An essential challenge in the development of nanosized metal organic framework (nanoMOF) materials in biomedicine is to develop a strategy to stabilize their supramolecular structure in biological media while being able to control drug encapsulation and release. We have developed a method to efficiently encapsulate topotecan (TPT, 1), an important cytotoxic drug, in biodegradable nanoMOFs. Once inside the pores, 1 monomers aggregate in a "ship in a bottle" fashion, thus filling practically all of the nanoMOFs' available free volume and stabilizing their crystalline supramolecular structures. Highly efficient results have been found with the human pancreatic cell line PANC1, in contrast with free 1. We also demonstrate that one- and two-photon light irradiation emerges as a highly promising strategy to promote stimuli-dependent 1 release from the nanoMOFs, hence opening new standpoints for further developments in triggered drug delivery.

The photochromic and thermochromic behavior of four commercially available Reversacol dyes are presented. The compounds studied belong to the class of spirooxazines and naphthopyrans, which are typically thermoreversible photochromic molecules. On stimulation with UV light, these compounds become colored and exhibit spectra which extend over the whole visible region. Increasing the temperature causes spontaneous coloration (thermochromism). Herein, absorption and fluorescence spectra, molar absorption coefficients of the colorless and colored forms, fluorescence and photochemical quantum yields, and kinetic parameters of thermal bleaching (rate constant, frequency factor, and activation energy) are determined in acetonitrile solution. The thermal ground-state reaction is exhaustively described in terms of thermodynamic parameters (equilibrium constant, free energy, enthalpy, and entropy). Temperature effects on photochemical and thermal colorabilities are evaluated. The results indicate that the two spirooxazines are good photochromes below room temperature, whereas they are efficient thermochromic compounds above room temperature. Naphthopyrans are better photochromes but worse thermochromic compounds than spirooxazines.

We report on the role of H-bonding interactions on the UV-visible absorption and emission (steady-state and time-resolved) spectroscopy of topotecan (TPT) in solution. In aprotic solvents, a very fast (less than 10 ps) excited-state intramolecular proton-transfer reaction occurs in the absorbing enol (E) form to give a zwitterion (Z) form, emitting with a large Stokes shift. In protic solvents like methanol, the time constant of Z* formation is longer (32 ps) due to the participation of solvent molecules in the proton-transfer reaction. In aqueous solution at near-neutral pH (6.24), a ground-state equilibrium is established between E, cation (C), and Z forms. Direct excitation of E leads to Z* through two channels: a very fast one (less than 10 ps) involving an intramolecular proton-tranfer and a slower one (680 ps) with the C* intermediate formation and reaction. A fast (42 ps) deprotonation of E* to give the excited anion (A*) also competes with the photoformation of Z* at the S(1) state. At pH =12.15, the A structures are the principal emitting species (τ(A) ~ 0.41 ns), showing the largest Stokes shift. In aqueous solutions, we cannot exclude the existence of an equilibrium between the lactone and carboxylate forms of TPT, whose spectroscopic (absorption and emission spectra) and dynamical behaviors should not be very different. Time-resolved emission anisotropy measurements in solvents of different viscosities suggest that the rotational relaxation time (φ) of TPT is mainly governed by the viscosity of the medium, increasing from 104 ps (in tetrahydrofuran, THF) to 156 ps (in water) and 338 ps (in dimethyl sulfoxide, DMSO). These results give spectroscopic and dynamical information on the structures, stability, and dynamics (picosecond to nanosecond time scale) of TPT in solution. They provide insights on the role of the intermolecular H-bonding surrounding medium on the ground- and excited-state structure and reaction of TPT. The finding should contribute to a better understanding of the relationship between the structures of the drug and its surroundings.

In this paper, we report on the absorption, emission, and photodynamics of a push−pull zinc porphyrin (YD2-o-C8) in solution and adsorbed on Al 2 O 3 (alumina) nanoparticles (NPs). The shift in the absorption and emission spectra for the dye adsorbed on alumina NPs with respect to the solution suggests the presence of an anchoring effect of the dye to the NPs' surface. Indeed, different molecular populations (monomers and aggregates) coexist for the solid film, as it is confirmed by both steady-state and timeresolved measurements. The emission decays, while monoexponential in acetonitrile (ACN) solution (τ = 1.5 ns) become multiexponential (140 and 550 ps) for the dye interacting with the alumina NPs. These lifetime values increase (210 and 930 ps) upon addition of CDCA (a disaggregating agent). The deactivation (vibrational cooling) of the hot S 1 to cold S 1 state, which occurs in 4−6 ps in ACN, shows a biexponential behavior in the solid state (2 ps, 20 ps). These two ps components, whose values upon excitation at 460 nm do not change with respect to those observed without CDCA, become longer in the presence of the coadsorbent and exciting at 640 nm (4 ps, 50−60 ps), where the aggregates mostly absorb. The charge separation (2 ps in ACN, 15 ps in toluene) becomes faster in the solid state in absence of CDCA (∼500 and 110 fs exciting at 460 and 640 nm, respectively), while, by adding the coadsorbent, the process occurs on a time scale that is below our time-resolution (<50 fs). The obtained results are of potential interest to understand the dynamics of the porphyrin used in the up-to-now world-record-efficient dye-sensitized solar cell (DSSC).

In this work the photochromic behaviors of four Reversacol compounds from James Robinson Ltd., previously investigated in solution, have been studied in a microcrystalline solid phase and embedded in a poly(methyl methacrylate) matrix (PMMA). The compounds studied belong to the classes of spirooxazines and naphthopyrans. In solution, they showed photocoloration and thermocoloration. Embedded into polymeric films, as well as in the microcrystalline phase, they have maintained their photochromic and thermochromic behavior. The photocoloration quantum yields in PMMA were determined and found to be fairly high. Compared to solution, the kinetic and thermodynamic activation parameters of thermal bleaching significantly changed. The solid environment has the effect of slowing down the bleaching rates in films as well in the pure crystalline phase and introduces multiexponential terms in the photocoloration and thermal bleaching kinetics. Activation enthalpy and entropy decrease, compared to solution, leaving the activation free energy substantially unchanged. Thermochromism was characterized through the equilibrium constant and the thermodynamic standard parameters, ∆H 0 , ∆S 0 , and ∆G 0 , of the thermal reactions. Time-dependent processes were also analyzed according to the maximum entropy method (MEM). Based on the data from the leastsquares and MEM treatments, the occurrence of multiexponential kinetics in polymers was attributed to the effects of the inhomogeneous distribution of free volume in the matrix.