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The ability to access panchromatic absorption and long-lived charge-transfer (CT) excited states is critical to the pursuit of abundant-metal molecular photosensitizers. Fe(II) complexes supported by benzannulated diarylamido ligands have been reported to broadly absorb visible light with nanosecond CT excited state lifetimes, but as amido donors exert a weak ligand field, this defies conventional photosensitizer design principles. Here, we report an aerobically stable Fe(II) complex of a phenanthridine/quinoline diarylamido ligand, Fe(ClL)2, with panchromatic absorption and a 3 ns excited-state lifetime. Using X-ray absorption spectroscopy (XAS) and resonant inelastic X-ray scattering (RIXS) at the Fe L-edge and N K-edge, we experimentally validate the strong Fe-Namido orbital mixing in Fe(ClL)2 responsible for the panchromatic absorption and demonstrate a previously unreported competition between ligand-field strength and metal-ligand (Fe-Namido) covalency that stabilizes the 3CT state over the lowest energy triplet metal-centered (3MC) state in the ground-state geometry. Single-crystal X-ray diffraction (XRD) and density functional theory (DFT) suggest that formation of this CT state depopulates an orbital with Fe-Namido antibonding character, causing metal-ligand bonds to contract and accentuating the geometric differences between CT and MC excited states. These effects diminish the driving force for electron transfer to metal-centered excited states and increase the intramolecular reorganization energy, critical properties for extending the lifetime of CT excited states. These findings highlight metal-ligand covalency as a novel design principle for elongating excited state lifetimes in abundant metal photosensitizers.

In this work, the spatially dependent recombination kinetics of mixed-halide hybrid perovskite CH3NH3Pb(Br1–xClx)3 (0 ≤ x ≤ 0.19) single crystals are investigated using time-resolved photoluminescence spectroscopy with one- and two-photon femtosecond laser excitation. The introduction of chloride by substituting a fraction of the bromide leads to a decreased lattice constant compared to pure bromide perovskite (x = 0) and a higher concentration of surface defects. The measured kinetics under one-photon excitation (1PE) shows that increasing the chloride addition quenches the photoluminescence (PL) lifetimes, due to substitution-induced surface defects. In stark contrast, upon 2PE, the PL lifetimes measured deeper in the bulk become longer with increasing chloride addition, until the halide substitution reaches the critical concentration of ∼19%. At x = 19% Cl concentration, a significant reversal of this behavior is observed indicating a change in crystal structure beyond the continuous trends observed at...

Electromigration of nanoparticles (NPs) is relevant to many technological and biological applications. We correlate the experimentally observed electromigration of Au NPs with a closed-form theoretical model that furnishes key NP characteristics, including the previously unknown values of Au NP core ζ-potential, PEG-corona permeability, and particle-hydrogel friction coefficient. More generally, the theory furnishes new understanding of NP electromigration in complex environments, establishing a robust and predictive model to guide the design and characterization of functionalized NPs.

Cu(2-x)S (x = 1, 0.2, 0.03) nanocrystals were synthesized with three different chemical methods: sonoelectrochemical, hydrothermal, and solventless thermolysis methods in order to compare their common optical and structural properties. The compositions of the Cu(2-x)S nanocrystals were varied from CuS (covellite) to Cu(1.97)S (djurleite) through adjusting the reduction potential in the sonoelectrochemical method, adjusting the pH value in the hydrothermal method and by choosing different precursor pretreatments in the solventless thermolysis approach, respectively. The crystallinity and morphology of the products were characterized by X-ray diffraction (XRD) and transmission electron microscopy (TEM), which shows that most of them might be of pure stoichiometries but some of them are mixtures. The obtained XRDs were studied in comparison to the XRD patterns of previously reported Cu(2-x)S. We found consistently that under ambient conditions the copper deficient Cu(1.97)S (djurleite) is more stable than Cu(2)S (chalcocite). Corroborated by recent computational studies by Lambrecht et al. and experimental work by Alivisatos et al. This may be the reason behind the traditionally known instability of the bulk Cu(2)S/CdS interface. Both Cu(2)S and the copper-deficient Cu(1.97)S have very similar but distinguishable electronic and crystal structure. The optical properties of these Cu(2-x)S NCs were characterized by UV-vis spectroscopy and NIR. All presented Cu(2-x)S NCs show a blue shift in the band gap absorption compared to bulk Cu(2-x)S. Moreover the spectra of these Cu(2-x)S NCs indicate direct band gap character based on their oscillator strengths, different from previously reported experimental results. The NIR spectra of these Cu(2-x)S NCs show a carrier concentration dependent plasmonic absorption.

