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Monitoring the biological fate of medicaments within the environments of cancer cells is an important challenge which is nowadays the object of intensive studies. In this regard, rhodamine-based supramolecular systems are one of the most suitable probes used in drug delivery thanks to their high emission quantum yield and sensitivity to the environment which helps to track the medicament in real time. In this work, we used steady-state and time-resolved spectroscopy techniques to investigate the dynamics of the anticancer drug, topotecan (TPT), in water (pH ~6.2) in the presence of a rhodamine-labeled methylated ß-cyclodextrin (RB-RM-ßCD). A stable complex of 1:1 stoichiometry is formed with a Keq value of ~4 × 104 M-1 at room temperature. The fluorescence signal of the caged TPT is reduced due to: (1) the CD confinement effect; and (2) a Förster resonance energy transfer (FRET) process from the trapped drug to the RB-RM-ßCD occurring in ~43 ps with 40% efficiency. These findings provide additional knowledge about the spectroscopic and photodynamic interactions between drugs and fluorescent functionalized CDs, and may lead to the design of new fluorescent CD-based host-guest nanosystems with efficient FRET to be used in bioimaging for drug delivery monitoring.

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Hydrogen-bonded organic frameworks (HOFs) have attracted renewed attention as another type of promising candidates for functional porous materials. In most cases of HOF preparation, the applied molecular design principle is based on molecules with rigid π-conjugated skeleton together with more than three H-bonding groups to achieve 2D- or 3D-networked structures. However, the design principle does not always work, but results in formation of unexpected structures, where subtle structural factors of which we are not aware dictate the entire structure of HOFs. In this contribution, we assess recent advances in HOFs, focusing on those composed of hexatopic building block molecules, which can provide robust frameworks with a wide range of topologies and properties. The HOFs described in this work are classified into three types, depending on their H-bonded structural motifs. Here in, we focus on: (1) the chemical aspects that govern their unique fundamental chemistry and structures; and (2) their photophysics at the ensemble and single-crystal levels. The work addresses and discusses how these aspects affect and orient their photonic applicability. We trust that this contribution will provide a deep awareness and will help scientists to build up a systematic series of porous materials with the aim to control both their structural and photodynamical assets.

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In this contribution, we report on the solid-state-photodynamical properties and further applications of a low dimensional composite material composed by the luminescent trans-4-(dicyanomethylene)-2-methyl-6-(4-dimethylaminostyryl)-4H-pyran (DCM) dye interacting with a two-dimensional-metal organic framework (2D-MOF), Al-ITQ-HB. Three different samples with increasing concentration of DCM are synthesized and characterized. The broad UV-visible absorption spectra of the DCM/Al-ITQ-HB composites reflect the presence of different species of DCM molecules (monomers and aggregates). In contrast, the emission spectra are narrower and exhibit a bathochromic shift upon increasing the DCM concentration, in agreeance with the formation of adsorbed aggregates. Time-resolved picosecond (ps)-experiments reveal multi-exponential behaviors of the excited composites, further confirming the heterogeneous nature of the samples. Remarkably, DCM/Al-ITQ-HB fluorescence is sensitive to vapors of electron donor aromatic amine compounds like aniline, methylaniline, and benzylamine due to a H-bonding-induced electron transfer (ET) process from the analyte to the surface-adsorbed DCM. These findings bring new insights on the photobehavior of a well-known dye when interacting with a 2D-MOF and its possible application in sensing aniline derivatives.

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Micro- and mesoporous silica-based materials are a class of porous supports that can encapsulate different guest molecules. The formation of these hybrid complexes can be associated with significant alteration of the physico-chemical properties of the guests. Here, we report on a photodynamical study of a push⁻pull molecule, trans-4-(dicyanomethylene)-2-methyl-6-(4-dimethylaminostyryl)-4H-pyran (DCM), entrapped within faujasite-type zeolites (HY, NaX, and NaY) and MCM-41 in dichloromethane suspensions. The complex formation gives rise to caged monomers and H- and J-aggregates. Steady-state experiments show that the nanoconfinement provokes net blue shifts of both the absorption and emission spectra, which arise from preferential formation of H-aggregates concomitant with a distortion and/or protonation of the DCM structure. The photodynamics of the hybrid complexes are investigated by nano- to picosecond time-resolved emission experiments. The obtained fluorescence lifetimes are 65⁻99 ps and 350⁻400 ps for H- and J-aggregates, respectively, while those of monomers are 2.46⁻3.87 ns. Evidences for the presence of a charge-transfer (CT) process in trapped DCM molecules (monomers and/or aggregates) are observed. The obtained results are of interest in the interpretation of electron-transfer processes, twisting motions of analogues push⁻pull systems in confined media and understanding photocatalytic mechanisms using this type of host materials.

