Functional Arrays for Light Energy Capture and Charge Separation.

This article draws, with a simplified but rigorous approach, the typical procedure for the design and optimization of functional multicomponent structures for light to chemical energy conversion for two series of multipartite structures based on prototypical chromophores: polypyridyl metal complexes and porphyrinoids. Starting from a photophysical study performed by steady-state and time-resolved spectroscopic methods, the full deactivation dynamics of the light-absorbing chromophore(s) are disclosed. The preferred deactivation step (electron transfer in this case) is then optimized. This can be done by simply operating on the solvent, but also by changing structure/components that can alter electronic and nuclear factors, via continuous feedback with the research groups in charge of the synthesis. With a presentation suitable for a wide audience, it is here discussed how the effective design of functional multicomponent structures for charge separation can be achieved.

A Theranostic Agent Combining a Two-Photon-Absorbing Photosensitizer for Photodynamic Therapy and a Gadolinium(III) Complex for MRI Detection.

The convergent synthesis and characterization of a potential theranostic agent, [DPP-ZnP-GdDOTA](-), which combines a diketopyrrolopyrrole-porphyrin component DPP-ZnP as a two-photon photosensitizer for photodynamic therapy (PDT) with a gadolinium(III) DOTA complex as a magnetic resonance imaging probe, is presented. [DPP-ZnP-GdDOTA](-) has a remarkably high longitudinal water proton relaxivity (19.94 mm(-1) s(-1) at 20 MHz and 25 °C) for a monohydrated molecular system of this size. The Nuclear Magnetic Relaxation Dispersion (NMRD) profile is characteristic of slow rotation, related to the extended and rigid aromatic units integrated in the molecule and to self-aggregation occurring in aqueous solution. The two-photon properties were examined and large two-photon absorption cross-sections around 1000 GM were determined between 910 and 940 nm in DCM with 1 % pyridine and in DMSO. Furthermore, the new conjugate was able to generate singlet oxygen, with quantum yield of 0.42 and 0.68 in DCM with 1 % pyridine and DMSO, respectively. Cellular studies were also performed. The [DPP-ZnP-GdDOTA](-) conjugate demonstrated low dark toxicity and was able to induce high one-photon and moderate two-photon phototoxicity on cancer cells.

Photoinduced Processes in Self-Assemblies of Bis-Porphyrinic Tweezers with an Axially Coordinated Bispyridinofullerene.

Self-assembled bis(zinc porphyrin)-bispyridinopyrrolidinofullerene coordination complexes were obtained in solution. Two [5]polynorbornane-bridged bis-porphyrins were used that differed in the arms containing porphyrin units: whereas 1 has rigid [5]polynorbornane linkers, compound 2 has additional flexible propyl chains. The different geometries of the two hosts affect both the complexation process and the photoreactivity of the final product. Formation of the complexes, characterized by absorption, emission, and NMR spectroscopy, occurs with association constants in the order of 104 and 106 m-1 for bis-porphyrin tweezers 1 and 2, respectively. The higher flexibility of tweezers 2 accounts for the greater association ability. Full photophysical characterization of the complexes, as well as of suitable models, has been performed by means of steady-state and time-resolved optical spectroscopy. Ultrafast luminescence detection and pump-probe transient absorption analysis were used to investigate photoinduced processes within the complexes. The results provide evidence that an electron-transfer process from the bis-porphyrin host to the fullerene guest occurs in both complexes, and a slightly longer lifetime of the charge-separated state is observed in the complex with more flexible host 2.

Model dyads for 2PA uncaging of a protecting group via photoinduced electron transfer.

