Photophysics of Soret-Excited Tetrapyrroles in Solution. II. Effects of Perdeuteration, Substituent Nature and Position, and Macrocycle Structure and Conformation in Zinc(II) Porphyrins

Motional Narrowing through Photonic Exchange: Rational Suppression of Excitonic Disorder from Molecular Cavity Polariton Formation

Maximizing the coherence between the constituents of molecular materials remains a crucial goal toward the implementation of these systems into everyday optoelectronic technologies. Here we experimentally assess the ability of strong light-matter coupling in the collective limit to reduce energetic disorder using porphyrin-based chromophores in Fabry-Pérot (FP) microresonator structures. Following characterization of cavity polaritons formed from chemically distinct porphyrin dimers, we find that the peaks corresponding to the lower polariton (LP) state in each sample do not possess widths consistent with conventional theories. We model the behavior of the polariton peak widths effectively using the results of spectroscopic theory. We correlate differences in the suppression of excitonic energetic disorder between our samples with microscopic light-matter interactions and propose that the suppression stems from photonic exchange. Our results demonstrate that cavity polariton formation can suppress disorder and show researchers how to design coherence into hybrid molecular material systems.

Synthetic Control of Exciton Dynamics in Bioinspired Cofacial Porphyrin Dimers

Understanding how the complex interplay among excitonic interactions, vibronic couplings, and reorganization energy determines coherence-enabled transport mechanisms is a grand challenge with both foundational implications and potential payoffs for energy science. We use a combined experimental and theoretical approach to show how a modest change in structure may be used to modify the exciton delocalization, tune electronic and vibrational coherences, and alter the mechanism of exciton transfer in covalently linked cofacial Zn-porphyrin dimers (meso-beta linked ABm-β and meso-meso linked AAm-m). While both ABm-β and AAm-m feature zinc porphyrins linked by a 1,2-phenylene bridge, differences in the interporphyrin connectivity set the lateral shift between macrocycles, reducing electronic coupling in ABm-β and resulting in a localized exciton. Pump-probe experiments show that the exciton dynamics is faster by almost an order of magnitude in the strongly coupled AAm-m dimer, and two-dimensional electronic spectroscopy (2DES) identifies a vibronic coherence that is absent in ABm-β. Theoretical studies indicate how the interchromophore interactions in these structures, and their system-bath couplings, influence excitonic delocalization and vibronic coherence-enabled rapid exciton transport dynamics. Real-time path integral calculations reproduce the exciton transfer kinetics observed experimentally and find that the linking-modulated exciton delocalization strongly enhances the contribution of vibronic coherences to the exciton transfer mechanism, and that this coherence accelerates the exciton transfer dynamics. These benchmark molecular design, 2DES, and theoretical studies provide a foundation for directed explorations of nonclassical effects on exciton dynamics in multiporphyrin assemblies.

Two-photon absorption spectrum and characterization of the upper electronic states of the dye IR780

In this work, the full two-photon absorption (2PA) spectrum of cyanine dye IR780 in methanol was measured and some important properties of the upper excited electronic states were investigated. Specifically, two IR780 2PA bands of intensities nearing 140 and 2800 Goeppert-Mayer (GM) were found. In order to determine the optical properties of the upper electronic singlet states, a deconvolution of the absorption peaks in the UV region of the spectrum was made. Based on this, properties such as transition dipole moments, oscillator strengths, absorption maxima in the UV-vis spectra, S₂-S₁ vibrational couplings and predictions of the lifetime of the second excited state were calculated. Moreover, by combining experimental and computational results, the 2PA transitions were assigned to the upper excited states S₂ and S₄. Cross-section magnitudes, positions and shapes of the 2PA bands have been satisfactorily explained with a four-state model that comprises the singlet states S₁, S₂ and S₄. From these results, the cyanine investigated in the present work could be used as a novel and interesting moiety for more complex systems that respond to two-photon excitation.

