Behavior of the rotational diffusion tensor of tetracene under subslip conditions

Wirth, M. J.; Chou, Shinn-Huey S.

doi:10.1021/j100157a054

Cited by 21 publications

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“…The slow disappearance of the fluorescence anisotropy was well fitted to a single-exponential function having a time constant of about 30 ps (the dotted curve). This time constant is ascribable to the orientational diffusion time of tetracene in hexadecane. , For comparison, the fluorescence anisotropy measurement was also carried out in heptane that is much less viscous than hexadecane. Owing to the poor solubility of tetracene in heptane (0.6 × 10 -5 mol dm -3 ), the signal-to-noise ratio of the time-resolved fluorescence data was significantly lower than that in hexadecane.…”

Section: Resultsmentioning

confidence: 99%

Vibronic Relaxation of Polyatomic Molecule in Nonpolar Solvent: Femtosecond Anisotropy/Intensity Measurements of the S_nand S₁Fluorescence of Tetracene

1999

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The electronic and vibrational relaxation of tetracene have been studied in solution by femtosecond timeresolved fluorescence spectroscopy. Tetracene was initially photoexcited to the highly excited singlet (S n ) state, 1 B b , and the dynamics of the fluorescence from the 1 B b state and the 1 L a state (S 1 ) were investigated by fluorescence up-conversion. The fluorescence from the 1 B b state was observed in the ultraviolet region, and its lifetime was determined as ∼120 fs. The anisotropy of the 1 B b fluorescence was close to 0.4, which assured that the fluorescence is emitted from the excited state that was prepared by photoexcitation. The visible fluorescence from the 1 L a state showed a finite rise that agreed well with the decay of the 1 B b fluorescence. Negative anisotropy was observed for the 1 L a fluorescence, reflecting that the 1 L a transition moment is parallel to the short axis of the molecule and hence perpendicular to the 1 B b transition moment. The anisotropy of the 1 L a fluorescence, however, showed a very characteristic temporal behavior in the femtosecond time region: it exhibited a very rapid change and reached a certain value that is deviated from -0.2. The anisotropy data indicate that the 1 L a fluorescence contains not only a short-axis polarized component but also a long-axis polarized component and that the ratio between the two components depends on both time and wavelength. The long-axis polarized component in the 1 L a fluorescence was assigned to the 1 B b -type fluorescence that appears as the result of the vibronic coupling between the 1 L a state and the 1 B b state. The observed initial rapid change of the anisotropy suggests that the highly excited vibrational states in the 1 L a state which are strongly coupled with the 1 B b state are first populated preferentially when the molecule is relaxed from the 1 B b state to the 1 L a state. The visible fluorescence anisotropy vanishes gradually because of the rotational diffusion in a few tens of picoseconds. In the picosecond region, we also observed additional dynamics in the fluorescence intensity whose time constant was about 12 ps. This dynamics was assigned to the vibrational relaxation (cooling) in the 1 L a state. A series of relaxation processes taking place after photoexcitation of the molecule in solution are discussed.

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Section: Resultsmentioning

confidence: 99%

Vibronic Relaxation of Polyatomic Molecule in Nonpolar Solvent: Femtosecond Anisotropy/Intensity Measurements of the S_nand S₁Fluorescence of Tetracene

1999

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“…Rotation faster than slip is described as “subslip”. Such behavior in alcohol solutions has been ascribed to the formation of a large cavity round the solute as a result of hydrogen-bonding in the solvent. However, the phenomenon is also seen in the alkanes. − ,,,, These studies further illustrate the importance of relative molecular size; rotation times rise less steeply with viscosity once the length of the solvent molecule exceeds that of the solute.…”

