Intramolecular charge transfer with the planarized 4-cyanofluorazene and its flexible counterpart 4-cyano-N-phenylpyrrole. Picosecond fluorescence decays and femtosecond excited-state absorption

Sergey I. Druzhinin, Sergey A. Kovalenko, Tamara A. Senyushkina, A. Demeter, Reinhard Machinek, Mathias Noltemeyer, Klaas A. Zachariasse

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Abstract

The fluorescence spectrum of the rigidified 4-cyanofluorazene (FPP4C) in n-hexane consists of a dual emission from a locally excited (LE) and an intramolecular charge-transfer (ICT) state, with an ICT/LE fluorescence quantum yield ratio of Φ′(ICT)/Φ(LE) = 3.3 at 25 °C. With the flexible 4-cyano-N-phenylpyrrole (PP4C) in n-hexane, such an ICT reaction also takes place, with Φ′(ICT)/Φ(LE) = 1.5, indicating that for this reaction, a perpendicular twist of the pyrrole and benzonitrile moieties is not required. The ICT emission band of FPP4C and PP4C in n-hexane has vibrational structure, but a structureless band is observed in all other solvents more polar than the alkanes. The enthalpy difference ΔH of the LE → ICT reaction in n-hexane, -11 kJ/mol for FPP4C and -7 kJ/mol for PP4C, is determined by analyzing the temperature dependence of Φ′(ICT)/Φ(LE). Using these data, the energy E(FC,ICT) of the Franck - Condon ground state populated by the ICT emission is calculated, 41 (FPP4C) and 40 kJ/mol (PP4C). These large values for E(FC,ICT) lead to the conclusion that with FPP4C and PP4C, direct ICT excitation, bypassing LE, does not take place. FPP4C has an ICT dipole moment of 15 D, similar to that of PP4C (16 D). Picosecond fluorescence decays allow the determination of the ICT lifetime, from which the radiative rate constant k′f(ICT) is derived, with comparable values for FPP4C and PP4C. This shows that an argument for a twisted ICT state of PP4C cannot come from k′f(ICT). After correction for the solvent refractive index and the energy of the emission maximum vmax(ICT), it appears that k′f(ICT) is solvent-polarity-independent. Femtosecond transient absorption with FPP4C and PP4C in n-hexane reveals that the ICT state is already nearly fully present at 100 fs after excitation, in rapid equilibrium with LE. In MeCN, the ICT state of FPP4C and PP4C is likewise largely developed at this delay time, and the reaction is limited by dielectric solvent relaxation, which shows that the ICT reaction is ultrafast, at the experimental time limit of 50 fs. PP4C and FPP4C have a similar planar ICT structure, without an appreciable twist of the pyrrole and benzonitrile subgroups. Their crystal structure is compared with calculations for the S0 ground state.

Original languageEnglish
Pages (from-to)8238-8253
Number of pages16
JournalJournal of Physical Chemistry A
Volume112
Issue number36
DOIs
Publication statusPublished - Sep 11 2008

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Excited states
Charge transfer
Fluorescence
charge transfer
fluorescence
decay
excitation
N-phenylpyrrole
4-cyanofluorazene
Pyrroles
pyrroles
Ground state
Alkanes
ground state

ASJC Scopus subject areas

  • Physical and Theoretical Chemistry

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Intramolecular charge transfer with the planarized 4-cyanofluorazene and its flexible counterpart 4-cyano-N-phenylpyrrole. Picosecond fluorescence decays and femtosecond excited-state absorption. / Druzhinin, Sergey I.; Kovalenko, Sergey A.; Senyushkina, Tamara A.; Demeter, A.; Machinek, Reinhard; Noltemeyer, Mathias; Zachariasse, Klaas A.

In: Journal of Physical Chemistry A, Vol. 112, No. 36, 11.09.2008, p. 8238-8253.

