronic excited state

Intramolecular and intermolecular hydrogen bonding in electronic excited states of calixarene building blocks bis(2-hydroxyphenyl)methane (2HDPM) monomer and hydrogen-bonded 2HDPM-H₂O complex were studied theoretically using the time-dependent density functional theory (TDDFT). Twenty-four stable conformations (12 pairs of enantiomers) of 2HDPM monomer have been found in the ground state. From the calculation results, the conformations 1a and 1b which both have an intramolecular hydrogen bond are the most stable ones. The infrared spectra of 2HDPM monomer and 2HDPM-H₂O complex in ground state and S₁ state were calculated. The stretching vibrational absorption band of O₂???H₃ group in the monomer and complex disappeared in the S₁ state. At the same time, a new strong absorption band appeared at the C=O stretching region. From the calculation of bond lengths, it indicates that the O₂???H₃ bond is significantly lengthened in the S₁ state. However, the C₁???O₂ bond is drastically shortened upon electronic excitation to the S₁ state and has the characteristics of C=O band. Furthermore, the intramolecular hydrogen bond O₂???H₃?·?·?·?O₄ of the 2HDPM monomer and the intermolecular hydrogen bonds O₂???H₃?·?·?·?O₇ and O₇???H₉?·?·?·?O₄ of 2HDPM-H₂O complex are all shortened and strengthened in the S₁ state.