Uncovering Structure–Property Relationships in Push–Pull Chromophores: A Promising Route to Large Hyperpolarizability and Two-Photon Absorption

Alessio Cesaretti,Paolo Foggi,Anna Spalletti,Benedetta Carlotti,Cosimo G Fortuna,Fausto Elisei

doi:10.1021/acs.jpcc.0c03536

Abstract

In this investigation, we report the first hyperpolarizabilities and two-photon absorption cross sections of a large series of 12 push–pull cationic chromophores. All of these dyes show a dipolar acceptor+–π–donor structure, where the nature of the donor and acceptor units and π-bridge was synthetically tuned to allow insightful comparisons among the molecules. The hyperpolarizability was obtained through a solvatochromic method, by exploiting the rare negative solvatochromism exhibited by the investigated compounds. The two-photon absorption cross sections were determined through two-photon excited fluorescence measurements by means of a tunable nanosecond laser system for sample excitation. The nonlinear optical properties were discussed relatively to the photoinduced intramolecular charge transfer occurring in these donor–acceptor systems, investigated by femtosecond transient absorption experiments. We found a strong increase in hyperpolarizability upon increasing the molecular conjugation. Unexpectedly, the hyperpolarizability is almost unaffected by an increase in donor/acceptor strength and intramolecular charge transfer degree. Differently, the two-photon absorption cross sections of these dyes are enhanced by an increase in both molecular conjugation and intramolecular charge transfer efficiency. Several recent literature works have reported at the same time scattered information about the hyperpolarizability and two-photon absorption of small organic molecules. Our investigation is, to the best of our knowledge, the first attempt to uncover detailed structure–property relationships for these two nonlinear optical properties. Our results represent a promising route to achieve large hyperpolarizability and two-photon absorption in push–pull dyes and may drive the design of new efficient nonlinear optical materials.

Highlights

Nonlinear optical materials are among the smartest materials of the current age owing to their frequency-tuning ability of laser light interacting with them.[1]
Great interest has lately been devoted to cationic chromophores for their surprisingly high nonlinear optical responses[4,5] and for their water solubility, which is really appealing in view of their possible use in biology and medicine.[6−9]
All of these results unambiguously show a significant increase in the hyperpolarizability of these chromophores upon increasing their molecular conjugation

Summary

Introduction

Nonlinear optical materials are among the smartest materials of the current age owing to their frequency-tuning ability of laser light interacting with them.[1] Development of nonlinear optical materials is an area of frontier research due to extensive applications in optoelectronics, photonics, and medicine Emerging technologies such as imaging, photodynamic therapy, and sensing are in continuous search for new, highly performing, low-cost materials. The nonlinear optical properties of D−π−A compounds can be finely tuned by selecting appropriate D, A units and π−bridges at suitable positions Such dipolar D−π−A structures have recently shown very high hyperpolarizabilities[2] and large two-photon absorption.[3] In this respect, great interest has lately been devoted to cationic chromophores for their surprisingly high nonlinear optical responses[4,5] and for their water solubility, which is really appealing in view of their possible use in biology and medicine.[6−9]

Objectives

Methods

Discussion

Conclusion