Accurate Vertical Excitation Energies of BODIPY/Aza-BODIPY Derivatives from Excited-State Mean-Field Calculations

We report a benchmark study of vertical excitation energies and oscillator strengths for the HOMO -> LUMO transitions of 17 boron-dipyrromethene (BODIPY) structures, showing a large variety of ring sizes and substituents. Results obtained at the time-dependent density functional theory (TDDFT) and at the delta-self-consistent-field (Delta SCF) by using 13 different exchange correlation kernels (within LDA, GGA, hybrid, and range-separated approximations) are benchmarked against the experimental excitation energies when available. It is found that the time-independent Delta SCF DFT method, when used in combination with hybrid PBE0 and B3LYP functionals, largely outperforms TDDFT and can be quite competitive, in terms of accuracy, with computationally more costly wave function based methods such as CC2 and CASPT2

Towards Prediction of Non-Radiative Decay Pathways in Organic Compounds I: The Case of Naphthalene Quantum Yields

Many emerging technologies depend on human’s ability to control and manipulate the excited-state properties of molecular systems. These technologies include fluorescent labeling in biomedical imaging, light harvesting in photovoltaics, and electroluminescence in light-emitting devices. All of these systems suffer from non-radiative loss pathways that dissipate electronic energy as heat, which causes the overall system efficiency to be directly linked to quantum yield (Φ) of the molecular excited state. Unfortunately, Φ is very difficult to predict from first principles because the description of a slow non-radiative decay mechanism requires an accurate description of long-timescale excited-state quantum dynamics. In the present study, we introduce an efficient semiempirical method of calculating the fluorescence quantum yield (Φfl) for molecular chromophores, which, based on machine learning, converts simple electronic energies computed using time-dependent density functional theory (TDDFT) into an estimate of Φfl. As with all machine learning strategies, the algorithm needs to be trained on fluorescent dyes for which Φfl’s are known, so as to provide a black-box method which can later predict Φfl’s for chemically similar chromophores that have not been studied experimentally. As a first illustration of how our proposed algorithm can be trained, we examine a family of 25 naphthalene derivatives. The simplest application of the energy gap law is found to be inadequate to explain the rates of internal conversion (IC) or intersystem crossing (ISC) – the electronic properties of at least one higher-lying electronic state (Sn or Tn) or one far-from-equilibrium geometry are typically needed to obtain accurate results. Indeed, the key descriptors turn out to be the transition state between the Franck–Condon minimum a distorted local minimum near an S0/S1 conical intersection (which governs IC) and the magnitude of the spin–orbit coupling (which governs ISC). The resulting Φfl’s are predicted with reasonable accuracy (±22%), making our approach a promising ingredient for high-throughput screening and rational design of the molecular excited states with desired Φ’s. We thus conclude that our model, while semi-empirical in nature, does in fact extract sound physical insight into the challenge of describing non-radiative relaxations

A density functional theory based analysis of photoinduced electron transfer in a triazacryptand based K+ sensor

The electronic structure and photoinduced electron transfer processes in a K+ fluorescent sensor that comprises a 4-amino-naphthalimide derived fluorophore with a triazacryptand lig- and is investigated using density functional theory (DFT) and time-dependent density functional theory (TDDFT) in order to rationalise the function of the sensor. The absorption and emission energies of the intense electronic excitation localised on the fluorophore are accurately described using a ∆SCF Kohn-Sham DFT approach, which gives excitation energies closer to experiment than TDDFT. Analysis of the molecular orbital diagram arising from DFT calculations for the isolated molecule or with implicit solvent cannot account for the function of the sensor and it is necessary to consider the relative energies of the electronic states formed from the local excitation on the fluorophore and the lowest fluorophore→chelator charge transfer state. The inclusion of solvent in these calculations is critical since the strong interaction of the charge transfer state with the solvent lowers it energy below the local fluorophore excited state making a reductive photoinduced electron transfer possible in the absence of K+, while no such process is possible when the sensor is bound to K+. The rate of electron transfer is quantified using Marcus theory, which gives a rate of electron transfer of k_ET=5.98 x 10^6 s−1

