Ground state electronic structures and spectra of zinc complexes of porphyrin, tetraazaporphyrin, tetrabenzoporphyrin, and phthalocyanine: A density functional theory study

The trials and triumphs of modelling X-ray absorption spectra of transition metal phthalocyanines

This study explores the electronic structure of Co, Fe and Mn phthalocyanines (TMPcs) as well as their perfluorinated counterparts through a series of electronic structure calculations utilizing multireference methods and by simulating their metal L-edge and ligand (nitrogen and fluorine) K-edge X-ray absorption spectra (XAS) in an angle-resolved manner. Simulations targeting different ground-state symmetries, where relevant, have been conducted to observe changes in the N K-edge lineshape. The applicability of the quasi-degenerate formulation of n-electron valence state perturbation theory (QD-NEVPT2) for L-edge X-ray absorption spectroscopy (XAS) is evaluated, alongside the use of a restricted active space (RAS) formalism to describe the final-state multiplets generated by L-shell X-ray processes. Our findings provide valuable insights into the electronic properties of TMPcs, in particular with respect to the effect of fluorination, and demonstrate the broad applicability of various formulations of NEVPT2 in spectral simulations. Moreover, this study highlights the utility of manual truncation of the configuration spaces in order to allow for large active orbital spaces in aforementioned calculations.

Efficient Protocol for Computing MCD Spectra in a Broad Frequency Range Combining Resonant and Damped CC2 Quadratic Response Theory

Coupled cluster response theory offers a path to high-accuracy calculations of spectroscopic properties, such as magnetic circular dichroism (MCD). However, divergence or slow convergence issues are often encountered for electronic transitions in high-energy regions with a high density of states. This is here addressed for MCD by an implementation of damped quadratic response theory for resolution-of-identity coupled cluster singles-and-approximate-doubles (RI-CC2), along with an implementation of the MCD A term from resonant response theory. Combined, damped and resonant response theory calculations provide an efficient strategy to obtain MCD spectra over a broad frequency range and for systems that include highly symmetric molecules with degenerate excited states. The protocol is illustrated by application to zinc tetrabenzoporphyrin in the energy region of 2-8 eV and comparison to experimental data. Timings are reported for the resonant and damped approaches, showing that a greater part of the calculation time is consumed by the construction of the building blocks for the final MCD ellipticity. A recommendation on how to use the procedure is outlined.

Comparison of the Performance of Density Functional Methods for the Description of Spin States and Binding Energies of Porphyrins

This work analyzes the performance of 250 electronic structure theory methods (including 240 density functional approximations) for the description of spin states and the binding properties of iron, manganese, and cobalt porphyrins. The assessment employs the Por21 database of high-level computational data (CASPT2 reference energies taken from the literature). Results show that current approximations fail to achieve the "chemical accuracy" target of 1.0 kcal/mol by a long margin. The best-performing methods achieve a mean unsigned error (MUE) <15.0 kcal/mol, but the errors are at least twice as large for most methods. Semilocal functionals and global hybrid functionals with a low percentage of exact exchange are found to be the least problematic for spin states and binding energies, in agreement with the general knowledge in transition metal computational chemistry. Approximations with high percentages of exact exchange (including range-separated and double-hybrid functionals) can lead to catastrophic failures. More modern approximations usually perform better than older functionals. An accurate statistical analysis of the results also casts doubts on some of the reference energies calculated using multireference methods. Suggestions and general guidelines for users are provided in the conclusions. These results hopefully stimulate advances for both the wave function and the density functional side of electronic structure calculations.

Chirogenesis in Zinc Porphyrins: Theoretical Evaluation of Electronic Transitions, Controlling Structural Factors and Axial Ligation

In the present work, sixteen different zinc porphyrins (possessing different meso substituents) with and without a chiral guest were modelled using DFT and TD-DFT approaches in order to understand the influence of various controlling factors on electronic circular dichroism (ECD) spectra. Two major aspects are influenced by these factors: excitation energy of the electronic transitions and their intensity. In the case of excitation energy, the influence increases in the following order: orientation of the peripheral substituents Collapse

Key Words

• circular dichroism
• density functional calculations
• porphyrins
• supramolecular chemistry
• transition metals

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MESH Headings Collapse

Grants

• PUTJD749 Estonian Research Council
• PRG399 Estonian Research Council
• 828779 European Union's H2020-FETOPEN
• Estonian Research Council

