Quantum Confinement-Tunable Ultrafast Charge Transfer at the PbS Quantum Dot and Phenyl-C_(61)-butyric Acid Methyl Ester Interface

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-C_(61)-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

Is pi-Stacking Prone To Accelerate Singlet-Singlet Energy Transfers?

pi-Stacking is the most common structural feature that dictates the optical and electronic properties of chromophores in the solid state. Herein, a unidirectional singlet-singlet energy-transfer dyad has been designed to test the effect of pi-stacking of zinc(II) porphyrin, [Zn-2], as a slipped dimer acceptor using a BODIPY unit, [bod], as the donor, bridged by the linker C6H4C equivalent to CC6H4. The rate of singlet energy transfer, k(ET)(S-1), at 298 K (k(ET)(S-1) = 4.5 X 10(10) s(-1)) extracted through the change in fluorescence lifetime, tau(F), of [bod] in the presence (27.1 ps) and the absence of [Zn-2] (4.61 ns) from Streak camera measurements, and the rise time of the acceptor signal in femtosecond transient absorption spectra (22.0 ps), is faster than most literature cases where no pi-stacking effect exists (i.e., monoporphyrin units). At 77 K, the tau(F), of [bod] increases to 45.3 ps, indicating that k(ET)(S-1) decreases by 2-fold (2.2 X 10(10) s(-1)), a value similar to most values reported in the literature, thus suggesting that the higher value at 298 K is thermally promoted at a higher temperature

What does it take to induce equilibrium in bidirectional energy transfers?

Two dyads built with a co-facial slipped bis(zinc( ii )porphyrin), a free base and a bridge, [Zn 2 ]–bridge–[Fb] (bridge = C 6 H 4 CC, 1 and C 6 H 4 CCC 6 H 4 , 2), exhibit S 1 energy equilibrium [Zn 2 ]* ↔ [Fb]* at 298 K, an extremely rare situation, which depends on the degree of MO coupling between the units. At 77 K, 2 becomes bi-directional due to the two large C 6 H 4 –[Zn 2 ] and C 6 H 4 –[Fb] dihedral angles

Convenient tautomeric forms in an amino-anthraquinone diimine for the generation of a mixed-valent push-pull conjugated polymer

The use of NH2 on the (Pt(C C)(2)(PBu3)(2))(n)/anthraquinone diimine)(n) polymer induces a tautomeric species which leads to a mixed-valent form reminiscent of emaraldine in polyaniline

Incorporation of zinc(II) porphyrins in polyaniline in its perigraniline form leading to polymers with the lowest band gap

Conjugated copolymers built upon quinone diimine-zinc(II) porphyrin units exhibit a very low lying charge transfer band at 800 nm and are strongly emissive from the S(2) and T(2) states

Platinum Complexes of N,N',N '',N'''-Diboronazophenines

Azophenine, (alpha-C6H5NH)(2)(C6H-N=C6H2=N-C6H6), well known to be non-emissive, was rigidified by replacing two amine protons by two difluoroboranes (BF2+) and further functionalized at the pars-positions of the phenyl groups by luminescent trans-ArC C-Pt(PR3)(2)-C C ([Pt]) arms [Ar = C6H4 (R = Et), hexa(n-hexyl)-truxene) (Tru; R = Bu)]. Two effects are reported. First, the linking of these [Pt] arms with the central azophenine (C6H4-N=C6H2(NH)(2)=N-C6H4; Q) generates very low energy charge-transfer (CT) singlet and triplet excited states ((3,1)([Pt]-to-Q)*) with absorption bands extending all the way to 800 nm. Second, the rigidification of azophenine by the incorporation of BF2+ units renders the low-lying CT singlet state clearly emissive at 298 and 77 K in the near-IR region. DFT computations place the triplet emission in the 1200-1400 nm range, but no phosphorescence was detected. The photophysical properties are investigated, and circumstantial evidence for slow triplet energy transfers, (3)Tru* -> Q is provided

Increasing the lifetimes of charge separated states in porphyrin-fullerene polyads

Two linear polyads were designed using zinc(II) porphyrin, [ZnP], and N-methyl-2-phenyl-3,4-fulleropyrrolidine (C-60) where C-60 is dangling either at the terminal position of [ZnP]-C6H4-R-C6H4-[ZnP]-C-60 (1) or at the central position of [ZnP]-C6H4-R-C6H4-[ZnP(C-60)]-C6H4-R-C6H4-[ZnP] (2) in order to test whether the fact of having one or two side electron donors influences the rate of electron transfer, k(et). These polyads were studied using cyclic voltammograms, DFT computations, steady state and timeresolved fluorescence spectroscopy, and femtosecond transient absorption spectroscopy (fs-TAS). Photo-induced electron transfer confirmed by the detection of the charge separated state [ZnP center dot+]/C-60(center dot-) from fs-TAS occurs with rates (k(et)) of 3-4 x 10(10) s(-1) whereas the charge recombinations (CRs) are found to produce the [ZnP] ground state via two pathways (central [(ZnP center dot+)-Zn-1]/C-60(center dot-) (ps) and terminal central [ZnP center dot+]/C-60(center dot-) (ns) producing [(ZnP)-Zn-1] (ground state) and [(ZnP)-Zn-3*]). The formation of the T1 species is more predominant for 2

Ph-Induced Surface Modification Of Atomically Precise Silver Nanoclusters: An Approach For Tunable Optical And Electronic Properties

Noble metal nanoclusters (NCs) play a pivotal role in bridging the gap between molecules and quantum dots. Fundamental understanding of the evolution of the structural, optical, and electronic properties of these materials in various environments is of paramount importance for many applications. Using state-of-the-art spectroscopy, we provide the first decisive experimental evidence that the structural, electronic, and optical properties of Ag44(MNBA)30 NCs can now be tailored by controlling the chemical environment. Infrared and photoelectron spectroscopies clearly indicate that there is a dimerization between two adjacent ligands capping the NCs that takes place upon lowering the pH from 13 to 7

Application of the boron center for the design of a covalently bonded closely spaced triad of porphyrin-fullerene mediated by dipyrromethane

Two novel triads had been designed through covalent bond connection of the boron dipyrromethane (BODIPY), free base porphyrin (H2P) or zinc(II) porphyrin (ZnP) and N-methyl-2-phenyl-3,4-fulleropyrrolidine (C-60) mediated by BODIPY. This closely spaced triad arrangement where porphyrin and fullerene are placed apart is anticipated to stabilize charge separation by separating the two radicals from each other. Two model polyads were synthesized with BODIPY and H2P or ZnP to investigate interaction between the two chromophores. Photo-excitation of the BODIPY triggered an efficient singlet energy transfer where the rates are found to be similar to 10(10)-10(11)s(-1). For triads with C-60 fast electron transfer was confirmed by the detection of the C-60(center dot-) signature from femtosecond transient absorption (fs-TA) in similar to 0.4-3 ps. The charge recombination is estimated to be in the nanosecond window. This indicates the convenience of this arrangement for stabilizing the charge-separated state