Zeolites as Scaffolds for Metal Nanoclusters

This chapter critically reviews the studies related to structural and photophysical properties of metal clusters within zeolites matrices and summarizes the progress made in understanding the host-guest interactions. The goal is to provide useful insight into the nature of such interactions and experiments used in identifying the excited state dynamics and the reaction mechanisms leading to the emitting species. Especially interesting are the combined experimental and computational approaches used to elucidate the structures and electronic transition of clusters inside the cavity. Although a number of excellent research articles have been published in the last years they only cover rather specific areas like organic photochemistry, confinement, charge transfer, theoretical modeling or photostimulated luminescence

One-Directional Antenna Systems: Energy Transfer from Monomers to JAggregates within 1D Nanoporous Aluminophosphates

A cyanine dye (PIC) was occluded into two 1D-nanopoporus Mg-containing aluminophosphates with different pore size (MgAPO-5 and MgAPO-36 with AFI and ATS zeolitic structure types, with cylindrical channels of 7.3 Å diameter and elliptical channels of 6.7 Å × 7.5 Å, respectively) by crystallization inclusion method. Different J-aggregates are photophysically characterized as a consequence of the different pore size of the MgAPO frameworks, with emission bands at 565 nm and at 610 nm in MgAPO-5 and MgAPO-36, respectively. Computational results indicate a more linear geometry of the J-aggregates inside the nanochannels of the MgAPO-36 sample than those in MgAPO-5, which is as a consequence of the more constrained environment in the former. For the same reason, the fluorescence of the PIC monomers at 550 nm is also activated within the MgAPO-36 channels. Owing to the strategic distribution of the fluorescent PIC species in MgAPO-36 crystals (monomers at one edge and J-aggregates with intriguing emission properties at the other edge) an efficient and one-directional antenna system is obtained. The unidirectional energy transfer process from monomers to J-aggregates is demonstrated by remote excitation experiments along tens of microns of distance.Financial support from Gobierno Vasco (IT912-16) and Ministerio de Economía y Competitividad “MINECO” (through Projects MAT2014-51937-C3-3-P, MAT2016-77496-R and MAT-2015-65767-P) is acknowledged. R.S.L. and V.M.M. acknowledge niversidad del PaísVasco (UPV-EHU) for a postdoctoral fellowship and MINECO for a “Ramón y Cajal” Contract RYC-2011-09505), respectively. H.U. gratefully acknowledges the financial support of the European Research Council (#280064), the FWO (G056314N, G0B5514N, G081916N), and JSPS KAKENHI (JP17H03003, JP17H05244, JP17H05458). Centro Técnico de Informática (CSIC) is acknowledged for running the calculations and Accelrys for providing the computational softwar

