Level of Theory and Solvent Effects on DASA Absorption Properties Prediction: Comparing TD-DFT, CASPT2 and NEVPT2

Abstract

Donor-acceptor Stenhouse adducts (DASAs) are a very recent class of organic photoswitches that combine excellent properties, such as color and polarity change, a large structural modification, and excellent fatigue resistance. Despite their potential applications in different fields, very few studies have focused on rationalizing their electronic structure properties. Here, by means of different state-of-the-art theoretical methods, including solvent and vibrational effects, we show that while time dependent-density functional theory (TD-DFT) can qualitatively describe DASAs' excited states, multiconfigurational quantum chemistry methods along with dynamic electron correlation (CASPT2, NEVPT2) are required for a quantitative agreement with the experiment. This finding is reasoned based on the different charge transfer characteristics observed. Moreover, the TD-DFT computed two-photon absorption properties are reported and suggested to red-shift the absorption band, as required for biological applications.

From Light Absorption to Cyclization: Structure and Solvent Effects in Donor‐Acceptor Stenhouse Adducts

“…For all acceptor moieties A1 – A3 , ca . 90 nm are recovered compared to TD‐DFT results, hence showing that a multiconfigurational treatment makes possible, in principle, a quantitative description of the absorption spectrum, as already found by some of the authors for a 1 st generation DASA …”

“…The ground state minimum of each DASA derivative under study has been optimized at the Density Functional Theory (DFT) level, using the M06‐2X functional and the 6–31+G(d) basis set, as their suitability for this type of chromophores has been already demonstrated, at least for an affordable qualitative description of both ground and excited state properties . Indeed, an extended benchmark with different functionals including range‐separated ones did not allow an improved description of the DASA charge‐transfer character . Then, the photochemical path has been studied at the Time Dependent (TD)‐DFT level of theory, considering two excited states.…”

“…The suitability of this active space has been checked by performing the same calculation with an active space of 14 electrons in 14 orbitals (see next section for active space details and Table S1). Moreover, the use of multireference perturbation theory methods was proven to recover some of the limits caused by TD‐DFT, possibly reaching a quantitative description of DASAs …”

Redesigning donor–acceptor Stenhouse adduct photoswitches through a joint experimental and computational study

“…Thus, we will only compute the S 0 -S 1 transition to model the spectrum, and spectral features will be reproduced entirely by the vibronic progression corresponding to this single electronic transition. 41 The computed spectra in Figure 3 were obtained using DLPNO-STEOM-CCSD transition energies and dipoles and assuming: a) the same B3LYP Hessian for both S 0 and S 1 , but with the S 1 geometry obtained from the S 0 one after a single optimization step (called Vertical Gradient approach, VG) or b) different B3LYP Hessians, with the S 1 Hessian computed at the new geometry (using the Adiabatic Hessian After Step, AHAS) 38 . In comparison with the experimental spectrum, the shape and the position of the bands are largely reproduced for both.…”

Steady-State Linear and Non-linear Optical Spectroscopy of Organic Chromophores and Bio-macromolecules

“…This symmetry reasonings explain why for both systems a low S 0 →S 1 TPA intensity is theoretically predicted [ ca . 2 and 3.32 GM for 11- cis retinal and DASA, respectively (Palczewska et al, 2014 ; García-Iriepa and Marazzi, 2017 )]. Even so, retinal isomerization by TPA is expected to play a role to trigger human infrared vision (Artal et al, 2017 ).…”