Section: Theory and Backgroundmentioning

confidence: 99%

Rotation of Aromatic Hydrocarbons in Viscous Alkanes. 1. Methylcyclohexane

Brocklehurst

Young

1999

J. Phys. Chem. A

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Time-resolved single-photon counting has been used to measure the decay of fluorescence anisotropy of a number of aromatic hydrocarbons in methylcyclohexane excited by synchrotron radiation. Changing the temperature (−60 to −140 °C) produces large changes in the viscosity, η. Most solutes were excited both parallel and perpendicular to the emission axis with the aim of determining all three principal rotational diffusion coefficients, D. The results are compared to predictions from hydrodynamic theory made using the “slip” and “stick” approximations and literature data for the n-alkanes and other solvents. Rotation times should be proportional to η/T, i.e., Dη/T should be constant. This is found to be approximately true for 9,10-disubstituted anthracenes; it does not hold for anthracene, perylene, and triphenylene for which Dη/T increases markedly on cooling. Also, against prediction, the ratios between D values for different molecular axes are found to change with temperature.

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“…Hydrodynamic theory has been widely used to describe the decay of fluorescence anisotropy, which results from molecular rotation in solution. − It is implied that the solvent can be treated as a continuum. This description breaks down when the solute molecule is smaller than those of the solvent. − This has been demonstrated in studies of series of n -alkanes where plots of rotation time against viscosity, η, show a change in slope when the solvent chain length exceeds the longest dimension of the solute.…”

Section: Introductionmentioning

confidence: 99%

Rotation of Aromatic Hydrocarbons in Viscous Alkanes. 2. Hindered Rotation in Squalane

Brocklehurst

Young

1999

J. Phys. Chem. A

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Time-resolved single-photon counting and the synchrotron radiation source at the CCLRC’s Daresbury Laboratory have been used to measure the decay of fluorescence anisotropy of solutions in squalane of the aromatic hydrocarbons triphenylene, coronene, benzoperylene, perylene, anthracene, and tetracene. For the last four, extra information was obtained by exciting both parallel and perpendicular to the fluorescence transition. The results differ markedly from those obtained in methylcyclohexane (part 1 of the study) probably because the solute molecules are smaller than those of the solvent. Contrary to the predictions of hydrodynamic theory, decay rates appeared to change with excitation wavelength, and three decays were observed in some cases. Temperature effects (+20 to −70 °C) provided evidence for bifurcation of the molecular rotations into two stages: slow overall motion and faster rotation over a restricted angular range. This resembles the split into α and β processes observed in many relaxation studies of highly viscous liquids.

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Behavior of the rotational diffusion tensor of tetracene under subslip conditions

Cited by 21 publications

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Vibronic Relaxation of Polyatomic Molecule in Nonpolar Solvent: Femtosecond Anisotropy/Intensity Measurements of the S_nand S₁Fluorescence of Tetracene

Vibronic Relaxation of Polyatomic Molecule in Nonpolar Solvent: Femtosecond Anisotropy/Intensity Measurements of the S_nand S₁Fluorescence of Tetracene

Rotation of Aromatic Hydrocarbons in Viscous Alkanes. 1. Methylcyclohexane

Rotation of Aromatic Hydrocarbons in Viscous Alkanes. 2. Hindered Rotation in Squalane

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Behavior of the rotational diffusion tensor of tetracene under subslip conditions

Cited by 21 publications

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Vibronic Relaxation of Polyatomic Molecule in Nonpolar Solvent: Femtosecond Anisotropy/Intensity Measurements of the Snand S1Fluorescence of Tetracene

Vibronic Relaxation of Polyatomic Molecule in Nonpolar Solvent: Femtosecond Anisotropy/Intensity Measurements of the Snand S1Fluorescence of Tetracene

Rotation of Aromatic Hydrocarbons in Viscous Alkanes. 1. Methylcyclohexane

Rotation of Aromatic Hydrocarbons in Viscous Alkanes. 2. Hindered Rotation in Squalane

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Vibronic Relaxation of Polyatomic Molecule in Nonpolar Solvent: Femtosecond Anisotropy/Intensity Measurements of the S_nand S₁Fluorescence of Tetracene

Vibronic Relaxation of Polyatomic Molecule in Nonpolar Solvent: Femtosecond Anisotropy/Intensity Measurements of the S_nand S₁Fluorescence of Tetracene