Abstract
We have investigated quadratic nonlinearity (β _{HRS}) and linear and circular depolarization ratios (D and D^{′}, respectively) of a series of 1:1 complexes of tropyliumtetrafluoroborate as a cation and methyl-substituted benzenes as π-donors by making polarization resolved hyper-Rayleigh scattering measurements in solution. The measured D and D^{′} values are much lower than the values expected from a typical sandwich or a T-shaped geometry of a complex. In the cation-π complexes studied here, the D value varies from 1.36 to 1.46 and D^{′} from 1.62 to 1.72 depending on the number of methyl substitutions on the benzene ring. In order to probe it further, β, D and D^{′} were computed using the Zerner intermediate neglect of differential overlap-correction vector self-consistent reaction field technique including single and double configuration interactions in the absence and presence of BF_{4} ^{−} anion. In the absence of the anion, the calculated value of D varies from 4.20 to 4.60 and that of D^{′} from 2.45 to 2.72 which disagree with experimental values. However, by arranging three cation-π BF_{4} ^{−} complexes in a trigonal symmetry, the computed values are brought to agreement with experiments. When such an arrangement was not considered, the calculated β values were lower than the experimental values by more than a factor of two. This unprecedented influence of the otherwise “unimportant” anion in solution on the β value and depolarization ratios of these cation-π complexes is highlighted and emphasized in this paper.
R.P. and S.M. thank the CSIR, Government of India, for Senior Research Fellowships. P.K.D. and S.R. are thankful to the DST, Government of India, for generous funding. We thank Professor Jean-Luc Brédas for the use of the ZOA package for plotting the molecular orbitals of the cation-π complexes.
I. INTRODUCTION
II. EXPERIMENTAL
A. UV-Vis spectroscopic measurements
B. Measurement of β, D, and D^{′}
III. RESULTS AND DISCUSSION
A. First hyperpolarizability of the cation-π complexes
B. The structure of the cation-π complexes
IV. CONCLUSION
Key Topics
- Tensor methods
- 15.0
- Charge transfer
- 14.0
- Scattering measurements
- 7.0
- Light scattering
- 6.0
- Polarization
- 5.0
G02F2/00
Figures
Schematic representation of the polarization resolved HRS measurement. The incident beam is propagating along the Z axis and is polarized along the X axis. The SH-scattered light is measured along the Y axis with the polarization along the X and the Z axes.
Schematic representation of the polarization resolved HRS measurement. The incident beam is propagating along the Z axis and is polarized along the X axis. The SH-scattered light is measured along the Y axis with the polarization along the X and the Z axes.
Tropylium cation and π donors chosen for this study.
Tropylium cation and π donors chosen for this study.
Normalized second harmonic intensity, I_{2ω}/I_{ω} ^{2} as a function of concentration of the TRO-MES complex.
Normalized second harmonic intensity, I_{2ω}/I_{ω} ^{2} as a function of concentration of the TRO-MES complex.
The centre of mass of the cationic complex consisting of the substituted benzene (R is either CH_{3} or H) donor and the tropylium acceptor pair at the origin of the reference frame. The centre of mass of the BF_{4} ^{−} counterion is placed at a distance of 6.5 Å along the x axis. The angle θ describes the rotation of BF_{4} ^{−} about the z axis whereas ϕ describes the same about the y axis.
The centre of mass of the cationic complex consisting of the substituted benzene (R is either CH_{3} or H) donor and the tropylium acceptor pair at the origin of the reference frame. The centre of mass of the BF_{4} ^{−} counterion is placed at a distance of 6.5 Å along the x axis. The angle θ describes the rotation of BF_{4} ^{−} about the z axis whereas ϕ describes the same about the y axis.
Trigonal arrangement in solution with 3 C-BF_{4} ^{−} moieties that is used for calculating the NLO coefficients of the cation-π complex. C stands for the complex between tropylium cation and the substituted methyl benzene.
Trigonal arrangement in solution with 3 C-BF_{4} ^{−} moieties that is used for calculating the NLO coefficients of the cation-π complex. C stands for the complex between tropylium cation and the substituted methyl benzene.
Tables
Experimental β _{HRS} values, wavelength of maximum absorption (λ _{max}), and molar extinction coefficient (ɛ) at λ _{max} for the cation-π complexes in acetonitrile.^{a}
Experimental β _{HRS} values, wavelength of maximum absorption (λ _{max}), and molar extinction coefficient (ɛ) at λ _{max} for the cation-π complexes in acetonitrile.^{a}
Measured linear and circular depolarization ratios for the cation-π complexes in acetonitrile.
Measured linear and circular depolarization ratios for the cation-π complexes in acetonitrile.
Selected structural parameters of the CT complexes in their ground state optimized geometries. The distances are between an atom in the donor molecule and an atom in the acceptor molecule in the MP2/6-31g* (CP) optimized geometry. The last column gives the angle between the molecular planes of the donor and the acceptor moieties.
Selected structural parameters of the CT complexes in their ground state optimized geometries. The distances are between an atom in the donor molecule and an atom in the acceptor molecule in the MP2/6-31g* (CP) optimized geometry. The last column gives the angle between the molecular planes of the donor and the acceptor moieties.
Vertical singlet excitation energies (in some cases lowest two energies) of all the CT complexes and their oscillator strengths (in parenthesis) are shown. Calculated tumbling averaged first hyperpolarizability ||β|| and the linear and circular depolarization ratios are also given in the table.
Vertical singlet excitation energies (in some cases lowest two energies) of all the CT complexes and their oscillator strengths (in parenthesis) are shown. Calculated tumbling averaged first hyperpolarizability ||β|| and the linear and circular depolarization ratios are also given in the table.
Calculated linear and nonlinear optical properties of the three cation-π-BF_{4} ^{−} complexes, arranged in a C_{3} symmetric fashion (Fig. 5). The oscillator strengths corresponding to excitation energies are given in parentheses next to the gaps. All the calculations are performed in acetonitrile using the SCRF model within a correction vector (CV) approach. D and D^{′} are the linear and circular depolarization ratios, respectively. Angles θ and ϕ are defined in Fig. 4.
Calculated linear and nonlinear optical properties of the three cation-π-BF_{4} ^{−} complexes, arranged in a C_{3} symmetric fashion (Fig. 5). The oscillator strengths corresponding to excitation energies are given in parentheses next to the gaps. All the calculations are performed in acetonitrile using the SCRF model within a correction vector (CV) approach. D and D^{′} are the linear and circular depolarization ratios, respectively. Angles θ and ϕ are defined in Fig. 4.
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