Geometries, IR and Raman spectra, nucleus independent chemical shifts, and static electronic (hyper)polarizabilities of the equilibrium conformations of 2,2′-biselenophene were determined in vacuum using density functional theory (DFT) computations. At the DFT-PBE0/6-31G+pdd′ level the antigauche structure characterized by the dihedral angle of 157° is the global minimum, whereas the syngauche rotamer ( = 40°) lies ca. 0.7？kcal·mol？1 above the antigauche form. The structural and spectroscopic properties as well as the electronic polarizability of the antigauche are similar to those of the syngauche structure. On the other hand, the dipole moments and first-order hyperpolarizabilities are strongly influenced by the conformational characteristics, increasing by ca. a factor of five when passing from the antigauche to the syngauche form. 1. Introduction Oligomers and polymers based on five-membered heterocycles have received great attention as promising conductive and nonlinear optical (NLO) materials [1–3]. Although the major efforts have been principally directed towards polyfurans, polypyrroles, and especially polythiophenes, recent studies have been also dedicated to the properties of polyselenophenes [4, 5]. Selenophene, owing to the intrinsic effects of the heavy atom, is considered an interesting building-block for the design of NLO devices [6–10]. Physicochemical properties of -conjugated oligomers and polymers are usually influenced by the twisting degree of the backbone as well as by the extension of the electron delocalization [11, 12]. However, structural and electronic properties of large oligomers and polymers can be extrapolated by using data of smaller oligomers [11, 12]. Differently from the monomer, little is known about the physicochemical properties of the smallest oligoselenophenes. Structures and torsional potentials of 2,2′-biselenophene, 2,2′:5′,2′′-terselenophene, and 2,2′:5′,2′′:5′′,2′′′-quaterselenophene have been previously investigated by using ab initio and density functional theory (DFT) methods [13, 14]. On the basis of the most recent theoretical results obtained in vacuum, 2,2′-biselenophene is predicted to exist in two nonplanar minimum-energy conformations, characterized by dihedral angles of ca. 150° (antigauche) and 40° (syngauche) [14]. The antigauche is the lowest-energy conformer, with the syngauche being predicted to lie above the antigauche structure by less than 1？kcal/mol [13, 14]. The torsional potentials of 2,2′-biselenophene for the 0°–360° rotation around the bond are described by flat four-well potentials,
References
[1]
R. D. McCullough, “The chemistry of conducting polythiophenes,” Advanced Materials, vol. 10, no. 2, pp. 93–116, 1998.
[2]
P. N. Prasad and D. J. Williams, Introduction to Nonlinear Optical Effects in Molecules and Polymers, Wiley, New York, NY, USA, 1991.
[3]
J. Zyss, Molecular Nonlinear Optics: Materials, Physics and Devices, Academic Press, London, UK, 1994.
[4]
A. Patra and M. Bendikov, “Polyselenophenes,” Journal of Materials Chemistry, vol. 20, no. 3, pp. 422–433, 2010.
[5]
D. Bhattacharyya and K. K. Gleason, “Low band gap conformal polyselenophene thin films by oxidative chemical vapor deposition,” Journal of Materials Chemistry, vol. 22, no. 2, pp. 405–410, 2012.
[6]
K. Kamada, M. Ueda, T. Sakaguchi, K. Ohta, and T. Fukumi, “Femtosecond optical Kerr study of heavy atom effects on the third-order optical nonlinearity of thiophene homologues: purely electronic contribution,” Chemical Physics Letters, vol. 263, no. 1-2, pp. 215–222, 1996.
[7]
K. Kamada, M. Ueda, H. Nagao et al., “Molecular design for organic nonlinear optics: polarizability and hyperpolarizabilities of furan homologues investigated by ab initio molecular orbital method,” Journal of Physical Chemistry A, vol. 104, no. 20, pp. 4723–4734, 2000.
[8]
S. Millefiori and A. Alparone, “(Hyper)polarizability of chalcogenophenes C4H4X (X = O, S, Se, Te) conventional ab initio and density functional theory study,” Journal of Molecular Structure: THEOCHEM, vol. 431, no. 1-2, pp. 59–78, 1998.
[9]
S. Millefiori and A. Alparone, “Theoretical determination of the vibrational and electronic (hyper)polarizabilities of C4H4X (X = O, S, Se, Te) heterocycles,” Physical Chemistry Chemical Physics, vol. 2, no. 11, pp. 2495–2501, 2000.
[10]
B. Jansik, B. Schimmelpfennig, P. Norman, P. Macak, H. ？gren, and K. Ohta, “Relativistic effects on linear and non-linear polarizabilities of the furan homologues,” Journal of Molecular Structure, vol. 633, no. 2-3, pp. 237–246, 2003.
