# Spirofluorene-Bridged Through-Space Charge-Transfer Radicals with 1-Phenyl-1H-indole Donor

## TL;DR

A new spirofluorene-bridged radical with a unique indole donor shows efficient light emission and stability for potential optoelectronic uses.

## Contribution

A novel TSCT radical with a spirofluorene bridge and indole donor is designed and analyzed for optoelectronic performance.

## Key findings

- PID-FR-TTM emits light at 609 nm with a 23.1% quantum yield and 90.1 ns lifetime.
- The compound shows high thermal and electrochemical stability with decomposition at ≈340 °C.
- Calculations confirm a TSCT-dominated excited state with SOMO–HOMO inversion.

## Abstract

Organic luminescent radicals with through-space charge-transfer (TSCT) excited states are attractive for optoelectronic applications, yet donor-dependent structure–property relationships remain underexplored. Here we report a new spirofluorene-bridged TSCT radical, PID-FR-TTM, employing 1-phenyl-1H-indole (PID) as the donor. Single-crystal X-ray diffraction confirms a carbon-centered TTM radical and a less bulky, more planar five-membered N-heterocycle in the donor region. PID-FR-TTM shows TSCT-type absorption and an emission at 609 nm with a photoluminescence quantum yield (PLQY) of 23.1% and a 90.1 ns emission lifetime in cyclohexane. Calculations indicate a TSCT-dominated excited state and a pronounced singly occupied molecular orbital–highest occupied molecular orbital (SOMO–HOMO) inversion. Notably, PID-FR-TTM exhibits markedly improved stability, including high decomposition temperatures (≈340 °C), excellent electrochemical stability, and enhanced photostability. These results provide donor-structure insights for designing high-performance TSCT radical emitters.