CH3NH3PbI3 perovskite layered films deposited on substrates with and without a titania support structure have been prepared and studied using time-resolved femtosecond transient absorption (fs-TA) spectroscopy in the visible light range (450-800 nm). The electron injection dynamics from the photoexcited perovskite layers to the neighboring film structures could be directly monitored via the transient bleaching dynamics of the perovskite at ∼750 nm and thus systematically studied as a function of the layer-by-layer architecture. In addition, for the first time we could spectrally distinguish transient bleaching at ∼750 nm from laser-induced fluorescence that occurs red-shifted at ∼780 nm. We show that an additional bleach feature at ∼510 nm appears when PbI2 is present in the perovskite film. The amplitudes of the PbI2 and perovskite TA peaks were compared to estimate relative amounts of PbI2 in the samples. Kinetic analysis reveals that perovskite films with less PbI2 show faster relaxation rates than those containing more PbI2. These fast dynamics are attributed to charge carrier trapping at perovskite grain boundaries, and the slower dynamics in samples containing PbI2 are due to a passivation effect, in line with other recently reported work.

The primary photochemistry of the trans isomer of a simple merocyanine dye of the stilbazolium betaine type 1-methyl-4-(4‘-hydroxytyryl)pyridinium betaine (Mtrans) and its conjugate acid MH+trans in aqueous solution is studied by femtosecond time-resolved pump probe spectroscopy. The measured rate of the primary photodynamics is determined to be k = 1.1 × 1012 s-1 for Mtrans at pH 10 and 0.8 × 1012 s-1 for MH+trans at pH 6. This was assigned to either conformational changes or a simple vibrational relaxation before the actual isomerization takes place. Wavelength excitation studies give support for the former assignment. These results are discussed in terms of the recent results found for the primary processes of retinal in bacteriorhodopsin. Time-resolved transient measurements show that no excited-state deprotonation of MH+trans occurs in aqueous solutions at pH 6 or pH 0, suggesting that the deprotonation occurs on a longer time scale than the picosecond time domain. This is in agreement with present t...

Flash photolysis of 1H-benzotriazole (1) has shown that N−N bond fission occurs within a few picoseconds of excitation (248 nm) giving 6-diazo-2,4-cyclohexadien-1-imine (2), λmax = 430 nm. Fluorescence and intersystem crossing do not compete measurably, and photodenitrogenation is a minor side reaction (

Supramolecular photoinduced electron transfer dynamics between coumarin 153 (C153) and 4,4'-dimethyl viologen dichloride (MV(2+)) across the molecular barrier of a host molecule, octa acid (OA), has been investigated with femtosecond time resolution. The ultrafast electron transfer from C153 to MV(2+) followed excitation with 150 fs laser pulses at a wavelength of 390 nm despite the fact that C153 was incarcerated within an OA(2) capsule. As a result, the photoexcited coumarin did not show any of the typical relaxation dynamics that is usually observed in free solution. Instead, the excited electron was transferred across the molecular wall of the capsuleplex within 20 ps. Likewise, the lifetime of the charge transfer state was short (724 ps), and electron back-transfer reestablished the ground state of the system within 1 ns, showing strong electronic coupling among the excited electron donor, host, and acceptor. When the donor was encapsulated into the host molecule, the electron transfer process showed significantly accelerated dynamics and essentially no solvent relaxation compared with that in free solution. The study was also extended to N-methylpyridinium iodide as the acceptor with similar results.

A highly efficient drug vector for photodynamic therapy (PDT) drug delivery was developed by synthesizing PEGylated gold nanoparticle conjugates, which act as a water-soluble and biocompatible "cage" that allows delivery of a hydrophobic drug to its site of PDT action. The dynamics of drug release in vitro in a two-phase solution system and in vivo in cancer-bearing mice indicates that the process of drug delivery is highly efficient, and passive targeting prefers the tumor site. With the Au NP-Pc 4 conjugates, the drug delivery time required for PDT has been greatly reduced to less than 2 h, compared to 2 days for the free drug.

