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

Abstract

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(pphenylene)s (ZnP−ph n −SWNT) or oligo(p-xylene)s (ZnP− xy n−1 −ph 1 −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 timeresolved 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.

Photophysical Properties of Porphyrin Dimer–Single-Walled Carbon Nanotube Linked Systems

“…The IPCE spectrum matches with that of the FTO/SnO 2 /pinB- p -ph-SWNT (Figure S13c). These indicate that the absorption arising from the porphyrin dimers does not generate the photocurrent, but one from the SWNT moiety solely generates the photocurrent in the FTO/SnO 2 /(ZnP- m -ph) 2 -SWNT device, as also seen in the devices with porphyrin monomer-linked SWNTs. − This result is in sharp contrast with analogous photoelectrochemical devices with the FTO/SnO 2 electrodes modified with porphyrin–fullerene linked systems that yield the complete charge-separated state and the resultant photocurrent response from the porphyrin absorption . These photoelectrochemical results also corroborate no formation of the complete charge-separated state by the photoexcitation of the (ZnP- m -ph) 2 -SWNT.…”

“…The fourth TA decay component spectrum exhibits negative signals with a negative peak at 1010 nm in the NIR region, but the sum of the spectra for ZnP 2 •+ and SWNT •– shows downward-sloping negative signals in the NIR region. In addition, the absence of the characteristic absorption bands for the ZnP monomer radical cation (ZnP •+ ) in 600–700 nm and radical anion (ZnP •– ) in 700–750 nm in the fourth decay component spectrum contradicts the formation of a symmetry-breaking charge-separated state , in the porphyrin dimer.…”

“…Given the intensive intermolecular π–π interaction between the two porphyrin rings, one can anticipate that the special pair mimics, i.e., porphyrin dimers, are grafted on the SWNT sidewall by using the direct addition reaction of the corresponding aryl radicals bearing a porphyrin moiety. In this regard, we also prepared selectively porphyrin monomer–SWNT linked systems with a rigid, short p -phenylene linker by the corresponding two-step reaction: initial addition reaction of the aryl radicals without the porphyrin moiety, followed by the low-yield Suzuki coupling of the porphyrin to the grafted aryl moiety. − They revealed that the photoexcitation of the porphyrin monomer on SWNT yielded an exciplex state, possessing partial positive and negative charges located on the porphyrin and SWNT, respectively. The exciplex rapidly decayed to the ground state without forming the complete charge-separated state.…”

Exciplex emissions derived from exceptionally long-distance donor and acceptor molecules

“…For exciplex formation, the distance between electron-donor and electron-acceptor molecules has to be very small, on the order of a few nanometers at most. [11][12][13] However, Nakanotani et al observed exciplex emissions derived from long-distance coupled donor and acceptor molecules with a 10 nm thick spacer layer, 14 and then, Chapran et al, Li et al, and Sych et al also observed similar exciplex emissions with a several nanometer spacer layer, using different types of donor and acceptor molecules. [15][16][17] Colella et al reported that donor and acceptor molecules diluted in a host matrix (i.e.…”

Unravelling Electroplex Emission from Long-Range Charge Transfer Based on a Phosphorescent Dendrimer as the Electron Donor

“…Regarding the electroplex, the decay of the CT excited states only takes place until holes and electrons are, respectively, injected into the donor and acceptor . In general, for both the exciplex and the electroplex, the interaction between donor–acceptor pairs requires intimate contact with each other, usually limited to the distance of a few nanometers. , Recently, it has been proven that an exciplex can be generated from long-range CT interactions, − even up to 70 nm . However, the long-range CT-induced electroplex has never been disclosed.…”