Three dyads with a fluorene derivative as an electron-donor and with electron-acceptors of variable redox potentials were synthesized as models for two-photon activated uncaging via electron transfer. A spectroscopic and photophysical study of the component units and the dyads in solvents of different polarities demonstrated an efficient electron transfer (efficiencies > 80%) followed by charge recombination in the arrays (30 ps < τ < 1.6 ns). Recombination takes place to the ground state in all cases except for the dyad displaying the highest driving force for charge recombination in the apolar solvent. The effects of changing the solvent polarity, as well as the driving force, for electron-transfer are discussed in the frame of the current theories of electron transfer.

Diketopyrrolopyrrole-porphyrin conjugates with high two-photon absorption and singlet oxygen generation for two-photon photodynamic therapy.

Two-photon photodynamic therapy is a promising therapeutic method which requires the development of sensitizers with efficient two-photon absorption and singlet-oxygen generation. Reported here are two new diketopyrrolopyrrole-porphyrin conjugates as robust two-photon absorbing dyes with high two-photon absorption cross-sections within the therapeutic window. Furthermore, for the first time the singlet-oxygen generation efficiency of diketopyrrolopyrrole-containing systems is investigated. A preliminary study on cell culture showed efficient two-photon induced phototoxicity.

Synthesis and solution studies of silver(I)-assembled porphyrin coordination cages.

The synthesis and the characterization of two porphyrin coordination cages are reported. The design of the cage formation is based on the coordination of silver(I) ions to the pyridyl units of 3-pyridyl appended porphyrins. (1)H/(109)Ag NMR spectroscopy, and diffusion-ordered spectroscopy (DOSY) experiments demonstrate that both the free base porphyrin 2H-TPyP and the Zn-porphyrin Zn-TPyP form the closed cages, [Ag4(2H-TPyP)2](4+) and [Ag4(Zn-TPyP)2](4+), respectively, upon addition of two equivalents of Ag(+). The complexation processes are characterized in details by means of absorption and emission spectroscopy in diluted CH2Cl2 solutions. The data are discussed in the frame of the point-dipole exciton coupling theory; the two porphyrin monomers, in fact, experience a rigid face-to-face geometry in the cages and a weak inter-porphyrin exciton coupling. An intermediate species is observed, for Zn-TPyP, in a porphyrin/Ag(+) stoichiometric ratio of about 1:0.5 and is tentatively ascribed to an oblique open form. The occurrence of a photoinduced electron-transfer reaction within the cages is excluded on the basis of the experimental outcomes and thermodynamic evaluations. Photophysical experiments evidence different reactivities of singlet and triplet excited states in the assemblies. A lower fluorescence quantum yield and triplet formation is discussed in relation to the constrained geometry of the complexes. Unusually long triplet excited state lifetimes are measured for the assemblies.

NIR dual luminescence from an extended porphyrin. Spectroscopy, photophysics and theory.

Spectroscopic and photophysical properties of an extended Zn porphyrin with fused bis(tetraazaanthracene) arms including a 2,9-diphenyl-1,10-phenanthroline incorporated in a polyether macrocycle are investigated in solvents of different polarity pointing to the presence of two emitting singlet excited states. The absorption and emission features are identified and ascribed, on the basis of solvent polarity dependence, to a π-π* and to a charge transfer (CT) state, respectively. Whereas the intraligand π-π* transition is assigned to the intense absorption observed at 442-455 nm, the CT states contribute to the bands at 521-525 nm and 472-481 nm. The theoretical analysis of the absorption spectrum confirms the presence of two strong bands centered at 536 and 437 nm corresponding to CT and π-π* states, respectively. Weak CT transitions are calculated at 657 and 486 nm. Two emission maxima are observed in toluene at 724 nm from a (1)π-π* state and at 800 nm from a (1)CT state, respectively. (1)CT bands shift bathochromically by increasing the solvent polarity whereas the energy of the (1)π-π band is less affected. Likewise, the emission yield and lifetime associated with the low energy (1)CT band are strongly affected by solvent polarity. This is rationalized by a (1)π-π* → (1)CT internal conversion driven by solvent polarity, this process being competitive with the (1)π-π* to ground state deactivation channel. Time resolved absorption spectra indicate the presence of two triplet states, a short-lived one (nanoseconds range) and a longer lived one (hundreds of microsecond range) ascribed to a (3)π-π* and a (3)CT, respectively. For them, a conversion mechanism similar to that of the singlet excited states is suggested.

