Prospects for useful fission from singlet states higher than S1 in aggregated organic chromophores.

The few known reports and the likely prospects of finding new efficient routes to exciton fission from higher excited singlet states, Sn (n > 1), are reviewed. Aggregates of molecules that have large S2-S1 electronic energy spacings and/or emit measurable "contra-Kasha" emission may offer further opportunities. Among these, electronically excited molecular systems that exhibit known efficient (T1 + T1) triplet-triplet annihilation producing S2 could exhibit efficient singlet fission in aggregates when appropriately substituted to meet the necessary energy requirements. The potential problem of loss of triplet excitons via 2T1 → Tn>1 + S0 triplet-triplet annihilation following (S2 + S0) singlet fission is addressed. Aggregates of substituted azulenes and aliphatic thiones and dithiones are particularly attractive and are discussed in detail.

Spectroscopic and Dynamic Properties of Electronically Excited Pendant Porphyrin Polymers with Backbones of Differing Flexibility.

A zinc porphyrin-pendant norbornene polymer with a rigid backbone characterized by a 2:1 E/Z isomeric structure ratio has been synthesized, and its spectroscopic and photophysical properties are examined. Zinc tetraphenylporphyrin, the porphyrin-substituted norbornene monomer, and a previously reported zinc porphyrin-pendant polymer with a flexible polymethylene backbone have been used as comparators. Unlike its flexible counterpart, the rigid norbornene polymer exhibits clear exciton splitting of its Soret band, much more rapid relaxation rates of its excited singlet states, and a very small yield of an unusually short-lived triplet state. Unlike the flexible pendant polymer, which exhibits excimeric S2 fluorescence as a result of chromophore rotation, anti-Kasha emission from the norbornene polymer originates primarily from the unperturbed porphyrin E region. The low triplet yield in the polymer is attributed to greatly increased rates of competing internal conversion within the singlet manifold. Nevertheless, upconverted delayed fluorescence that is quenched by oxygen is observed upon intense steady-state Q-band excitation of degassed polymer solutions, signaling direct triplet involvement. Consistent with the polymer's rigid structure, this biexcitonic process is assigned to ultrafast singlet exciton migration and triplet-triplet annihilation following absorption of a second photon by the small steady-state concentration of polymer triplets.

Exciton Dynamics of Photoexcited Pendant Porphyrin Polymers in Solution and in Thin Films.

Several new polymers with rotatable zinc porphyrin pendants have been synthesized and their optical spectroscopic and photophysical properties, including upconversion efficiencies, determined in both fluid solution and thin films. Comparisons made with the ß-substituted zinc tetraphenylporphyrin monomers and ZnTPP itself reveal that the yield of triplets resulting from either Q-band or Soret-band excitation of the polymers is surprisingly small. A detailed kinetic analysis of the fluorescence decays and transient triplet absorptions of the substituted monomers and their corresponding polymers reveals that this phenomenon is due to two parallel internal singlet quenching processes assigned to transient intrachain excimer formation. Consequently, the yields of upconverted S2 fluorescence resulting from Q-band excitation in the degassed polymers are significantly diminished in both fluid solution and thin films. Implications of these results for the design of polymer upconverting systems are discussed.

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, 6, 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.

Photophysics of Zinc Porphyrin Aggregates in Dilute Water-Ethanol Solutions.

Dimeric and multimeric aggregates of a model metalloporphyrin, zinc tetraphenylporphyrin (ZnTPP), have been produced in a controlled manner by incrementally increasing the water content of dilute aqueous ethanol solutions. Steady state absorption, fluorescence emission, and fluorescence excitation spectra have been measured to identify the aggregates present as a function of solvent composition. The dynamics of the excited states of the aggregates produced initially by excitation in the Soret region have been measured by ultrafast fluorescence upconversion techniques. Only the monomer produces measurable emission from S2 with a picosecond lifetime; all Soret-excited aggregates, including the dimer, decay radiationlessly on a femtosecond time scale. The S1 state is the only significant product of the radiationless decay of the S2 state of the excited monomer, and the aggregates also produce substantial quantum yields of S1 fluorescence when initially excited in the Soret region. The resulting fluorescent aggregates all decay on a subnanosecond time scale, likely by a mechanism that involves dissociation of the excited monomer from the excitonic multimer. The ZnTPP dimers excited at their ground state geometries in the Soret region exhibit a dynamic behavior that is quite different from those produced following noncoherent triplet-triplet annihilation under the same conditions. The important implications of these observations in determining the aggregation conditions promoting efficient photon upconversion by excitonic annihilation in a variety of media are thoroughly discussed.

