Thiazole and Thiophene Analogues of Donor−Acceptor Stilbenes:  Molecular Hyperpolarizabilities and Structure−Property Relationships

We calculate the first hyperpolarizability (β) of several thiazole and thiophene analogues of donor−acceptor stilbene compounds using the ZINDO (sum-over-states) formalism. Because of the inherent dipolar nature of thiazole, in which C2 is electron-poor and C5 is electron-rich, the relative orientation of the thiazole subunit in the dipolar chromophore dramatically affects the nonlinear optical properties. In the “mismatched” case, the dipole of the thiazole ring opposes the molecular dipole created by the donor−acceptor substituents, while in the “matched” case, the dipole of the thiazole ring reinforces the molecular dipole. The hyperpolarizability of the “mismatched” monothiazole 2 (βμ = 68 × 10-30 cm5 esu-1) exceeds that of stilbene 1 (βμ = 34 × 10-30 cm5 esu-1) but is smaller than that of monothiophene 4 (βμ = 90 × 10-30 cm5 esu-1). By contrast, the hyperpolarizability of the “matched” monothiazole 3 (βμ = 177 × 10-30 cm5 esu-1) exceeds not only that of the “mismatched” monothiazole 2, but also that of monothiophene 4. Substituting thiazole for both aryl rings of stilbene produces very large hyperpolarizabilities in the “matched−matched” case (e.g., bis-thiazole 24, βμ = 254 × 10-30 cm5 esu-1). The nonlinear optical response of heterocyclic analogues of donor−acceptor stilbene derivatives is discussed in terms of the difference in aromatic delocalization energy between phenyl, thiophene, and thiazole, the electronic nature of the heteroaromatic rings, and conformational factors.