Abstract
Visible light-induced charge transfer dynamics were investigated in a novel transition metal triad acceptor-chromophore-donor, (NDI-phen)Pt(II)(-C≡C-Ph-CH2-PTZ)2 (1), designed for photoinduced charge separation using a combination of time-resolved infrared (TRIR) and femtosecond electronic transient absorption (TA) spectroscopy. In 1, the electron acceptor is 1,4,5,8-naphthalene diimide (NDI), and the electron donor is phenothiazine (PTZ), and [(phen)Pt(-C≡C-Ph-)], where phen is 1,10-phenanthroline, represents the chromophoric core. The first excited state observed in 1 is a (3)MLCT/LL'CT, with {Pt(II)-acetylide}-to-phen character. Following that, charge transfer from the phen-anion onto the NDI subunit to form NDI(-)-phen-[Pt-(C≡C)2](+)-PTZ2 occurs with a time constant of 2.3 ps. This transition is characterised by appearance of the prominent NDI-anion features in both TRIR and TA spectra. The final step of the charge separation in 1 proceeds with a time constant of ∼15 ps during which the hole migrates from the [Pt-(C≡C)2] subunit to one of the PTZ groups. Charge recombination in 1 then occurs with two distinct time constants of 36 ns and 107 ns, corresponding to the back electron transfer to each of the two donor groups; a rather rare occurrence which manifests that the hole in the final charge-separated state is localised on one of the two donor PTZ groups. The assignment of the nature of the excited states and dynamics in 1 was assisted by TRIR investigations of the analogous previously reported ((COOEt)2bpy)Pt(C≡C-Ph-CH2-PTZ)2 (2), (J. E. McGarrah and R. Eisenberg, Inorg. Chem., 2003, 42, 4355; J. E. McGarrah, J. T. Hupp and S. N. Smirnov, J. Phys. Chem. A, 2009, 113, 6430) as well as (bpy)Pt(C≡C-Ph-C7H15)2, which represent the acceptor-free dyad, and the chromophoric core, respectively. Thus, the step-wise formation of the full charge-separated state on the picosecond time scale and charge recombination via tunnelling have been established; and the presence of two distinct charge recombination pathways has been observed.
Highlights
Transition metal complexes have been at the forefront of developments of artificial light-harvesting systems
For the charge-separated states involving naphthalene diimide (NDI) as an electron acceptor and phen–Pt–acetylide or PTZ as electron donors, energy levels of those with respect to the ground state were estimated using a simplified Rehm–Weller equation: ECS = E+1//02 À E01//2À À (De)2/(4pe0er) where E+1//02 is the first oxidation potential, E01//2À is the first reduction potential, and the last term is introduced to correct for the Coulombic interaction between the electron donor and acceptor counterparts in the charge-separated state; De is the amount of transferred charge between the donor and acceptor, e0 is the permittivity of vacuum, e is the static dielectric constant of the solvent, and r is the distance between the donor and acceptor subunits.[72]
The present paper reports a time-resolved spectroscopic study of excited state dynamics in transition metal complexes 1, 2 and 4 developed for photoinduced charge separation
Summary
Transition metal complexes have been at the forefront of developments of artificial light-harvesting systems. Paper between the separated charges without altering its light-absorbing properties. In such systems, absorption of light will lead to the formation of an initial donor–[chromophore]*– acceptor state, with the D+–[chromophore]–AÀ being an ultimate target state. Much attention has been recently directed towards square-planar, Pt(II)-based systems, which are characterised by intense absorption of light in the visible region as well as by the directionality of the electron transfer. Pt(II) diimine (bis)acetylide chromophores in particular have been a subject of intense research in this regard, owing to the presence of a charge-transfer, ML/LLCT, excited state that usually possesses a relatively long lifetime due to the strong-field acetylide ligand,[6,7,8,9,10,11,12,13,14,15,16,17,18,19,20,21,22,23,24,25,26,27] which is generally sufficient to initiate an electron transfer cascade
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