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

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.

Figures (12)

• Figure 1: Schematic illustration of the molecular structure of anilino-squaraines. The electron-donating and electron-accepting regions are highlighted in orange and blue, respectively. In the experiments, SQIB ($R =\mathrm{isobutyl}$) was used, whereas for the calculations, the simplified model compound MeSQ ($R =\mathrm{methyl}$) was employed.
• Figure 2: (a) Frontier $\pi$-orbitals of MeSQ with $D_{2h}$ symmetry (isovalue of 0.02) calculated at the DFT (CAM-B3LYP/cc-pVTZ) level. Each orbital is labeled according to its irreducible representation. Additionally, we label them by their energetic order in the DFT calculation as HOMO$-n$ and LUMO$+n$. Occupied orbitals are shown with black labels and unoccupied orbitals with grey labels. These molecular orbitals comprise the CAS(12,12) active space in our CASSCF calculations. (b) Comparison of orbital energy levels obtained from DFT, GW, and CASSCF calculations. Each horizontal line represents the energy of a corresponding orbital, and dashed lines connect orbitals of similar symmetry and character across the three computational methods.
• Figure 3: (a) Comparison of the experimental optical density of SQIB solvated in acetonitrile with the absorption spectrum of a single MeSQ molecule in the gas phase, calculated using TDDFT, GW/BSE, and CASSCF/NEVPT2. The top panel shows the experimental spectrum grouped into three spectral regions: I (1.5–3.0 eV), II (3.0–5.0 eV), and III (5.0–7.0 eV). The magnified inset highlights weak features in Region II of the experimental spectrum. (b–d) Enlarged sections of the three energy regions showing the theoretical assignment of the experimental peak positions. Theoretical transition energies are indicated by colored bars, where red and blue correspond to transition dipoles polarized along the $x$- and $y$-directions, respectively. The grey arrows in panels (a), (c), and (d) mark weak peaks to make them more visible and to indicate the positions discussed in the text.
• Figure 4: (a) Comparison between the experimental transient absorbance spectra of SQIB molecules dispersed in a PMMA matrix. Experimental spectra recorded at a 0.1 ps delay time between the pump and probe pulses and the theoretical spectra of an isolated MeSQ molecule in the gas phase, obtained from QR-TDDFT, GW/BSE, and CASSCF/NEVPT2 calculations, respectively. The zero of the energy scale corresponds to the ground state, $S_0$. The QR-TDDFT spectrum is scaled by a factor of 20 for clarity. (b) Gaussian fit of the experimental ESA signal, overlaid with the calculated excitation energies from QR-TDDFT, GW/BSE, and CASSCF/NEVPT2. Theoretical assignments of the main transitions are indicated by red bars, where $x$- and $y$-polarized transition dipoles are represented by red and blue labels, respectively. (c) Transient absorption anisotropy at $\Delta t = 0$ ps (dark-green triangles) and $\Delta t = 10$ ps (light-green circles) as a function of absolute energy. Both delay times correspond to independent experimental data sets, illustrating the overall trend rather than exact quantitative values. The dashed red and purple lines indicate the limiting cases of purely parallel and purely perpendicular ESA transition dipole moments with respect to the pump--probe polarization direction.
• Figure 5: Two-photon absorption (2PA) spectrum of SQIB in toluene and acetonitrile. The vibronic peaks of S$_1$ appear at 2.06 eV and 2.27 eV, followed by the main 2-A$_g$ transition at 2.98 eV in toluene and 2.95 eV in acetonitrile. The negligible energy shift between the two solvents indicates that the 2PA response of SQIB is largely insensitive to the solvent environment.