Spectroscopic Properties of Porphyrin-Like Photosensitizers: Insights from Theory

Application of TD-DFT Theory to Studying Porphyrinoid-Based Photosensitizers for Photodynamic Therapy: A Review

An important focus for innovation in photodynamic therapy (PDT) is theoretical investigations. They employ mostly methods based on Time-Dependent Density Functional Theory (TD-DFT) to study the photochemical properties of photosensitizers. In the current article we review the existing state-of-the-art TD-DFT methods (and beyond) which are employed to study the properties of porphyrinoid-based systems. The review is organized in such a way that each paragraph is devoted to a separate aspect of the PDT mechanism, e.g., correct prediction of the absorption spectra, determination of the singlet-triplet intersystem crossing, and interaction with molecular oxygen. Aspects of the calculation schemes are discussed, such as the choice of the most suitable functional and inclusion of a solvent. Finally, quantitative structure-activity relationship (QSAR) methods used to explore the photochemistry of porphyrinoid-based systems are discussed.

Reduced scaling extended multi-state CASPT2 (XMS-CASPT2) using supporting subspaces and tensor hyper-contraction

We present a reduced scaling formulation of the extended multi-state CASPT2 (XMS-CASPT2) method, which is based on our recently developed state-specific CASPT2 (SS-CASPT2) formulation using supporting subspaces and tensor hyper-contraction. By using these two techniques, the off-diagonal elements of the effective Hamiltonian can be computed with only O(N³) operations and O(N²) memory, where N is the number of basis functions. This limits the overall computational scaling to O(N⁴) operations and O(N²) memory. Thus, excited states can now be obtained at the same reduced (relative to previous algorithms) scaling we achieved for SS-CASPT2. In addition, we also investigate how the energy denominators can be factorized with the Laplace quadrature when some of the denominators are negative, which is critical for excited state calculations. An efficient implementation of the method has been developed using graphical processing units while also exploiting spatial sparsity in tensor operations. We benchmark the accuracy of the new method by comparison to non-THC formulated XMS-CASPT2 for the excited states of various molecules. In our tests, the THC approximation introduces negligible errors (≈0.01 eV) compared to the non-THC reference method. Scaling behavior and computational timings are presented to demonstrate performance. The new method is also interfaced with quantum mechanics/molecular mechanics (QM/MM). In an example study of green fluorescent protein, we show how the XMS-CASPT2 potential energy surfaces and excitation energies are affected by increasing the size of the QM region up to 278 QM atoms with more than 2300 basis functions.

TD-DFT benchmark for UV-visible spectra of fused-ring electron acceptors using global and range-separated hybrids

The accuracy of Time-Dependent Density Functional Theory in predicting the vertical absorption wavelength of 50 widely-used fused-ring electron acceptors (FREAs) has been investigated by considering the solvent effects.

The influence of antenna and anchoring moieties on the improvement of photoelectronic properties in Zn(ii)-porphyrin-TiO2 as potential dye-sensitized solar cells

A systematic study for the rational design of porphyrins (P4 spider-shaped derivatives) with potential application in dye-sensitized solar cells is presented. Using density functional theory (DFT) (B3LYP/6-31G*) and time-dependent DFT (M06/6-31G*) we show that the UV-vis absorption properties of a spider-shaped Zn(ii) porphyrin, previously synthesized by Stangel et al., may be greatly improved by applying some push-pull strategies in meso positions. We found that the selected triphenylamine push group induces a remarkable improvement in the absorption bands of P4 spider-shaped derivatives. The pull effect reached through the π-electron-rich phenyl group and the benzodithiazole (BTD) group allowed the Q bands to be red-shifted up to 689 nm, much longer than the 593 nm reported experimentally for the original spider-shaped porphyrin. The adsorption results of the P4 spider-shaped derivatives onto a TiO2-anatase surface model [Ti16O34H4] through the carboxylic acid group showed that the adsorptions energies were favourable and very similar in all cases. Natural bond orbitals (NBO) indicated a two-center bond (BD) O(carboxyl)-Ti(TiO2) for the porphyrin with the highest adsorption energy (8.27 kcal mol-1), and donor acceptor interactions from LP O(carboxyl) to Ti(TiO2) for the other porphyrins. The natural transition orbitals (NTO) for P4-derivatives-TiO2 confirm the nature of the excited states associated with Q and Soret bands. Finally, the frontier molecular orbitals revealed charge-separated states between those occupied and unoccupied, indicating a favourable charge-transfer process between the dyes and the surface conduction bands. In conclusion, this work showed a systematic study based on the push-pull strategy that improves the performance of porphyrins with the purpose to be used in dye-sensitized solar cells.

