Dioxaborine- and Indole-Terminated Polymethines: Effects of Bridge Substitution on Absorption Spectra and Third-Order Polarizabilities

Abstract

Cyanine-like dyes are promising candidates for third-order nonlinear optical (NLO) applications such as all-optical switching. Here, we examine the consequences for linear and nonlinear optical properties of varying substituents on the central methine unit of bis(dioxaborine)-terminated anionic pentamethines and bis(indole)-terminated cationic heptamethines. The variation in absorption maxima and electrochemical potentials with structure can generally be rationalized using the Dewar-Knott rules, providing that mesomeric and inductive electron-withdrawal and donation are explicitly considered. In the case of nitro- and (dioxaborinyl)vinyl-substituted bis(dioxaborine) pentamethines, the low-energy transitions are significantly broadened, consistent with (1)H NMR spectra indicating deviation from cyanine-like geometry. Real and imaginary parts of the third-order polarizabilities, Re(γ) and Im(γ), were measured at 1.3 μm. The values of Im(γ) indicate that the values of Re(γ) are significantly resonantly enhanced, while the positive value of Re(γ) found for a nitro-substituted dioxaborine example is atypical for a symmetrical polymethine and suggests that a two-state treatment is inadequate. The relevance of these results to chromophore design for third-order NLO applications is discussed.

NMR Spin–Spin Coupling Constants in Polymethine Dyes as Polarity Indicators

“…This result has been attributed to alternating electron density along the conjugation pathway, which has been observed experimentally 49. A similar oscillatory behavior has also been reported for the 13 C NMR chemical shifts of carbon atoms in the conjugation pathway of penta‐ and heptamethines 46. Every intermediate second carbon atom (i.e., atoms with the numbering 2, 4, 6, and 8) that is connected to an acceptor group (NR) has a relatively large positive charge and so the chemical shifts for these carbon atoms are shifted upfield.…”

“…However, this value only conveys the information that the shielding of the donor‐group carbon atom increases with solvent polarity and that the shielding of the acceptor‐group carbon atom decreases with increasing solvent polarity and, hence, it cannot be used to identify the conformational nature of the probe in different solvents. There have also been some studies on the solvent‐dependence of the 13 C NMR and 1 H NMR chemical shifts for atoms in the hyperconjugation pathway of donor–acceptor polyenes 46–49. Interestingly, an oscillatory behavior has been seen in the chemical shifts of the odd‐row protons, starting from the donor terminus.…”

Conjugated Polycyanines: A New Class of Materials with Large Third‐Order Optical Nonlinearities

“…Extrapolating the aforementioned values to zero irradiance yields cm 2 W −1 , corresponding to esu ( mks). We note here that the magnitude of γ values (Table ) of AJMPC02 represents one of the largest values ever reported, which are much greater than those observed in most indolium or TCF‐based[15a] monomeric cyanines with the same conjugation length (C7) such as Indo‐C7‐H and AJBC1722 ( Scheme ), and even larger than that of DOB‐C9 (Scheme ),[8a] which has a longer conjugation length C9. This confirms our hypothesis that connecting cyanine molecules into fully conjugated polycyanines significantly enhances molecular third‐order optical nonlinearities without increasing the conjugation bridge length, but via an extended delocalization of the exciton over several cyanine units in the polycyanine.…”

Assessment of range‐separated exchange functionals and nonempirical functional tuning for calculating the static second hyperpolarizabilities of streptocyanines

“…Polymethines and cyanines are becoming the research spot for the application of all‐optical signal processing (AOSP) due to the fast nonlinear response and large third‐order nonlinear susceptibilities (χ (3) ) . A large number of experimental and theoretical studies has been contributed to understand the molecular structure–property relationships . The well‐known sum‐over‐states (SOS) method has been regarded as one of the accurate model on the evaluation of static molecular second hyperpolarizabilities γ for polymethines .…”