Long-Range Observation of Exciplex Formation and Decay Mediated by One-Dimensional Bridges

We report herein unprecedented long-range observation of both formation and decay of the exciplex state in donor (D)–bridge (B)–acceptor (A) linked systems. Zinc porphyrins (ZnP) as a donor were tethered to single-walled carbon nanotube (SWNT) as an acceptor through oligo­(p-phenylene)­s (ZnP–ph_n–SWNT) or oligo­(p-xylene)­s (ZnP–xy_n–1–ph₁–SWNT) with systematically varied lengths (n = 1–5) to address the issue. Exponential dependencies of rate constants for the exciplex formation (k_FEX) and decay (k_DEX) on the edge-to-edge separation distance between ZnP and SWNT through the bridges were unambiguously derived from time-resolved spectroscopies. Distance dependencies (i.e., attenuation factor, β) of k_FEX and k_DEX in ZnP–ph_n–SWNT were found to be considerably small (β = 0.10 for k_FEX and 0.12 Å^–1 for k_DEX) compared to those for charge separation and recombination (0.2–0.8 Å^–1) in D–B–A systems with the same oligo­(p-phenylene) bridges. The small β values may be associated with the exciplex state with mixed characters of charge-transfer and excited states. In parallel, the substantially nonconjugated bridge of oligo­(p-xylene)­s exhibited larger attenuation values (β = 0.12 for k_FEX and 0.14 Å^–1 for k_DEX). These results provide deep insight into the unique photodynamics of electronically strongly coupled D–B–A systems involving exciplex.