Chemical Control of Channel Interference in Two-Photon Absorption Processes

Abstract

The two-photon absorption (TPA) process is the simplest and hence the most studied nonlinear optical phenomenon, and various aspects of this process have been explored in the past few decades, experimentally as well as theoretically. Previous investigations have shown that the two-photon (TP) activity of a molecular system can be tuned, and at present, performance-tailored TP active materials are easy to develop by monitoring factors such as length of conjugation, dimensionality of charge-transfer network, strength of donor-acceptor groups, polarity of solvents, self-aggregation, H-bonding, and micellar encapsulation to mention but a few. One of the most intriguing phenomena affecting the TP activity of a molecule is channel interference. The phrase "channel interference" implies that if the TP transition from one electronic state to another involves more than one optical pathway or channel, characterized by the corresponding transition dipole moment (TDM) vectors, the channels may interfere with each other depending upon the angles between the TDM vectors and hence can either increase (constructive interference) or decrease (destructive interference) the overall TP activity of a system to a significant extent. This phenomenon was first pointed out by Cronstrand, Luo, and Ågren [Chem. Phys. Lett. 2002, 352, 262-269] in two-dimensional systems (i.e., only involving two components of the transition moment vectors). For three-dimensional molecules, an extended version of this idea was required. In order to fill this gap, we developed a generalized model for describing and exploring channel interference, valid for systems of any dimensionality. We have in particular applied it to through-bond (TB) and through-space (TS) charge-transfer systems both in gas phase and in solvents with different polarities. In this Account, we will, in addition to briefly describing the concept of channel interference, discuss two key findings of our recent work: (1) how to control the channel interference by chemical means, and (2) the role of channel interference in the anomalous solvent dependence of certain TP chromophores. For example, we will show that simple structurally induced changes in certain dihedral angles of the well-known betaine dye (TB type) will help fine-tune the constructive channel interference and hence increase the overall TP activity of molecules with this general TP channel structure. Another intriguing result we will discuss is observed for a tweezer-trinitrofluorinone complex (TS type) where, on moving from polar to essentially nonpolar solvents, the nature of the channel interference switches from destructive to constructive, leading to a net abnormal solvent dependence of the TP activity of the system. The present Account highlights the usefulness of the channel interference effect and establishes it as a new and unique way of controlling the TP transition probability in different types of three-dimensional molecules.

Interplay of twist angle and solvents with two-photon optical channel interference in aryl-substituted BODIPY dyes

“…This analysis was first proposed by Ågren and co-workers 19 for two-dimensional systems wherein only two components of the transition moment vectors are involved. Alam et al [20][21][22] proposed a general model that can be used for any molecule, irrespectively of its dimensionality.…”

Strong Dipolar Effects on an Octupolar Luminiscent Chromophore: Implications on their Linear and Nonlinear Optical Properties

“…All materials and solvents are commercially available.Infrared spectra were obtained on a FTIR Varian Spectrometer ATR. Melting points were obtained in an Electrothermal-9200 1. H and 13 C NMR spectra, were taken in a JEOL eclipse ECA +500 spectrometer, the chemical shifts in ppm are relative to (CH 3 )4 Si for1 H and 13 C. High resolution TOF-MS mass spectra were recorded on an Agilent G1969A spectrometer.…”

A Theoretical Study of One- and Two-Photon Activity of D-Luciferin

“…Chattopadhyaya et al [18] have shown that the number of D/A groups cannot be the sole criteria for larger TPA cross-section; rather, the D/A groups at a suitable position can lead to a large dipole moment difference between the ground and excited state, which is ultimately responsible for the high TP activity of bisanthene and D/A substituted bisanthene molecule. Apart from all these factors, Cronstrand et al [19,20] and Alam et al [21,22] have shown that as the excitation from ground to excited state takes place through several optical pathways by the involvement of several intermediate states, the interference between different optical channels also have a crucial role in determining the overall TP activity of a molecule. Cronstrand et al have scrutinized the role of interference between different optical channels and derived a two-dimensional four-state model formula to demonstrate the role of some selective chromophores on TP activity.…”