The Low Lying Double-Exciton State of Conjugated Diradicals: Assessment of TDUDFT and Spin-Flip TDDFT Predictions

Abstract

Conjugated singlet ground state diradicals have received remarkable attention owing to their potential applications in optoelectronic devices. A distinctive character of these systems is the location of the double-exciton state, a low lying excited state dominated by the doubly excited HOMO,HOMOLUMO,LUMO configuration, (where HOMO=highest occupied molecular orbital, LUMO=lowest unoccupied molecular orbital) which may influence optical and other photophysical properties. In this contribution we investigate this specific excited state, for a series of recently synthesized conjugated diradicals, employing time dependent density functional theory (TDDFT) based on the unrestricted parallel spin reference configuration in the spin-flip formulation (SF-TDDFT) and standard TD calculations based on the unrestricted antiparallel spin reference configuration (TDUDFT). The quality of computed results is assessed considering diradical and multiradical descriptors, and the excited state wavefunction composition.

Addressing the Elusive Polaronic Nature of Multiple Redox States in a π‐Conjugated Ladder‐Type Polymer

“…[28] Charged states, P±(S) and B±(D) show higher N FODs than the neutral species, with values for holes larger than electrons. The FOD analysis justifies the presence of BS solutions, [38] for BBL charged species (Figure 1), pointing out the role of SEC/MR effects in determining the electronic wavefunction of charged states.…”

Elucidating the Relationship between Multiradical Character and Predicted Singlet Fission Activity

“…[40][41][42][43] One can include analysis of other descriptors such as the singlet-triplet gap~E(ST), spin density or N FOD i. e. the integral over all the space of the fractional orbital density from finite-temperature DFT. [44][45][46][47][48] This work will focus only on results assessed by the NOON method using CASSCF as multiconfigurational methods are more naturally designed to describe fractional occupation numbers of orbitals. We note that diradical descriptors evaluated using PUHF may be different for the same molecule computed with CASSCF, however we have not investigated the structures using PUHF for comparison as it is not uncommon for triplet calculations to converge to an incorrect triplet state.…”

“…One can include analysis of other descriptors such as the singlet‐triplet gap ▵E(ST), spin density or N FOD i. e . the integral over all the space of the fractional orbital density from finite‐temperature DFT . This work will focus only on results assessed by the NOON method using CASSCF as multiconfigurational methods are more naturally designed to describe fractional occupation numbers of orbitals.…”

Investigating the (Poly)Radicaloid Nature of Real-World Organic Compounds with DFT-Based Methods

“…The Spin-Flip Time-Dependent DFT (SF-TDDFT) method is recognized to uniformly describe excited states of single, double, and mixed excitation character in molecular systems, and more specifically in conjugated molecules featuring diradical or (poly)­radical character, , starting from a high-spin (e.g., triplet) reference state. The formalism is based on the (linear response) TDDFT equations for excitation energies: with X ( Y ) the set of (de)­excitation amplitudes, A and B the linear-response matrices, and Ω the excitation energies.…”