[11]
J. L. Brédas, G. B. Street, B. Thémans, and J. M. André, “Organic polymers based on aromatic rings (polyparaphenylene, polypyrrole, polythiophene): evolution of the electronic properties as a function of the torsion angle between adjacent rings,” Journal of Chemical Physics, vol. 83, no. 3, pp. 1323–1329, 1985.
[12]
V. Hernandez, C. Castiglioni, M. Del Zoppo, and G. Zerbi, “Confinement potential and π-electron delocalization in polyconjugated organic materials,” Physical Review B, vol. 50, no. 14, pp. 9815–9823, 1994.
[13]
S. Millefiori and A. Alparone, “Theoretical investigation of the structure and conformational behaviour of small selenophene oligomers,” Synthetic Metals, vol. 95, no. 3, pp. 217–224, 1998.
[14]
G. Sánchez-Sanz, I. Alkorta, and J. Elguero, “A theoretical study of the conformation of 2, -bifuran, 2, -bithiophene, 2, -bitellurophene and mixed derivatives: chalcogen-chalcogen interactions or dipole-dipole effects?” Computational and Theoretical Chemistry, vol. 974, no. 1–3, pp. 37–42, 2011.
[15]
G. J. M. Velders, J.-M. Gillet, P. J. Becker, and D. Feil, “Electron density analysis of nonlinear optical materials. An ab initio study of different conformations of benzene derivatives,” Journal of Physical Chemistry, vol. 95, no. 22, pp. 8601–8608, 1991.
[16]
J. Lipiński and W. Bartkowiak, “Conformation and solvent dependence of the first and second molecular hyperpolarizabilities of charge-transfer chromophores. Quantum-chemical calculations,” Chemical Physics, vol. 245, no. 1-3, pp. 263–276, 1999.
[17]
J. S. Salafsky, “Second-harmonic generation as a probe of conformational change in molecules,” Chemical Physics Letters, vol. 381, no. 5-6, pp. 705–709, 2003.
[18]
A. Plaquet, B. Champagne, F. Castet et al., “Theoretical investigation of the dynamic first hyperpolarizability of DHA-VHF molecular switches,” New Journal of Chemistry, vol. 33, no. 6, pp. 349–356, 2009.
[19]
Z.-M. Su, H.-L. Xu, Z.-R. Li, S. Muhammad, F. L. Gu, and K. Harigaya, “Knot-isomers of m？bius cyclacene: how does the number of knots influence the structure and first hyperpolarizability,” Journal of Physical Chemistry C, vol. 113, no. 34, pp. 15380–15383, 2009.
[20]
A. Alparone, “Comparative study of CCSD(T) and DFT methods: electronic (hyper)polarizabilities of glycine,” Chemical Physics Letters, vol. 514, no. 1–3, pp. 21–25, 2011.
[21]
M. J. Frisch, G. W. Trucks, H. B. Schlegel, et al., GAUSSIAN 03, Revision E.01. Gaussian, J. A. Pople.
[22]
C. Adamo and V. Barone, “Toward reliable density functional methods without adjustable parameters: the PBE0 model,” Journal of Chemical Physics, vol. 110, no. 13, pp. 6158–6170, 1999.
[23]
K. Kamada, M. Ueda, H. Nagao et al., “Effect of heavy atom on the second hyperpolarizability of tetrahydrofuran homologs investigated by ab initio molecular orbital method,” International Journal Quantum Chemistry, vol. 70, pp. 737–743, 1998.
[24]
H. Sekino and R. J. Bartlett, “Frequency dependent nonlinear optical properties of molecules,” Journal of Chemical Physics, vol. 85, no. 2, pp. 976–989, 1986.
[25]
Z. Chen, C. S. Wannere, C. Corminboeuf, R. Puchta, and P. von Ragué Schleyer, “Nucleus-independent chemical shifts (NICS) as an aromaticity criterion,” Chemical Reviews, vol. 105, no. 10, pp. 3842–3888, 2005.
[26]
N. S. Mills and K. B. Llagostera, “Summation of nucleus independent chemical shifts as a measure of aromaticity,” Journal of Organic Chemistry, vol. 72, no. 24, pp. 9163–9169, 2007.
[27]
F. Fringuelli, G. Marino, and A. Taticchi, “Tellurophene and related compounds,” Advances in Heterocyclic Chemistry, vol. 21, pp. 119–173, 1977.
[28]
G. R. Dennis, I. R. Gentle, G. L. D. Ritchie, and C. G. Andrieu, “Field-gradient-induced birefringence in dilute solutions of furan, thiophen and selenophen in cyclohexane,” Journal of the Chemical Society, Faraday Transactions 2, vol. 79, no. 4, pp. 539–545, 1983.
