A Bis-Porphyrin Cavitand Breathing-In to Constrict Bucky Balls.

Bis-porphyrin cages have long been exploited to bind fullerenes selectively for various applications. The major consideration for an effective binding here had been the cavity size. Herein, we structurally demonstrate that a bis-Ni-porphyrin cavitand having even a smaller cavity can host a larger fullerene by a breathing and ruffling mechanism. It has also been shown that both the electronic and steric influence at the meso- positions of the porphyrin in fact dictate the binding character. The smaller cavity of 2NiD exhibits preferential binding for C70 over C60; however, surprisingly, the larger cavities in 2HD and 2NiTD display stronger affinities for C60 over the larger fullerene. We show here that the structural elasticity infused both by the metalloporphyrins and the connecting bridges play a major role in directing the binding. These conclusions have adequately been supported by structural and spectroscopic investigations. Additionally, the suitability of one of the conjugates for photoinduced charge-separation has been investigated using ultrafast transient absorption measurements. 2NiD⊃C60 has a charge separation timescale of ~0.8 ps, while charge recombination occurs at a longer timescale of ~920 ps.

Single- and two-photon-induced Förster resonance energy transfer in InP-mCherry bioconjugates.

Indium phosphide (InP) quantum dots (QDs) have recently garnered considerable interest in the design of bioprobes due to their non-toxic nature and excellent optical properties. Several attempts for the conjunction of InP QDs with various entities such as organic dyes and dye-labeled proteins have been reported, while that with fluorescent proteins remains largely uncharted. This study reports the development of a Förster resonance energy transfer pair comprising glutathione-capped InP/GaP/ZnS QDs [InP(G)] and the fluorescent protein mCherry. Glutathione on InP(G) undergoes effective bioconjugation with mCherry consisting of a hexahistidine tag, and the nonradiative energy transfer is investigated using steady-state and time-resolved measurements. Selective one-photon excitation of InP(G) in the presence of mCherry shows a decay of the emission of the QDs and a concomitant growth of acceptor emission. Time-resolved investigations prove the nonradiative transfer of energy between InP(G) and mCherry. Furthermore, the scope of two-photon-induced energy transfer between InP(G) and mCherry is investigated by exciting the donor in the optical transparency range. The two-photon absorption is confirmed by the quadratic relationship between the emission intensity and the excitation power. In general, near-infrared excitation provides a path for effective light penetration into the tissues and reduces the photodamage of the sample. The two-photon-induced energy transfer in such assemblies could set the stage for a wide range of biological and optoelectronic applications in the foreseeable future.

Non-condon Effect on Ultrafast Excited-State Intramolecular Proton Transfer.

The reaction dynamics of excited-state intramolecular proton transfer (ESIPT) of 2,2'-dihydroxyazobenzene (2,2'-DHAB) was investigated by means of white-light supercontinuum femtosecond transient absorption spectroscopy. A coherent in-phase oscillation was observed in the entire wavelength range where stimulated emission of the photoproduct is dominant. This result indicates that the transition strength of the product state is dynamically modulated by a nuclear wavepacket motion (non-Condon effect). The observed vibration was assigned to the mode which modulates the distance between oxygen and hydrogen atoms. By integrating the result of time-dependent density functional theory calculation, the origin of the non-Condon effect was attributed to a dynamical change of configuration interaction between enol and keto characters along the vibrational coordinate, indicating that this vibration is strongly related to the reaction coordinate of ESIPT.