Luminescent blue emissive bis(alkynyl) borane compounds with a N,O-coordinated ligand

Five bis(alkynyl)boranes with a (imidazo[1,5-a]pyridin-3-yl)phenolate ligand have been synthesized and characterized both in solution (1H, 13C, 11B, 19F NMR) and in the solid state (X-ray). All derivatives, differing for the substituent R (H, Me, OMe, CF3, NMe2) in the para position of the phenylacetylene moieties, displayed blue fluorescence emission in solution, linearly correlated to the electronic properties of the substituent R (i.e., its σp Hammett constant). High Stokes shifts and good quantum yields were recorded. Time-Dependent Density Functional Theory (TD-DFT) calculations were performed to describe the percentage contribution of each fragment of the molecule to the frontier orbitals. Electron Density Difference Maps (EDDMs) calculated for all derivatives allowed to explain the emissive properties of the studied compounds

Photoactive nanostructured hybrid materials for optical and biomedical applications

198 p.Along this manuscript different hybrid materials are synthesized and extensively characterized for several used: from optical to therapeutic applications. First, by the incorporation of different dyes, styryl 722 and pyronine into several smectite clay films, macroscopically ordered systems are obtained. The effect of the clay on the dye is deeply analysed and its preferential orientation is studies by anisotropic response of the films to the linear polarized light. Second, large monoliths with embedded laser dyes with strong absorption and fluorescence bands in different region of the visible spectrum are attained by sol gel chemistry to obtain solid-state dye laser (SSDL) with good photo, thermal and chemical stabilities. Third, silica NP (NP) with suitable size (50 nm) and functionalized external surface are also synthesized by sol gel chemistry. Through the encapsulation of fluorescent dye molecules in their core and by the grafting of the photosensitizers on their shell, biocompatible nanoparticles for bio-imaging and photodynamic therapy (PDT) applications are prepared. In order to optimize their properties, a careful investigation of the photophysical properties and mainly the singlet oxygen generation of a large range of new photosensitizer based on chromophores known as BODIPYs, is previously carried out. Based on the results, some efficient BODIPYs are selected for grafting on silica nanoparticles in order to use them for PDT.Université de Pau et des pays de L'Adour CNRS Laboratory for Molecular Spectroscopy IPREM. Institut des sciences analytiques et de physico-chimie pour l'environnement et des matériau

Synthesis and photophysical studies of crown ether-bodipy dyes and the fabrication of bodipy embedded fluorescent nanofibers

This study has three major objectives: 1) to synthesize a series of structurally related BODIPY dyes, 2) to fabricate BODIPY embedded electrospun nanofibers, and 3) to investigate and characterize the photophysical properties of all synthesized BODIPY dyes with a special focus on their ability to generate singlet oxygen. This thesis first explores the acid catalysed condensation reaction to produce two structurally analogous meso-substituted BODIPY dyes based on cuminaldehyde and 4-dimethylaminobenzaldehdye. In order to enhance the rate of ISC and promote the generation of reactive oxygen species bromine atoms were then attached to the BODIPY 2,6-positions. These BODIPY dyes were then embedded in a polystyrene solution and electrospun into nanofibers. The resulting nanofibers were found to be highly fluorescent, but were no longer able to generate singlet oxygen. Ion-sensitive BODIPYs were prepared from the dibrominated BODIPY dyes by employing a modified Knoevenagel condensation reaction to form a styryl bond with 4’-formylbenzo-15-crown-5 at the 3,5-position of the BODIPY core. Changes in the morphology and position of the absorption and emission spectra of these crown ether-styryl BODIPY dyes were observed in the presence of sodium ions. These results imply that crown ether-substituted BODIPY dyes could function as ion sensors