Self-Sorting of cyclic peptide homodimers into a heterodimeric assembly featuring an efficient photoinduced intramolecular electron-transfer process.

We describe the thermodynamic characterisation of the self-sorting process experienced by two homodimers assembled by hydrogen-bonding interactions through their cyclopeptide scaffolds and decorated with Zn-porphyrin and fullerene units into a heterodimeric assembly that contains one electron-donor (Zn­porphyrin) and one electron-acceptor group (fullerene). The fluorescence of the Zn-porphyrin unit is strongly quenched upon heterodimer formation. This phenomenon is demonstrated to be the result of an efficient photoinduced electron-transfer (PET) process occurring between the Zn-porphyrin and the fullerene units of the heterodimeric system. The recombination lifetime of the charge-separated state of the heterodimer complex is in the order of 180 ns. In solution, both homo- and heterodimers are present as a mixture of three regioisomers: two staggered and one eclipsed. At the concentration used for this study, the high stability constant determined for the heterodimer suggests that the eclipsed conformer is the main component in solution. The application of the bound-state scenario allowed us to calculate that the heterodimer exists mainly as the eclipsed regioisomer (75-90 %). The attractive interaction that exists between the donor and acceptor chromophores in the heterodimeric assembly favours their arrangement in close contact. This is confirmed by the presence of charge-transfer bands centred at 720 nm in the absorption spectrum of the heterodimer. PET occurs in approximately 75% of the chromophores after excitation of both Zn-porphyrin and fullerene chromophores. Conversely, analogous systems, reported previously, decorated with extended tetrathiafulvalene and fullerene units showed a PET process in a significantly reduced extent (33%). We conclude that the strength (stability constant (K) x effective molarity (EM)) of the intramolecular interaction established between the two chromophores in the Zn-porphyrin/fullerene cyclopeptide-based heterodimers controls the regioisomeric distribution and regulates the high extent to which the PET process takes place in this system.

Coordination chemistry-assembled multicomponent systems built from a gable-like bis-porphyrin: synthesis and photophysical properties.

Multiporphyrinic assemblies were quantitatively formed, in one step, from a gable-like zinc(II) bis-porphyrin ZnP2 and free-base porphyrins bearing pyridyl groups. The different fragments are held together by axial 4'-N(pyridyl)-Zn interactions. Formation of a macrocycle ZnP2 •(4'-cisDPyP) and a bis-macrocycle (ZnP2 )2 •(TPyP) is discussed. The macrocycle and the bis-macrocycle were crystallized and studied by X-ray diffraction, which confirmed the excellent complementarity between the various components. Spectrophotometric and spectrofluorimetric titrations and studies reveal high association constants for both multiporphyrinic assemblies due to the almost perfect geometrical match between the interacting units. As expected, energy transfer from the zinc porphyrin component to the free-base porphyrin quenches the fluorescence of the zinc porphyrin components in both compounds. But while in ZnP2 •(4'-cis DPyP) sensitization of the emission of the free-base porphyrin was observed, in (ZnP2)2 •(TPyP) excitation of the peripheral Zn porphyrin units does not lead to quantitative sensitization of the luminescence of the free-base porphyrin acceptor. An unusual HOMO-HOMO electron transfer reaction from ZnP2 to the excited TPyP unit was detected and studied.

Photoinduced processes in a dyad made of a linear and an angular perylene bisimide.

