Impact of Bulk Aggregation on the Electronic Structure of Streptocyanines: Implications for the Solid-State Nonlinear Optical Properties and All-Optical Switching Applications

Abstract

Polymethine dyes in dilute solutions show many of the electronic and optical properties required for all-optical switching applications. However, in the form of thin films, their aggregation and interactions with counterions do generally strongly limit their utility. Here, we present a theoretical approach combining molecular-dynamics simulations and quantum-chemical calculations to describe the bulk molecular packing of streptocyanines (taken as representative of simple polymethines) with counterions of different hardness (Cl − and BPh 4 −) and understand the impact on the optical properties. The accuracy of the force field we use is verified by reproducing experimental crystal parameters as well as the configurations of polymethine/counterion complexes obtained from electronicstructure calculations. The aggregation characteristics can be understood in terms of both polymethine−counterion and polymethine−polymethine interactions. The counterions are found to localize near one end of the streptocyanine backbones, and the streptocyanines form a broad range of aggregates with significant electronic couplings between neighboring molecules. As a consequence, the linear and nonlinear optical properties are substantially modified in the bulk. By providing an understanding of the relationship between the molecular interactions and the bulk optical properties, our results point to a clear strategy for designing polymethine and counterion molecular structures and optimizing the materials properties for all-optical switching applications.

25th Anniversary Article: Design of Polymethine Dyes for All‐Optical Switching Applications: Guidance from Theoretical and Computational Studies

“…This transition dipole moment is determined by a sum of contributions due to the transitions from the HOMO→LUMO configuration in state e to each of the three major excitations in e ′. In unsubstituted polyenes and cyanines, the excitations from the HOMO→LUMO configuration to each of the singly excited configurations in e ′ have the same sign, whereas the excitation to the double excitation in e ′ has the opposite sign . Owing to this partial cancellation of terms of opposite signs, μ ee′ is typically relatively small …”

Linear polarizabilities and second hyperpolarizabilities of streptocyanines: Results from broken‐Symmetry DFT and new CCSD(T) benchmarks

“…In the past 50 years, these NLO properties have been harnessed in a variety of advanced materials and technologies such as electro‐optic waveguide devices, memory devices, three dimension fluorescence microscopy, and nanofabrication . To further enhance their usage in next‐generation optical technologies, significant effort has been focused on achieving a detailed understanding of the electronic structure–property relationships that govern their performance in realistic materials. As such, predictive computational methods, particularly quantum chemical methods, offer a rational approach for guiding future experimental efforts to obtain tailored NLO properties in these functional π‐conjugated systems.…”

Zwitterionic Cyanine–Cyanine Salt: Structure and Optical Properties

“…Higher order nonlinear refraction appears to be present for both salts, as values of the Re­(γ) measured from the lowest energy (Table ) clearly underestimate the signals with higher energies. As suggested by theoretical calculations, , the electronic coupling between adjacent cyanine molecules can broaden the excited state spectrum, resulting in an enhancement of Im­(γ) and a sequential reduction of the |Re­(γ)/Im­(γ)| ratio. However, it can be seen that both Re­(γ) and Im­(γ) values of 1 are quite similar to those of 2 , in spite of the former having demonstrated electronic coupling.…”

“…Typically, an isolated cyanine possesses a well-defined molecular symmetry with a charge fully delocalized over the polymethine backbone, endowing them with the aforementioned attractive optical properties. − However, the strong Coulombic interactions (i.e., ion pairing) between chromophores and counterions tend to localize the charges close to one side of the end groups, resulting in a significant distortion of the delocalization and subsequent symmetry breaking of their electronic structures. − This can adversely affect the favorable NLO properties due to the appearance of undesirable one- and two-photon states, which is more pronounced in particular for those with small and hard (less polarizable) counterions such as halides. , …”

“…To mitigate the ion-pairing effect, both the experimental and the theoretical data have suggested using bulky, soft (easily polarizable) ions as counterions such as tetraaryl–borate anions and–phosphonium cations. − Another more attractive approach is to chemically lock the position of the counterions at the center of the polymethine backbone to form a highly symmetrical zwitterionic salt. Many rhodamine derivatives, squaraine, − and quinonoid − can be regarded as such zwitterionic salts; however, they have limited molecular nonlinearities for AOSP applications due to relatively short conjugation lengths.…”