Intramolecular charge transfer with crystal violet lactone in acetonitrile as a function of temperature

Reaction is not solvent-controlled

Sergey I. Druzhinin, A. Demeter, Klaas A. Zachariasse

Research output: Contribution to journalArticle

2 Citations (Scopus)

Abstract

Intramolecular charge transfer (ICT) with crystal violet lactone (CVL) in the excited singlet state takes place in solvents more polar than n-hexane, such as ethyl acetate, tetrahydrofuran, and acetonitrile (MeCN). In these solvents, the fluorescence spectrum of CVL consists of two emission bands, from a locally excited (LE) and an ICT state. The dominant deactivation channel of the lowest excited singlet state is internal conversion, as the quantum yields of fluorescence (0.007) and intersystem crossing (0.015) in MeCN at 25 C are very small. CVL is a weakly coupled electron donor/acceptor (D/A) molecule, similar to an exciplex 1(A-D+). A solvatochromic treatment of the LE and ICT emission maxima results in the dipole moments μe(LE) = 17 D and μe(ICT) = 33 D, much larger than those previously reported. This discrepancy is attributed to different Onsager radii and spectral fluorimeter calibration. The LE and ICT fluorescence decays of CVL in MeCN are double exponential. As determined by global analysis, the LE and ICT decays at 25 C have the times τ2 = 9.2 ps and τ1 = 1180 ps, with an amplitude ratio of 35.3 for LE. From these parameters, the rate constants ka = 106 × 109 s -1 and kd = 3.0 × 109 s-1 of the forward and backward reaction in the LE ⇠ICT equilibrium are calculated, resulting in a free enthalpy difference ΔG of -8.9 kJ/mol. The amplitude ratio of the ICT fluorescence decay equals -1.0, which signifies that the ICT state is not prepared by light absorption in the S0 ground state, but originates exclusively from the directly excited LE precursor. From the temperature dependence of the fluorescence decays of CVL in MeCN (-45 to 75 C), activation energies Ea = 3.9 kJ/mol (LE → ICT) and E d = 23.6 kJ/mol (ICT → LE) are obtained, giving an enthalpy difference ΔH (= Ea - Ed) of -19.7 kJ/mol, and an entropy difference ΔS = -35.5 J mol-1 K-1. These data show that the ICT reaction of CVL in MeCN is not barrierless. The ICT reaction time of 9.2 ps is much longer than the mean solvent relaxation time of MeCN (0.26 ps), indicating, in contrast with earlier reports in the literature, that the reaction is not solvent controlled. This conclusion is supported by the observation of double exponential LE and ICT fluorescence with the same decay times.

Original languageEnglish
Pages (from-to)7721-7736
Number of pages16
JournalJournal of Physical Chemistry A
Volume117
Issue number33
DOIs
Publication statusPublished - Aug 22 2013

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acetonitrile
Charge transfer
charge transfer
crystals
Fluorescence
Temperature
temperature
fluorescence
decay
crystal violet lactone
Excited states
Enthalpy
enthalpy
Fluorometers
Dipole moment
internal conversion
Quantum yield
electromagnetic absorption
tetrahydrofuran
reaction time

ASJC Scopus subject areas

  • Physical and Theoretical Chemistry

Cite this

Intramolecular charge transfer with crystal violet lactone in acetonitrile as a function of temperature : Reaction is not solvent-controlled. / Druzhinin, Sergey I.; Demeter, A.; Zachariasse, Klaas A.

In: Journal of Physical Chemistry A, Vol. 117, No. 33, 22.08.2013, p. 7721-7736.

