Rational Design of Molecules with Large Hyperpolarizabilities. Electric Field, Solvent Polarity, and Bond Length Alternation Effects on Merocyanine Dye Linear and Nonlinear Optical Properties

Abstract

The effects of applied electric fields and solvent polarity on the linear and nonlinear optical (NLO) properties of three prototypical merocyanine (donor−conjugated pathway−acceptor) chromophores of differing architectural types are analyzed using semiempirical INDO/1 calculations in the presence of imposed static electric fields, using finite-field self-consistent field (FFSCF), and in the presence of solvent dielectrics, self-consistent reaction field (SCRF), models. The NLO properties are computed using the computationally efficient correction vector approach. Both applied electric fields and solvent are found to affect the extent of charge separation induced in the ground states of these molecules. This charge separation leads to a geometric distortion, measured by the bond-length alternation (BLA) parameter, which indexes the geometrical evolution of the molecular architecture from a neutral polyenic structure to a partially ionic cyanine-like structure to an ionic polymethine-like structure. These geometric variations lead to large changes in the linear as well as NLO response properties, which are different, both qualitatively and quantitatively, at the three different structural limits. The applied electric field is found to produce large variations in the structural, electronic, linear, and NLO properties. However, an analysis based on the Onsager reaction field model shows that in general the electric field produced by even the most polar solvents is inadequate to produce large geometric distortions. This trend is also observed experimentally in solvent-dependent electronic and optical properties, where even the most polar solvents produce effects equivalent only to those produced by a very small electric field. However, variations in structural and optical properties are also found to be highly architecture specific and large variations are possible in chromophore structures stabilized on charge separation. The field at which the chromophore geometry attains the cyanine-like structure is not coincident with the field at which the SHG coefficient goes to zero; an asymmetry in the BLA pattern is required to attain the zero-β limit. Optimal NLO response occurs when the geometry of the conjugation pathway lies between the polyenic and cyanine-like structures.

Aromatic/Proaromatic Donors in 2‐Dicyanomethylenethiazole Merocyanines: From Neutral to Strongly Zwitterionic Nonlinear Optical Chromophores

“…Nevertheless, the central region and the right-hand side (RHS) of the plot offer many possibilities. Thus, although b vanishes at (or close to) [7] the cyanine limit (at which BLA = 0), the a parameter peaks at that coordinate, with the result that, for instance, photorefractive effects [8] dominated by the orientational birefringence contribution are maximised close to [9] or somewhat beyond [10] the cyanine limit. Moreover, strongly polar molecules, with ground states adequately described by a balanced contribution of the CL and the ZW forms can display high negative b values, but comparatively minor efforts have been devoted to these RHS chromophores.…”

Breakdown of the first hyperpolarizability/bond-length alternation parameter relationship

“…In theoretical calculations, molecular structures with varying BLA parameter have been generated using negative and positive charges placed close to the donor and acceptor groups and a subsequent optimization of the structure in the finite field due to the charges or "sparkles" (3,5,6,9,18). It has been indicated by semiempirical calculations that the field strength due to stronger polar solvents is less influential and only brings smaller changes in solute molecular geometry and in its BLA parameter (6, 18).…”

“…In theoretical calculations, molecular structures with varying BLA parameter have been generated using negative and positive charges placed close to the donor and acceptor groups and a subsequent optimization of the structure in the finite field due to the charges or "sparkles" (3,5,6,9,18). It has been indicated by semiempirical calculations that the field strength due to stronger polar solvents is less influential and only brings smaller changes in solute molecular geometry and in its BLA parameter (6,18). Experimentally, many molecules have been synthesized possessing varying BLA parameters (with negative and positive values) and where the corresponding hyperpolarizability has been used to derive the first hyperpolarizability/BLA parameter relationship (7).…”

“…The BLA parameter dependence of the first hyperpolarizability has been studied theoretically for many organic molecules including both negative and positive solvatochromic molecules such as phenol blue, stilbazolium merocyanine, 9-(dimethylamino)-nona-2,4,6,8-tetraenal that all show a significant change in structure, and so in BLA parameter, with varying solvent polarity (12,18). In these cases, the molecular structures with different BLA values were generated using an electric field or using charges near the acceptor or donor groups.…”

Solvent Effect on Color, Band Shape, and Charge‐Density Distribution for Merocyanine Dyes Close to the Cyanine Limit

“…[6] In recent years, this model with interdependent ground and excited states has been widely applied to evaluate donoracceptor chromophores for nonlinear optics. [7] Although the donor and acceptor strengths and the influence of the pconjugated bridge have been experimentally addressed in great detail, the impact of the solvent on the p charge-density distribution of dipolar dyes and its contribution to solvatochromism have only been elucidated by indirect experimental methods (e.g., IR frequencies, NMR chemical shifts, hyperpolarizability) [8] or by calculations [9] owing to the difficulty of determining dipole moments in polar media. [10] In this work we focused our attention on solvent-dependent electrooptical absorption (EOA) and UV/Vis measurements to determine the solvent-dependent dipole moments of dye 3 in the ground and excited states.…”