Femtosecond transient absorption of donor-acceptor systems

The results presented in this thesis concern the photophysical properties of several perylenemonoimide functionalized penthaphenylene systems in solution. The experimental investigation of the excited state dynamics performed by means of the ultrafast laser spectroscopy revealed the occurrence of electron and energy transfer. These processes lead to the formation of excited state intermediates with distinct photophysical properties. In Chapter 1 theoretical aspects of photo-induced electron and excitation energy transfer are described with the focus on intramolecular interactions. The relevant concepts and equations related to electron transfer are briefly outlined to serve as a basis for interpretation. Particular attention is given to the Förster model for energy transfer with the theoretical investigations of dipole-dipole interaction and its boundary conditions. Chapter 2 is devoted to a brief literature review of the photophysical properties of a few rigid organic π-conjugated donor-acceptor systems. Several examples of dynamic processes taking place in π-conjugated systems were presented, including charge transport, charge recombination and various electronic interactions. The elucidation of the details of excited-state dynamics in molecular models is essential in understanding the photophysics of the natural and synthetic multichromophoric systems. Chapter 3 addresses the femtosecond spectrometer setup, detection principles and the different methods used to obtain the experimental data. The generation, amplification and conversion as parts of the femtosecond system are briefly outlined. It is shown that the improvements made to the apparatus have boosted its accuracy and flexibility as a spectroscopic tool for sensitive investigations including biological samples. The data analysis section receives a particular consideration due to the complexity of the data resulting from a substantial number of competing processes involved. A novel spectroscopic tool has been implemented by incorporating a third independent spectrally and temporally tunable femtosecond pulse in the conventional transient absorption. To achieve this, a part of the regenerative amplifier output was branched off and routed into a second OPA were the pulses were spectrally tuned to the desired wavelength. A second delay line was mounted at this OPA exit to allow for the necessary timing relative to the first excitation pulse. Successively, the pulse was adjusted in power, polarized under required orientation, routed and directed into the sample where all three pulses overlap. Collecting the data with the CCD camera ensures that within a single measurement the evolution of spectrally different excited state species are monitored at the same time and under the same conditions. Three-pulse transient absorption was used to intervene in the photoinduced dynamics of two molecular systems by modifying either the excited or ground state population. Chapter 4 presents an experimental and systematic study on time-dependent spectral properties of a rigid, extended system consisting of one [PI-(pPh)1-PI], two [PI-(pPh)2-PI] and three [PI-(pPh)3-PI] pentaphenylene units end-capped with perylenemonoimide. These systems are investigated in detail by femtosecond transient absorption and single photon timing experiments and compared to analogous model systems. The obtained results here are compared to those obtained for PI-(pPh)1 and PI-(pPh)2. Exciton-exciton annihilation occurs in both PI-(pPh)1-PI and PI-(pPh)2-PI systems investigated upon high power excitation. These results are in line with the kinetic results obtained for PI-(pPh)3-PI and show that the annihilation becomes faster and more efficient as the exciton coupling increases. The annihilation was found to promote one chromophore into a higher excited singlet state which then rapidly relaxes to S1 via a charge transfer state intermediate. Starting from the higher excited state the PI radical anion is formed even in a low polar environment and decays with a time constant of about 1 ps. The mechanism observed suggests an elegant way to explore reactions in the upper excited states, as in this case an ultrafast charge transfer occurred above the lowest singlet excited state. For the largest system PI-(pPh)3-PI, the experiments demonstrated that a CT-CT annihilation takes place after formation of two independent charge transfer-like states. This novel excited state interaction was observed in a solvent of medium polarity by comparing the transient decay traces recorded with five different excitation powers. Based on quantum-chemical calculations the PI excited state wavefunction was found to significantly spread over the neighboring pentaphenylene skeleton in polar solvents. This leads to a shorter center-to-center separation between the two PI transition dipoles. Both theoretical and experimental results furthermore yield a larger overlap between the excited state emission and absorption spectra upon increasing the solvent polarity. In Chapter 5 the excited state properties of PI-(pPh)1 and PI-(pPh)2 are explored using pump-re-pump-probe and pump-dump-probe transient absorption technique in the visible region. Although the data are complex, this allowed gaining new insights of the dynamics of the excited states. For instance, upon pre-exciting the PI subunit an additional relaxation pathway is revealed in the deactivation process of the pPh. By appropriate selection of wavelength and timing, the extra pulse initially promotes the energy acceptor into an excited state thereby opening a new pathway in transferring the excitation energy from the pPh to the excited state. The results presented demonstrate the complexity of the excited state properties and dynamics in perylene end-capped phentaphenylenes. These open the opportunity for studying other complex photophysical systems using similar experimental approaches.status: publishe

The BOPHY fluorophore with double boron chelation: Synthesis and spectroscopy

© 2018 Elsevier B.V. In this review, the literature on the new fluorophore BOPHY is covered. This doubly boron-centered fluorophore resembles the well-studied BODIPY, but has its own characteristics, both in synthesis and spectroscopy. The general synthesis and properties of this fluorophore, the possibilities for postmodifications, the literature on aromatic ring-fused BOPHYs and boron substitution are discussed.status: publishe

QM/MM-Based Calculations of Absorption and Emission Spectra of LSSmOrange Variants