A dyad (PI0-PIa) made of a linear (PI0) and an angular (PIa) perylene biscarboximide is synthesized and its spectroscopic, electrochemical and photophysical properties investigated in solvents of various polarity. PIa is characterized by a high intersystem crossing. The spectroscopy and electrochemistry data point to a modest electronic coupling. LUMO-LUMO electron transfer from the singlet excited state PI0-¹PIa is thermodynamically feasible in polar solvents but its occurrence is precluded by a very fast energy transfer to yield ¹PI0-PIa, k(en) ≥ 10(11) s(-1). A HOMO-HOMO electron transfer in the latter state in polar solvents is precluded by the poor driving force, the reaction being unable to compete with the radiative deactivation of the excited state. The efficient energy transfer process is quantitatively examined in the frame of current theories and ascribed to a dipole-dipole (Förster) mechanism.

Blue-green emitting sulphonamido-imidazole derivatives: ESIPT based excited state dynamics.

Six imidazole derivatives characterized by the presence in their molecules of a sulphonamido group and able to display excited state intramolecular proton transfer (ESIPT) have been synthesized in a straightforward manner and the dynamics of their excited states investigated in detail in solvents of different polarity and proticity: toluene (TOL), dichloromethane (DCM) and methanol (MeOH). With the exception of one compound, these ESIPT-capable molecules are highly luminescent. The major emitting species at room temperature is the ketimine (K) tautomer but the weak emission from the enamide (E) form was detected in several cases. In general, the luminescence quantum yields (φfl) of the K form range between 0.4 and 0.6 with lifetimes of several nanoseconds, with radiative rate constants kr of the order of 10(8) s(-1). The lifetime of the E form, and hence of the ESIPT process, range between τ <10 ps up to 190 ps. At 77 K, in addition to hypsochromically shifted fluorescence bands from the E and/or K tautomers, E phosphorescence emissions with lifetimes in the range of seconds (0.4-2.5 s) are also detected. The triplet excited state absorbance of these molecules was probed in DCM and MeOH and both E (between 450-520 nm) and K (at about 420 nm) forms were identified. The triplet lifetimes at room temperature in air-free solutions are in the microsecond range, whereas the reaction rates with oxygen are of the order of 10(9) M(-1) s(-1).

Bright, fluorescent dyes based on imidazo[1,2-a]pyridines that are capable of two-photon absorption.

A library of imidazo[1,2-a]pyridines was synthesized by using the Gevorgyan method and their linear and non-linear optical properties were studied. Derivatives that contained both electron-donating and electron-withdrawing groups at the 2 position were comprehensively investigated. Their emission quantum yield ranged between 0.2-0.7 and it was shown to depend on the substitution pattern, most notably that on the phenyl ring. Electron-donating substituents improved the luminescence performance of these compounds, whereas electron-withdrawing substituents led to a more erratic behavior. Substitution on the six-membered ring had less effect on the fluorescence properties. Extension of the delocalization increased the luminescence quantum yield. A new quadrupolar system was designed that contained two imidazo[1,2-a]pyridine units on its periphery and a 1,4-dicyanobenzene unit at its center. This system exhibited a large Stokes-shifted luminescence that was affected by the polarity and rigidity of the solvent, which was ascribed to emission from an excited state with strong charge-transfer character. This quadrupolar feature also led to an acceptable two-photon absorption response in the NIR region.

Light energy collection in a porphyrin-imide-corrole ensemble.