Ultrafast Dynamics of Nonrigid Zinc-Porphyrin Arrays Mimicking the Photosynthetic "Special Pair"

Conjugated porphyrin arrays are heavily investigated as efficient molecular systems for photosynthesis and photocatalysis. Recently, a series of one-, two-, and six-zinc-porphyrin arrays, noncovalently linked through benzene-based hubs, have been synthesized with the aim of mimicking the structure and function of the bacteriochlorophyll "special pair" in photosynthetic reaction centers. The excitonically coupled porphyrin subunits are expected to activate additional excited state relaxation channels with respect to the monomer. Here, we unveil the appearance of such supramolecular electronic interactions using ultrafast transient absorption spectroscopy with sub-25 fs time resolution. Upon photoexcitation of the Soret band, we resolve energy trapping within ∼150 fs in a delocalized dark excitonic manifold. Moreover, excitonic interactions promote an additional fast internal conversion from the Q-band to the ground state with an efficiency of up to 60% in the hexamer. These relaxation pathways appear to be common loss channels that limit the lifetime of the exciton states in noncovalently bound molecular aggregates.

Diphenyl functional porphyrins and their metal complexes as visible-light photoinitiators for free-radical, cationic and thiol–ene polymerizations

Diphenyl functional porphyrin derivatives as new visible-light photoinitiators for efficient free-radical, cationic and thiol–ene polymerizations.

A quercetin-based flavanoid (rutin) reverses amyloid fibrillation in β-lactoglobulin at pH 2.0 and 358 K

β-lactoglobulin (BLG) is a well characterized milk protein and a model for folding and aggregation studies. Rutin is a quercetin based-flavanoid and a famous dietary supplement. It is a potential protector from coronary heart disease, cancers, and inflammatory bowel disease. In this study, amyloid fibrillation is reported in BLG at pH 2.0 and temperature 358 K. It is inhibited to some extent by rutin with a rate of 99.3 h^-1 M^-1. Amyloid fibrillation started taking place after 10 h of incubation and completed near 40 h at a rate of 16.6 × 10^-3 h^-1, with a plateau during 40-108 h. Disruption of tertiary structure of BLG and increased solvent accessibility of hydrophobic core seem to trigger intermolecular assembly. Increase in 7% β-sheet structure at the cost of 10% α-helical structures and the electron micrograph of BLG fibrils at 108 h further support the formation of amyloid. Although it could not block amyloidosis completely, and even the time required to reach plateau remains the same, a decrease of growth rate from 16.6 × 10^-3 to 13.5 × 10^-3 h^-1 was observed in the presence of 30.0 μM rutin. Rutin seems to block solvent accessibility of the hydrophobic core of BLG. A decrease in the fibril population was observed in electron micrographs, with the increase in rutin concentration. All evidences indicate reversal of fibrillation in BLG in the presence of rutin.

Prospects for efficient solar energy upconversion using metalloporphyrins as dual absorber-upconverters

The novel potential use of selected metalloporphyrins as dual absorber-upconverters in solar photovoltaics is discussed. Additional efficiencies are available if use can be made of the porphyrin's short-lived S2 state, which is formed directly by excitation in the strong Soret transition in the blue-violet and also by absorption in the Q bands followed by rapid intersystem crossing and upconversion by triplet-triplet annihilation. The main challenge in realizing a working photovoltaic based on such a protocol is that energy must be extracted from the S2 state of the porphyrin within its picosecond lifetime. The structure-property relationships that may be used to select metalloporphyrins with the longest possible intrinsic lifetimes are outlined. The prospects for energy extraction from S2via ultrafast electron transfer or ultrafast resonant electronic energy transfer within a solid structure designed to maximize efficiency are discussed. Both MOF and pendant porphyrin polymer structures offer reasonable possibilities.

Comment on "Metal-organic green dye: chemical and physical insight into a modified Zn-benzoporphyrin for dye-sensitized solar cells" by G. Zanotti, N. Angelini, G. Mattioli, A. M. Paoletti, G. Pennesi, G. Rossi, D. Caschera, L. de Marcoc and G. Giglide, RSC Adv., 2016, , 5123

The emission spectroscopy and photophysics reported in the title paper are shown to be untenable in light of previously reported experimental results and widely accepted theories of electronic excited state relaxation. The published results strongly suggest that the newly synthesized dye is contaminated with one of more highly fluorescent impurities.