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Affiliation(s)

Irina Osadchuk Department of Chemistry and BiotechnologySchool of ScienceTallinn University of Technology AddressAkadeemia tee 1512618TallinnEstonia ICGMUniv MontpellierCNRS, ENSCMMontpellierFrance
Riina Aav Department of Chemistry and BiotechnologySchool of ScienceTallinn University of Technology AddressAkadeemia tee 1512618TallinnEstonia
Victor Borovkov Department of Chemistry and BiotechnologySchool of ScienceTallinn University of Technology AddressAkadeemia tee 1512618TallinnEstonia
Eric Clot ICGMUniv MontpellierCNRS, ENSCMMontpellierFrance

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Distorted Porphyrins with High Stability: Synthesis and Characteristic Electronic Properties of Mono- and Di-Nuclear Tricarbonyl Rhenium Tetraazaporphyrin Complexes

Mono- and di-nuclear tricarbonyl Re(I) tetraazaporphyrin complexes (Re₁ TAP and Re₂ TAP) are investigated and compared with Re(I) phthalocyanine complexes (Re₁ Pc and Re₂ Pc). Although Re₂ Pc is unstable in polar solvents, and easily undergoes demetallation reaction, the coordination of the TAP ligand significantly improves the tolerance toward polar solvents, affording more stability to Re₂ TAP. Additionally, the incorporation of [Re(CO)₃ ]⁺ unit(s) and the TAP ligand results in remarkable positive shifts in both oxidation and reduction potentials. Consequently, the more positive oxidation potentials of the ReTAP complexes significantly increase the tolerance toward oxidation, while the reduction potential indicates that Re₂ TAP is suitable for a soluble electron acceptor. In contrast to Re₁ Pc and Re₂ Pc, Re₁ TAP and Re₂ TAP show unique broad Q bands, which can be attributed to the admixture of the π-π* and metal-to-ligand charge transfer characters, owing to the lowered π orbital energy in the TAP complexes. This study is useful for controlling electronic properties and realizing high stability in Pc analogues.

Peculiarities of electronic structure and chemical bonding in iron and cobalt metal complexes of porphyrazine and tetra(1,2,5-thiadiazole)porphyrazine

The geometrical and electronic structures of iron and cobalt metal complexes of porphyrazine and tetra(1,2,5-thiadiazole)porphyrazine in ground and low-lying excited electronic states were determined by DFT calculations and the complete active space (CASSCF) method with following accounting dynamic correlation by multiconfigurational quasidegenerate second-order perturbation theory (MCQDPT2). A geometrical structure of D[Formula: see text] symmetry has been obtained for all four complexes. According to data obtained by the MCQDPT2 method, the complexes of cobalt and iron possess the ground states 2A[Formula: see text] and 3A[Formula: see text], respectively, and wave functions of the ground states have the form of a single determinant. It is shown that the crystal field theory (CFT) can be used to describe the sequence of electronic states of the investigated complexes. The nature of the bonds between metal atoms and nitrogen atoms has been described using the analysis of the electron density distribution in the frame of Bader’s quantum theory of atoms in molecule (QTAIM).

Screen-Printed Electrodes Modified with Metal Phthalocyanines: Characterization and Electrocatalysis in Chlorinated Media

Metal phthalocyanines are well-known sensing phases with applications in different scientific fields due to their interesting properties. Detailed characterization by Raman spectroscopy was performed in order to study the shifting of the vibrational bands related to the coordination sphere of each metal phthalocyanine. In this work, a study involving the use of screen-printed electrodes (SPEs) with various metal phthalocyanines to electrochemically detect and quantify chlorine (Cl₂) gas is presented. The Cl₂ gas was generated in-situ via oxidation of the chloride present in form of aqueous salt solutions. The developed method offers not only the possibility to quantify chlorine, but also to discriminate among several chlorinated species due to the changes observed in the voltammetric profiles associated with the interaction between the specie assayed and the phthalocyanine metallic center. Optimization of detecting parameters was also performed to apply this procedure for the quantification of chlorine generated from commercial chlorine tablets. The development of this proof of concept shows interesting possibilities and easy-to-use applications with novel on metal phthalocyanines based SPE sensors.