Quantifying Electronic Phenomena in Organic Chromophores

Challenging ground and excited state problems in the chemistry of common organic chromophores are investigated with state-of-the-art quantum chemical methods. We present a comprehensive excited state molecular dynamics analysis of (a) fundamental building blocks in organic electronics (thiophene and its derivatives), (b) aggregation-induced emission systems (tetraphenylethylene), and (c) organic fluorophores used for imaging and sensing applications (BODIPY and its derivatives). We identify the efficient excited state deactivation pathways which are essential to understanding the photochemical stability and emissive properties of these compounds. The internal conversion mechanisms of theoretically challenging thiophene and bithiophene molecules are investigated with a trajectory surface hopping approach utilizing reliable electronic structure methods. We gain new insights into the photochemistry and photophysics of these systems, including a new mechanism in thiophene excited state decay and the increased photostability of bithiophene, thereby complementing earlier theoretical and experimental literature. The origin of the non-emissive behavior of tetraphenylethylene in the gas phase is explained by identifying energetically accessible conical intersections which promote radiationless decay. It is implied that restricted access to the conical intersection induces strong emission upon aggregation - a phenomenon that attracted significant research attention recently. Finally, the concept of conical intersection accessibility is utilized to explain the fluorescence quenching in certain meso-substituted BODIPY derivatives. We deliver a full mechanistic picture of the nonradiative decay of these molecules, invoking the role of excited state charge transfer and weak intramolecular interactions. Understanding the failures of quantum chemical electronic structure methods is crucial for subsequent improvements of commonly applied theoretical approximations. To elucidate the origins behind the unbalanced description of the low-lying pipi* excited states of heteroaromatic molecules (including thiophene and its derivatives, or in general fused heteroaromatics), we employ a range of quantum chemical approaches, from more approximate time-dependent density functional theory (TDDFT) to highly accurate wavefunction-based methods. The drawbacks of standard TDDFT were ascribed to the ubiquitous adiabatic approximation, rather than different functional approximations. On the other hand, the performance of wavefunction-based methods is found to be largely dependent on the treatment of electron correlation, which is key for a balanced description of excited states with disparate electronic character. Non-covalent molecular interactions are at the origin of many chemical and physical phenomena. While quantifying intermolecular interactions has become a routine task, intramolecular interactions are still considered particularly difficult to treat by theoretical methods. Here, we develop an original wavefuction-based method for quantifying intermolecular and intramolecular interactions on equal footing. The method, intra-SAPT, makes use of a single Slater determinant wavefunction, subject to perturbational corrections. The scheme decomposes interaction energies into physically meaningful components: electrostatics-exchange, induction and dispersion

A Computational Investigation of BODIPY Excited State Properties and Photosensitization of Molecular Oxygen

Cancer has long been a significant problem that has affected our world’s population for years and continues to this day. With the number of cases expected to increase annually there is a societal pressure to find effective treatment methods for eliminating cancer. Current forms of cancer treatment tend to cause detrimental effects to the human body and are usually quite expensive and long lasting, some costing upwards of $30,000 over an 8 week period. A more recently established form of cancer treatment known as photodynamic therapy is an effective treatment option for ridding cancers that lie on or just below the surface of the skin. Photodynamic therapy is usually done as an outpatient procedure, on average costing between$2,500-3,000 and can eliminate all traces of cancer in as little as a single visit. A major drawback to this form of cancer treatment is the lack of efficient photosensitizers, the light absorbing organic compounds which initiate the destruction of cancer cells. Our research is based on establishing a computational strategy for predicting the effectiveness of new photodynamic therapy photosensitizers. We focus our study on a set of photosensitizers known as boron-dipyrromethene (BODIPY) dyes. These dyes are fluoresecent compounds used throughout a variety of photochemical applications such as photovoltaics, biological imaging, and more recently photodynamic therapy. We apply computational chemistry methods to calculate electronic properties we can use to rate the performance of these photosensitizers. First, we begin with a fundamental understanding of what photodynamic therapy is and the components that make up the treatment method. Then we move to descriptions of the computational methods we implement, including density functional theory (DFT), time- dependent density functional theory (TDDFT), restricted open-shell Kohn-Sham method (ROKS), and constrained density functional theory (CDFT). Next we investigate the parallelity between the S1 excited state potential energy surfaces predicted by TDDFT and ROKS. Finally, we investigate the singlet oxygen photosensitization characteristics of a particular BODIPY derivative. This study will help future scientists approach the issue of finding the top candidate photosensitizers for use in photodynamic therapy through a rational design process rather than a repetitive trial and error based approach

