Probing the Structure and Photophysics of Porphyrinoid Systems for Functional Materials

Porphyrins (Pors) and their many cousins, including phthalocyanines (Pcs), corroles (Cors), subphthalocyanines (SubPcs), porphyrazines (Pzs), and naphthalocyanines (NPcs), play amazingly diverse roles in biological and non-biological systems because of their unique and tunable physical and chemical properties. These compounds, collectively known as porphyrinoids, can be employed in any number of functional devices that have the potential to address the challenges of modern society. Their incorporation into such devices, however, depends on many structural factors that must be well understood and carefully controlled in order to achieve the desired behavior. Self-assembly and self-organization are key processes for developing these new technologies, as they will allow for inexpensive, efficient, and scalable designs. The overall goal of this dissertation is to elucidate and ultimately control the interplay between the hierarchical structure and the photophysical properties of these kinds of systems. This includes several case studies concerning the design and spectroscopic analysis of supramolecular systems formed through simple, scalable synthetic methods. We also present detailed experimental and computational studies on some porphyrin and phthalocyanine compounds that provide evidence for fundamental changes in their molecular structure. In addition to their impact on the photophysics, these changes also have implications for the organization of these molecules into higher order materials and devices. It is our hope that these findings will help to drive chemists and engineers to look more closely at every level of hierarchical structure in the search for the next generation of advanced materials

Computational and Spectroscopic Studies in the Design of Tetrapyrrole Dyes for Dye-Sensitized Solar Cells

Cyclic tetrapyrroles, like porphyrins, phthalocyanines, and chlorins, are of great interest for dye-sensitized solar cell (DSSC) applications due to their highly versatile structure, tunable π based spectroscopic and electrochemical properties, and excellent stabilities. As well, they have a structural analogy with chlorophyll, a natural photosensitizer. Chlorophylls exhibit a red and intense lowest energy absorption band that is one of the ideal properties of a dye for application in DSSCs. However, because chlorophylls are unstable, it is necessary to design similar but more stable tetrapyrroles with these ideal properties. The relationship between chlorophyll’s geometric structure and spectral properties were first explored using density functional theory (DFT) calculations. Understanding the unique electronic structure of the chlorophylls will help guide future designs of synthetic tetrapyrroles. The electronic structure of synthetic and fictive porphyrins and chlorins with β-substitutions were then probed using magnetic circular dichroism and DFT calculations

Unravelling the ultrafast dynamics of a N-BODIPY compound

Although the photophysics of BODIPY compounds has been widely investigated in the last few years, their analogues N-BODIPY, with nitrogen substitution at the boron center, did not receive comparable attention. In this work we report the synthesis and photochemical characterization of a substituted N-BODIPY compound, by means of a combined theoretical and spectroscopic approach. Compared to a standard BODIPY, the compound under investigation presents a lower fluorescence quantum yield (QY) in the visible region. The excited state relaxation dynamics of the dye was studied in different solvents, showing further fluorescence quenching in polar solvents, and excited state decay rates strongly dependent on the environment polarity. The role of the pendant moieties and the involvement of charge transfer states in the excited state dynamics was experimentally addressed by transient absorption spectroscopy, and further analyzed with TD-DFT calculations, which allowed precise assignment of the transient signals to the correspondent electronic configuration. The complete picture of the N-BODIPY behavior shows the presence of both charge transfer and localized states, influencing the observed photophysics to different amounts, depending on the excitation conditions and the surrounding environment

Advances and Challenges in Organic Electronics

Organic Electronics is a rapidly evolving multidisciplinary research field at the interface between Organic Chemistry and Physics. Organic Electronics is based on the use of the unique optical and electrical properties of π-conjugated materials that range from small molecules to polymers. The wide activity of researchers in Organic Electronics is testament to the fact that its potential is huge and its list of potential applications almost endless. Application of these electronic and optoelectronic devices range from Organic Field Effect Transistors (OFETs) to Organic Light Emitting Diodes (OLEDs) and Organic Solar Cells (OSCs), sensors, etc. We invited a series of colleagues to contribute to this Special Issue with respect to the aforementioned concepts and keywords. The goal for this Special Issue was to describe the recent developments of this rapidly advancing interdisciplinary research field. We thank all authors for their contributions

Organoimido-Polyoxometalate Nonlinear Optical Chromophores: A Structural, Spectroscopic, and Computational Study

