Lucia Flamigni

Photochemistry and Photophysics of Coordination Compounds: Iridium

Mononuclear Ir(III)-polyimine complexes show outstanding luminescence properties, i.e., high intensities, lifetimes in the μs time range, and emission wavelengths that can be tuned so as to cover a full range of visible colors, from blue to red. We discuss the approaches for the use of ligands that afford control on luminescence features. Emphasis is placed on subfamilies of cyclometalated complexes, whose recent enormous expansion is motivated by their potential for applications, including that as phosphorescent dopants in OLEDs fabrication. The interplay of the different excited states associated with the luminescence, usually of MLCT and/or LC nature, is examined and the possible detrimental role of MC levels toward the luminescence properties is outlined. Ir(III)-polyimine moieties can be incorporated within multicomponent arrays where they can play as photoactive and/or electroactive units in photoinduced energy and electron transfer processes. The field is reviewed with attention to the processes of light collection and conversion into chemical energy.

Photoactive molecular wires based on metalcomplexes

Molecular wires incorporating polypyridine metal complexes are amenable to studies of directional energy and electron transfer. The complexes are chromophores mainly based on Ru(II), Os(II), Rh(III), and Re(I) centres, which usually exhibit luminescence and can play as donor (D) or acceptor (A) units. A bridging ligand (B) provides both the structural and electronic connectivity between D and A and the DAB wires are flexible or rigid, depending on the spacers included within the bridge. Developments regarding multicentre systems and stereochemically interesting systems are taken into account.

A family of luminescent coordination compounds: iridium(III) polyimine complexes

30 years of IrIII coordination chemistry with polyimine-type ligands are summarized. Over the years, milder reaction conditions have been used for their synthesis, allowing the incorporation of various functional substituents. Complexes are described with bidentate and terdentate ligands, with both N- and C-donor sites. All complexes are luminescent, with predominantly charge-transfer or ligand-centred emissive states depending on the charge density donated from the ligands to the metal. IrIII excited state lifetimes range from nanoseconds to microseconds. A wide range of properties are obtained: [IrN6]3+ complexes are strong photooxidants while tris-cyclometallated [IrN3C3] complexes are strong photoreductants.

Iridium Terpyridine Complexes as Functional Assembling Units in Arrays for the Conversion of Light Energy

In photosynthesis, sunlight energy is converted into a chemical potential by an electron transfer sequence that is started by an excited state and ultimately yields a long-lived charge-separated state. This process can be reproduced by carefully designed multicomponent artificial arrays of three or more components, and the stored energy can be used to oxidize or reduce molecules in solution, to inject electrons or holes, or to create an electron flow. Therefore, the process is important both for artificial-photosynthesis research and for photovoltaic and optoelectronic applications. Molecular arrays for photoinduced charge separation often use chromophores that resemble the natural ones. However, new synthetic components, including transition metal complexes, have had some success. This Account discusses the use of bis-terpyridine (tpy) metal complexes as assembling and functional units of such multicomponent arrays. M(tpy)2(n+) complexes have the advantage of yielding linear arrays with unambiguous geometry. Originally, Ru(tpy)2(2+) and Os(tpy)2(2+) were used as photosensitizers in triads containing typical organic donors and acceptors. However, it soon became evident that the relatively low excited state of these complexes could act as an energy drain of the excited state of the photosensitizer and, thus, seriously compete with charge separation. A new metal complex that preserved the favorable tpy geometry and yet had a higher energy level was needed. We identified Ir(tpy)2(3+), which displayed a higher energy level, a more facile reduction that favored charge separation, a longer excited-state lifetime, and strong spectroscopic features that were useful for the identification of intermediates. Ir(tpy)2(3+) was used in arrays with electron-donating gold porphyrin and electron-accepting free-base porphyrins. A judicious change of the free-base porphyrin photosensitizer with zinc porphyrin allowed us to shape the photoreactivity and led to charge separation with unity yield and a lifetime on the order of a microsecond. In a subsequent approach, an Ir(tpy)2(3+) derivative was connected to an amine electron donor and a bisimide electron acceptor in an array 5 nm long. In this case, the complex acted as photosensitizer, and long-lived charge separation over the extremities (>100 micros, nearly independent of the presence of oxygen) was achieved. The efficiency of the charge separation was modest, but it was improved later, after a modification aiming at decoupling the donor and photosensitizer components. This study represents an example of how the performances of an artificial photofunctional array can be modeled by a judicious design assisted by a detailed knowledge of the systems.

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.

Photophysical characterization of free-base corroles, promising chromophores for light energy conversion and singlet oxygen generation

A detailed photophysical characterization of a series of six free-base corroles with different substitution patterns at meso-positions is presented. In air-free toluene at 295 K, the luminescence quantum yields range from 0.13 to 0.22 and lifetimes vary from 4.1 ns to 6.3 ns whereas in air-saturated toluene the lifetimes range from 3.8 ns to 5.6 ns. In glassy toluene at 77 K the lifetimes have values in the range 5.2 ns to 7.9 ns. Transient absorption spectra of the singlet excited states have positive bands art λ 680–700 nm. The transient absorption spectra of the lowest triplet display a band around 460–470 nm and bleaching features in the Q bands region, no absorption above 700 nm. The triplet lifetimes in air-free toluene are in the interval 50–150 μs. The lowest excited states react with oxygen and sensitized singlet oxygen (1Δg) yields of 0.51–0.77 are determined via its luminescence. Electron and energy transfer from the excited states to molecular oxygen is discussed. Effect of polar solvents on the luminescence parameters indicate that charge transfer (CT) excited states play a very minor role. Thermal and photo stability is excellent for most of the examined compounds in toluene and dichloromethane.

