The Intriguing Case of the One‐Photon and Two‐Photon Absorption of a Prototypical Symmetric Squaraine: Comparison of TDDFT and Wave‐Function Methods

Abstract

We face the challenging description of the excited states responsible for one photon (OPA) and two photon (TPA) absorption in squaraine dyes, adopting both time-dependent density functional theory and a large variety of post Hartree Fock methods, including coupled cluster and adiabatic diagrammatic construction methods (ADC, to the second and third order), symmetry adapted cluster configuration interaction (CI), CASSCF including second-order perturbative corrections (PT2) with standard CASPT2 recipe and according to n-electron valence PT2 (NEVPT2) approach, and an additional multireference CI with PT2 corrections (CI-MRPT2). We selected a small prototypical symmetric squaraine dye for which these accurate computations are feasible and experimental data are available.We show that while all methods reasonably reproduce the OPA spectrum, an acceptable description and assignment of the states responsible for TPA is only possible through ADC(3) and multireference calculations, due to the strong involvement of double-excitations. The nature of ground state and OPA and TPA states is investigated with a rigorous reading of the CASSCF wavefunctions in terms of localized molecular orbitals on the donor and acceptor fragments, showing that these states arise from a balance of many different contributions including biradical configurations, local excitations and charge-transfer states where both side aromatic rings and the carbonyls of the squarylium moiety act as donors.[a] Dr.

Is ADC(3) as Accurate as CC3 for Valence and Rydberg Transition Energies?

“…As ADC(3) enjoys a lower formal computational scaling [ ] than CC3 [ ], and is generally regarded as the logical path for improvement over ADC(2), this finding contributed to enhancing the popularity of ADC(3) in the electronic structure community. ADC(3) was subsequently employed to perform theory versus experiment comparisons − and to define benchmark values for assessing lower-order methods. − …”

Is ADC(3) as Accurate as CC3 for Valence and Rydberg Transition Energies?

“…As ADC(3) enjoys a lower formal computational scaling [O(N 6 )] than CC3 [O(N 7 )], and is generally regarded as the logical path for improvement over ADC (2), this finding contributed to enhance the popularity of ADC(3) in the electronic structure community. ADC(3) was subsequently employed to perform theory vs experiment comparisons, [20][21][22][23][24][25][26] and to define benchmark values for assessing lower-order methods. [27][28][29][30] Given, on the one hand, that ADC(3) was advocated as a benchmark method and, on the other hand, the recent availability of high-accuracy reference energies for a large panel of ES, 18,31,32 we believe that the time has come to perform a new performance assessment of this method.…”

Supramolecular Approach to Tuning the Photophysical Properties of Quadrupolar Squaraines

“…Since the bis(N,N-dimethylamine) squaraine molecule represents the challenge for the computational methods due to the double excitation effects balanced with the charge transfer and moderate biradicaloid character, the literature recommendations have been applied for the computational methodology for the electronic excitations (Quartarolo et al, 2009;Alberto et al, 2014;Bassal et al, 2017;Ferrer et al, 2019). It has been proven that the conventional long-range-corrected functionals correctly predict the one-photon absorption spectrum of bis(N,N-dimethylamine) squaraine and despite strong overestimation of the energy of the states involved in two-photon transitions allow the correct assignment of the experimentally observed states.…”

“…The presence of the D-A-D motif may lead to the efficient two-photon absorption (TPA) of squaraine systems. The simple symmetrical dibutylaniline squaraine dye (SQ) has been shown to exhibit the strong two-photon absorption with three bands: vibronic coupling band of the TPA cross section equal to 200 GM, second band at 850 nm with σ TPA 2000 GM corresponding to the S 2 excited state, and the third one at 700 nm with the cross section of 15,000 GM is governed by the excitation to the S 4 state (Webster et al, 2010;Ferrer et al (2019)). Squaraines of the extremely large values of σ TPA have been obtained (for instance, 27 ,000 GM for heterocyclic pyrrolesubstituted squaric acid ring) (Pawlicki et al, 2009).…”

“…One need to underline that the prototypical dimethylaniline squaraine dye is challenging from the theoretical point of view due to the interplay of several competing effects: the charge transfer between the electron-accepting central squaric acid ring and the electron-donating side groups, the mild biradicaloid character, and the significant double excitations affecting the two-photon theoretical description (Prabhakar et al, 2005b;Prabhakar et al, 2010;Prabhakar et al, 2005a;Yesudas et al, 2006;Srinivas et al, 2007;Ferrer et al, 2019).…”