The Stability of Cyclodextrin Complexes in Solution.

We present measurements of the photoexcited states of poly(p-phenylene vinylene), PPV, and a model oligomer, trans,trans-distyrylbenzene, probed by detection of photoluminescence and photoinduced absorption. Photoluminescence is strong in both materials and due to radiative recombination of the singlet exciton. Triplet excitons are also photogenerated, and in the case of the oligomer are seen directly in light-induced ESR measurements. We find a signal with a peak-to-peak width of 12 G at 1580 G for a probe frequency of 9.14 GHz. We assign this to the ``forbidden'' \ensuremath{\Delta}${\mathit{m}}_{\mathit{s}}$=2 transition between the triplet sublevels. From the position of the minimum field ${\mathit{H}}_{\mathrm{min}}$ for this microwave transition, we determine the zero-field splitting of the triplet sublevels to be 0.07 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$. We find an associated photoinduced absorption at 1.95 eV in the oligomer which we attribute to a triplet-triplet transition of the excited state. For the polymer we find a photoinduced absorption response at 1.45 eV, very similar to that of the oligomer, and we consider that this is similarly due to a triplet exciton, although we have not been able yet to find direct ESR evidence. In the case of the polymer, charged photoexcitations are also seen in photoinduced absorption measurements, characteristic of bipolarons, with induced absorption at 0.6 and 1.6 eV. We do not find similar excitations in the oligomer, and we consider that intermolecular charge transport is necessary for the generation of the long-lived charged excitations probed in our experiments.

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Photoexcited states in poly(p-phenylene vinylene): Comparison with trans,trans-distyrylbenzene, a model oligomer.

Intersystem crossing involving photogenerated strongly spin exchange-coupled radical ion pairs in a series of donor-bridge-acceptor molecules was examined. These molecules have a 3,5-dimethyl-4-(9-anthracenyl)-julolidine (DMJ-An) donor either connected directly or connected by a phenyl bridge (Ph), to pyromellitimide (PI), 1 and 2, respectively, or naphthalene-1,8:4,5-bis(dicarboximide) (NI) acceptors, 3 and 4, respectively. Femtosecond transient optical absorption spectroscopy shows that photodriven charge separation produces DMJ(+•)-PI(-•) or DMJ(+•)-NI(-•) quantitatively in 1-4 (τ(CS) ≤ 10 ps), and that charge recombination occurs with τ(CR) = 268 and 158 ps for 1 and 3, respectively, and with τ(CR) = 2.6 and 10 ns for 2 and 4, respectively. Magnetic field effects (MFEs) on the neutral triplet state yield produced by charge recombination were used to measure the exchange coupling (2J) between DMJ(+•) and PI(-•) or NI(-•), giving 2J > 600 mT for 1-3 and 2J = 170 mT for 4. Time-resolved electron paramagnetic resonance (TREPR) spectroscopy revealed that the formation of (3)*An upon charge recombination occurs by spin-orbit charge transfer intersystem crossing (SOCT-ISC) and/or radical-pair intersystem crossing (RP-ISC) mechanisms with the magnitude of 2J determining which triplet formation mechanism dominates. SOCT-ISC is the exclusive triplet formation mechanism in 1-3, whereas both RP-ISC and SOCT-ISC are active for 4. The triplet sublevels populated by SOCT-ISC in 1-4 depend on the donor-acceptor geometry in the charge separated state. This is consistent with the fact that the SOCT-ISC mechanism requires the relevant donor and acceptor orbitals to be nearly perpendicular, so that electron transfer results in a large orbital angular momentum change that must be compensated by a fast spin flip to conserve overall system angular momentum.

Intersystem crossing involving strongly spin exchange-coupled radical ion pairs in donor-bridge-acceptor molecules.

Photochemistry in Cyclodextrin Cavities

Steady state and time-resolved studies on the fluorescence of 4-aminophthalimide (AP) in aqueous β-cyclodextrin (CD) solutions are reported. A blue shift of the fluorescence maximum, enhancement of the fluorescence quantum yield and increase in the lifetime of the emitting intramolecular charge-transfer state are observed on addition of CD to an aqueous solution of AP. It is suggested that the elimination of water molecules from the surrounding of the fluorescent probe, as a result of encapsulation, leads to the changes in the photophysical properties. The formation constant of the complex is estimated from the pre-exponential factors of the biexponential decay of the fluorescence and is found to agree reasonably well with that obtained from fluorescence intensity measurements.

Yoshinori Yamamoto

Papers

Intermolecular energy transfer of the spin-polarized triplet state in a γ-cyclodextrin cavity

Intermolecular energy transfer of the spin polarized triplet state in frozen SDS micelles

Photosensitized triplet state of stilbene detected by a time-resolved ESR method