CdSe quantum dot (QD)-phthalocyanine (Pc) conjugates were prepared as energy transfer donor-acceptor pairs, and the efficiency of the energy transfer process in this system was investigated as a function of QD size and under different surface chemistry conditions. The kinetics and efficiency of the energy transfer process were studied by femtosecond time-resolved laser spectroscopy. We observed that the energy transfer efficiency does not follow a linear dependence on spectral overlap integrals as predicted by the Forster theory for molecules. This observation is found to be due to the involvement of QD surface states in the energy transfer process from the photoexcited QDs to the molecular energy acceptor.

The effect of linker chain length on the energy transfer from CdSe quantum dots (QDs) to silicon phthalocyanine (Pc) photodynamic therapy agents was investigated by steady-state and femtosecond time-resolved spectroscopy with 500 nm light for the specific excitation of the QD energy donor. The conjugation between the QD and the Pc was achieved with linker chains varying from 4 to 9 bond lengths by incorporating 1−6 methylene groups into the axial ligand of the Pc. With increasing chain length, the energy-transfer efficiency increased, which appears to be opposed to a purely Forster-type resonance energy-transfer behavior that is commonly discussed for the energy transfer in QD conjugates. The obtained results provide strong evidence for a capping-layer-mediated energy transfer in the QD-based donor−acceptor conjugates.

Investigation of the growth of CdSe nanocrystals ( approximately 160 atoms) to the uniquely stable size of 2 nm allows the monitoring of the crystallization process in semiconductor quantum dots. By using a combination of optical techniques, high-resolution transmission electron microscopy (HRTEM), and powder X-ray diffractometry (XRD), new phenomena were explored during the CdSe nanocrystal growth process, which involved significant morphological reconstruction and crystallization of the initially formed amorphous nanoparticles. During the crystallization, the absorption onset of the CdSe quantum dots blue shifted toward higher energies at 3 eV (414 nm), while the photoluminescence red shifted to lower energies. Furthermore, an apparent increasing Stokes shift was observed during the formation of small CdSe nanoparticles. On the other hand, the photoluminescence excitation spectra showed constant features over the reaction time. Additionally, results from HRTEM and XRD studies show that the CdSe nanoparticles were amorphous at early reaction stages and became better crystallized after longer reaction times, while the particle size remained the same during the crystallization process. These observations demonstrate the important role of the surface on the optical properties of small CdSe quantum dots and facilitated the spectroscopic monitoring of the crystallization process in quantum dots.

Single and multijunction nanowires with diameters of 10-40 nm were synthesized by an electrically assisted sonochemical method in aqueous solution at room temperature. By properly controlling the reaction conditions and reactants, this method opens a new strategy towards the fabrication of one-dimensional superlattices in nanowires under ambient conditions. Such a chemical approach provides a promising and simple way to synthesize high-performance building blocks for device applications.

The applicability of semiconductor QDs in photodynamic therapy (PDT) was evaluated by studying the interaction between CdSe QDs with a known silicon phthalocyanine PDT photosensitizer, Pc4. The study revealed that the QDs could be used to sensitize the PDT agent through a fluorescence resonance energy transfer (FRET) mechanism, or interact directly with molecular oxygen via a triplet energy-transfer process (TET). Both mechanisms result in the generation of reactive singlet oxygen species that can be used for PDT cancer therapy.

The initial relaxation dynamics of the photoexcited fullerenes C60, C70, C76, C84, C86, and C90 were investigated by dispersion-free femtosecond pump−probe spectroscopy. Under identical excitation conditions, the formation of the lowest excited state slows down for the larger fullerenes. This trend in dynamics, monitored throughout the visible and NIR range, is found to correlate with the number of isomers in accordance with the isomerization mechanism suggested by Stone and Wales. The Stone−Wales isomerization was calculated as thermally inaccessible but photoinduced barrierless. The energy difference of the isomers is in the 1 meV range, and back-isomerization is observed on the picosecond time scale. The characteristic spectrally broad transient absorption of the investigated fullerenes is promising for fast optical gating applications.

This study presents the potential of quantum dots as two-photon sensitizers for attached energy acceptors. With two-photon absorption cross sections, which are orders of magnitudes higher than those of any molecule, quantum dots constitute outstanding NIR sensitizers. The successful energy transfer to conjugated phthalocyanines was demonstrated using two-photon excitation of the quantum dots with a 1100 nm wavelength and was monitored with femtosecond time-resolved pump−probe spectroscopy.