Determinants of the efficiency of photon upconversion by triplet-triplet annihilation in the solid state: zinc porphyrin derivatives in PVA

Spectroscopic, photophysical and computational studies designed to expose and explain the differences in the efficiencies of non-coherent photon upconversion (NCPU) by triplet-triplet annihilation (TTA) have been carried out for a new series of alkyl-substituted diphenyl and tetraphenyl zinc porphyrins, both in fluid solution and in solid films. Systematic variations in the alkyl-substitution of the phenyl groups in both the di- and tetraphenyl porphyrins introduces small, but well-understood changes in their spectroscopic and photophysical properties and in their TTA efficiencies. In degassed toluene solution TTA occurs for all derivatives and produces the fluorescent S₂ product states in all cases. In PVA matrices, however, none of the di-phenylporphyrins exhibit measurable NCPU whereas all the tetraphenyl-substituted compounds remain upconversion-active. In PVA the NCPU efficiencies of the zinc tetraphenylporphyrins vary significantly with their steric characteristics; the most sterically crowded tetraphenyl derivative exhibits the greatest efficiency. DFT-D computations have been undertaken and help reveal the sources of these differences.

Interaction of indomethacin with calf thymus DNA: a multi-spectroscopic, thermodynamic and molecular modelling approach

Indomethacin belongs to the acetic acid derivative class of non-steroidal anti-inflammatory drugs with diverse pharmacological and biological activities. Understanding the mechanism of interaction of drugs with possible target and off-target biomolecules can prove useful in the development of a rational drug designing system. In this paper, we have attempted to ascertain the mode of binding of indomethacin with calf thymus DNA (Ct-DNA) through various biophysical techniques and in silico molecular docking. Analysis of the UV-visible absorbance spectra and fluorescence emission profile of indomethacin upon addition of Ct-DNA indicates the formation of a drug-DNA complex. UV-visible absorbance and steady state fluorescence experiments revealed a binding constant on the order of 103 L mol-1, which is consistent with those of well-known groove binders. Competitive displacement studies with ethidium bromide, acridine orange and Hoechst 33258 further suggested that indomethacin binds to the minor groove of the Ct-DNA. The above observations were further confirmed by KI induced quenching experiments, DNA melting studies, CD spectral analysis and viscosity measurements. The thermodynamic parameters like spontaneous free energy (ΔG < 0) and large favourable enthalpy (ΔH < 0) obtained from isothermal calorimetry indicated the involvement of hydrogen bonding and van der Waals forces in the binding process. Molecular docking further corroborated the experimental results.

Elucidating the interaction of sulindac with calf thymus DNA: biophysical and in silico molecular modelling approach

Sulindac is one of the most effective NSAIDs and belongs to the arylalkanoic acid class.

Ultrafast Relaxation Dynamics of Photoexcited Zinc-Porphyrin: Electronic-Vibrational Coupling

Cyclic tetrapyrroles are the active core of compounds with crucial roles in living systems, such as hemoglobin and chlorophyll, and in technology as photocatalysts and light absorbers for solar energy conversion. Zinc-tetraphenylporphyrin (Zn-TPP) is a prototypical cyclic tetrapyrrole that has been intensely studied in past decades. Because of its importance for photochemical processes the optical properties are of particular interest, and, accordingly, numerous studies have focused on light absorption and excited-state dynamics of Zn-TPP. Relaxation after photoexcitation in the Soret band involves internal conversion that is preceded by an ultrafast process. This relaxation process has been observed by several groups. Hitherto, it has not been established if it involves a higher lying "dark" state or vibrational relaxation in the excited S2 state. Here we combine high time resolution electronic and vibrational spectroscopy to show that this process constitutes vibrational relaxation in the anharmonic S2 potential.