Ten organoimido polyoxometalate (POM) based chromophores have been synthesized and studied by hyperRayleigh Scattering (HRS), Stark and Resonance Raman spectroscopies and DFT calculations. HRS β0 values for chromophores with resonance electron donors are significant (up to 139 × 10-30 esu, ca. 5 × that of the DAS+ cation), but systems with no donor, or the –NO2 acceptor show no activity, in some cases despite large DFT-predicted β-values. In active systems with short (phenyl) π-bridges, β0-values comfortably exceed that of the purely organic structural analogue N,N-dimethyl-4-nitroaniline (DMPNA), and intrinsic β-values, β0/N3/2 (N = number of bridge π-electrons) thus appear to break empirical performance limits (β0/N3/2 vs λmax) for planar organic systems. However, β0-values obtained for extended systems with a diphenylacetylene bridge are comparable to or lower than that of their nitro analogue N,N-dimethyl-4-[(4-nitrophenyl)ethynyl]-aniline (DMNPEA). Resonance Raman spectroscopy confirms involvement of the POM in the electronic transitions whether donor groups are present or not, but Stark spectroscopy indicates that in their absence the transitions have little dipolar character (hence NLO inactive), consistent with DFT-calculated frontier orbitals which extend over both POM and organic group. Stark and DFT also suggest that β is enhanced in the short compounds because extension of charge transfer (CT) onto the POM increases excited state dipole moment changes. With extended π-systems this effect does not increase CT distances relative to a –NO2 acceptor, so β0-values do not exceed that of DMNPEA. Overall, our results show that: (i) the organoimido–POM unit is an efficient acceptor for 2nd order NLO, but an ineffective donor; (ii) the nature of electronic transitions in arylimido-POMs is strongly influenced by the substituents of the aryl group; and (iii) organoimido-POMs outperform organic acceptors with short π-bridges, but lose their advantage with extended π-conjugation

Singlet Oxygen Generation by Porphyrins and Metalloporphyrins Revisited: a Quantitative Structure-property Relationship (QSPR) Study

state followed by formation of singlet oxygen (1O2), which is a highly reactive species and mediates various oxidative processes. The design of advanced sensitizers based on porphyrin compounds have attracted significant attention in recent years. However, it is still difficult to predict the efficiency of singlet oxygen generation for a given structure. Our goal was to develop a quantitative structure-property relationship (QSPR) model for the fast virtual screening and prediction of singlet oxygen quantum yields for pophyrins and metalloporphyrins. We performed QSPR analysis of a dataset containing 32 compounds, including various porphyrins and their analogues (chlorins and bacteriochlorins). Quantum-chemical descriptors were calculated using Density Functional Theory (DFT), namely B3LYP and M062X functionals. Three different machine learning methods were used to develop QSPR models: random forest regression (RFR), support vector regression (SVR), and multiple linear regression (MLR). The optimal QSPR model «structure – singlet oxygen generation quantum yield» obtained using RFR method demonstrated high determination coefficient for the training set (R2 = 0.949) and the highest predicting ability for the test set (pred_R2 = 0.875). This proves that the developed QSPR method is realiable and can be directly applied in the studies of singlet oxygen generation both for free base porphyrins and their metal complexes. We believe that QSPR approach developed in this study can be useful for the search of new poprhyrin photosensitizers with enhanced singlet oxygen generation ability

Synthesis and study of fluorescent molecular dyes

PhD ThesisUses for fluorescent dyes are diverse and increasingly important with compounds having many uses in medicinal, chemical and physical fields - amongst others. The creation of new fluorescent dyes helps to push the boundaries of molecular photonics alongside the further study of the underlying principles involved in the systems. This thesis concentrates largely on the synthesis and characterisation aspects of novel fluorescent dyes, though the analysis of the resultant photophysical data also features prominently. Chapter 1 is an introduction to fluorescence from some of the more basic principles involved in the field. A discussion and comparison of intrinsic and extrinsic fluorescent dyes is followed by a brief discussion of a series of examples of fluorescent molecular sensors. As bodipy dyes feature heavily throughout the thesis the second half of the introduction is focused solely on this topic. This half of the chapter centres around the synthetic approaches towards bodipy, modifications to the bodipy core and the resulting photophysics. Photo-induced electron transfer and fluorescence energy transfer is introduced from basic principles along with selected literature examples that demonstrate these processes in systems that incorporate bodipy. Chapter 2 discusses the synthesis and photophysics of a new class of fluorescent dyes based on a highly substituted terephthalate core. The initial aim of the chapter was to create fluorescent systems based on a xanthene core, this was found to be non-fluorescent. As such attention was turned towards a terephthalate intermediate which demonstrated strong and highly red shifted fluorescence in solution, as well as solid state fluorescence. A meso-perfluorinated phenyl ring causes a marked increase in fluorescence quantum yield, along with a pronounced red-shift, relative to the equivalent meso-phenyl variant. This observation lead to a series of Fn-aryl (n = 1,2,3,5) bodipy dyes being synthesised. Chapter 3 subsequently investigates the relationship between the number and position of fluorine atoms on the aryl moiety, and the resultant photophysical measurements. High fluorescence quantum yields were observed with ortho-substitution of fluorine atoms, a trend that was mirrored with the fluorescence lifetime. Mono-ortho fluorine substitution iii of the aryl group was also found to make the bodipy prochiral pathing the way towards axially chiral bodipy compounds. Chapter 4 follows on from chapter 4 by taking prochiral bodipy compounds to there chiral conclusion. In this chapter several synthetic approaches towards axially chiral (AxC) bodipy compounds are discussed. Included in these synthetic approaches is a completely novel route towards asymmetric bodipy cores thus AxC-bodipy compounds. This chapter represents the first examples of AxC bodipy compounds to exist with future developments in the field aimed at enhanced fluorescence sensing in chiral media and facile enantiomeric determination via circularly-polarised fluorescence measurements. Chapter 5 is an in-depth experimental section where the synthesis and characterisation of each compound is detailed. Also provided are the details for each chemical used, purification and drying methods for each solvent used and techniques used for proper characterisation of all of the compounds