Research output: Contribution to journalArticle

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title = "Intramolecular charge transfer with crystal violet lactone in acetonitrile as a function of temperature: Reaction is not solvent-controlled",
abstract = "Intramolecular charge transfer (ICT) with crystal violet lactone (CVL) in the excited singlet state takes place in solvents more polar than n-hexane, such as ethyl acetate, tetrahydrofuran, and acetonitrile (MeCN). In these solvents, the fluorescence spectrum of CVL consists of two emission bands, from a locally excited (LE) and an ICT state. The dominant deactivation channel of the lowest excited singlet state is internal conversion, as the quantum yields of fluorescence (0.007) and intersystem crossing (0.015) in MeCN at 25 C are very small. CVL is a weakly coupled electron donor/acceptor (D/A) molecule, similar to an exciplex 1(A-D+). A solvatochromic treatment of the LE and ICT emission maxima results in the dipole moments μe(LE) = 17 D and μe(ICT) = 33 D, much larger than those previously reported. This discrepancy is attributed to different Onsager radii and spectral fluorimeter calibration. The LE and ICT fluorescence decays of CVL in MeCN are double exponential. As determined by global analysis, the LE and ICT decays at 25 C have the times τ2 = 9.2 ps and τ1 = 1180 ps, with an amplitude ratio of 35.3 for LE. From these parameters, the rate constants ka = 106 × 109 s -1 and kd = 3.0 × 109 s-1 of the forward and backward reaction in the LE {\^a}‡ ICT equilibrium are calculated, resulting in a free enthalpy difference ΔG of -8.9 kJ/mol. The amplitude ratio of the ICT fluorescence decay equals -1.0, which signifies that the ICT state is not prepared by light absorption in the S0 ground state, but originates exclusively from the directly excited LE precursor. From the temperature dependence of the fluorescence decays of CVL in MeCN (-45 to 75 C), activation energies Ea = 3.9 kJ/mol (LE → ICT) and E d = 23.6 kJ/mol (ICT → LE) are obtained, giving an enthalpy difference ΔH (= Ea - Ed) of -19.7 kJ/mol, and an entropy difference ΔS = -35.5 J mol-1 K-1. These data show that the ICT reaction of CVL in MeCN is not barrierless. The ICT reaction time of 9.2 ps is much longer than the mean solvent relaxation time of MeCN (0.26 ps), indicating, in contrast with earlier reports in the literature, that the reaction is not solvent controlled. This conclusion is supported by the observation of double exponential LE and ICT fluorescence with the same decay times.",
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N2 - Intramolecular charge transfer (ICT) with crystal violet lactone (CVL) in the excited singlet state takes place in solvents more polar than n-hexane, such as ethyl acetate, tetrahydrofuran, and acetonitrile (MeCN). In these solvents, the fluorescence spectrum of CVL consists of two emission bands, from a locally excited (LE) and an ICT state. The dominant deactivation channel of the lowest excited singlet state is internal conversion, as the quantum yields of fluorescence (0.007) and intersystem crossing (0.015) in MeCN at 25 C are very small. CVL is a weakly coupled electron donor/acceptor (D/A) molecule, similar to an exciplex 1(A-D+). A solvatochromic treatment of the LE and ICT emission maxima results in the dipole moments μe(LE) = 17 D and μe(ICT) = 33 D, much larger than those previously reported. This discrepancy is attributed to different Onsager radii and spectral fluorimeter calibration. The LE and ICT fluorescence decays of CVL in MeCN are double exponential. As determined by global analysis, the LE and ICT decays at 25 C have the times τ2 = 9.2 ps and τ1 = 1180 ps, with an amplitude ratio of 35.3 for LE. From these parameters, the rate constants ka = 106 × 109 s -1 and kd = 3.0 × 109 s-1 of the forward and backward reaction in the LE ⇠ICT equilibrium are calculated, resulting in a free enthalpy difference ΔG of -8.9 kJ/mol. The amplitude ratio of the ICT fluorescence decay equals -1.0, which signifies that the ICT state is not prepared by light absorption in the S0 ground state, but originates exclusively from the directly excited LE precursor. From the temperature dependence of the fluorescence decays of CVL in MeCN (-45 to 75 C), activation energies Ea = 3.9 kJ/mol (LE → ICT) and E d = 23.6 kJ/mol (ICT → LE) are obtained, giving an enthalpy difference ΔH (= Ea - Ed) of -19.7 kJ/mol, and an entropy difference ΔS = -35.5 J mol-1 K-1. These data show that the ICT reaction of CVL in MeCN is not barrierless. The ICT reaction time of 9.2 ps is much longer than the mean solvent relaxation time of MeCN (0.26 ps), indicating, in contrast with earlier reports in the literature, that the reaction is not solvent controlled. This conclusion is supported by the observation of double exponential LE and ICT fluorescence with the same decay times.

AB - Intramolecular charge transfer (ICT) with crystal violet lactone (CVL) in the excited singlet state takes place in solvents more polar than n-hexane, such as ethyl acetate, tetrahydrofuran, and acetonitrile (MeCN). In these solvents, the fluorescence spectrum of CVL consists of two emission bands, from a locally excited (LE) and an ICT state. The dominant deactivation channel of the lowest excited singlet state is internal conversion, as the quantum yields of fluorescence (0.007) and intersystem crossing (0.015) in MeCN at 25 C are very small. CVL is a weakly coupled electron donor/acceptor (D/A) molecule, similar to an exciplex 1(A-D+). A solvatochromic treatment of the LE and ICT emission maxima results in the dipole moments μe(LE) = 17 D and μe(ICT) = 33 D, much larger than those previously reported. This discrepancy is attributed to different Onsager radii and spectral fluorimeter calibration. The LE and ICT fluorescence decays of CVL in MeCN are double exponential. As determined by global analysis, the LE and ICT decays at 25 C have the times τ2 = 9.2 ps and τ1 = 1180 ps, with an amplitude ratio of 35.3 for LE. From these parameters, the rate constants ka = 106 × 109 s -1 and kd = 3.0 × 109 s-1 of the forward and backward reaction in the LE ⇠ICT equilibrium are calculated, resulting in a free enthalpy difference ΔG of -8.9 kJ/mol. The amplitude ratio of the ICT fluorescence decay equals -1.0, which signifies that the ICT state is not prepared by light absorption in the S0 ground state, but originates exclusively from the directly excited LE precursor. From the temperature dependence of the fluorescence decays of CVL in MeCN (-45 to 75 C), activation energies Ea = 3.9 kJ/mol (LE → ICT) and E d = 23.6 kJ/mol (ICT → LE) are obtained, giving an enthalpy difference ΔH (= Ea - Ed) of -19.7 kJ/mol, and an entropy difference ΔS = -35.5 J mol-1 K-1. These data show that the ICT reaction of CVL in MeCN is not barrierless. The ICT reaction time of 9.2 ps is much longer than the mean solvent relaxation time of MeCN (0.26 ps), indicating, in contrast with earlier reports in the literature, that the reaction is not solvent controlled. This conclusion is supported by the observation of double exponential LE and ICT fluorescence with the same decay times.

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