DFT Study of Molecular and Electronic Structure of Ca(II) and Zn(II) Complexes with Porphyrazine and tetrakis(1,2,5-thiadiazole)porphyrazine

Electronic and geometric structures of Ca(II) and Zn(II) complexes with porphyrazine (Pz) and tetrakis(1,2,5-thiadiazole)porphyrazine (TTDPz) were investigated by density functional theory (DFT) calculations and compared. The perimeter of the coordination cavity was found to be practically independent on the nature of a metal and a ligand. According to the results of the natural bond orbital (NBO) analysis and quantum theory of atoms in molecules (QTAIM) calculations, Ca-N bonds possess larger ionic contributions as compared to Zn-N. The model electronic absorption spectra obtained with the use of time-dependent density functional theory (TDDFT) calculations indicate a strong bathochromic shift (~70 nm) of the Q-band with a change of Pz ligand by TTDPz for both Ca and Zn complexes. Additionally, CaTTDPz was synthesized and its electronic absorption spectrum was recorded in pyridine and acetone.

Excited State Orbital Optimization via Minimizing the Square of the Gradient: General Approach and Application to Singly and Doubly Excited States via Density Functional Theory

We present a general approach to converge excited state solutions to any quantum chemistry orbital optimization process, without the risk of variational collapse. The resulting square gradient minimization (SGM) approach only requires analytic energy/Lagrangian orbital gradients and merely costs 3 times as much as ground state orbital optimization (per iteration), when implemented via a finite difference approach. SGM is applied to both single determinant ΔSCF and spin-purified restricted open-shell Kohn-Sham (ROKS) approaches to study the accuracy of orbital optimized DFT excited states. It is found that SGM can converge challenging states where the maximum overlap method (MOM) or analogues either collapse to the ground state or fail to converge. We also report that ΔSCF/ROKS predict highly accurate excitation energies for doubly excited states (which are inaccessible via TDDFT). Singly excited states obtained via ROKS are also found to be quite accurate, especially for Rydberg states that frustrate (semi)local TDDFT. Our results suggest that orbital optimized excited state DFT methods can be used to push past the limitations of TDDFT to doubly excited, charge-transfer, or Rydberg states, making them a useful tool for the practical quantum chemist's toolbox for studying excited states in large systems.

Benchmarking computational methods and influence of guest conformation on chirogenesis in zinc porphyrin complexes

Different computational methods and influence of the guest conformation and solvent effect to analyze chirogenesis in zinc porphyrins by several chiral compounds have been investigated.

A simple and facile bioinspired catalytic strategy to decolorize dye wastewater by using metal octacarboxyphthalocyanine particles

To explore the simple, facile, environmental friendly and low cost catalytic technique to decolorize harmful dye contaminants in solution and understand the mechanism is an interesting and practical research. In this paper, we provide a highly efficient and convenient method for fast decolorization of dyes (methylene blue and rhodamine B) in aqueous solution catalyzed by iron octacarboxyphthalocyanine (FeOCPc) or cobalt octacarboxyphthalocyanine (CoOCPc). Compared to the traditional methods, our method is very simple. The 30 mg/L methylene blue could be decolorized almost absolutely less than 30 min just by dispersing FeOCPc powders into the dye solution. The decolorization of rhodamine B at high concentration (30 mg/L) could be achieved to 100% decolorization degree less than 20 min in the presence of FeOCPc and tert-butyl hydroperoxide (BuOOH). Moreover, the ESR and HPLC-MS measurement were performed to determine the active radicals and various intermediates in decolorization processes and the possible catalytic mechanism was proposed. It is noted that both FeOCPc and CoOCPc catalysts show the different catalytic oxidation behaviors depending on the oxidant (O₂ or BuOOH). Our investigation provides a novel, low cost and convenient strategy to purify the environmental pollutions.

Strong, near-infrared absorbing porphyrins: a DFT study

A systematic density functional theory study of electronic properties and optical spectra of four potential fused porphyrins with extended π-conjugation has been reported employing the M06 functional along with 6-31G(d)+LanL2DZ basis set. Symmetrical tetra fusion of the benzene rings at the β-meso-β position of porphyrins, resulted in four quadruply fused porphyrins. Such studies revealed the fact that the ring fusion with extended π-electrons causes remarkable red-shifts with narrowed band gaps. Thus, such systems can be used to extend the absorption deeper in to near-infrared (NIR) to even better utilize the photon rich region of solar spectrum. These fused porphyrins have emerged as promising NIR absorbing materials, having utility in molecular solar-conversion systems.

