Photoinduced Electron Transfer in Os(terpyridine)-biphenylene-(bi)pyridinium Assemblies

Abstract

A series of linearly arranged donor-spacer-acceptor (D-S-A) systems 1-3, has been prepared and characterized. These dyads combine an Os(II)bis(terpyridine) unit as the photoactivable electron donor (D), a biphenylene (2) or phenylene-xylylene (3) fragment as the spacer (S), and a N-aryl-2,6-diphenylpyridinium electrophore (with aryl = 4-pyridyl or 4-pyridylium in 1 or 2/3, respectively) as the acceptor (A). Their absorption spectra, redox behavior, and luminescence properties (both at 77 K in rigid matrix and at 298 K in fluid solution) have been studied. The electronic structure and spectroscopic properties of a representative compound of the series (i.e., 2) have also been investigated at the theoretical level, performing Density Functional Theory (DFT)-based calculations. Time-dependent transient absorption spectra of 1-3 have also been recorded at room temperature. The results indicate that efficient photoinduced oxidative electron transfer takes place in the D-S-A systems at room temperature in fluid solution, for which rate constants (in the range 4 × 10(8)-2 × 10(10) s(-1)) depend on the driving force of the process and the spacer nature. In all the D-S-A systems, charge recombination is faster than photoinduced charge separation, in spite of the relatively large energy of the D(+)-S-A(-) charge-separated states (between 1.47 and 1.78 eV for the various species), which would suggest that the charge recombination occurs in the Marcus inverted region. Considerations based on superexchange mechanism suggest that the reason for the fast charge recombination is the presence of a virtual D-S(+)-A(-) state at low energy--because of the involvement of the easily oxidizable biphenylene spacer--which is beneficial for charge recombination via superexchange but unsuitable for photoinduced charge separation. To further support the above statement, we prepared a fourth D-S-A species, 4, analogous to 2 but with a (hardly oxidizable) single phenylene fragment serving as the spacer. For such a species, charge recombination (about 3 × 10(10) s(-1)) is slower than photoinduced charge separation (about 1 × 10(11) s(-1)), thereby confirming our suggestions.

Shining a New Light on an Old Game – An OsII‐Based Near‐IR Light Switch

“…A single reversible anodic wave at +0.89 V vs. SCE is shown. This value is similar to that reported for Os–terpy‐based species in CH 3 CN and can be attributed to a monoelectronic metal‐centered oxidation, E 1/2 (Os III/II ), of the complex 16…”

“…In addition, four well‐resolved reversible cathodic waves are observed, while in the same experimental window [Os(ptpy) 2 ] 2+ (ptpy = 4′‐phenyl‐2,2′:6′,2″‐terpyridine) exhibits a single reduction wave centered at –1.22 V vs. SCE in CH 3 CN 16. The potential at which the first reduction process is observed for 1 (–0.88 V vs. SCE) is significantly less negative (by ca.…”

“…Table 1 collects all the oxidation and reduction potentials recorded for 1. [16] Figure 1. A single reversible anodic wave at +0.89 V vs. SCE is shown.…”

“…[16] The potential at which the first reduction process is observed for 1 (-0.88 V vs. SCE) is significantly less negative (by ca. [16] The potential at which the first reduction process is observed for 1 (-0.88 V vs. SCE) is significantly less negative (by ca.…”

Spectral signature of a Ru(ii, iii, iv) complex: a combined experimental and theoretical investigation

“…Several works have already demonstrated the capability and the accuracy of DFT for the description of the electronic and geometrical features of complexes containing transition metal ions and, more particularly of Ru complexes. As a matter of fact several studies reported in literature, both fully theoretical and mixed theoretical and experimental, focused on Ru(II) complexes of technological (such as dyes for Dye Sensitized Solar Cells) [16][17][18] , biological (such as DNA intercalators) 19,20 and fundamental interest (such as compounds for photoinduced charge separation) [21][22][23][24] Since TD-DFT methods and computationally more expensive ab-initio approaches, such as post-Hartree Fock methods, provide a similar description of the photophysical properties of Ru containing complexes, not surpringly the former are currently extensively used for the investigation of the electronic properties at both the ground and the excited state of large d-transition metal complexes. [25][26] Remarkably, theoretical works concerning Ru(III) and Ru(IV) complexes are less numerous than those devoted to Ru(II) complexes and mainly related to the investigation of catalytically active compounds (both of synthetic or biomimetic relevance).…”