Excited State Intramolecular Charge Transfer Suppressed Proton Transfer Process in 4‑(Diethylamino)-2-hydroxybenzaldehyde

In this work, we report intramolecular
charge transfer (ICT) suppressed
excited state intramolecular proton transfer (ESIPT) process in 4-(diethylamino)-2-hydroxybenzaldehyde
(DEAHB). Photophysical properties of DEAHB have been extensively studied
in different solvents with varying pH, polarity, and hydrogen bonding
capability of the solvent using steady state and time-resolved spectroscopy.
To establish the competition between the ICT and ESIPT processes in
DEAHB, we have synthesized and studied the photophysical properties
of 4-(diethylamino)-2-methoxybenzaldehyde (DEAMB) molecule where only
the charge transfer process has been observed. Recently, we have reported
simple Schiff base molecules (J. Phys. Chem. A 2012, 116, 10948) formed by condensation of DEAHB
and hydrazine (5-(diethylamino)-2-[(4-(diethylamino)­benzylidene)­hydrazonomethyl]­phenol
(DDBHP) and N,N′-bis­(4-N,N-(diethylamino)­salisalidene)­hydrazine
(DEASH)), where charge transfer is assisted by the proton transfer
process. In the present case, the DEAHB molecule shows the reverse
phenomenon; i.e., charge transfer is suppressed by the proton transfer
process. Comparing the photophysical properties of DEAHB with DEAMB
it is also found that ICT process in DEAHB is suppressed by the ESIPT
process.