Methodological Survey of Simplified TD-DFT Methods for Fast and Accurate Interpretation of UV-Vis-NIR Spectra of Phthalocyanines

A methodological survey of density functional theory (DFT) methods for the prediction of UV-visible (vis)-near-infrared (NIR) spectra of phthalocyanines is reported. Four methods, namely, full time-dependent (TD)-DFT and its Tamm-Dancoff approximation (TDA), together with their simplified modifications (sTD-DFT and sTDA, respectively), were tested by using the examples of unsubstituted and alkoxy-substituted metal-free ligands and zinc complexes. The theoretical results were compared with experimental data derived from UV-visible absorption and magnetic circular dichroism spectroscopy. Seven popular exchange-correlation functionals (BP86, B3LYP, TPSSh, M06, CAM-B3LYP, LC-BLYP, and ωB97X) were tested within these four approaches starting at a relatively modest level using 6-31G(d) basis sets and gas-phase BP86/def2-SVP optimized geometries. A gradual augmentation of the computational levels was used to identify the influence of starting geometry, solvation effects, and basis sets on the results of TD-DFT and sTD-DFT calculations. It was found that although these factors do influence the predicted energies of the vertical excitations, they do not affect the trends predicted in the spectral properties across series of structurally related substituted free bases and metallophthalocyanines. The best accuracy for the gas-phase vertical excitations was observed in the lower-energy Q-band region for calculations that made use of range-separated hybrids for both full and simplified TD-DFT approaches. The CAM-B3LYP functional provided particularly accurate results in the context of the sTD-DFT approach. The description of the higher-energy B-band region is considerably less accurate, and this demonstrates the need for further advances in the accuracy of theoretical calculations. Together with a general increase in accuracy, the application of simplified TD-DFT methods affords a 2-3 orders of magnitude speedup of the calculations in comparison to the full TD-DFT approach. It is anticipated that this approach will be widely used on desktop computers during the interpretation of UV-vis-NIR spectra of phthalocyanines and related macrocycles in the years ahead.

CASSCF-based explicit ligand field models clarify the ground state electronic structures of transition metal phthalocyanines (MPc; M = Mn, Fe, Co, Ni, Cu, Zn)

Multireference electronic structure methods are used to assign ground state electronic configurations for a series of metallophthalocyanines. Ligand orbital occupancies remain constant across the period and are consistent with a formal 2– charge on the ligand. The d electron configurations of some metallophthalocyanines are straightforward and can be unambiguously assigned, (dxy)2(dxz,dyz)2,2( [Formula: see text])2([Formula: see text])n, with n = 2, 1, 0, respectively, for ZnPc, CuPc, and NiPc. Controversies over ground state electronic structure assignments for other metallophthalocyanines arise due to multiple complicating factors: accidental near-degeneracies, environmental effects, and different ligand field models used in interpreting experimental spectra. We demonstrate that explicit ligand field models provide more reliable and consistent interpretations of experimental data than implicit, parameterized alternatives. On this basis, we assign gas-phase electronic ground states for MnPc, (dxy)2(dxz,dyz)1,1([Formula: see text])1 and CoPc, (dxy)2(dxz,dyz)2,2([Formula: see text])1, and show that the ground state of FePc cannot be resolved to a single state, with two near-degenerate states that are likely spin-orbit coupled: (dxy)2(dxz,dyz)1,1( [Formula: see text])2 and (dxy)2(dxz,dyz)2,1([Formula: see text])1. Remaining differences between computational predictions and experimental observations are small and may be ascribed primarily to environmental effects but are also partly due to incomplete modelling of electron correlation.

Excitonic Coupling and Femtosecond Relaxation of Zinc Porphyrin Oligomers Linked with Triazole Bridge: Dynamics and Modeling

The synthesis of new zinc porphyrin oligomers linked by a triazole bridge was carried out via "click" reaction. A split in the porphyrin oligomer B-band was observed. It was considered as evidence of exciton-excitonic coupling. The relaxation of excited states in Q-band porphyrin oligomers was studied by the femtosecond laser spectroscopy technique with a 20 fs pump pulse. The transient oscillations of two B-band excitonic peaks have a π-radian shift. For explanation of the coherent oscillation, a theoretical model was developed. The model considered the combination of the exciton-excitonic coupling between porphyrin rings in dimer and weak exciton-vibronic coupling in one porphyrin ring. By varying the values of the structural parameters of porphyrins (the strength values of this couplings and measure of symmetry breaking), we obtained correspondence between the experimental data (phase shift and amplitudes of the spectrum oscillations) and the predictions of the model developed here.