An assembly consisting of three units, that is, a meso-substituted corrole (C3), 1,8 naphthaleneimide (NIE), and a Zn porphyrin (ZnP), has been synthesized. NIE is connected to C3 through a 1,3-phenylene bridge and to the ZnP unit through a direct C-C bond. The convergent synthetic strategy includes the preparation of a trans-A2B-corrole possessing the imide unit, followed by Sonogashira coupling with a meso-substituted A3B-porphyrin. The photophysical processes in the resulting triad ZnP-NIE-C3 are examined and compared with those of the corresponding C3-NIE dyad and the constituent reference models C3, NIE, and ZnP. Excitation of the NIE unit in C3-NIE leads to a fast energy transfer of 98 % efficiency to C3 with a rate k(en) =7.5×10(10) s(-1), whereas excitation of the corrole unit leads to a reactivity of the excited state identical to that of the model C3, with a deactivation rate to the ground state k=2.5×10(8) s(-1). Energy transfer to C3 and to ZnP moieties follows excitation of NIE in the triad ZnP-NIE-C3. The rates are k(en) =7.5×10(10) s(-1) and k(en) =2.5×10(10) s(-1) for the sensitization of the C3 and ZnP unit, respectively. The light energy transferred from NIE to Zn porphyrin unit is ultimately funneled to the corrole component, which is the final recipient of the excitation energy absorbed by the different components of the array. The latter process occurs with a rate k(en) =3.4×10(9) s(-1) and 89 % efficiency. Energy transfer processes take place in all cases by a Förster (dipole-dipole) mechanism. The theory predicts quite satisfactorily the rate for the ZnP/C3 couple, where components are separated by about 23 Å, but results in calculated rates that are one to two orders of magnitude higher for the couples NIE/ZnP (D/A) and NIE/C3, which are separated by distances of about 14 and 10 Å, respectively.

Photoinduced electron transfer in an amine-corrole-perylene bisimide assembly: charge separation over terminal components favoured by solvent polarity.

An assembly has been synthesised that consists of four units: a meso-substituted corrole (C3), perylene bisimide (PI), and two electron-rich triphenylamine (DPA) units. PI is connected through a 1,4-phenylene bridge to C3, whereas the two DPA units are linked to C3 through a diphenyl ether linkage, which is used for the first time to connect the various moieties. Various synthetic strategies were elaborated, and the chosen one afforded the final system in six steps in an overall yield of 6 %. The resulting assembly, made of three different units, was named a "triad". Excitation of the corrole (C3) or perylene bisimide (PI) units led to the charge-separated state DPA-C3(+)-PI(-) with a rate k>10(11) s(-1) in benzonitrile and dichloromethane (CH(2) Cl(2) ) or with k of the order of 10(10) s(-1) in toluene. The latter charge-separated state decayed to the ground state with a rate k=1.8×10(9) s(-1) in toluene. In the polar solvents benzonitrile and dichloromethane, recombination to the ground state competes with a charge shift to form the distal charge-separated state, DPA(+)-C3-PI(-), the formation of which occurs with a yield of 50 %. Recombination to the ground state of DPA(+)-C3-PI(-) occurs with a rate k=5×10(7) s(-1) in CH(2)Cl(2) and k=2×10(7) s(-1) in benzonitrile.

NIR emission of cyclic [4]rotaxanes containing π-extended porphyrin chromophores.

The photophysical properties of a Cu(I) [4]rotaxane 4(4+) and of the demetalated [4]rotaxane 3 have been determined and compared to those of the component Zn porphyrin 2. All samples emit in the NIR region (700-1200 nm). The luminescence from the interlocked structures is bathochromically shifted with respect to 2 and displays a lower emission quantum yield, much lower for 4(4+) than for 3. The occurrence of intra-molecular electron or energy transfer is excluded and the decrease in luminescence yield is discussed in terms of the energy gap law and of electronic interactions between components of the cyclic interlocked structure. In toluene a dual emission behavior, similar to that of 2, is observed for 3 and ascribed to the presence of two non-equilibrated excited states, π-π* and CT in nature with lifetimes of 0.80 and 0.14 ns, respectively.

Phosphorescent perylene imides.

Asymmetrically substituted perylene imide derivatives PIa and PIx display phosphorescence in glassy matrices at 77 K. The lifetime is 49.0 ms for PIa and 13.5 ms for PIx. The triplet energy is 1.79 eV for PIa and 1.68 eV for PIx as confirmed by sensitization experiments of the C(60) triplet.

Photophysical and redox properties of perylene bis- and tris-dicarboximide fluorophores with triplet state formation: transient absorption and singlet oxygen sensitization.