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.

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

Computational modeling of selected artificial special pairs has been carried out. The structures chosen are bio-inspired molecular models of the light harvesting system II that have been previously investigated experimentally. Time-dependent density functional theory calculations have been employed to characterize the inter-macrocycle interactions resulting from two zinc porphyrins that are covalently linked with rigid linkers that vary the inter-porphyrin distance and the inter-planar angle in a C2v framework. The effects of varying the linker structure have been explored for electronic states with energies up to and including the Soret-correlated states in the dimer. An expansion of the Gouterman four orbital model for the monomers to an eight orbital model in the dimers provides a reasonable explanation of the inter-macrocycle interactions and provides insight into their experimental properties.

White light generation using Förster resonance energy transfer between 3-hydroxyisoquinoline and Nile Red.

Simple composite films consisting of a polymer blended with organic emitters have the potential for broad-band "white" light emission that can be used for general lighting applications. In the present work, a simple mixture of 3-hydroxyisoquinoline (HIQ) with Nile Red (NR) in a polymeric matrix of polyvinyl alcohol (PVA) is used to generate white light through a non-radiative excitation energy transfer (NREET) mechanism. NREET between HIQ and NR doped in PVA films is investigated using a combination of steady state and time resolved fluorescence spectroscopic methods. It is observed that NR has very weak fluorescence in the PVA film upon excitation at 400 nm, but upon mixing NR with HIQ, sensitized emission of NR is observed with decreased emission of HIQ. The behavior of the sensitized emission of NR is consistent with Förster resonance energy transfer (FRET) between the donor HIQ and acceptor NR. By adjusting the relative fractions of HIQ and NR in the films, the extent of FRET could be regulated and the overall film emission color could be manipulated to enable overall "white" (CIE color coordinates 0.34, 0.38) emission. The films showed excellent photostability with 405 nm diode illumination, along with mechanical flexibility, suggesting good potential utility as a down converting element for lighting applications.

Spectroscopic and Structural Studies of a Surface Active Porphyrin in Solution and in Langmuir-Blodgett Films.

Controlling aggregation of the dual sensitizer-emitter (S-E) zinc tetraphenylporphyrin (ZnTPP) is an important consideration in solid state noncoherent photon upconversion (NCPU) applications. The Langmuir-Blodgett (LB) technique is a facile means of preparing ordered assemblies in thin films to study distance-dependent energy transfer processes in S-E systems and was used in this report to control the aggregation of a functionalized ZnTPP on solid substrates. This was achieved by synthetic addition of a short polar tail to one of the pendant phenyl rings in ZnTPP in order to make it surface active. The surface active ZnTPP derivative formed rigid films at the air-water interface and exhibited mean molecular areas consistent with approximately vertically oriented molecules under appropriate film compression. A red shift in the UV-vis spectra as well as unquenched fluorescence emission of the LB films indicated formation of well-ordered aggregates. However, NCPU, present in the solution phase, was not observed in the LB films, suggesting that NCPU from ZnTPP as a dual S-E required not just a controlled aggregation but a specific orientation of the molecules with respect to each other.

Spectroscopic and photophysical study of the demetallation of a zinc porphyrin and the aggregation of its free base in a tetraalkylphosphonium ionic liquid.

Dissolving zinc tetraphenylporphyrin in the tetraalkylphosphonium chloride ionic liquid P4448Cl results in progressive demetallation of the solute and quantitative production of the free base porphyrin. Aggregation of the free base occurs in which the monomer and J aggregates are in fully reversible thermal equilibrium in the ionic liquid. The thermodynamic, kinetic and spectroscopic behaviour of this system is described based on absorption, emission and excited state lifetime measurements. Both the thermodynamics of the ground state aggregation and the kinetics of the excited state relaxation processes are unusual due to the particular role played by the ionic liquid solvent.