Modelling excitation energy transfer in covalently linked molecular dyads containing a BODIPY unit and a macrocycle

We model the singlet–singlet Excitation Energy Transfer (EET) process in a panel of large BODIPY–macrocycle dyads, including some azacalixphyrin derivatives.

Synergistic Effects in PtII -Porphyrinoid Dyes as Candidates for a Dual-Action Anticancer Therapy: A Theoretical Exploration

The combination of a photosensitizer (PS) with a cisplatin-like unit represents a challenging strategy to increase the effectiveness of photodynamic therapy and to afford a dual-action anticancer treatment. Recently, new tetra-Pt^II -porphyrin conjugates have been proposed as promising multitarget agents. To reveal the effect of the Pt^II center on the chemical and physical properties of the PS and to explore the effect of the PS on the activation mechanism of Pt^II ligand before reaching its biological target, we carried out a first-principle investigation on these tetra-Pt^II -porphyrin conjugates. To propose a further advance in this novel field and to gain useful insights for the design of new, more efficient Pt^II -PS conjugates, we introduced structural modifications into the porphyrin dye, which involved the synthesis of the tetra-Pt^II -chlorin and tetra-Pt^II -bacteriochlorin derivatives. Results showed that the designed dyes better met the criteria to be successful in a dual-action therapy, as they displayed improved optical properties and reduced the hydrolysis rate of the Pt^II moiety, the latter being a desirable feature to avoid many side reactions of the conjugate during their transport to the biological target.

Synthesis and photophysical properties of halogenated derivatives of (dibenzoylmethanato)boron difluoride

A series of (dibenzoylmethanato)boron difluoride (BF₂DBM) derivatives with a halogen atom in one of the phenyl rings at the para-position were synthesized and used to elucidate the effects of changing the attached halogen atom on the photophysical properties of BF₂DBM. The room-temperature absorption and fluorescence maxima of fluoro-, chloro-, bromo- and iodo-substituted derivatives of BF₂DBM in THF are red-shifted by about 2-10nm relative to the corresponding peaks of the parent BF₂DBM. The fluorescence quantum yields of the halogenated BF₂DBMs (except the iodinated derivative) are larger than that of the unsubstituted BF₂DBM. All the synthesized compounds are able to form fluorescent exciplexes with benzene and toluene (emission maxima at λ_em=433 and 445nm, respectively). The conformational structure and electronic spectral properties of halogenated BF₂DBMs have been modeled by DFT/TDDFT calculations at the PBE0/SVP level of theory. The structure and fluorescence spectra of exciplexes were calculated using the CIS method with empirical dispersion correction.

Recent Advances in Subporphyrins and Triphyrin Analogues: Contracted Porphyrins Comprising Three Pyrrole Rings

Subporphyrinato boron (subporphyrin) was elusive until the syntheses of tribenzosubporphine in 2006 and meso-aryl-substituted subporphyrin in 2007. These novel contracted analogues possess a 14π-electron conjugated system embedded in a bowl-shaped structure. They exhibit absorption and fluorescence in the UV/vis region and nonlinear optical properties due to their octupolar structures. The unique coordination geometry around the central boron atom in the structure of subporphyrin enabled investigation of rare boron species, such as borenium cations, boron hydrides, and boron peroxides. Along with the burgeoning development of the chemistry of subporphyrins, analogous triphyrin systems have also emerged. Their rich coordination chemistry as a result of their free-base structures, which are different from the boron-coordinating structure of subporphyrins, has been intensively investigated. On the basis of the unique structures and reactivities of subporphyrins and their related triphyrin analogues, supramolecular architectures and covalently linked multicomponent systems have also been actively pursued. This Review provides an overview of the development of subporphyrin and triphyrin chemistry in the past decade and future prospects in this field, which may inspire molecular design toward applications based on their unique properties.

