Photoinduced charge separation in two bis(phenylethynyl)anthracene-based triads: inverted region effect vs distance effect on back electron transfer

Abstract

Photoinduced electron transfer processes in two bis(phenylethynyl)anthracene (BPEA)-based triads were investigated to identify the dominating factors that lead to long-lived charge-separated (CS) states in BPEA-based donor-acceptor systems. In both systems studied, phenothiazine moieties acted as final donors. Nitrotoluene or pyromellitic diimide units acted as final acceptors. Two possible electron transfer pathways could be identified in these systems. Fluorescence of the BPEA chromophore was highly quenched in both systems due to the photoinduced electron transfer process. Picosecond transient absorption studies suggested that excitation of BPEA leads to electron transfer from ¹BPEA^∗ to the acceptor followed by a second electron transfer from phenothiazine to the BPEA radical cation. In both systems formation of long-lived CS states was confirmed by nanosecond flash photolysis. A comparison showed that the BPEA-based triads exhibited lower CS state lifetimes compared to the BPEA-phenothiazine dyad. Analysis of the -Δ G° and λ values showed that for both triads -Δ G° ≤ λ, and hence, the inverted region effect cannot operate. Hence, the long lifetime of the final CS state is attributed to the long distance separating the donor and acceptor components in the CS state. This study supports the contention that if the CS state in a dyad is long-lived due to the inverted region effects, the CS state lifetime will decrease if the dyad is converted to a triad.