Solvent effects on the charge transfer excited states of 4-dimethylaminobenzonitrile (DMABN) and 4-dimethylamino-3,5-dimethylbenzonitrile (TMABN) studied by time-resolved infrared spectroscopy: a direct observation of hydrogen bonding interactions

Abstract

Time-resolved infrared absorption spectra of the C≡N bands of photoexcited TMABN and DMABN have been measured in non-polar hexane, polar aprotic THF and polar protic butanol with high temporal and spectral resolution (<0.5 ps and 5 cm⁻¹, respectively). In butanol, the intramolecular charge transfer (ICT) state C≡N infrared absorption bands of DMABN and TMABN both develop from an initial singlet into a doublet, demonstrating the co-existence of two charge transfer excited states, one of which is hydrogen-bonded and the other similar to the state formed in aprotic solvents. The ICT C≡N absorption band of TMABN is already strong at the earliest measurement time of 2 ps in THF, hexane, and butanol, indicating prompt population of ICT by a barrierless process, as expected from the pre-twisted structure of this molecule. There are little or no subsequent fast kinetics in hexane and THF but the signal observed in butanol continues to grow substantially at later times, prior to decay, indicating population transfer from a second state excited at 267 nm. No CN absorption band attributable to this state is observed, consistent with it being similar to the LE state of DMABN. The kinetics of the later stages of the hydrogen-bonding of both DMABN and TMABN in butanol takes place on timescales consistent with known values for dipolar solvation relaxation and result in a ratio of the hydrogen-bonded to non-bonded species of ∼3:1 at equilibrium for both molecules. The contrast between the prompt population of the charge transfer state of TMABN in all three solvents and charge transfer rates in DMABN limited to 13 ps⁻¹ in THF and 9 ps⁻¹ in butanol is fully consistent with the TICT description for the ICT state structure.