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.

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.

Efficiency of noncoherent photon upconversion by triplet-triplet annihilation: the C60 plus anthanthrene system and the importance of tuning the triplet energies.

As part of a continuing effort to find noncoherent photon upconversion (NCPU) systems with improved energy conversion efficiencies, the photophysics of the blue emitter, anthanthrene (An), and the fullerene absorber-sensitizer, C60, have been examined by both steady-state and pulsed laser techniques. An is a promising candidate for NCPU by homomolecular triplet-triplet annihilation (TTA) because its triplet state lies ∼800 cm(-1) below the triplet energy of the C60 donor (thereby improving efficiency by reducing back triplet energy transfer), and its fluorescent singlet state lies in near resonance with double its triplet energy (thus minimizing thermal energy losses in the annihilation process). In fluid solution, efficient triplet-triplet donor-acceptor energy transfer is observed, and rate constants for homomolecular TTA in the An acceptor are estimated to approach the diffusion limit. NCPU is also observed in An + C60 in poly(methylmethacrylate) thin films.

Concerning the dual emission of porphyrazines employed in biomedical imaging.

Previous reports of unusual dual fluorescence in several porphyrazines proposed for use in photodynamic therapy and biomedical imaging have been re-examined. The blue-violet emissions previously assigned to S2-S0 fluorescence of these porphyrazines are shown to exhibit fluorescence excitation spectra that do not coincide with their absorption spectra, and the photophysical properties of the emission calculated from measured quantum yields and decay parameters do not match those calculated from the absorption spectra. In addition, the blue-violet emission intensities increase on exposure of aerated solutions of the porphyrazines to a broad spectrum of UV-visible light. The blue-violet emissions previously assigned to S2-S0 fluorescence of these porphyrazines themselves are therefore re-assigned to S1-S0 fluorescence of small amounts of photolysis products strongly absorbing in the same near UV spectral regions.

Photophysics and halide quenching of a cationic metalloporphyrin in water.

In this work, the steady state S0-S1 and S0-S2 absorption and emission behaviour of the water-soluble tetrakis(N-methyl-4-pyridyl)porphyrin zinc(ii) tetrachloride (ZnTMPyP) in media of constant and high ionic strength, both with and without iodide ions as a fluorescence quencher, was measured. The quenching of the ZnTMPyP S1 state by iodide ions proceeds primarily through diffusion-limited interaction in an encounter pair but the formation of a loose association between the ZnTMPyP S1 state and iodide ions also provides a minor quenching pathway. The ZnTMPyP S2 state was quenched minimally by iodide, likely through an electron transfer mechanism at an average donor-acceptor distance of ~0.7 nm. The results presented here highlight the notion that significant iodide quenching of the ZnTMPyP S1 state can be a source of inefficiencies in porphyrin-based dye-sensitized solar cells. The minimal quenching of the S2 state indicates that no significant loss of efficiency will be introduced by using iodide as component of the electrolyte system in upconversion by triplet-triplet annihilation (UC-TTA) enhanced solar cells.

Dendron to central core S1-S1 and S2-S(n) (n>1) energy transfers in artificial special pairs containing dendrimers with limited numbers of conformations.

Two dendrimers consisting of a cofacial free-base bisporphyrin held by a biphenylene spacer and functionalized with 4-benzeneoxomethane (5-(4-benzene)tri-10,15,20-(4-n-octylbenzene)zinc(II)porphyrin) using either five or six of the six available meso-positions, have been synthesized and characterized as models for the antenna effect in Photosystems I and II. The presence of the short linkers, -CH2O-, and long C8H17 soluble side chains substantially reduces the number of conformers (foldamers) compared with classic dendrimers built with longer flexible chains. This simplification assists in their spectroscopic and photophysical analysis, notably with respect to fluorescence resonance energy transfer (FRET). Both steady-state and time-resolved spectroscopic measurements indicate that the cofacial free bases and the flanking zinc(II)-porphyrin antennas act as energy acceptor and donor, respectively, following excitation in either the Q or Soret bands of the dendrimers. The rate constants for singlet electronic energy transfer (k(EET)) extracted from the S1 and S2 fluorescence lifetimes of the donor in the presence and absence of the acceptor are ≤ (0.1-0.3)×10(9) and ∼2×10(9)  s(-1) for S1→S1 (range from a bi-exponential decay model) and about 1.5×10(12)  s(-1) for S2→S(n) (n>1). Comparisons of these experimental data with those calculated from Förster theory using orientation factors and donor-acceptor distances extracted from computer modeling suggest that a highly restricted number of the many foldamers facilitate energy transfer. These foldamers have the lowest energy by molecular modeling and consist of one or at most two of the flanking zinc porphyrin antennas folded so they lie near the central artificial special pair core with the remaining antennas located almost parallel to and far from it.

