Mapping the Optical Landscape of a Squaraine Molecule in the Visible and Ultraviolet Energy Range

TL;DR

This study comprehensively maps the excited state landscape of a squaraine dye across visible and ultraviolet spectra using advanced spectroscopy and ab initio calculations, revealing twelve excited states.

Contribution

It extends the understanding of squaraine dyes' excited states into ultraviolet energies using combined experimental and theoretical approaches, which was previously limited.

Findings

Identified twelve electronically excited states of SQIB.

Mapped transition dipole moments and anisotropy in UV-visible range.

Provided a consistent optical behavior model for SQIB.

Abstract

Although squaraine dyes are commonly praised as candidates for light-based applications, little is known about their excited state landscape beyond the low-energy visible light region. Our work aims for an improved understanding of the photophysical properties of squaraines at the example of N-isobutyl substituted anilino-squaraine (SQIB) by extending ground-state and excited-state absorption spectroscopy of the molecule into the ultraviolet up to 6.5~eV. In addition, we distinguish the relative transition dipole moments of the excited state absorption peaks with the help of transient absorption anisotropy experiments. To relate experimental features to specific states, we employ a set of ab initio methods including time-dependent density functional theory (TDDFT), the Bethe-Salpeter equation (BSE) and n-electron valence perturbation theory on top of a self-consistent complete active space (CASSCF/NEVPT2). Our assignment is complemented by vibronic simulations and a discussion of two-photon absorption measurements. Through this joint effort, we are able to provide a consistent picture of the optical behavior of SQIB across the visible and ultraviolet light regime, and assign a total of twelve electronically excited states to our experimental data.