Evaluation of Single-Reference DFT-Based Approaches for the Calculation of Spectroscopic Signatures of Excited States Involved in Singlet Fission

Abstract

Singlet fission (SF) has the potential to dramatically increase solar cell efficiency by converting one singlet exciton to two free triplet excitons via a correlated triplet pair intermediate. Identification and characterization of excited states involved in SF are of great importance for understanding the fundamentals of SF. Despite their importance, it is still nontrivial to distinguish various species in transient absorption spectra due to their spectral overlaps and ultrashort lifetimes. Theoretical modeling of SF and its electronically excited state absorption (ESA) is generally challenging due to the multiexciton nature of the correlated triplet pair, which usually requires description by expensive high-level ab initio methods. In this work, taking the bis­((triisopropylsilyl)­ethynyl) (TIPS)-pentacene monomer and its covalently linked dimer as representative examples, we demonstrate the use of single-reference DFT-based approaches to simulate the ESA spectra during SF. In particular, the singlet and triplet ESA are evaluated by TDDFT, QR-TDDFT, SLR-TDDFT, SF-TDDFT, and UTDDFT, in combination with ten different exchange–correlation functionals. The correlated triplet pair and its ESA are characterized by broken-symmetry DFT and TDDFT, and the role of orbital relaxation is highlighted. With a rational choice of exchange–correlation functionals, we found the resulting spectra to show good agreement with transient absorption experiments and certain improvements over high-order CI methods.

Structure Set in Stone: Designing Rigid Linkers to Control the Efficiency of Intramolecular Singlet Fission

“…The weak electronic coupling is reinforced by quantum chemical calculations, indicating an absence of NIR absorption bands of the correlated triplet pair. 37,59,60 This contrasts reports on tetracene and pentacene, for which a significant stabilization of 1 (TT) with regard to 2T 1 is observed. 61 While these observations point toward T 1 T 1 , we note that an unambiguous assignment would require the employment of time-resolved electron spin-resonance measurements.…”

“…This is based on the weak binding energy of 1 (TT), that is, E (2T 1 ) – E ( 1 (TT)) (Table ), which facilitates the loss of the interstate coupling. The weak electronic coupling is reinforced by quantum chemical calculations, indicating an absence of NIR absorption bands of the correlated triplet pair. ,, This contrasts reports on tetracene and pentacene, for which a significant stabilization of 1 (TT) with regard to 2T 1 is observed . While these observations point toward T 1 T 1 , we note that an unambiguous assignment would require the employment of time-resolved electron spin-resonance measurements. − However, regardless of the exact nature of the triplet pair species, its presence is operative for the mechanism invoked for our dimers and supported by the SADS (applying a kinetic model to the global analysis, Figure S59).…”

“…All quantum chemical calculations were performed using Gaussian 16 and Multiwfn 3.7 . The detailed explanation for the choice of the method is given in refs – . The ground-state structures of 8a–c , as well as their homodimers 1a–c , 2a–c , and 3a–c were optimized using the density functional theory (DFT) in combination with the ωB97X-D functional and the 6-31G* basis set .…”

Second Linear Response Theory and the Analytic Calculation of Excited-State Properties

“…In previous work, we applied SLR methods to ES and two-photon absorption , (where the dipoles involving ES-to-ES transitions are required). Han et al also applied this idea to ES absorption of the TIPS-pentacene system. Previously, we developed two methods: (i) a procedure that uses numerical differentiation to determine transition dipoles for excitations from a selected excited state (if N excited states are selected, then this method requires a solution for N + 1 Casida equations) and (ii) a faster method where only the standard Casida equation is solved.…”

Machine-learning assisted design principle search for singlet fission: an example study of cibalackrot

“…However, since the training reference values were obtained from theoretical calculations themselves, such deviations can be considered negligible compared to the errors inherent to the density functional theory (DFT) reference itself. Judging from benchmark studies on the performance of DFT methods for the description of singlet fission molecules [31][32][33] , it is thus fair to assume that the actual excitation energies of our predicted overall distribution will be globally shifted by somewhere between 0.1 and 0.3 eV with respect to possible experimental values, while the trends when comparing individuals inside the distribution are predicted correctly.…”