Synthesis of novel asymmetrically substituted phthalocyanines

Phthalocyanines are among the more promising second generation photosensitizers for photodynamic therapy. Our research group has consistently shown that the more amphiphilic of these compounds display improved biological properties as photosensitizers for photodynamic therapy. However, synthetic approaches towards such asymmetrically substituted amphiphilic phthalocyanines are quite limited. As such, we have examined different methodologies for imparting amphiphilicity to phthalocyanine-based photosensitizers. Boron subphthalocyanines are the lower homologs of phthalocyanines and the reactivity of boron subphthalocyanines allows them to react with 1,3-diiminoisoindolines in a Kobayashi ring expansion reaction to give 3:1 asymmetrically substituted phthalocyanines. While several literature examples demonstrate that this protocol can lead to a mixture of substituted phthalocyanine products, the ring expansion reaction of halogenated boron subphthalocyanines in the current study has proven to proceed smoothly to selectively produce the desired 3:1 asymmetrically substituted products. Fluorinated photosensitizers have been previously demonstrated to have interesting properties for PDT and a series of 3:1 asymmetrically substituted dodecafluorinated phthalocyanines have been synthesized by the Kobayashi ring expansion reaction of (dodecafluorosubphthalocyaninato)boron(III) bromide. The asymmetry in these lipophilic compounds improves the photodynamic efficiency of these photosensitizers compared to previously examined symmetrically substituted fluorinated phthalocyanine derivatives. The chemical versatility of aryl iodides, in particular towards palladium-catalyzed reactions, allows for the controlled addition of novel functionality to 3:1 asymmetrically substituted iodinated phthalocyanines prepared by the Kobayashi ring expansion reaction of iodinated boron subphthalocyanines. Palladium-catalyzed reactions have thus been employed in the preparation of new amphiphilic anionic and cationic water-soluble photosensitizers. These compounds should have interesting properties for photodynamic therapy. Lastly, boron subnaphthalocyanines absorb light at a wavelength around 660-680 nm. Their cone-shaped structure prevents aggregation and may impart amphiphilicity to the molecule depending on the nature of the substituents on the subnaphthalocyanine macrocycle and the axial ligand on the central boron. A series of boron subnaphthalocyanines has been synthesized and this class of photosensitizers has been shown to effectively generate singlet oxygen in an aqueous, biologically relevant environment while undergoing rapid photobleaching

New nitrogen-rich azo-bridged porphyrin conjugated microporous networks for high performance of gas capture and storage

A series of new conjugated microporous polymers (Azo-1, Azo-2 and Azo-3) based on a nitrogen-rich porphyrin building unit and an azo bond linkage were synthesized by KOH assisted condensation. These materials were characterized by Fourier transform infrared spectroscopy (FT-IR), solid-state 13C NMR, XPS, scanning electron microscopy (SEM), high-resolution transmission electron microscopy (TEM), X-ray diffraction (XRD), thermogravimetric analysis (TGA), and tested for gas (N2, CO2 and H2) adsorption. It was revealed that the azos presented the formation of porous polymer networks affording amorphous particles with rough surfaces and irregular morphology with excellent thermal stability under nitrogen conditions. The Brunauer–Emmett–Teller (BET) model of the N2 adsorption gave apparent surface area ranges of 520–675 m2 g-1. The results from non-local density functional theory (NL-DFT) calculations suggested a pore size distribution between 1.6 and 4.0 nm. The gas (CO2, H2) adsorption isotherms demonstrated outstanding CO2 uptake up to 17.5 wt% (3.98 mmol g-1 for Azo-2) and moderate H2 storage. The isosteric heats of adsorption (Qst) are high, with values of 36–37 kJ mol-1 for the azo polymers. Moreover, the azo-polymer networks exhibited excellent selectivity with CO2/N2 up to 64.3 for Azo-2 at 273 K/1 bar. It was suggested that the nitrogen-rich active sites of the polymers play an important role for CO2 capture and storage