J. Phys. Chem. C 2008, 112, 3109-3120
3109
Electronic, Optical, and Vibrational Properties of Bridged Dithienylethylene-Based NLO Chromophores M. Carmen Ruiz Delgado,† Juan Casado, Victor Herna´ ndez, and Juan T. Lo´ pez Navarrete* Department of Physical Chemistry, UniVersity of Ma´ laga, 29071-Ma´ laga, Spain
Jesu´ s Orduna Department of Organic Chemistry, ICMA, UniVersity of Zaragoza-CSIC, E-50009 Zaragoza, Spain
Bele´ n Villacampa and Raquel Alicante Department of Condensed Matter Physics, ICMA, UniVersity of Zaragoza-CSIC, E-50009 Zaragoza, Spain
Jean-Manuel Raimundo, Philippe Blanchard, and Jean Roncali Groupe Syste` mes Conjugue´ s Line´ aires, CIMMA UMR CNRS 6200, UniVersite´ d’Angers, 2 Bd LaVoisier, 49045 Angers Cedex, France ReceiVed: October 30, 2007; In Final Form: NoVember 29, 2007
The vibrational, optical, and nonlinear optical (NLO) properties of a series of push-pull chromophores built around dithienylethylene-based π-conjugating spacers have been investigated by UV-vis, IR, and Raman spectroscopies and electric field-induced second harmonic generation (EFISH) measurements. The effects of the strength of the acceptor group on the molecular electronic properties of these conjugated NLO chromophores have been addressed. The magnitude of the intramolecular charge transfer has been tested as a function of the acceptor strength and of the bridging of the spacer. Density functional theory (DFT) calculations have been performed to help the assignment of the main electronic and vibrational features of the NLO chromophores and to derive useful information about their molecular structures. EFISH measurements show that push-pull systems that contain strong electron-acceptor groups connected via a rigidified dithienylene spacer exhibit large values of µβ0. Theoretical NLO calculations are in excellent agreement with experimental results. The geometrical and electronic properties calculated in solution reveal that chromophores become highly polarized as the dielectric constant of the solvent increases. The spectroscopic properties/structure relationships agree with the observation that β0 increases upon the covalent bridging of the spacer and with the increase of the acceptor strength.
1. Introduction Dipolar push-pull chromophores constitute the widest class of molecules investigated for their nonlinear optical (NLO) properties.1-8 These push-pull NLO chromophores are basically constituted by an electron-donor (D) group connected to an electron-acceptor (A) via a conjugated π bridge. The NLO activity of this class of compounds depends not only on the strength of the D-A pair but also on the nature of the π-conjugated spacer.2 Recent research has also shown that organotransition metal NLO systems,9 ferrocenyl-based chromophores,10 and systems with a highly charge-separated zwitterionic structure display large first hyperpolarizability (β) values.11 When comparing typical spacers such as polyenes, oligophenylenes, or oligothiophenes of the same chain length and bearing the same D-A pair, a more pronounced redshift is commonly observed for the low energy absorption maximum, λmax, in polyenes than in oligophenylenes or oligothiophenes, indicative of an efficient electron transmission from the donor to the * Corresponding author. E-mail:
[email protected]. † Current address: School of Chemistry and Biochemistry, Georgia Institute of Technology, Atlanta, GA 30332-0400.
acceptor. The bathochromic shift of the visible absorption of the push-pull chromophore is even more intensified in high polar solvents. This positive solvatochromism has been commonly regarded as an indication of molecular nonlinearity (µβ) of NLO chromophores.12-14 However, it was recently reported that some polyenes attached to a strong D-A pair display an inverted solvatochromism.15,16 Clearly, such spectroscopic behavior is not a simple attribute of the D-A strength but it is also associated with the electronic properties of the π-conjugated spacer. Although D-A polyenes have been shown to exhibit huge nonlinearities,2c,16 the well-known limited thermal stability of extended polyenes might represent an obstacle for device applications. On the other hand, the large aromaticity of the benzene ring has a detrimental effect on the second-order polarizability. In comparison with oligophenylenes, oligothiophenes show efficiencies almost comparable to those of polyenes,2,17 which is ascribed to their lower resonance energy as compared with that of benzene, and have been shown to give larger contributions to µβ(0).13,18,19 Another merit of oligothiophenes is their inherent stability compared with polyenes.20,21 It has already been demonstrated that rigidification of thiophene-based π-conjugated systems by means of covalent
10.1021/jp710459c CCC: $40.75 © 2008 American Chemical Society Published on Web 02/05/2008
3110 J. Phys. Chem. C, Vol. 112, No. 8, 2008 bridging represents a powerful approach for reducing the band gap of conjugated oligomers and polymers.22 More recently, this concept has been extended to the molecular engineering of push-pull NLO chromophores; UV-vis spectroscopic data and the results of electric field-induced second harmonic generation (EFISH) experiments have shown that, when used as π-conjugating spacer in push-pull NLO chromophores, bridged dithienylethylenes lead to a large bathochromic shift of the absorption spectrum and to a huge enhancement of the efficiency of second harmonic generation.23-25 Raman spectroscopy has been shown to be well-suited for determining precise relationship between structure and properties of π-conjugated systems.26-30 In this respect, the effective conjugation coordinate (ECC) theory justifies the selective enhancement of particular scatterings associate with the collective CdC/CsC stretching vibrations of the π-conjugated backbone of the molecule on the basis of the existence of an effective electron-phonon coupling (or electronic delocalization) in the π-conjugated systems due to their quasi-onedimensional structures.31 Thus, Raman frequencies and intensities are experimental observables closely related to the π-conjugated properties of oligothiophenes and related systems, which account for the main optical and electronic signatures of these new molecular materials. For instance, some of us have recently demonstrated that the presence of the vinylene bridge strongly influences those properties in comparison with data for other linear thiophene-based oligomers.27 The present work is intended for a better understanding of the molecular structure/properties relationships of a series of NLO chromophores with a common donor group and acceptors of various strength, containing a central π-conjugated dithienylene (DTE) spacer or its rigidified version (i.e., covalently bridged DTE), from their electronic, vibrational, and optical spectra. The interest and significance of this study is to analyze their molecular level features by means of a few spectroscopic techniques (UV-vis, IR, Raman), EFISH and quantum-chemical calculations. Comparisons of the Raman features will be made between the rigidified DTE-based NLO chromophores and its open chain homologues, pursuing the estimation of the degree of intramolecular charge transfer (ICT) from the electron donor to the electron acceptor groups. Density functional theory (DFT) calculations in the framework of the polarized continuum model (PCM) developed by Tomasi have been carried out for guidance in the analysis of the solvent effects on the molecular and electronic properties.32 The effects of the acceptor strength and rigidification of the π spacer on the second-order NLO properties have also been studied theoretically and experimentally. 2. Experimental and Theoretical Details 2.1. Experimental Techniques. The synthesis of the DTEbased NLO chromophores under study has already been reported elsewhere.23-25,33 Spectroscopic Measurements. UV-vis absorption spectra were recorded at room temperature with an Agilent 8453 spectrophotometer equipped with a diode-array for the fast scanning of all electromagnetic absorptions in the 190-1100 nm wavelength region with a spectral resolution of better than 2 nm and using solvents of the highest quality (supplied by Aldrich) with a sufficiently low concentration (