Time dependent DFT investigation of the optical properties of artificial light harvesting special pairs

Simulated absorption spectra of (ZnTriPP)₂DPB dimer in which Q band is enhanced 50 times for visibility.

Fine-Tuning of β-Substitution to Modulate the Lowest Triplet Excited States: A Bioinspired Approach to Design Phosphorescent Metalloporphyrinoids

Learning nature's approach to modulate photophysical properties of NIR porphyrinoids by fine-tuning β-substituents including the number and position, in a manner similar to naturally occurring chlorophylls, has the potential to circumvent the disadvantages of traditional "extended π-conjugation" strategy such as stability, molecular size, solubility, and undesirable π-π stacking. Here we show that such subtle structural changes in Pt(II) or Pd(II) cis/trans-porphodilactones (termed by cis/trans-Pt/Pd) influence photophysical properties of the lowest triplet excited states including phosphorescence, Stokes shifts, and even photosensitization ability in triplet-triplet annihilation reactions with rubrene. Prominently, the overall upconversion capability (η, η = ε·Φ(UC)) of Pd or Pt trans-complex is 10(4) times higher than that of cis-analogue. Nanosecond time-resolved infrared (TR-IR) spectroscopy experiments showed larger frequency shift of ν(C═O) bands (ca. 10 cm(-1)) of cis-complexes than those of trans-complexes in the triplet excited states. These spectral features, combining with TD-DFT calculations, suggest the strong electronic coupling between the lactone moieties and the main porphyrin chromophores and thus the importance of precisely positioning β-substituents by mimicking chlorophylls, as an alternative to "extended π-conjugation", in designing NIR active porphyrinoids.

Electronic, bonding, and optical properties of 1d [CuCN]n (n = 1-10) chains, 2d [CuCN]n (n = 2-10) nanorings, and 3d [Cun (CN)n ]m (n = 4, m = 2, 3; n = 10, m = 2) tubes studied by DFT/TD-DFT methods

The electronic, bonding, and photophysical properties of one-dimensional [CuCN](n) (n = 1-10) chains, 2-D [CuCN](n) (n = 2-10) nanorings, and 3-D [Cu(n)(CN)(n)](m) (n = 4, m = 2, 3; n = 10, m = 2) tubes are investigated by means of a multitude of computational methodologies using density functional theory (DFT) and time-dependent-density-functional theory (TD-DFT) methods. The calculations revealed that the 2-D [CuCN](n) (n = 2-10) nanorings are more stable than the respective 1-D [CuCN](n) (n = 2-10) linear chains. The 2-D [CuCN](n) (n = 2-10) nanorings are predicted to form 3-D [Cun (CN)(n)](m) (n = 4, m = 2, 3; n = 10, m = 2) tubes supported by weak stacking interactions, which are clearly visualized as broad regions in real space by the 3D plots of the reduced density gradient. The bonding mechanism in the 1-D [CuCN](n) (n = 1-10) chains, 2-D [CuCN](n) (n = 2-10) nanorings, and 3-D [Cu(n)(CN)(n)](m) (n = 4, m = 2, 3; n = 10, m = 2) tubes are easily recognized by a multitude of electronic structure calculation approaches. Particular emphasis was given on the photophysical properties (absorption and emission spectra) of the [CuCN](n) chains, nanorings, and tubes which were simulated by TD-DFT calculations. The absorption and emission bands in the simulated TD-DFT absorption and emission spectra have thoroughly been analyzed and assignments of the contributing principal electronic transitions associated to individual excitations have been made.

