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(<i>p</i>-phenylene)­s (ZnP–ph<sub><i>n</i></sub>–SWNT) or oligo­(<i>p</i>-xylene)­s (ZnP–xy<sub><i>n</i>–1</sub>–ph<sub>1</sub>–SWNT) with systematically varied lengths (<i>n</i> = 1–5) to address the issue. Exponential dependencies of rate constants for the exciplex formation (<i>k</i><sub>FEX</sub>) and decay (<i>k</i><sub>DEX</sub>) 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 <i>k</i><sub>FEX</sub> and <i>k</i><sub>DEX</sub> in ZnP–ph<sub><i>n</i></sub>–SWNT were found to be considerably small (β = 0.10 for <i>k</i><sub>FEX</sub> and 0.12 Å<sup>–1</sup> for <i>k</i><sub>DEX</sub>) compared to those for charge separation and recombination (0.2–0.8 Å<sup>–1</sup>) in D–B–A systems with the same oligo­(<i>p</i>-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(<i>p</i>-xylene)­s exhibited larger attenuation values (β = 0.12 for <i>k</i><sub>FEX</sub> and 0.14 Å<sup>–1</sup> for <i>k</i><sub>DEX</sub>). These results provide deep insight into the unique photodynamics of electronically strongly coupled D–B–A systems involving exciplex.