Influences of protonation and hydrogen bonding on intramolecular charge transfer compounds possessing different spacers

This work investigated the influence of protonation and hydrogen bonding on intramolecular charge transfer (ICT) through different spacers. Three compounds, DS-NH2 (with a conjugated double bond), TS-NH2 (with a conjugated triple bond), and SS-NH2 (with a flexible saturated bond), possessing the same donor group (aniline) and the same acceptor group (naphthalene) but with different linkages were synthesized for this purpose. The double bond and triple bond spacers provided a more efficient ICT pathway than a flexible saturated bond in the ground state, so an ICT absorption band was observed for compounds DS-NH2 and TS-NH2. Although SS-NH2 did not exhibit any ICT absorption band, two kinds of intramolecular exciplex emissions were observed depending on the solvent polarity. The excited-state dipole moment of DS-NH2 is less than that of TS-NH2. The ICT absorption and emission bands are quenched by protonation in this system. The influence of hydrogen bonding for DS-NH2 and TS-NH2 only occurs in the ground state and disappears in the excited state. The intermolecular hydrogen-bond between SS-NH2 and CF3CH2OH molecules block the reorientation of the amino group and prevent the occurrence of the ICT. Therefore, no exciplex emission was recorded for SS-NH2 in the mixed (CH2Cl2:CF3CH2OH) solvents.