Bright, Modular, and Switchable Near-Infrared II Emission from Compact Tetrathiafulvalene-Based Diradicaloid Complexes

Abstract

Near-infrared (NIR)-emitting molecules are promising candidates for biological sensing and imaging applications; however, many NIR dyes are large conjugated systems which frequently have issues with stability, solubility, and tunability. Here, we report a novel class of compact and tunable fluorescent diradicaloid complexes which are air-, water-, light-, and temperature-stable. These properties arise from a compressed π manifold which promotes an intense ligand-centered π–π transition in the NIR II (1000–1700 nm) region and which subsequently emits at ∼1200 nm. This emission is among the brightest known for monomolecular lumiphores with deep NIR II (>1100 nm) emission, nearly an order of magnitude brighter than the commercially available NIR II dye IR 26. Furthermore, this fluorescence is electrochemically sensitive, with efficient switching upon addition of redox agents. The brightness, stability, and modularity of this system distinguish it as a promising candidate for the development of new technologies built around NIR emission.

Interplay of Electronic and Geometric Structure Tunes Organic Biradical Character in Bimetallic Tetrathiafulvalene Tetrathiolate Complexes

“…To resolve any trends in the biradical character and triplet–singlet (T–S) gaps of bimetallic TTFtt 2– bridged complexes, we investigate the electronic structures of a series of complexes that have been recently reported (Pt, PtCF3, Pd, Ni, Sn, Fe), as well as the newly proposed and yet to be experimentally characterized PtCOD. These compounds are of particular interest in the development of optically addressable molecular qubits or switches, and a recent experimental and computational investigation by the authors has shown them to exhibit fluorescent behavior driven by a π* → π transition of the TTFtt ligand . The chemical structures of all investigated compounds are displayed in Figure .…”

“…First, broken symmetry (BS) DFT geometry optimizations were performed with the B3LYP functional in combination with the LANL2DZ basis set for Pt, Pd, Sn and the 6-31G* basis set for all remaining atoms, relaxing the geometry from the one obtained in the solid state via single crystal X-ray diffraction (XRD). ,, The use of unrestricted Kohn–Sham DFT calculations, allowing them to break the spin-symmetry, is essential for the accurate capture of the geometry and properties of systems displaying multireference character. , The DFT optimizations were followed by high-level CASSCF and NEVPT2 calculations allowing us to capture accurately the effects arising from both the multireference and biradical character and dynamic correlation. A minimal active space comprising four electrons distributed in four spatial orbitals, [4,4], was chosen for the CASSCF calculations as the strong correlation in these systems has been shown to be limited to two frontier natural orbitals, largely comprised of TTFtt-based π orbitals that give rise to the biradical character.…”

“… a R (C–C) is the length of the central TTF C–C bond in Å with R (C–C) BS obtained from broken-symmetry singlet DFT geometry and R (C–C) XRD obtained from experimental XRD data, ,, S 2 is the unrestricted singlet DFT spin contamination in the optimized geometry, Δ E T–S denotes the triplet–singlet energy gap in kcal/mol, and λ HONO and λ LUNO are the occupation numbers of the HONO and LUNO, respectively. …”

“…These compounds are of particular interest in the development of optically addressable molecular qubits or switches, and a recent experimental and computational investigation by the authors has shown them to exhibit fluorescent behavior driven by a π* → π transition of the TTFtt ligand. 33 The chemical structures of all investigated compounds are displayed in Figure 1. We note that while all complexes considered in this study are doubly charged cations, they are referred to solely by the identity of the metal centers and ligands as indicated in Figure 1 with their charges being omitted in the following sections of this article.…”

“…photoluminescence quantum yield (PLQY),33 as well as high conductivity in amorphous, glassy coordination polymers 14. In this article we perform high-level ab initio electronic structure calculations on a series of TTFtt 2− -bridged bimetallic complexes.…”