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This work reports on photophysical studies of the irinotecan (IRT) anti-cancer drug in water solutions of different acidities (pH = 1.11-9.46). We found that IRT co-exists as mono-cationic (C1), di-cationic (C2), or neutral (N) forms. The population of each prototropic species depends on the pH of the solution. At pH = 1.11-3.01, the C1 and C2 structures are stabilized. At pH = 7.00, the most populated species is C1, while at pH values larger than 9.46 the N form is the most stable species. In the 1.11-2.61 pH range, the C1* emission is efficiently quenched by protons to give rise to the emission from C2*. The dynamic quenching constant, KD, is ∼32 M-1. While the diffusion governs the rate of excited-state proton-transfer (ESPT) under these conditions, the reaction rate increases with the proton concentration. A two-step diffusive Debye-Smoluchowski model was applied at pH = 1.11-2.61 to describe the protonation of C1*. The ESPT time constants derived for C1* are 382 and 1720 ps at pH = 1.11 and 1.95, respectively. We found that one proton species is involved in the protonation of C1* to give C2*, in the analyzed acidic pH range. Under alkaline conditions (pH = 9.46), the N form is the most stable structure of IRT. These results indicate the influence of the pH of the medium on the structural and dynamical properties of IRT in water solution. They may help to provide a better understanding on the relationship between the structure and biological activity of IRT.

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We report on time-resolved fluorescence anisotropy studies of poly(9-vinylcarbazole) (PVK) nanoparticles (NPs) encapsulating Coumarin 153 (C153) and Nile Red (NR). The wobbling-in-a-cone model successfully describes the restricted movements of the encapsulated molecules. For C153-doped PVK NPs, when increasing the C153 content, the diffusional relaxation (τD) times become shorter (τD = 4.6-1.6 ns), following its increased preference for less rigid environments. On the other hand, for NR, τD is affected by the dopant content (τD = 12.2-3.09 ns), thus suggesting a more rigid environment, which is in agreement with its higher ability to interact with the polymer chains. For the two-dye-doped PVK NPs, where the content of one of the trapped guests is kept fixed while varying the concentration of the other one slows down (NR; τR = 0.30-0.37 ns and τD = 3.09-7 ns) or accelerates (C153: τR = 0.14-0.03 ns and τD = 1.57-0.69 ns). This suggests that the guest molecules assume different positions, with C153 being preferentially in the less rigid environment closer to the NP surface, while NR is located in the more rigid ones, closer to the core. We suggest that the dye distribution within PVK NPs is governed by the combination of the Marangoni effect and the consecutive particle swelling. Furthermore, in the two-dye-doped systems, competition for the available less rigid sites is observed upon an increase in the C153 co-dopant content, while in the case of NR as the variable co-dopant this effect is smaller. These findings are relevant for improving our knowledge for a better design of nanophotonic devices based on dye-doped polymer NPs.

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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.

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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.

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In this work, we report on photophysical studies of the anticancer drug topotecan (TPT) in aqueous solutions at different pHs. We used steady-state (UV-visible absorption and emission) and time-resolved picosecond (ps) emission spectroscopies to investigate the role of the H-bonding interactions as well as the proton concentration (pH = 0.48-7.40) on the behavior of topotecan (TPT) in its ground- (S0) and electronically first (S1) excited-states. At physiological conditions (pH = 7.40), the drug exists at S0 in equilibrium between the enol (E), cation (C), and zwitterion (Z) forms. The photoformation of Z* (τZ = 5.83 ns) occurs from directly excited (λexc = 371 nm) E as the two-step reaction: E*→C*→Z*. In this process, a very fast (less than 10 ps) protonation of E* leads to C*, which subsequently undergoes fast (580 ps) deprotonation to give Z* as the final photoproduct. At higher proton concentrations (pH = 0.48-1.31), a ground-state equilibrium exists between three different cationic species (C1, C2, and C3). The proton motion from the acidic solution to the C forms of TPT to give the reactions C1*→C2*→C3* is governed by the proton diffusion. In these conditions, both dynamic and static quenching occurs. The rate constant values k*(DPT1) and k*(DPT2) for the direct protonation of C1* and C2*, respectively, depend on the pH of the surrounding. The number of protons implicated in the reaction changes from ∼2 (pH = 0.48-0.78) to ∼1 (pH = 0.78-1.31), thus indicating the existence of two different reactions and proton-transfer dynamics. These results evidence the conformational, structural, and dynamical changes of aqueous solutions of TPT with the pH of the environment. They should help to understand the molecular structure/activity of TPT at cellular level.

