Control of Intramolecular Electron Transfer in Perylene Dihydrazides and Perylene Diimides: A Comparative Study by Time-Resolved Spectroscopy

TL;DR

This study compares intramolecular electron transfer in perylene dihydrazides and diimides using time-resolved spectroscopy, revealing how donor-acceptor distance and excitation energy influence charge transfer efficiency.

Contribution

It introduces new soluble perylene dyes with specific donor groups and systematically compares their electron transfer dynamics with classical diimides, highlighting the effects of molecular design.

Findings

Electron transfer rates depend on donor-acceptor distance and excitation energy.

Efficient intersystem-crossing occurs without heavy elements.

Charge-transfer efficiency is strongly influenced by surplus excitation energy.

Abstract

Electron transfer (ET) in molecular donor-acceptor dye systems is crucial for charge transport in organic semiconductors. Classically, ET rates should decrease with increasing donor-acceptor distance while the microscopic mechanism is more complex and shows intricate dependencies on the excitation conditions. In this paper, we introduce highly soluble N,N'-dialkyl perylene dihydrazides (PDH) - perylene dyes with a dialkylamino -NR$_2$ donor functionality directly bonded to both of their imide nitrogen atoms. We compare the PDH electron-transfer dynamics with a group of classical N,N'-bisalkylperylene diimides (PDI) equipped with a -NR$_2$ donor linked to the PDI acceptor core via a varying number of alkylene -(CH$_2$)- spacer groups, thus at distinctively different distance. Special physicochemical design features of our study objects include: i) amine moieties as donor group to minimize spin-orbit coupling; ii) substitution solely at both imide positions to avoid major impact on HOMO and LUMO levels and distortions of the PDI backbone; iii) control of donor-acceptor separation by non-conjugated alkylene groups to exclude any additional effects due to delocalized $\pi$ electron systems. All materials show non-single-exponential photoluminescence decay dynamics. A rate equation analysis supported by electrochemical and absolute photoluminescence efficiency measurements yields evidence for efficient intersystem-crossing without heavy elements and reveals that the charge-transfer efficiency across the intramolecular interface strongly depends on the surplus excitation energy.