Ground and excited states of zinc phthalocyanine, zinc tetrabenzoporphyrin, and azaporphyrin analogs using DFT and TDDFT with Franck-Condon analysis

The electronic structure of eight zinc-centered porphyrin macrocyclic molecules are investigated using density functional theory for ground-state properties, time-dependent density functional theory (TDDFT) for excited states, and Franck-Condon (FC) analysis for further characterization of the UV-vis spectrum. Symmetry breaking was utilized to find the lowest energy of the excited states for many states in the spectra. To confirm the theoretical modeling, the spectroscopic result from zinc phthalocyanine (ZnPc) is used to compare to the TDDFT and FC result. After confirmation of the modeling, five more planar molecules are investigated: zinc tetrabenzoporphyrin (ZnTBP), zinc tetrabenzomonoazaporphyrin (ZnTBMAP), zinc tetrabenzocisdiazaporphyrin (ZnTBcisDAP), zinc tetrabenzotransdiazaporphyrin (ZnTBtransDAP), and zinc tetrabenzotriazaporphyrin (ZnTBTrAP). The two latter molecules are then compared to their phenylated sister molecules: zinc monophenyltetrabenzotriazaporphyrin (ZnMPTBTrAP) and zinc diphenyltetrabenzotransdiazaporphyrin (ZnDPTBtransDAP). The spectroscopic results from the synthesis of ZnMPTBTrAP and ZnDPTBtransDAP are then compared to their theoretical models and non-phenylated pairs. While the Franck-Condon results were not as illuminating for every B-band, the Q-band results were successful in all eight molecules, with a considerable amount of spectral analysis in the range of interest between 300 and 750 nm. The π-π(∗) transitions are evident in the results for all of the Q bands, while satellite vibrations are also visible in the spectra. In particular, this investigation finds that, while ZnPc has a D4h symmetry at ground state, a C4v symmetry is predicted in the excited-state Q band region. The theoretical results for ZnPc found an excitation energy at the Q-band 0-0 transition of 1.88 eV in vacuum, which is in remarkable agreement with published gas-phase spectroscopy, as well as our own results of ZnPc in solution with Tetrahydrofuran that are provided in this paper.

TD-DFT calculations of the excited states of metalloporphyrins relevant to organic solar photovoltaic cells

The molecular orbital energies and symmetries, electronic state energies and symmetries, and orbital compositions and oscillator strengths for one-photon radiative transitions up to an energy of 4 eV have been calculated by DFT and TD-DFT methods for 15 d0 and d10 metalloporphyrins. Data for both singlet and triplet excited states are reported and used to identify potential candidates for use as photon upconverters by homomolecular triplet–triplet annihilation.

Computational study on zinc porphyrin analogs for use in dye-sensitized solar cells

The density functional theory (DFT) and time-dependent DFT (TD-DFT) approaches have been applied to obtain the optimized geometries, electronic structures, molecular orbitals and absorption spectra of a series of meso-substituted zinc porphyrin analogs with phenyl and thiophene groups as the π bridging unit and cyanoacrylic acid as the acceptor unit. The results showed that the introduction of thiophene group increases the orbital splitting and changes the absorption spectra properties significantly. It is indicated that when there is only one thiophene group included in the π bridge, the oscillator strength of B absorption band is much stronger. The increasing length of thiophene chain just changes the scope of specific absorption enhancement. The effect of attaching an additional electron-donating group diphenylamine instead of phenyl to the porphyrin core also has been shown. It is found that the diphenylamine group reduces the band gap, and leads to facile intramolecular charge transfer from diphenylamine and porphyrin ring unit to acceptor unit. These kinds of zinc porphyrin analogs have the LUMO energy close to the conduction band of TiO 2 and more red-shifted absorption spectrum compared with phenyl substituted analogs.

A systematic theoretical study of the electronic structures of porphyrin dimers: DFT and TD-DFT calculations on diporphyrins linked by ethane, ethene, ethyne, imine, and azo bridges