Can Expanded Bacteriochlorins Act as Photosensitizers in Photodynamic Therapy? Good News from Density Functional Theory Computations

The main photophysical properties of a series of expanded bacteriochlorins, recently synthetized, have been investigated by means of DFT and TD-DFT methods. Absorption spectra computed with different exchange-correlation functionals, B3LYP, M06 and ωB97XD, have been compared with the experimental ones. In good agreement, all the considered systems show a maximum absorption wavelength that falls in the therapeutic window (600-800 nm). The obtained singlet-triplet energy gaps are large enough to ensure the production of cytotoxic singlet molecular oxygen. The computed spin-orbit matrix elements suggest a good probability of intersystem spin-crossing between singlet and triplet excited states, since they result to be higher than those computed for 5,10,15,20-tetrakis-(m-hydroxyphenyl)chlorin (Foscan©) already used in the photodynamic therapy (PDT) protocol. Because of the investigated properties, these expanded bacteriochlorins can be proposed as PDT agents.

Effects of Strong Electronic Coupling in Chlorin and Bacteriochlorin Dyads

Achieving tunable, intense near-infrared absorption in molecular architectures with properties suitable for solar light harvesting and biomedical studies is of fundamental interest. Herein, we report the photophysical, redox, and molecular-orbital characteristics of nine hydroporphyrin dyads and associated benchmark monomers that have been designed and synthesized to attain enhanced light harvesting. Each dyad contains two identical hydroporphyrins (chlorin or bacteriochlorin) connected by a linker (ethynyl or butadiynyl) at the macrocycle β-pyrrole (3- or 13-) or meso (15-) positions. The strong electronic communication between constituent chromophores is indicated by the doubling of prominent absorption features, split redox waves, and paired linear combinations of frontier molecular orbitals. Relative to the benchmarks, the chlorin dyads in toluene show substantial bathochromic shifts of the long-wavelength absorption band (17-31 nm), modestly reduced singlet excited-state lifetimes (τS = 3.6-6.2 ns vs 8.8-12.3 ns), and increased fluorescence quantum yields (Φf = 0.37-0.57 vs 0.34-0.39). The bacteriochlorin dyads in toluene show significant bathochromic shifts (25-57 nm) and modestly reduced τS (1.6-3.4 ns vs 3.5-5.3 ns) and Φf (0.09-0.19 vs 0.17-0.21) values. The τS and Φf values for the bacteriochlorin dyads are reduced substantially (up to ∼20-fold) in benzonitrile. The quenching is due primarily to the increased S1 → S0 internal conversion that is likely induced by increased contribution of charge-resonance configurations to the S1 excited state in the polar medium. The fundamental insights gained into the physicochemical properties of the strongly coupled hydroporphyrin dyads may aid their utilization in solar-energy conversion and photomedicine.

NIR bacteriochlorin chromophores accessed by Heck and Sonogashira cross-coupling reactions on a tetrabromobacteriochlorin derivative

The synthesis of a new tetrabromobacteriochlorin BCBr4 is reported. Pd cross-coupling reactions yielded tetra-coupled products with a significant red shift in the UV-Vis bands.

Photophysical comparisons of PEGylated porphyrins, chlorins and bacteriochlorins in water

Synthesis of a bioconjugatable water-soluble (PEGylated) trans-AB-porphyrin enables photophysical comparisons (τ_S, k_f, k_ic, k_isc, Φ_f, Φ_ic, Φ_isc) with analogous chlorins and bacteriochlorins in DMF and water.

Dye chemistry with time-dependent density functional theory

In this perspective, we present an overview of the determination of excited-state properties of "real-life" dyes, and notably of their optical absorption and emission spectra, performed during the last decade with time-dependent density functional theory (TD-DFT). We discuss the results obtained with both vertical and adiabatic (vibronic) approximations, choosing relevant examples for several series of dyes. These examples include reproducing absorption wavelengths of numerous families of coloured molecules, understanding the specific band shape of amino-anthraquinones, optimising the properties of dyes used in solar cells, mimicking the fluorescence wavelengths of fluorescent brighteners and BODIPY dyes, studying optically active biomolecules and photo-induced proton transfer, as well as improving the properties of photochromes.

