Iridium terpyridine complexes as functional assembling units in arrays for the conversion of light energy

Istituto ISOF-CNR, Via P. Gobetti 101, 40129 Bologna, Italy.
Accounts of Chemical Research (Impact Factor: 24.35). 07/2008; 41(7):857-71. DOI: 10.1021/ar700282n
Source: PubMed

ABSTRACT 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.

  • [Show abstract] [Hide abstract]
    ABSTRACT: A pure and stable copper(I)-based donor?Cu(I)?acceptor triad was synthesized featuring an efficient stepwise photoinduced charge separation upon excitation of the copper(I) metal-to-ligand charge transfer (MLCT) excited state. The heteroleptic copper(I) complex is composed of two phenanthrolines, one substituted by a naphthalene bisimide (NDI) as electron acceptor and the other by a ferrocene (Fc) as electron donor. The synthesis of two dyads with different spacers between the electron acceptor and Cu(I) center and the charge separation mechanism and dynamics were determined by electrochemical and femtosecond transient experiments, which show that two parallel electron-transfer routes occur from the unrelaxed 1MLCT and flattened 3MLCT states with time constants of 540 fs and 162 ps, respectively. The final charge-separated state Fc+?Cu(I)?NDI? has a 34 ns lifetime in acetonitrile and is formed with a quantum yield of 90% upon excitation on the MLCT transition of the copper(I) complex.
    The Journal of Physical Chemistry C 12/2014; 118(49):28388-400. DOI:10.1021/jp507984s · 4.84 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: Photoinduced intramolecular electron transfer of dyad PTZ3-PTZ2-PTZ1-B-AQ consisting of phenothiazine trimer (PTZ3-PTZ2-PTZ1), bicyclo[2.2.2]octane (B), and anthraquinone (AQ) was investigated. After excitation (similar to 20 ps) of the AQ moiety in THF, a metastable radical ion pair (RIP) PTZ3-PTZ2-PTZ1(+)-B-AQ(-) appeared at similar to 620 nm. From 500 ps to 6 ns the spectrum changed to a new absorption (similar to 950 nm), which was assigned to the hole-shifted stable RIP state PTZ3-PTZ2(+)-PTZ1-B-AQ(-). The time constant of the hole-shift process was determined to be 6.0 ns. The hole-shifted RIP state had a lifetime (t) of 250 ns and was characterized by spin-polarized signals as a spin-correlated radical pair (SCRP) by means of time-resolved ESR. These results were compared with those for the phenothiazine monomer analog PTZ-B-AQ, which also produced the RIP state PTZ(+)-B-AQ( with t = 1.9 mu s. Time-resolved ESR showed an all emission signal pattern showing the triplet mechanism of PTZ-B-(3)AQ* -> (3)[PTZ(+)-B-AQ(-)]. The origin of the difference in the lifetimes between the trimer and the monomer RIP states was discussed from various points of view, including free energy difference in the RIP states, reorganization energy difference in the charge recombination process, and the spin-state difference. Of these, the spin-state difference effect provided the most reasonable explanation.
    The Journal of Physical Chemistry A 10/2014; 118(47):11262. DOI:10.1021/jp509643q · 2.78 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: Iridium(III) bis(terpyridine) complexes are known as excellent triplet emitters with emission lifetimes in the order of microseconds. We report the homoleptic complex [IrL2](3+) (L = 4'-(4-2,5-bis(octyloxy)-4-styrylphenyl)ethinyl)phenyl)-2,2':6',2″-terpyridine) that shows no detectable phosphorescence at room temperature but shows fluorescence. Emission spectra of [IrL2](3+) depend on the excitation wavelength. The origin of this behavior is studied with the help of results from (TD-)DFT calculations and is attributed to the selective excitation of different rotamers and isomers. Femtosecond-transient absorption experiments give further support for this interpretation as the specific excited-state absorption features of Z- and E-stilbene motives can be identified.
    The Journal of Physical Chemistry A 12/2014; 118(51). DOI:10.1021/jp5081252 · 2.78 Impact Factor