Research output: Contribution to journalArticle

Druzhinin, Sergey I. ; Kovalenko, Sergey A. ; Senyushkina, Tamara A. ; Demeter, A. ; Machinek, Reinhard ; Noltemeyer, Mathias ; Zachariasse, Klaas A. / Intramolecular charge transfer with the planarized 4-cyanofluorazene and its flexible counterpart 4-cyano-N-phenylpyrrole. Picosecond fluorescence decays and femtosecond excited-state absorption. In: Journal of Physical Chemistry A. 2008 ; Vol. 112, No. 36. pp. 8238-8253.
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abstract = "The fluorescence spectrum of the rigidified 4-cyanofluorazene (FPP4C) in n-hexane consists of a dual emission from a locally excited (LE) and an intramolecular charge-transfer (ICT) state, with an ICT/LE fluorescence quantum yield ratio of Φ′(ICT)/Φ(LE) = 3.3 at 25 °C. With the flexible 4-cyano-N-phenylpyrrole (PP4C) in n-hexane, such an ICT reaction also takes place, with Φ′(ICT)/Φ(LE) = 1.5, indicating that for this reaction, a perpendicular twist of the pyrrole and benzonitrile moieties is not required. The ICT emission band of FPP4C and PP4C in n-hexane has vibrational structure, but a structureless band is observed in all other solvents more polar than the alkanes. The enthalpy difference ΔH of the LE → ICT reaction in n-hexane, -11 kJ/mol for FPP4C and -7 kJ/mol for PP4C, is determined by analyzing the temperature dependence of Φ′(ICT)/Φ(LE). Using these data, the energy E(FC,ICT) of the Franck - Condon ground state populated by the ICT emission is calculated, 41 (FPP4C) and 40 kJ/mol (PP4C). These large values for E(FC,ICT) lead to the conclusion that with FPP4C and PP4C, direct ICT excitation, bypassing LE, does not take place. FPP4C has an ICT dipole moment of 15 D, similar to that of PP4C (16 D). Picosecond fluorescence decays allow the determination of the ICT lifetime, from which the radiative rate constant k′f(ICT) is derived, with comparable values for FPP4C and PP4C. This shows that an argument for a twisted ICT state of PP4C cannot come from k′f(ICT). After correction for the solvent refractive index and the energy of the emission maximum vmax(ICT), it appears that k′f(ICT) is solvent-polarity-independent. Femtosecond transient absorption with FPP4C and PP4C in n-hexane reveals that the ICT state is already nearly fully present at 100 fs after excitation, in rapid equilibrium with LE. In MeCN, the ICT state of FPP4C and PP4C is likewise largely developed at this delay time, and the reaction is limited by dielectric solvent relaxation, which shows that the ICT reaction is ultrafast, at the experimental time limit of 50 fs. PP4C and FPP4C have a similar planar ICT structure, without an appreciable twist of the pyrrole and benzonitrile subgroups. Their crystal structure is compared with calculations for the S0 ground state.",
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T1 - Intramolecular charge transfer with the planarized 4-cyanofluorazene and its flexible counterpart 4-cyano-N-phenylpyrrole. Picosecond fluorescence decays and femtosecond excited-state absorption

AU - Druzhinin, Sergey I.

AU - Kovalenko, Sergey A.

AU - Senyushkina, Tamara A.

AU - Demeter, A.

AU - Machinek, Reinhard

AU - Noltemeyer, Mathias

AU - Zachariasse, Klaas A.