Theoretical calculations of the geometries, electronic structures and electronic absorption spectra of a series of covalently-linked porphyrin dimers are reported. The diporphyrins comprise 5,10,15-triphenylporphyrinatozinc(II) (ZnTriPP) units linked through the meso carbons by two-atom bridges, namely 1,2-ethanediyl (1), trans-1,2-ethenediyl (2), ethynediyl (3), 1,2-iminomethenediyl (4), and transdiazenediyl (5). The structures were optimised in toluene solvent by Density Functional Theory (DFT), using the integral equation formalism variant of the polarizable continuum model. The calculations were performed using the B3LYP functional and the 6-31G(d,p) basis set. The complete molecules were modelled, with no substitution of smaller groups on the periphery. In parallel, the compounds 2–5 were prepared by known or novel synthetic routes, to enable comparisons of experimental electronic absorption spectra with those calculated using time dependent-DFT at the same level of theory. As the ethane dimer 1 is not yet synthetically accessible, the model monomer meso-2-phenylethylZnTriPP was used for comparisons with the theoretical predictions. The results form a self-consistent set, enabling for the first time legitimate comparisons of the electronic structures of the series, especially regarding the degree to which the porphyrin p-systems interact by conjugation across the bridges. The theoretical calculations of the electronic transitions match the observed spectra in toluene to a remarkable degree, especially with respect to the peak maximum of the Q band, which represents to a large degree the energy of the HOMO–LUMO transition. The imine 4 is intrinsically polar due to the asymmetric bridge, and the HOMO is located almost exclusively on the ZnTriPP unit attached to the nitrogen of the imine, and the LUMO on the C-attached ring. Thus the Q-band transition is mapped as a comprehensive charge-transfer from the former ring to the latter. This may have consequences for the non-linear optical properties of the system. The azoporphyrin 5 exhibits the largest splittings between the interacting MOs via the conjugated bridge, vindicating a prediction by Anderson and co-workers in 2002, and confirmed experimentally by our synthesis of 5. The collected results also indicate that this level of theory is more thanadequate as a model with which to handle these large delocalised molecules.

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.

Photophysics of Soret-excited free base tetraphenylporphyrin and its zinc analog in solution

Photophysical properties of free base tetraphenylporphyrin and its zinc analog are investigated in detail in solvents of varying polarity by using steady state and time-resolved techniques. Both the porphyrins are excited at the Soret band to have better signal-to-noise ratio. Also, the fluorescence emission measurements are carried out by using dilute solutions (~10(-7)mol/L) of the fluorophores in order to minimize the self-quenching effect. It is observed that the steady state absorption and emission characteristics of the porphyrin molecules are mainly affected by polarizability (via refractive index) rather than polarity (via dielectric constant) of the moderate to highly polar solvents. As the molecules are highly symmetric in the ground state, the associated dipole moments are found to be very low from quantum chemical calculations performed by density functional theory method by using Gaussian 03 package. The dipole moments associated with the first excited singlet state of the porphyrins are computed by applying solvatochromic Stokes' shift method. To the best of our knowledge, this is the first attempt to calculate the excited state dipole moments of the porphyrins used in the present investigations. Also, fluorescence quantum yield, fluorescence lifetime of the first excited singlet state, radiative and non-radiative rate constants of the porphyrins are reported in solvents of varying polarity.

Hydrophilic annulated dinuclear zinc(II) phthalocyanine as Type II photosensitizers for PDT: a combined experimental and (TD)-DFT investigation

The design, synthesis and theoretical calculations of a mononuclear and its corresponding annulated dinuclear hydrophilic zinc(II) phthalocyanines (ZnPcs) are reported here, the hydrophilic substituent being 1,3-bis((2,2-dimethyl-1,3-dioxolan-4-yl)methoxy)propan-2-ol. These novel compounds have been characterized by UV-vis spectroscopy in ethanol and ethanol:water (50:50) solutions. The electronic spectra of the mononuclear derivatives retain the main spectral features of ZnPc with a strong Q-band absorbance at 678 nm in ethanol and a less intense B-band at 370 nm. The annulated dinuclear derivatives show a broad Q-band absorbance with main wavelength maxima between 600 and 720 nm. From time-dependent density functional (TDDFT) UV-vis spectra simulation performed on selected isomers, Q-band absorption maxima of the non-peripherally substituted forms are more red-shifted compared to those of peripheral forms.

Tetraaryltetraanthra[2,3]porphyrins: synthesis, structure, and optical properties

A synthetic route to symmetrical tetraaryltetraanthra[2,3]porphyrins (Ar(4)TAPs) was developed. Ar(4)TAPs bearing various substituents in meso-phenyls and anthracene residues were prepared from the corresponding pyrrolic precursors. The synthesized porphyrins possess high solubility and exhibit remarkably strong absorption bands in the near-infrared region (790-950 nm). The scope of the method, selection of the peripheral substituents, choice of the metal, and their influence on the optical properties are discussed together with the first X-ray crystallographic data for anthraporphyrin.