Control of local structures and photophysical properties of zinc porphyrin-based supramolecular assemblies structurally organized by regioselective ligand coordination

Nano- and micro-sized molecular assemblies of zinc porphyrins [5,10,15,20-tetrakis(4-carboxyphenyl)porphyrinato-zinc(II) (ZnTCPP)] utilizing bridging nitrogen ligands such as diazabicycro[2.2.2]octane (DABCO) were prepared to demonstrate the regioselective coordination by two different synthetic strategies such as (I) the solvothermal method and (II) the colloidal metal organic framework (MOF) method. The initial organization process is a planar checkerboard patterned formation (2D platform) of zinc porphyrins organized by paddlewheel secondary building units (PSBUs) between carboxylate and zinc ions. Then, DABCO moieties are decorated on zinc atoms in the metal centres of the porphyrin rings (m-cPDC) in the solvothermal method, whereas the metal centres in the porphyrin rings (n-uPDC) remain uncoordinated in the colloidal MOF method. These internal structural changes between m-cPDC and n-uPDC are in sharp contrast with the corresponding reference systems using ZnTCPP and a 4,4'-bipyridine (BPY) ligand (i.e., m-cPBC and n-cPBC). Concretely, the metal centres of zinc porphyrins in n-uPDC were unsaturated and uncoordinated with the DABCO ligands, which was confirmed by XRD and steady-state spectroscopic measurements. These different coordination features have great effect on the spectroscopic and photophysical properties. For example, the average fluorescence lifetime of m-cPDC is much smaller than that of n-uPDC because of the acceleration of nonradiative processes, which are highly related with the coordination of DABCO to the Zn(II) centre of the ZnTCPP unit. Finally, fluorescence quenching experiments via photoinduced electron transfer (PET) utilizing an electron acceptor: benzoquinone (BQ) were performed. The apparent association constant (Kapp) of n-uPDC is larger than that of m-cPDC. This suggested that the unsaturated ZnTCPP units embedded in n-uPDC easily accommodate guest molecules as compared to the other systems.

Substrate Dependent Native Luminescence from Cytochromes P450 3A4, 2C9, and P450cam

Metalloporphyrin containing proteins, such as cytochrome P450, play a key role in biological systems. The spectroscopic properties of metalloporphyrins have been a subject of intense interest and intense debate for over 50 years. Iron-porphyrins are usually believed to be nonfluorescent. Herein we report that, contrary to this belief, cytochrome P450 heme groups luminesce with enough intensity to be of use in the characterization of these enzymes. To confirm that the emission is from the heme, we destroyed the heme by titration with cumene hydroperoxide and measured the changes in emission upon titration with compounds known to bind to the distal face of the heme in two human cytochrome P450 enzymes, known as CYP3A4 and CYP2C9. The titration curves gave spectral dissociation constants that were not significantly different from those reported using the Soret UV/vis absorbance changes. We have tentatively assigned the broad luminescence at ∼500 nm to a (1)ππ* → gs fluorescence and the structured luminescence above 600 nm to a (3)ππ* → gs phosphorescence. These assignments are not associated with the Q-band, and are in violation of Kasha's rule. To illustrate the utility of the emission, we measured spectral dissociation constants for testosterone binding to P450 3A4 in bilayers formed on glass coverslips, a measurement that would be very difficult to make using absorption spectroscopy. Complementary experiments were carried out with water-soluble P450cam.

Influence of Phenylethynylene of Push-Pull Zinc Porphyrins on the Photovoltaic Performance

A series of zinc porphyrin dyes YD22-YD28 were synthesized and used for dye-sensitized solar cells. Dyes YD26-YD28 consist of zinc porphyrin (ZnP) as core unit, arylamine (Am) as electron-donating group, and p-ethynylbenzoic acid (EBA) as an electron-withdrawing/-anchoring group. The dyes YD22-YD25 contain additional phenylethynylene group (PE) bridged between Am and ZnP units. The influence of the PE unit on molecular properties as well as photovoltaic performances were investigated via photophysical and electrochemical studies and density functional calculations. With the insertion of PE unit, the dyes YD22-YD25 possess better light-harvesting properties in terms of significantly red-shifted Q-band absorption. The conversion efficiencies for dyes YD22-YD25 are better than those of dyes YD26-YD28 owing to larger J(SC) output. Natural transition orbitals and Mulliken charge analysis were used to analyze the electron injection efficiency for porphyrin dyes upon time-dependent DFT calculations. The results indicated that insertion of additional PE unit is beneficial to higher J(SC) by means of improved light-harvesting property due to broadened and red-shifted absorption.

