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

W. M. Kwok, C. Ma, M. W. George, David Grills, P. Matousek, A. W. Parker, D. Phillips, W. T. Toner, M. Towrie

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23 Citations (Scopus)

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 (-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.

Original languageEnglish
Pages (from-to)987-994
Number of pages8
JournalPhotochemical and Photobiological Sciences
Volume6
Issue number9
DOIs
Publication statusPublished - 2007

Fingerprint

Butanols
Hydrogen Bonding
Excited states
Charge transfer
Infrared spectroscopy
Spectrum Analysis
Hydrogen bonds
infrared spectroscopy
charge transfer
Observation
Hexanes
hydrogen
Absorption spectra
excitation
absorption spectra
interactions
Hydrogen
Infrared absorption
Population
infrared absorption

ASJC Scopus subject areas

  • Physical and Theoretical Chemistry
  • Cell Biology
  • Biochemistry, Genetics and Molecular Biology(all)
  • Biochemistry
  • Biophysics

Cite this

@article{cbf01f8f8e0649a9bd70c347ba5baa46,
title = "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 (-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.",
author = "Kwok, {W. M.} and C. Ma and George, {M. W.} and David Grills and P. Matousek and Parker, {A. W.} and D. Phillips and Toner, {W. T.} and M. Towrie",
year = "2007",
doi = "10.1039/b708414e",
language = "English",
volume = "6",
pages = "987--994",
journal = "Photochemical and Photobiological Sciences",
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TY - JOUR

T1 - 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

T2 - A direct observation of hydrogen bonding interactions

AU - Kwok, W. M.

AU - Ma, C.

AU - George, M. W.

AU - Grills, David

AU - Matousek, P.

AU - Parker, A. W.

AU - Phillips, D.

AU - Toner, W. T.

AU - Towrie, M.

PY - 2007

Y1 - 2007

N2 - 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 (-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.

AB - 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 (-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.

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