PHOTOINDUCED PROCESSES IN MULTICOMPONENT ARRAYS CONTAINING TRANSITION METAL COMPLEXES

Abstract The authors’ recent activity in the study of photoinduced energy and electron transfer in linear arrays containing porphyrins assembled around a Ru(II) ion, is reviewed. The effect of substituents and distance and the role of the heavy metal ion is discussed. The photophysical and electrochemical properties of Ir(III) terpyridine complexes indicate that Ir(III) ion is a good candidate to successfully replace Ru(II) in the construction of multiporphyrinic linear arrays to achieve efficient photoinduced electron transfer. Preliminary results on multi-porphyrinic systems based on Ir(III) bis-terpyridine are presented.

Bis(BF2)-2,2′-bidipyrrins, a class of BODIPY dyes with new spectroscopic and photophysical properties

The photophysical and spectroscopic characterization of four new dimeric bis(BF2)-2,2′-bidipyrrins (BisBODIPYs) recently synthesized and characterized (Chem. Eur. J., 2008, 14, 2976–2983) has been undertaken along with that of the component BODIPY monomers. The monomers display the typical photophysical properties of this family; (i) narrow and intense single absorption band in the visible range at 530 nm; (ii) intense, solvent independent emission (Φflca. 1); (iii) narrow emission band with a 200–400 cm−1 Stokes shifted emission, independent of solvent polarity; (iv) no absorption features for the singlet excited state; (v) very little triplet yield and no ability to sensitize singlet oxygen. The absorption spectra of the corresponding new dimers exhibit split band maxima in the visible range at about 490 and 560 nm, corresponding to an exciton splitting of ca. 2600 cm−1. The luminescence in toluene is strong (Φflca. 0.7, τ =3.4 ns), broad and Stokes shifted by ca. 2200 cm−1, but both luminescence yield and Stokes shift are solvent polarity dependent; Φfl < 0.1, τ < 1 ns and the Stokes shift is close to 2700 cm−1 in acetonitrile. Solvent viscosity does not appear to play an important role and freezing of the solvents to 77 K in a solid matrix cancels the differences in luminescence parameters. Singlet and triplet excited state absorbance was measured in toluene and acetonitrile and the ability of these new dyes to sensitize singlet oxygen was examined. The nature and dynamics of the excited state is discussed in comparison with the monomers properties and with some intra- or inter-molecular BODIPY dimers reported in the literature. Potential applications of these new dyes with respect to BODIPYs are pointed out on the basis of their spectroscopic and photophysical properties.

A new pyridyl-substituted methanofullerene derivative. Photophysics, electrochemistry and self-assembly with zinc(II) meso-tetraphenylporphyrin (ZnTPP)

A new methanofullerene derivative with a pyridine residue at the methano bridge (PC60) has been synthesized. Its electrochemical and photophysical properties have been investigated in toluene solution along with those of the analogous fullerene mono-malonate adduct RC60, which does not contain the pyridyl moiety. PC60 and RC60 display almost identical steady state absorption and luminescence properties and transient absorption spectra, as shown by means of nano- and picosecond laser flash photolysis. Their markedly different spectroscopic features with respect to plain C60 are discussed, with the aid of semi-empirical calculations. PC60 binds zinc(II) meso-tetraphenylporphyrin (ZnTPP) through coordination of the pyridyl moiety to the zinc ion giving, to the best of our knowledge, the first non-covalent assembly of a porphyrin–fullerene diad. The association constant of the 1:1 complex between PC60 and ZnTPP has been determined at 300 K by 1H NMR (Ka=3600±150 L mol-1, C6D6 solution) and fluorescence titrations (Ka=3000±400 L mol-1, toluene solution). Mixtures of RC60 and ZnTPP have also been investigated, giving no evidence of association. Extremely fast ZnTPP luminescence quenching in the PC60–ZnTPP complex was observed by time-resolved luminescence spectroscopy (k>5×1010 s-1), tentatively attributed to an energy transfer mechanism. The bimolecular quenching constant of the lowest triplet excited state of ZnTPP by PC60 was determined via a Stern–Volmer analysis (kq=6.7×109 s-1).

Photoinduced energy and electron transfer in 1,8-naphthalimide–corrole dyads

A series of corrole-1,8-naphthalimide dyads has been synthesized. The dyads were assembled in a convergent fashion from two fragments via a corrole forming reaction. Central to the success of the synthetic strategy was the preparation of suitably functionalized derivatives of naphthalene-1,8-carboxymide. Six different dyads possessing either a different linker (a meta-phenylene or a para-phenylmethylene) or a corrole with different substituents at the 5 and 15 positions were prepared. A photophysical and spectroscopic characterization of the dyads and the reference models show that whereas upon selective excitation of the corrole component no photo-induced process occurs, excitation of the naphthalimide unit results in very efficient energy or electron transfer processes. The electron transfer contributes to the quenching process with a ratio between 0% and 85% depending on the nature of the corrole accepting unit. The processes are discussed in the frame of current theories. This is the first report of stable corrole-based dyads with interesting photo-activity at ambient temperature.