Concerning correct and incorrect assignments of Soret (S2-S0) fluorescence in porphyrinoids: a short critical review

The relaxation dynamics of electronically excited porphyrinoids are often measured by steady-state and time-resolved fluorescence methods. The unusual occurrence of measurable fluorescence from an upper excited singlet state (often identified as the second electronically excited singlet state, S2) of some porphyrins has, in recent years, prompted a spate of mis-assignments of observed emission from other porphyrinoids excited in the near UV-violet regions of the spectrum. The criteria for correctly assigning fluorescence to a Soret excited state are reviewed. Questionable and mis-assigned reports are identified.

DFT and TD-DFT calculations of axially substituted tin porphyrins and an ethynyl-linked tin porphyrin dimer

Time-dependent density functional theory calculations have been employed to characterize the inter-macrocycle interactions introduced when two tin(IV) porphyrins are axially covalently bonded via an ethynyl linker. The effect of changing the relative orientations of the two metalloporphyrin macrocycles is explored, with particular attention given to electronic states at energies up to the Soret-correlated states in the dimer. Expansion of the Gouterman four orbital approach for the monomer to an eight orbital approach in the dimer produces a satisfactory analysis. The results are used to examine the feasibility of using such linked structures in photon harvesting schemes.

Photophysical properties of electron-deficient free-base corroles bearing meso-fluorophenyl substituents

The ultrafast photophysical behaviors of a series of meso-flurophenyl substituted electron-deficient free base corroles F₀C, F₅C, F₁₀C and F₁₅C in toluene have been investigated using femtosecond time resolved absorption spectroscopy and steady spectroscopies.

Synthesis of 2-nitro-3-(pyrrol-1-yl)-5,10,15,20-tetraarylporphyrins via a Clauson-Kaas reaction and the study of their electronic properties

Versatile synthesis of new porphyrin building blocks, 2-nitro-3-(pyrrol-1-yl)-5,10,15,20-tetraarylporphyrins is described. These porphyrins demonstrate red shifted absorption bands and S₂ emission.

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.

Photocurrent enhancement by multilayered porphyrin sensitizers in a photoelectrochemical cell

Multilayer Zn(II) tetraphenylporphyrin chromophores, assembled using copper-catalyzed azide-alkyne cycloaddition (CuAAC), provide a new sensitization scheme that could be useful in dye-sensitized solar cells (DSSCs). We report on the photoelectrochemical responses of multilayer films of Zn(II) 5,10,15,20-tetra(4-ethynylphenyl)porphyrin (1) assembled on planar ITO substrates operating as a p-type DSSC using three different redox mediators. The traditional I(-)/I3(-) redox couple results in the greatest short circuit current densities (JSC) but very low open circuit potentials (VOC). The use of cobalt sepulchrate ([Co(sep)](2+/3+)) and cobalt tris-bipyridine ([Co(bpy)3](2+/3+)) as redox mediators generates higher VOC values, but at the expense of lower photocurrents. These results highlight the inherent differences in the interactions between the redox mediator and Zn(II) tetraphenylporphyrin multilayer films. Increasing the porphyrin content through multilayer growth proved to be effective in increasing the performance of photoelectrochemical cells with all three redox mediators. Cells using I(-)/I3(-) reached maximum performance (power output) at five porphyrin layers, [Co(bpy)3](2+/3+) at five layers, and [Co(sep)](2+/3+) at three layers. For all mediators, JSC increases with the addition of porphyrin layers beyond a monolayer. However, JSC reaches a maximum value at a point greater than one layer after which it decreases, presumably due to exciton diffusion limitations and the insulating effects of the multilayer film. Similarly, all cells also reach a maximum VOC beyond one porphyrin layer. We show that porphyrin arrays assembled using newly developed CuAAC layer-by-layer growth may be useful as a multilayer sensitization scheme for use in photoelectrochemical cells.