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Steady-state and time-resolved picosecond emission studies were carried out to study the role of the proton concentration in the acid-base properties of the anticancer drug camptothecin (CPT) in its ground and electronically first excited states. The results show that, under acidic conditions, the excited-state proton-transfer (ESPT) reaction is irreversible, in contrast to previous literature data. We found that the prototropic species are equilibrated at the excited state (pK(a)* = 1.85) only in a restricted range of pH (1.5 < pH < 3), whereas only one species, either the neutral form (τ(N) = 3.76 ns) or the protonated form (τ(C) = 2.83 ns), can be detected at pH > 3 and pH < 1.5, respectively. The proton motion from the acidic solution to the neutral form in the pH 1-2 domain is diffusion-controlled. Within the range of pH 1-2, the reaction rate constant for the formation (k(d)) of the encounter complex between the proton and the neutral form ranges from 1.17 × 10(10) to 7.33 × 10(10) M(-1) s(-1), respectively. Under more acidic conditions (pH 0.9-0.95), the protonation of CPT does not depend on the diffusive step, because of the large amount of protons. The direct proton-transfer rate constant (k(DPT)*) increases with the proton concentration (time constants change from 24 ps to ∼1 ns at pH 0.9 and 2, respectively). The number of protons involved in the proton transfer changes from approximately one, for the diffusive regime, to approximately four, for the static regime. We found good agreement between the Birks model for equilibrated flourophores and the Debye-Smoluchowski equation (DSE) to accurately explain the ESPT reaction of CPT with acidic water in the reversible range. The proton motion at pH 2 (equilibrium range) exhibits diffusion-controlled behavior and can be explained using the Smoluchowski model. Our results show that the interaction of CPT with acidic water depends on the concentration of the acid, which changes the nature of both the structure and dynamics.

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A great variety of technological applications makes photochromism a currently appealing theme for basic studies. In this work, excited state dynamics of two spirooxazines and two naphthopyrans, that upon UV irradiation undergo thermally reversible conversion to coloured photomerocyanines, have been investigated by using pump-probe techniques (femtosecond time resolution). The breakage of the C-O bond, involved in the photoreaction, has been found to occur within a few hundreds of femtoseconds producing a first transient that evolved on picosecond time-scale to the most stable isomer through a number of intermediates that depended on the solvent and the structure of the photochrome. The peculiar behaviour of one of the molecules studied (1,3-dihydro-3,3-dimethyl-1-isobutyl-6'-(2,3-dihydro-1H-indol-1-yl)spiro [2H-indole-2,3'-3H-naphtho[2,1-b][1,4]oxazine]) has been investigated in depth in various media because it revealed an unusual dual photochemistry pathway. This finding is traced to reactivity of π,π* and ICT excited states whose relative populations are controlled by the polarity of the solvent.

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In this article, we report a study on the singlet and triplet excited-state properties of a spirooxazine (1,3-dihydro-3,3-dimethyl-1-isobutyl-6'-(2,3-dihydro-1H-indol-1-yl)spiro[2H-indole-2,3'-3H-naphtho[2,1-b][1,4]oxazine]). The singlet state of this molecule is photoreactive: upon UV light stimulation, it produces a colored merocyanine that thermally reverts to the starting compound. A double-way radiative relaxation path was found for singlet-state excitation. Experimental observations on the absorption and fluorescence spectra were in excellent agreement with TD-DFT calculations for the singlet state. The triplet state, which could not be directly populated by intersystem crossing from the singlet, when reached by energy transfer from a suitable sensitizer (camphorquinone), yielded the colored merocyanine with quantum yield close to unity. However, the donor/acceptor interaction also originated a new photochromic system as a consequence of the competition of hydrogen abstraction with energy transfer in the interplay of the sensitizer with the substrate. The newly produced photochrome was structurally, spectrally, and photochemically characterized. It exhibited excellent colorability in both directly excited and triplet-sensitized photoreactions by virtue of high photoreaction quantum yield and rather slow bleaching rate of the colored form but also underwent significant degradation in the presence of oxygen that led to the destruction of the photochromic functionality.

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