The steady-state and femtosecond (fs) time-resolved data clearly demonstrate that the charge transfer (CT) process at the porphyrin-graphene carboxylate (GC) interfaces can be tuned from zero to very sufficient and ultrafast by changing the electronic structure of the meso unit and the redox properties of the porphyrin cavity.

Quantum dot (QD) solar cells have emerged as promising low-cost alternatives to existing photovoltaic technologies. Here, we investigate charge transfer and separation at PbS QDs and phenyl-C61-butyric acid methyl ester (PCBM) interfaces using a combination of femtosecond broadband transient absorption (TA) spectroscopy and steady-state photoluminescence quenching measurements. We analyzed ultrafast electron injection and charge separation at PbS QD/PCBM interfaces for four different QD sizes and as a function of PCBM concentration. The results reveal that the energy band alignment, tuned by the quantum size effect, is the key element for efficient electron injection and charge separation processes. More specifically, the steady-state and time-resolved data demonstrate that only small-sized PbS QDs with a bandgap larger than 1 eV can transfer electrons to PCBM upon light absorption. We show that these trends result from the formation of a type-II interface band alignment, as a consequence of the size distribution of the QDs. Transient absorption data indicate that electron injection from photoexcited PbS QDs to PCBM occurs within our temporal resolution of 120 fs for QDs with bandgaps that achieve type-II alignment, while virtually all signals observed in smaller bandgap QD samples result from large bandgap outliers in the size distribution. Taken together, our results clearly demonstrate that charge transfer rates at QD interfaces can be tuned by several orders of magnitude by engineering the QD size distribution. The work presented here will advance both the design and the understanding of QD interfaces for solar energy conversion.

Three conjugated polymers of the type (−spacer−CC−PtL2−CC−) n , with spacer = para- and ortho-bis(diphenyl(tetramethoxy)quinone diimine) and L = PEt3, were synthesized as the cis- and trans-isomers about the Pt. The three combinations (spacer/PtL2Cl2), ortho/trans, para/trans, and para/cis led to polydispersed polymers, which upon fractionation provided more monodispersed materials with M n ranging from 3600 to 32400 (GPC). The spacer ortho-bis(diphenylethynyl)tetramethoxyquinone diimine reacts with cis-PtL2Cl2 to form small oligomers and a chelate cyclic complex (7), which was used as a model compound. All materials along with the aryl−CC−PtL2− CC−aryl models (L = PEt3; aryl = C6H5, 2,4,6-C6H2Me3; 2,4,5-C6H2Me3) were studied by 1H and 31P NMR, IR, UV–vis, TGA, DSC, luminescence spectroscopy, photophysics, and cyclic voltammetry. The UV–vis spectra exhibit an intramolecular low-energy and low-intensity charge transfer band (CT) assigned to π(Ph−CC) (for organics) and π/d xy ((Ph−CC)2Pt) (for organometallics) to the π*(quinone diimine), as demonstrated by comparison with a corresponding amine derivative (i.e., no CT band) and corroborated by DFT and TDDFT. The polymers and 7 are not luminescent at room temperature (solid and solution). However in 2MeTHF at 77 K, the polymers bearing the combination ortho/trans and para/cis as well as the model compound 7 exhibit higher energy phosphorescence (T n → S0) originating from the ππ*/d xy ((Ph−CC)2Pt) excited states. A correlation between the structural parameters (angle made by the PtP2(CC)2 vs the aryl planes, angle made by the aryl and the quinone diimine planes, molecular weight) with the calculated oscillator strength, absorptivities, and the observed position of the lowest energy absorption bands is demonstrated. Finally, upper energy delayed fluorescence (S n → S0) was detected for the polymers and 7 at 77 K and was assigned to T1−T1 interactions via aggregation as its intensity varies with the concentration.

Reaction of CuI with bis(phenylthio)propane in a 1:1 ratio yields the two-dimensional coordination polymer [{Cu(μ(2)-I)(2)Cu}{μ-PhS(CH(2))(3)SPh}(2)](n) (1). The 2D-sheet structure of 1 is built up by dimeric Cu(2)I(2) units, which are connected via four bridging 1,3-bis(phenylthio)propane ligands. In contrast, treatment of 2 equiv of CuI with 1,3-bis(phenylthio)propane in MeCN solution affords in a self-assembly reaction the strongly luminescent metal-organic 2D-coordination polymer [Cu(4)I(4){μ-PhS(CH(2))(3)Ph}(2)](n) (2), in which cubane-like Cu(4)(μ(3)-I)(4) cluster units are linked by the dithioether ligands. The crystallographically characterized one-dimensional (1D) compound [{Cu(μ(2)-Br)(2)Cu}{μ-PhS(CH(2))(3)SPh}(2)](n) (3) is obtained using CuBr. The outcome of the reaction of PhS(CH(2))(5)SPh with CuI also depends of the metal-to-ligand ratio employed. Mixing CuI and the dithioether in a 2:1 ratio results in formation of [Cu(4)I(4){μ-PhS(CH(2))(5)Ph}(2)](n) (4) in which cubane-like Cu(4)(μ(3)-I)(4) clusters are linked by the bridging dithioether ligand giving rise to a 1D necklace structure. A ribbon-like 1D-polymer with composition [{Cu(μ(2)-I)(2)Cu}{μ-PhS(CH(2))(5)SPh}(2)](n) (5), incorporating rhomboid Cu(2)I(2) units, is produced upon treatment of CuI with 1,5-bis(phenylthio)pentane in a 1:1 ratio. Reaction of CuBr with PhS(CH(2))(5)SPh produces the isomorphous 1D-compound [{Cu(μ(2)-Br)(2)Cu}{μ-PhS(CH(2))(5)SPh}(2)](n) (6). Strongly luminescent [Cu(4)I(4){μ-p-TolS(CH(2))(5)STol-p}(2)](n) (7) is obtained after mixing 1,5-bis(p-tolylthio)pentane with CuI in a 1:2 ratio, and the 2D-polymer [{Cu(μ(2)-I)(2)Cu}(2){μ-p-TolS(CH(2))(5)STol-p}(2)](n) (8) results from reaction in a 1:1 metal-to-ligand ratio. Under the same reaction conditions, 1D-polymeric [{Cu(μ(2)-Br)(2)Cu}{μ-p-TolS(CH(2))(5)STol-p}(2)](n) (9) is formed using CuBr. This study reveals that the structure of the self-assembly process between CuX and ArS(CH(2))(m)SAr ligands is hard to predict. The solid-state luminescence spectra at 298 and 77 K of 2 and 4 exhibit very strong emissions around 535 and 560 nm, respectively, whereas those for 1 and 5 display weaker ones at about 450 nm. The emission lifetimes are longer for the longer wavelength emissions (>1.0 μs arising from the cubane species) and shorter for the shorter wavelength ones (<1.4 μs arising from the rhomboid units). The Br-containing species are found to be weakly fluorescent.