Photophysics of untethered ZnTPP-fullerene complexes in solution.

The spectroscopy and dynamic behavior of the self-assembled, Soret-excited zinc tetraphenylporphyrin (ZnTPP) plus fullerene (C(60)) model system in solution has been examined using steady state fluorescence quenching, nanosecond time-correlated single photon counting, picosecond fluorescence upconversion, and picosecond transient absorption methods. Evidence of ground state complexation is presented. Steady-state quenching of the S(2) and S(1) fluorescence of ZnTPP by C(60) reveals that the quenching processes only occur in the excited complexes, are ultrafast, and proceed at different rates in the two states. Only uncomplexed ZnTPP is observed by fluorescence lifetime methods; the locally excited complexes are either dark or, more likely, rapidly relax to products that do not radiate strongly. Both short-range (Dexter) energy transfer and electron transfer relaxation mechanisms are evaluated. Picosecond transient absorption data obtained from the subtle differences between the spectra of Soret-excited ZnTPP with and without a large excess of added C(60) reveal the formation, on a subpicosecond time scale, of relatively long-lived charge-separated species. Soret excitation of ZnTPP···C(60) does not produce a quantitative yield of species in the lower S(1) excited state.

Spectroscopic and photophysical properties of ZnTPP in a room temperature ionic liquid.

The steady-state absorption and emission spectra and the time-resolved Soret- and Q-band excited fluorescence profiles of the model metalloporphyrin, ZnTPP, have been measured in a highly purified sample of the common room temperature ionic liquid, [bmim][PF6]. S2-S0 emission resulting from Soret-band excitation behaves in a manner completely consistent with that of molecular solvents of the same polarizability. The ionic nature of the solvent and its slow solvation relaxation times have no significant effect on the nature of the radiationless decay of the S2 state, which decays quantitatively to S1 at a population decay rate that is consistent with the weak coupling case of radiationless transition theory (energy gap law). The ratio of the intensities of the Qα:Qß (0-0:1-0) bands is consistent with the solvatochromic shift correlation data obtained for molecular solvents. The temporal S1 fluorescence decay profiles measured at a single emission wavelength are biexponential; the longer-lived major component is similar to that observed for ZnTPP in molecular solvents, and the minor shorter-lived component is attributed to solvent relaxation processes on a nanosecond time scale.

Photophysics of self-assembled zinc porphyrin-bidentate diamine ligand complexes.

The effects of complexation--by bidentate nitrogen-containing ligands such as pyrazine and 4,4'-bipyridine commonly used for porphyrin self-assembly--on the photophysics of the model metalloporphyrin, ZnTPP, are reported. Ligation to form the 5-coordinate species introduces an intramolecular charge transfer (ITC) state that, depending on the oxidation and reduction potentials of the electron donor and acceptor, can become involved in the excited state relaxation processes. For ZnTPP, ligation with pyridine has little effect on excited state relaxation following either Q-band or Soret band excitation. However, coordination of ZnTPP with pyrazine and bipyridine causes the S(2) (Soret) state of the ligated species to decay almost exclusively via an S(2)-ICT-S(1) pathway, while affecting the S(1) decay route only slightly. In these 5-coordinate species the S(2)-ICT-S(1) decay route is ultrafast and nearly quantitative. Literature redox data for other bidentate ligands such as DABCO and multidentate ligands commonly used for pophyrin assembly suggest that the ITC states introduced by them could also modify the excited state relaxation dynamics of a wide variety of multiporphyrin arrays.