Donor-acceptor effects on the optical limiting properties of BODIPY dyes

The main objectives of the research described in this thesis were firstly to synthesize and characterize a series of structurally related BODIPY dyes that are potentially suitable for use in applications, secondly to conjugate a carboxylic acid substituted BODIPY dye to amine-functionalized upconversion nanoparticles (UCNPs) through an amide bond to enable singlet oxygen production upon irradiation at 978 nm in the biological window for tissue penetration for biomedical applications, and thirdly to compare the nonlinear optical (NLO) properties of various BODIPY dyes to determine whether push-pull effects enhance their utility for optical limiting (OL) applications. Halogenated BODIPY cores with high singlet oxygen quantum yields were prepared, which absorb in the green portion of the visible region and making it difficult to treat deeper skin tumors in the context of photodynamic therapy (PDT) applications. UCNPs generally absorb in the near-infrared (NIR) region (978 nm), and this is advantageous because, this is where absorption by water, cells and tissues is minimized. NaYF4: Yb/Er/Gd UCNPs were synthesized, amine functionalized and successfully conjugated to a halogenated carboxylic acid functionalized BODIPY. This allowed for favorable Förster resonance energy transfer (FRET) since one of the emission wavelengths of the NaYF4: Yb/Er/Gd UCNPs overlaps with the main absorption band of the BODIPY at 540 nm. The conjugate was irradiated at 978 nm, but instability of the BODIPY dye was observed, which made singlet oxygen quantum yield determination impossible. An enhanced singlet oxygen quantum yield value was observed upon irradiation of the conjugate at 540 nm, suggesting that further studies of this system are warranted. The OL properties of BODIPY cores and dyes, which are π-extended at the 3,5-positions with styryl groups, were studied in a series of different organic solvents at 532 nm by using the z-scan technique on a nanosecond timescale. Many of the dyes were used to compare the effects of introducing electron donor and acceptor groups on the OL properties of the dyes. The dipole moments of these dyes were found to correlate with the OL response. The OL results indicate that BODIPY dyes with push-pull properties, which are π-extended at the 3,5-positions with styryl groups, can be considered as viable candidates for use in OL applications. The studies sought to establish the effect of ESA in the triplet manifold as compared to the singlet manifold in as far as the OL response is concerned. The most promising dyes were embedded in polystyrene thin films, and this was found to significantly enhance their OL properties

Synergism between organic and inorganic moieties: in the search of new hybrid materials for optics and biomedicine

232 p.In this work different versatile photoactive hybrid materials with interesting features for optics and biomedicine are achieved and exhaustively characterized. Firstly, by means of the occlusion of different fluorescent dyes, by the crystallization inclusion method, into several 1D-channeled magnesium aluminophosphate (MgAPO) hosts (with different sized and shaped pores), optically dense fluorescent hybrid materials which show highly anisotropic response to linearly polarized light are obtained. Depending on the dye embedded within the selected MgAPO framework, interesting applications have been attained, such as one-directional artificial photonic antenna systems covering the whole UV-Visible spectral range, Second Harmonic Generators under NIR radiation, optically switchable hybrid systems and white light emitters. White light emission is also obtained from the luminescence that arises from embedding simultaneously different small aromatic molecules into a Metal Organic Framework (MOF), named [Zn2(bdc)2(dpNDI)]n, which contains a photoactive naphtalemediimide as organic pillar. The incorporation of halide-substituted aromatic molecules into this MOF also promotes phosphorescence at room temperature. Finally, the attachment of a BODIPY chromophore to a cyclometalated Ir(III) metallic centre results in the achievement of efficient photosensitizers for singlet oxygen generation upon visible excitation light. Moreover, these hybrid compounds also show fluorescence emission, thus, they are interesting for bioimaging as well, and as a consequence, extensible to their use in theragnosis