Competitive binding of Chlorin p6 and Dansyl-l-Proline to Sudlow's site II of human serum albumin

The binding of chlorin p6, a model photosensitizer for photodynamic therapy (PDT), to the Sudlow's site II of Human Serum Albumin (HSA) has been monitored by different spectroscopic methods. Displacement of Dansyl-l-Proline (DP) from its conjugate with HSA is manifested in the spectral shift and decrease in its fluorescence intensity as well as the emergence of component with lifetime of 2-3ns, which is characteristic of free DP. As DP is known to bind specifically to the Sudlow's site II of human serum albumin, its displacement by chlorin p6 indicates the residence of the photosensitizer in the same site, in addition to Sudlow's site I. The binding constants for Sudlow's site II, determined by the stopped-flow technique, are found to be two orders of magnitude smaller than that for Sudlow's site I.

High-pressure and theoretical studies reveal significant differences in the electronic structure and bonding of magnesium, zinc, and nickel ions in metalloporphyrinoids

High pressure in combination with optical spectroscopy was used to gain insights into the interactions between Mg(2+), Zn(2+), and Ni(2+) ions and macrocyclic ligands of porphyrinoid type. In parallel, the central metal ion-macrocycle bonding was investigated using theoretical approaches. The symmetry properties of the orbitals participating in this bonding were analyzed, and pigment geometries and pressure/ligation effects were computed within DFT. Bacteriopheophytin a was applied as both a model chelator and a highly specific spectroscopic probe. The analysis of solvent and pressure effects on the spectral properties of the model Mg(2+), Zn(2+), and Ni(2+) complexes with bacteriopheophytin a shows that various chemical bonds are formed in the central pocket, depending on the valence configuration of the central metal ion. In addition, the character of this bonding depends on symmetry of the macrocyclic system. Since in most cases it is not coordinative bonding, these results challenge the conventional view of metal ion bonding in such complexes. In (labile) complexes with the main group metals, the metal ion-macrocycle interaction is mostly electrostatic. Significantly, water molecules are not preferred as a second axial ligand in such complexes, mainly due to the entropic constraints. The metal ions with a closed d shell may form (stable) complexes with the macrocycle via classical coordination bonds, engaging their p and s orbitals. Transition metals, due to the unfilled d shell, do form much more stable complexes, because of strong bonding via both coordination and covalent interactions. These conclusions are confirmed by DFT computations and theoretical considerations, which altogether provide the basis to propose a consistent and general mechanism of how the central metal ion and its interactions with the core nitrogens govern the physicochemical properties of metalloporphyrinoids.

Design of novel luminescent porphyrins bearing donor–acceptor groups

In this work, we report the synthesis and characterization of a novel series of porphyrins, some of them bearing donor-acceptor groups. meso-substituted free-base porphyrins:5-(4-amino-phenyl)-10,15,20-triphenylporphyrin (TPPNH2) and 5-(4-acetamidophenyl)-10,15,20-triphenylporphyrin (TPPNHAc), as well as their tribromo (Br3TPPNH2 and Br3TPPNHAc) and trimethylsilyl-substituted homologs (TMS3TPPNH2 and TMS3TPPNHAc were synthesized and characterized. The optical properties of all compounds have been studied by absorption and fluorescence spectroscopy. Theoretical calculations were performed in order to recognize the optimized geometry and the correlation with the optical properties. On the other hand, an azobenzene containing porphyrin (TPPN2PhC14H29) was also prepared to study the influence of the trans-cis photoisomerization on its optical properties. Almost all porphyrin derivatives exhibited a Soret band at ca. λ = 419–422 nm followed by four Q-bands in the range between 500–700 nm. Besides, these porphyrins showed two emission bands located at ca. λ = 654 nm and 714 nm with different intensities, depending on the substituents. The relative quantum yields were calculated for these compounds with respect to the most emissive species.