Photophysics of soret-excited tin(IV) porphyrins in solution

The photophysics of low-chlorin tin(IV) tetraphenylporphyrin dihydroxide, a core building block for axially substituted supramolecular tin porphyrin constructs, has been studied in a variety of hydrogen-bonding, nonpolar, and aprotic polar solvents using steady-state, nanosecond, and femtosecond time-resolved emission, and femtosecond time-resolved absorption methods. In hydrogen-bonding solvents the metalloporphyrin exists as solvated monomers, and its Soret-excited S2 state in these solvents exhibits the expected linear energy gap law relationship with first-order population decay times in the 0.8 to 1.7 ps range. Evidence is presented that this metalloporphyrin aggregates in other solvents at the concentrations typically used for these ultrafast measurements and yields species-averaged time-resolved data. Cw laser excitation in the Q-band under deaerated conditions produces weak S2-S0 fluorescence (photon upconversion) as a result of triplet-triplet annihilation.

Photophysical and ligand binding studies of metalloporphyrins bearing hydrophilic distal superstructure

UV-vis absorption and emission studies on zinc and iron porphyrin complexes bearing H-bonding distal superstructures have been performed in two different organic solvents- tetrahydrofuran (THF) (coordinating) and dichloromethane (DCM) (non-coordinating). Quantum yields and lifetimes have been measured for these complexes which are in good agreement with the other reported metalloporphyrins. Binding affinities with anionic ligands such as N3- , CN- , S-2 , F- were monitored for these two complexes in aqueous media and the respective binding constant values were calculated. The Zn complex shows more selectivity towards cyanide while the Fe complex shows more selectivity towards azide.

Ultrafast long-distance excitation energy transport in donor-bridge-acceptor systems

The excited-state dynamics of two energy donor-bridge-acceptor (D-B-A) systems consisting of a zinc tetraphenylporphyrin (ZnP) and a free base tetraphenylporphyrin (FbP) bridged by oligo-p-phenyleneethynylene units with different substituents has been investigated using ultrafast spectroscopy. These systems differ by the location of the lowest singlet excited state of the bridge, just above or below the S(2) porphyrin states. In the first case, Soret band excitation of the porphyrins is followed by internal conversion to the local S(1) state of both molecules and by a S(1) energy transfer from the ZnP to the FbP end on the 10 ns time scale, as expected for a center-to-center distance of about 4.7 nm. On the other hand, if the bridge is excited, the energy is efficiently transferred within 1 ps to both porphyrin ends. Selective bridge excitation is not possible with the second system, because of the overlap of the absorption bands. However, the time-resolved spectroscopic data suggest a reversible conversion between the D*(S(2))-B-A and D-B*(S(1))-A states as well as a transition from the D-B*(S(1))-A to the D-B-A* states on the picosecond time scale. This implies that the local S(2) energy of the ZnP end can be transported stepwise to the FbP end, i.e., over about 4.7 nm, within 1 ps with an efficiency of more than 0.2.

Photophysics of a cis axially disubstituted macrocycle: rapid intersystem crossing in a tin(IV) phthalocyanine with a half-domed geometry

We have studied the photophysical properties of a tin(IV) phthalocyanine which coordinates two myristate groups through their carboxylate functionalities in a cis disposition at the tin center. Such a coordination mode, anisobidentate through the same side of the macrocycle, makes this phthalocyanine acquire a capped or half-domed shape. This bis myristate tin(IV) molecule shows an intersystem crossing channel which populates the triplet manifold with high efficiency and with a time constant of 300 ps, about an order of magnitude faster than planar phthalocyanines, including some previously reported tin(IV) phthalocyanines. For comparison purposes, we also include the description of a planar silicon(IV) phthalocyanine that keeps the more common stereochemistry, of trans type, with the same axial myristate groups. The characterization of these systems included steady state and time-resolved spectroscopy through femtosecond fluorescence up-conversion and transient absorption. We also studied the initial S(n) → S(1) internal conversion dynamics when these compounds are excited to upper states with 387.5 nm light. In addition, we include measurements of the rate for singlet oxygen production through the formation of an ESR-active adduct in aerated solutions. Such measurements indicate that, associated to its photophysics, the tin(IV) phthalocyanine produces (1)O(2) with an efficiency significantly larger than the silicon(IV) counterpart, making it an interesting option for sensitization applications. Finally, we performed excited state calculations at the TD-DFT level which describe the effects of the reduced symmetry together with the state ordering and indicate the presence of near dark intermediate states between the Q and B transitions for both of these macrocycles.