Above the Energy Gap Law: Heavy Chalcogenide Substitution in NIR II-Emissive Diradicaloid Qubits

Abstract

Near-infrared (NIR, 700-1700 nm)- and telecom (∼1260-1625 nm)-emissive molecules are good candidates for biological imaging and quantum sensing applications, respectively; however, bright low energy emission is rare due to exponentially increasing nonradiative decay rates in these regions, a phenomenon known as the energy gap law. Recent literature has emphasized the importance of minimizing skeletal modes to prevent increased nonradiative decay rates, but most organic lumiphores in these regions utilize large, conjugated scaffolds containing many CC modes. Here we report a compact, telecom-emissive scaffold, tetrathiafulvalene-2,3,6,7-tetraselenate, or TTFts, that displays remarkable air, water, and acid stability, exhibits record quantum yields and brightness values, and retains quantum coherence under ambient conditions. These properties are enabled through methodical selenium substitution, which bathochromically shifts emission while shifting skeletal vibrations to lower energies. This new scaffold validates heavy heteroatom substitution strategies and establishes a new class of bright telecom emitters and robust qubits.

1

(a) Synthetic scheme for complexes 1 and 2. (b) Single-crystal structure of 1, with hydrogens, solvent, and counteranions omitted for clarity. Thermal ellipsoids shown at 50%. (c) Absorption (solid) and emission (dashed) spectra for 1 and 2 shown in blue and red, respectively.

2

(a) NIR TA spectra of 2 in DCM at 298 K. (b) Transient kinetics of 2 in DCM at 298 K, corresponding to the three main GSB (orange), ESA (teal), and SE (magenta) features. (c) SWIR TA spectra of 2 in DCM at 298 K, showing decay of initial excited state (S ₁). (d) SWIR TA spectra of 2 in DCM at 298 K, showing decay of second excited state (putatively T ₂).

3

(a) Comparison of nonradiative decay rates across previously reported organic lumiphores, phosphine-capped TTFtt analogs, ,, and 1 and 2. Dashed line indicates predicted C–H mode limitation on nonradiative decay rates. (b) Comparison of IR spectra of 1 (teal) and TTFtt (black) analog, normalized to the NIR feature, dropcast onto KBr plates from a DCM solution. Gray boxes highlight changes between the spectra. Inset: IR spectra showing bathochromic shift of NIR feature.

4

(a) Jablonski diagram of 2, showing the natural transition orbitals (NTOs) obtained via TD-DFT simulations for both the singlet and triplet manifolds. Wavelengths shown are TD-DFT predicted values, except where noted. (b) Variable-temperature PL of 2 in 1:1 DBrM:toluene. (c) Variable-temperature lifetime measurements of 2 in 1:1 DBrM:toluene. Inset: lifetime fits of 2 across temperatures in 1:1 DBrM:toluene.

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(a) Continuous-wave EPR of 1 in 1:1 DCM:toluene at 295 K. (b) Spin–lattice relaxation (T ₁) and phase memory (T _m) times of 0.1 mM solution of 2 in 1:1 DCM:toluene across various temperatures. (c) Transient EPR spectrum of 2 in 1:1 DBrM:toluene at 85 K. (d) Transient kinetics of 2 in 1:1 DBrM:toluene at 85 K.