Alternative selection rules for one- and two-photon transitions in tribenzotetraazachlorin: quasi-centrosymmetrical π-conjugation pathway of formally non-centrosymmetrical molecule

We compare the two-photon absorption (2PA) spectra of non-centrosymmetrical metal-free tribenzo-tetraazachlorin (H2TBTAC) and analogous symmetrical tetra-tert-butyl-phthalocyanine (H2TtBuPc). Surprisingly, despite formal lack of center of inversion, the 2PA spectrum of H2TBTAC displays a two-photon allowed transition at 935 nm, similar to gerade-gerade (g-g) transitions observed in H2TtBuPc and in other symmetrical phthalocyanines. This transition is even better resolved in the singlet-singlet excited-state absorption spectrum. We tentatively explain the survival of the g-g transition in H2TBTAC by assuming that the main π-electron conjugation pathway in the tetraaza-substituted tetrapyrrole macrocycle bypasses the outer parts of the two oppositely located isoindole rings and thus renders the optically responsive core of the chromophore quasi-centrosymmetrical. By using the independently measured ground- and excited-state absorption extinction coefficients, we also show that the two-photon absorptivity can be quantitatively explained by a simple three-level model with the lowest energy Q1 state serving as an intermediate level.

Theoretical study of novel azo-tetraphenylporphyrins: potential photovoltaic materials

A density functional theory study was performed to analyze the electron donor-acceptor properties of the cis and trans isomers of a novel azobenzene-containing tetraphenylporphyrin (TPPN2PhC14H29) with different substituents (Br or TMS). In general, the trans isomers are better electron acceptors than the correspondent cis homologues. Their UV-vis spectra were also obtained and a comparison with available experimental results is included. According to these results, the azo compounds reported here are promising materials for the elaboration of dye-sensitized solar cells because their HOMO-LUMO gaps are close to 2 eV. Moreover, the energy of the high intensity absorption bands also fulfills the requirements needed for the operation of a solar cell built with TiO2 and the I(-)/I3(-) pair.

Theoretical investigation of the absorption spectra and singlet-triplet energy gap of positively charged tetraphenylporphyrins as potential photodynamic therapy photosensitizers

DFT and TD-DFT calculations have been performed on a series of positively charged tetraphenylporphyrins with the aim to check whether these compounds can be used as photosensitizers in photodynamic therapy (PDT). Absorption spectra and singlet-triplet energy gaps have been computed by using the BP86, PBE0, and ωB97XD exchange-correlation functionals. Polarizable continuum model (PCM) has been used to take into account solvent effects. A careful analysis has been made on the Q-band, localized in the near-red visible region of the spectrum, since it plays a fundamental role in the drug design of new photodynamic therapy photosensitizers. Our results show that all the examined molecules have an adsorption band that falls in the so-called therapeutic window and possess a singlet-triplet energy gap able to promote the excitation of molecular oxygen from its ground triplet to the excited singlet state. The use of the ωB97XD exchange correlation functional, which accounts for dispersion interactions, allows calculating excitation energy values close to the experimental values.

Bisanthracene bis(dicarboxylic imide)s as potential photosensitizers in photodynamic therapy: a theoretical investigation

The electronic structures and spectroscopic properties of four bisanthracene bis(dicarboxylic imide)s (M1-M4) have been investigated theoretically by using density functional theory (DFT) and its time-dependent extension (TDDFT) in view of their potential use as photosensitizers in photodynamic therapy (PDT). The optimized geometries, electronic absorption transitions, singlet-triplet energy gaps, spin-orbit matrix elements, ionization potentials, and electron affinities have been determined in gas phase and in solvent. Both type I and II PDT mechanisms have been considered. In addition, the variation of a series of relevant properties upon heavy atom substitution (Br and I) have been determined and discussed. Results show that only M4 is able to support the type I reaction, and one of its brominated and iodinated derivatives can produce cytotoxic singlet oxygen (type II reaction).

Metallobacteriochlorophylls as potential dual agents for photodynamic therapy and chemotherapy

A theoretical analysis of bacteriochlorophyll a containing its non-native divalent metal ions: Co, Ni, Cu, Zn, Ru, Rh, Pd, and Pt, has been carried out by means of density functional theory (DFT) calculations. The main stress was put on the derivatives with metals, which already found applications as coordination compounds in anti-tumor therapy (Ru, Pt, Pd, and Rh). The idea was to combine their cytotoxic properties with the known suitability of bacteriochlorophylls macrocycle for photodynamic therapy. The geometries of the studied systems are compared and reveal a number of similarities. The cores of the modified bacteriochlorophylls are flat, and the introduced metal ions lie in plane of the macrocycle, showing its large ability to accommodate metal ions of different sizes. However, four metal–nitrogen bonds, linking the central ions with the macrocycle ligand, are not equivalent. Metals are the strongest attached to nitrogens, which come from the pyrrole, which is fused with isocyclic ring. Based on the known spectroscopic data, the absorption properties of the proposed systems are predicted. Finally, it is found that all studied metal–macrocycle adducts are stable in aqueous media. The only exceptions are Mg-BChla (the finding is reflected by experimental facts) and Zn-BChla. The predicted high stability of Ru-, Rh-, Pt- and Pd-bacteriochlorophylls might turn out beneficial for therapeutic purposes.