A detailed photophysical characterization of a couple of new perylene imide derivatives, a carboxylic trisimide PIx, and an asymmetrically substituted carboxylic bisimide PIa is presented. PIx and PIa have the lowest singlet excited state just below 2.6 eV. The dyes are remarkably fluorescent (ϕ(f) = 0.37 ± 0.03 for PIa and ϕ(f) = 0.58 ± 0.04 for PIx in toluene), but they also display an efficient intersystem crossing. This leads to typical excited triplet photophysics/photochemistry, with intense triplet state absorption spectra and efficient singlet oxygen ((1)Δ(g)) photosensitization (ϕ(Δ) = 0.68 ± 0.02 for PIa and 0.44 ± 0.02 for PIx in toluene). On the basis of the measured ϕ(Δ), a ϕ(isc) of 0.65 ± 0.02 for PIa and 0.43 ± 0.02 for PIx in toluene is derived. PIx reduces at -0.58 eV vs SCE, almost similarly to the corresponding symmetrically substituted perylene bisimide PI0, but unlike the latter, it has the first oxidation potential above +1.9 V. PIa is more electron rich and displays a more difficult first reduction at -0.95 V with a more facile oxidation at +1.75 V, similar to that of the parent PI0. The absorption spectra of the excited singlet and triplet states and that of electrochemically generated monoanions are reported.

Improving the photoinduced charge separation parameters in corrole-perylene carboximide dyads by tuning the redox and spectroscopic properties of the components.

A couple of corrole-perylene carboximide dyads (C2-PIa and C2-PIx) have been synthesized and their photoreactivity has been evaluated. We aimed at obtaining better performances for photoinduced charge separation, both in terms of efficiency and in terms of lifetime, with respect to formerly studied systems. The energy level of the charge-separated state was tuned by selecting perylene and corrole components with diverse redox and spectroscopic properties. High spectroscopic energy levels of the perylene carboximide derivatives (PIs) allow a fast charge separation to be maintained in competition with an energy-transfer process from the PI to the corrole unit. Yields and lifetimes of charge separation in toluene are, respectively, 75% and 2.5 µs for C2-PIa and 65% and 24 ns for C2-PIx. The results and the effect of solvent polarity are discussed in the framework of current energy- and electron-transfer theories.

Near-infrared dual luminescence from an extended zinc porphyrin.

The photophysical characterization of an extended zinc porphyrin is reported. Fusion of bis-tetraazaanthracene on the porphyrin ring causes an unusual dual luminescence ascribed to two non-equilibrated singlet excited states.

Supramolecular inclusion complexes of two cyclic zinc bisporphyrins with C60 and C70: structural, thermodynamic, and photophysical characterization.

The formation of thermodynamically stable inclusion complexes between two cyclic zinc bisporphyrins, differing in the saturation degree of the hydrocarbon linkers that connect their porphyrin units, and the fullerenes C(60) and C(70) is described. Binding and photophysical studies were performed in two solvents of very different polarity: toluene and dichloromethane. UV/Vis and fluorescence titration experiments showed π-π interactions between the cyclic zinc bisporphyrins and the fullerenes. Solid-state structures were determined by X-ray diffraction analysis and gave valuable insight into the different complexation behaviors of the two macrocyclic systems towards the fullerenes. NMR titrations were also helpful in understanding the geometry of the complexes in solution. Upon fullerene complexation, the two macrocyclic bisporphyrins adopt very distinct conformations. Charge-transfer absorption bands point to ground-state interactions, and quenching of the porphyrin component luminescence indicates fast reactivity in the excited states. Energy transfer plus HOMO-HOMO and LUMO-LUMO electron-transfer processes occur within the complexes. Charge-separated states characterized by a reduced fullerene and an oxidized porphyrin radical, with lifetimes in the order of several hundred picoseconds, are detected.