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.
Highlights
Conjugated diradical systems have attracted considerable interest in recent years owing to their potential electronic applications [1] in organic field effect transistors (OFETs) [2,3,4], organic photodetectors (OPDs) [5], and near-infrared (NIR) dyes [6,7], among others
The quantum-chemical description of the singlet ground state of conjugated diradicals calls for multireference methods to include static correlation effects, a very common approach is the use of DFT in its unrestricted formulation (UDFT) which, for the significant diradical character, leads to broken symmetry (BS) molecular orbitals
The results of TDUDFT and SF-time dependent density functional theory (TDDFT) calculations can be critically analyzed by considering (a) the diradical character and the reliability of the 2e-2o model for the systems investigated; (b) the derivation of the wavefunction describing the doubly excited state at TDUDFT level when the 2e-2o model holds; (c) the indication provided by NFOD parameter, and (d) the role of spin-contamination
Summary
Conjugated diradical systems have attracted considerable interest in recent years owing to their potential electronic applications [1] in organic field effect transistors (OFETs) [2,3,4], organic photodetectors (OPDs) [5], and near-infrared (NIR) dyes [6,7], among others. Significant efforts have been devoted to stabilize the active open-shell molecules, and a large number of stable diradicals with an open-shell singlet ground state have been synthetized with different conjugated cores and varying diradical character [8,9,10,11,12]. The quantum-chemical description of the singlet ground state of conjugated diradicals calls for multireference methods to include static correlation effects, a very common approach is the use of DFT in its unrestricted formulation (UDFT) which, for the significant diradical character, leads to broken symmetry (BS) molecular orbitals. A distinctive signature of singlet ground state conjugated diradical systems is the presence of a low lying double-exciton state [14], which in analogy to polyenes—that display diradical character, especially the longer members [15]—becomes the lowest excited state for large diradical character, as shown in our previous work [16,17,18]. The double-exciton state is an electronically excited state whose wavefunction is dominated by a large contribution of the HOMO,HOMO→LUMO,LUMO
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