# Concurrent Generation of Tight and Loose Ion Pairs upon Charge-Transfer Excitation of Electron Donor–Acceptor Complexes in Solution

## TL;DR

This study shows that charge-transfer excitation in electron donor–acceptor complexes creates both fluorescent and nonfluorescent ion pairs, explaining differences in measured lifetimes.

## Contribution

The paper introduces a revised mechanism where CT excitation generates a structurally diverse ion-pair ensemble with parallel relaxation pathways.

## Key findings

- CT-state lifetimes from TRFL are shorter than those from TA by factors of ∼2–5 for all DACs studied.
- Fluorescence lifetimes of ∼5–30 ps in dichloromethane contradict the assumption of only emissive tight ion pairs.
- The results support a concurrent mechanism with fluorescent tight and nonfluorescent loose ion pairs.

## Abstract

Ultrafast time-resolved fluorescence (TRFL) and visible
transient
absorption (TA) measurements were performed to investigate excited-state
dynamics of several electron donor–acceptor complexes (DACs)
following charge-transfer (CT) excitation in solution. For all DACs
studied, including benzene–tetracyanoethylene (BZ–TCNE),
toluene–TCNE, fluorobenzene–TCNE, and (BZ)2–TCNE, the CT-state lifetimes obtained from TRFL are consistently
shorter than those from TA by factors of ∼2–5. This
disparity, together with fluorescence lifetimes of ∼5–30
ps in dichloromethane, cannot be reconciled with the conventional
assumption that CT excitation initially yields only emissive tight
ion pairs (TIPs). The results instead support a revised concurrent
mechanism in which CT excitation generates a locally hot and structurally
diverse ion-pair ensemble that bifurcates into parallel relaxation
pathways, leading to fluorescent TIPs and nonfluorescent loose ion
pairs. The two species undergo independent charge recombination at
different rates, yielding the distinct lifetimes observed in the TRFL
and TA measurements.