A RASSCF study of free base, magnesium and zinc porphyrins: accuracy versus efficiency

The restricted-active-space self-consistent-field methodology is successfully applied to the study of free base- and regular metalloporphyrins, the latter containing magnesium and zinc central ions. It is shown that inclusion of all excitations involving the Gouterman frontier orbitals in the RAS2 subspace results in a numerically stable approach, producing highly accurate results at a fraction of the computational cost of the complete-active-space self-consistent-field method, whereas increasing RAS2 beyond this size leads to only modest improvement. Topological and orbital analysis shows that the approach is also able to give a highly accurate description of the electronic wavefunction. Inclusion of the entire π-conjugated subsystem in the active space results in more accurate excitation energies and a reduction in the dependence on the exact form of the perturbational Hamiltonian used to include dynamic correlation. The larger active space also resolves a quantitative disagreement in results obtained with and without the inclusion of dynamic correlation.

Insight into the electronic structure, optical properties, and redox behavior of the hybrid phthalocyaninoclathrochelates from experimental and density functional theory approaches

An insight into the electronic structure of several hafnium(IV), zirconium(IV), and lutetium(III) phthalocyaninoclathrochelates has been discussed on the basis of experimental UV-vis, MCD, electro- and spectroelectrochemical data as well as density functional theory (DFT) and time-dependent DFT (TDDFT) calculations. On the basis of UV-vis and MCD spectroscopy as well as theoretical predictions, it was concluded that the electronic structure of the phthalocyninoclathrochelates can be described in the first approximation as a superposition of the weakly interacting phthalocyanine and clathrochelate substituents. Spectroelectrochemical data and DFT calculations clearly confirm that the highest occupied molecular orbital (HOMO) in all tested complexes is localized on the phthalocyanine ligand. X-ray crystallography on zirconium(IV) and earlier reported hafnium(IV) phthalocyaninoclathrochelate complexes revealed a slightly distorted phthalocyanine conformation with seven-coordinated metal center positioned ∼1 Å above macrocyclic cavity. The geometry of the encapsulated iron(II) ion in the clathrochelate fragment was found to be between trigonal-prismatic and trigonal-antiprismatic.

Electronic structure, spin-states, and spin-crossover reaction of heme-related Fe-porphyrins: a theoretical perspective

The electronic structures, spin-states, and geometrical parameters of tetra-, penta-, and hexa-coordinated iron-porphyrins are investigated applying density functional theory (DFT) based calculations, utilizing the plane-wave pseudopotential as well as localized basis set approaches. The splitting of the spin multiplet energies are investigated applying various functionals including recently developed hybrid meta-GGA (M06 family) functionals. Almost all of the hybrid functionals accurately reproduce the experimental ground state spins of the investigated Fe-porphyrins. However, the energetic ordering of the spin-states and the energies between them are still an issue. The widely used B3LYP provides consistent results for all chosen systems. The GGA+U functionals are found to be equally competent. After assessing the performance of various functionals in spin-state calculations, the potential energy surfaces of the oxygen binding process by heme is investigated. This reveals a "double spin-crossover" feature for the lowest energy reaction path that is consistent with previous CASPT2 calculations but predicting a lowest energy singlet state. The calculations have hence captured the spin-crossover as well as spin-flip processes. These are driven by the intra-atomic orbital polarization on the central metal atom due to the atomic and orbitals rearrangements. The nature of the chemical bonding and a molecular orbital analysis are also performed for the geometrically simple but electronic structurally complicated system tetra-coordinated planar Fe porphyrin in comparison to the penta-coordinated systems. This analysis explains the observed paradoxical appearance of certain peaks in the local density of states (DOS).