The goal of this computational work is to gain new insight into the photochemistry of the fluorescent protein (FP) LSSmOrange. This FP is of interest because besides exhibiting the eponymous large spectral shift (LSS) between the absorption and emission energies, it has been experimentally observed that it can also undergo a photoconversion process, which leads to a change in the absorption wavelength of the chromophore (from 437 to 553 nm). There is strong experimental evidence that this photoconversion is caused by decarboxylation of a glutamate located in the close vicinity of the chromophore. Still, the exact chemical mechanism of the decarboxylation process as well as the precise understanding of structure-property relations in the measured absorption and emission spectra is not yet fully understood. Therefore, hybrid quantum mechanics/molecular mechanics (QM/MM) calculations are performed to model the absorption and emission spectra of the original and photoconverted forms of LSSmOrange. The necessary force-field parameters of the chromophore are optimized with CGenFF and the FFToolkit. A thorough analysis of QM methods to study the excitation energies of this specific FP chromophore has been carried out. Furthermore, the influence of the size of the QM region has been investigated. We found that QM/MM calculations performed with time-dependent density functional theory (CAM-B3LYP/D3/6-31G*) and QM calculations performed with the semiempirical ZIndo/S method including a polarizable continuum model can describe the excitation energies reasonably well. Moreover, already a small QM region size seems to be sufficient for the study of the photochemistry in LSSmOrange. Especially, the calculated ZIndo spectra are in very good agreement with the experimental ones. On the basis of the spectra obtained, we could verify the experimentally assigned structures.status: publishe

Plasmonic Dicke Effect in Ag-Nanoclusters-Doped Oxyfluoride Glasses

© 2015 American Chemical Society. We report the observation of picosecond kinetics (ps) of the luminescence decay in Ag nanoclusters dispersed in oxyfluoride glass host. Calculated normal oscillator strengths for the Ag nanoclusters cannot account for such fast kinetics of luminescence, suggesting its cooperative origin. This confirmed by the concentration dependence of the intensity of picosecond component. Further analysis shows that cooperativity does not relate to conventional superradiance but is rather mediated by plasmons of dispersed Ag nanoparticles around which Ag nanoclusters coordinated. The plasmon Dicke effect may be applied for the generation of picosecond pulses across a wide range of wavelengths in these glasses. (Figure Presented).status: publishe

Fluorescence modulation by fast photochromism of a [2.2]paracyclophane-bridged imidazole dimer possessing a perylene bisimide moiety

© 2018 The Royal Society of Chemistry. The development of single-molecule imaging and super-resolution microscopy techniques has promoted the study of fluorescence switchable molecules that have been important for the in-depth understanding of the activities of organelles and the geometries of materials in the nano- and microscale. The utilization of photochromic compounds as the photo-switching trigger is an efficient strategy to reversibly control the fluorescent "ON" and "OFF" states. In this study, we demonstrated the red-color fluorescence switching of a perylene bisimide (PBI) derivative by using a fast photochromic [2.2]paracyclophane-bridged imidazole dimer. The transient colored biradical species as the fluorescence quencher is generated upon UV light irradiation. Because the biradical species has broad absorption bands in the whole visible light and the near-infrared regions (500-900 nm), the fluorescence of PBI could be efficiently quenched by Förster resonance energy transfer (FRET). The fluorescence intensity was switched by means of fast photochromic cycles within a few tens of milliseconds. The potential capability of the transient biradical species to switch the fluorescence in the visible and NIR regions will open up new possibilities in multicolor fluorescence imaging.status: publishe

Host and guest joining forces : a holistic approach for metal-organic frameworks in nonlinear optics

Metal-organic frameworks (MOFs) are interesting candidates for applications in nonlinear optics (NLO). However, current design strategies for MOFs in NLO are typically limited to either engineering the MOF itself or using the MOF to align NLO-active molecules within its pores. But more design factors can be considered when engineering MOFs and choosing guest molecules. The NLO emission of the host and guests can be combined instead of using only either one of the two. The interaction between the host and guest can be a source for further improvement by changing the symmetry, dipole moment, bond lengths, charge distribution, etc. Additionally, unstable NLO molecules can potentially be stabilized by accommodation in the MOF. Here, we demonstrate a new strategy for MOFs by combining MOF-177 as a host and Li@C-60 as a guest for NLO, meaning that both the MOF itself as well as the guest molecules are emitting NLO signals, as well as further increasing the emission using their interaction elongating bonds within the MOF, thus fully using the MOF's potential for second harmonic generation. Using this approach, the overall emission can be boosted by 40% compared to MOF-177 alone. Furthermore, the accommodation into MOF-177 stabilizes Li@C-60 that is normally unstable under ambient conditions without a counterion