PY - 2008/9/11

Y1 - 2008/9/11

N2 - The fluorescence spectrum of the rigidified 4-cyanofluorazene (FPP4C) in n-hexane consists of a dual emission from a locally excited (LE) and an intramolecular charge-transfer (ICT) state, with an ICT/LE fluorescence quantum yield ratio of Φ′(ICT)/Φ(LE) = 3.3 at 25 °C. With the flexible 4-cyano-N-phenylpyrrole (PP4C) in n-hexane, such an ICT reaction also takes place, with Φ′(ICT)/Φ(LE) = 1.5, indicating that for this reaction, a perpendicular twist of the pyrrole and benzonitrile moieties is not required. The ICT emission band of FPP4C and PP4C in n-hexane has vibrational structure, but a structureless band is observed in all other solvents more polar than the alkanes. The enthalpy difference ΔH of the LE → ICT reaction in n-hexane, -11 kJ/mol for FPP4C and -7 kJ/mol for PP4C, is determined by analyzing the temperature dependence of Φ′(ICT)/Φ(LE). Using these data, the energy E(FC,ICT) of the Franck - Condon ground state populated by the ICT emission is calculated, 41 (FPP4C) and 40 kJ/mol (PP4C). These large values for E(FC,ICT) lead to the conclusion that with FPP4C and PP4C, direct ICT excitation, bypassing LE, does not take place. FPP4C has an ICT dipole moment of 15 D, similar to that of PP4C (16 D). Picosecond fluorescence decays allow the determination of the ICT lifetime, from which the radiative rate constant k′f(ICT) is derived, with comparable values for FPP4C and PP4C. This shows that an argument for a twisted ICT state of PP4C cannot come from k′f(ICT). After correction for the solvent refractive index and the energy of the emission maximum vmax(ICT), it appears that k′f(ICT) is solvent-polarity-independent. Femtosecond transient absorption with FPP4C and PP4C in n-hexane reveals that the ICT state is already nearly fully present at 100 fs after excitation, in rapid equilibrium with LE. In MeCN, the ICT state of FPP4C and PP4C is likewise largely developed at this delay time, and the reaction is limited by dielectric solvent relaxation, which shows that the ICT reaction is ultrafast, at the experimental time limit of 50 fs. PP4C and FPP4C have a similar planar ICT structure, without an appreciable twist of the pyrrole and benzonitrile subgroups. Their crystal structure is compared with calculations for the S0 ground state.

AB - The fluorescence spectrum of the rigidified 4-cyanofluorazene (FPP4C) in n-hexane consists of a dual emission from a locally excited (LE) and an intramolecular charge-transfer (ICT) state, with an ICT/LE fluorescence quantum yield ratio of Φ′(ICT)/Φ(LE) = 3.3 at 25 °C. With the flexible 4-cyano-N-phenylpyrrole (PP4C) in n-hexane, such an ICT reaction also takes place, with Φ′(ICT)/Φ(LE) = 1.5, indicating that for this reaction, a perpendicular twist of the pyrrole and benzonitrile moieties is not required. The ICT emission band of FPP4C and PP4C in n-hexane has vibrational structure, but a structureless band is observed in all other solvents more polar than the alkanes. The enthalpy difference ΔH of the LE → ICT reaction in n-hexane, -11 kJ/mol for FPP4C and -7 kJ/mol for PP4C, is determined by analyzing the temperature dependence of Φ′(ICT)/Φ(LE). Using these data, the energy E(FC,ICT) of the Franck - Condon ground state populated by the ICT emission is calculated, 41 (FPP4C) and 40 kJ/mol (PP4C). These large values for E(FC,ICT) lead to the conclusion that with FPP4C and PP4C, direct ICT excitation, bypassing LE, does not take place. FPP4C has an ICT dipole moment of 15 D, similar to that of PP4C (16 D). Picosecond fluorescence decays allow the determination of the ICT lifetime, from which the radiative rate constant k′f(ICT) is derived, with comparable values for FPP4C and PP4C. This shows that an argument for a twisted ICT state of PP4C cannot come from k′f(ICT). After correction for the solvent refractive index and the energy of the emission maximum vmax(ICT), it appears that k′f(ICT) is solvent-polarity-independent. Femtosecond transient absorption with FPP4C and PP4C in n-hexane reveals that the ICT state is already nearly fully present at 100 fs after excitation, in rapid equilibrium with LE. In MeCN, the ICT state of FPP4C and PP4C is likewise largely developed at this delay time, and the reaction is limited by dielectric solvent relaxation, which shows that the ICT reaction is ultrafast, at the experimental time limit of 50 fs. PP4C and FPP4C have a similar planar ICT structure, without an appreciable twist of the pyrrole and benzonitrile subgroups. Their crystal structure is compared with calculations for the S0 ground state.

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