Hole Localization and Ultrafast Electron Transfer Dynamics in CuInS₂/Organic Molecule Composite

Exploring the dynamics of photoexcited electrons and holes in green I–III–VI ternary quantum dots (QDs) is of utmost necessity to design and improve their functionality and applicability in devices. Here, we have studied the dynamics of photoexcited nascent carriers in CuInS2/ZnS (CISZ) core–shell QDs as potential excited state electron donors using picosecond time-resolved emission and femtosecond transient absorption (fsTA) spectroscopy techniques. The emission decay measurements conducted in the 460–660 nm wavelength range allowed us to distinguish between emission due to electron–hole recombination at conduction (CB) and valence bands (VB), respectively, and due to recombination of the CB electrons and holes at the self-trapped states (Cu1+ centers). The excited state electron transfer (ET) was studied by attaching an anthraquinone-2-carboxylic acid (AQ) electron acceptor to the QDs. The ET from the CB of CISZ QD to AQ was confirmed by the fsTA studies, which revealed the formation of a characteristic anion (AQ–) band at 600 nm. The ET time constant was estimated to be 1.5 ps in ideal one-to-one CISZ-AQ complex, and the charge recombination takes place with time constant >5 ns at delay times beyond the fsTA Instruments reach. We propose that such a long lifetime of the charge-separated state is achieved after hole localization at the (Cu1+) defect, which effectively decouples the hole at the QD and the electron at AQ

Exciplex−Exciplex Energy Transfer and Annihilation in Solid Films of Porphyrin−Fullerene Dyads

Exciplex−exciplex annihilation was observed for the first time in porphyrin−fullerene molecular films. The films were prepared using Langmuir−Blodgett and drop casting methods. The exciplex−exciplex interactions were studied using femtosecond pump−probe method. The exciplex−exciplex annihilation can be seen as a fast (within few picoseconds) decay of the transient absorption at excitation densities higher than 0.4 mJ/cm2. Analysis of the excitation density dependences indicates that in average four dyads are involved in the exciplex−exciplex interaction, suggesting that an exciplex−exciplex energy transfer may precede the annihilation

Photoinduced Electron Transfer in Double-Bridged Porphyrin−Fullerene Triads

Electron and energy transfer reactions of porphyrin−porphyrin−fullerene triads (P2P1C) with controllable sandwich-like structures have been studied using spectroscopic and electrochemical methods. The stabile, stacked structure of the molecules was achieved applying a two-linker strategy developed previously for porphyrin−fullerene dyads. Different triad structures with altered linker positions, linker lengths, and center atoms of the porphyrin rings were studied. The final charge-separated (CS) state and the different transient states of the reactions have been identified and energies of the states estimated based on the experimental results. In particular, a complete CS state P2+P1C- was achieved in a zinc porphyrin−free-base porphyrin−fullerene triad (ZnP2t9P1C) in both polar (benzonitrile) and nonpolar (toluene) solvents. The lifetime of this state was longer living in the nonpolar solvent. An outstanding feature of the ZnP2t9P1C triad is the extremely fast formation of the final CS state, P2+P1C-. This state is formed after primary excitation of either zinc porphyrin or free-base porphyrin chromophores in less than 200 fs. Although the intermediate steps between the locally excited states and the final CS state were not time-resolved for this compound, the process is clearly multistep and the fastest ever observed for porphyrin-based compounds

Photoinduced Electron Transfer in CdSe/ZnS Quantum Dot–Fullerene Hybrids

Photoinduced electron transfer (ET) in CdSe/ZnS core–shell quantum dot (QD) – fullerene (COOH–C₆₀) hybrids was studied by the means of time-resolved emission and absorption spectroscopy techniques. A series of four QDs with emission in the range 540–630 nm was employed to investigate the dependence of the electron transfer rate on the QD size. Emission of the QDs is quenched upon hybrid formation, and the quenching mechanism is identified as photoinduced electron transfer from the QD to the fullerene moiety due to the fullerene anion signature observed in transient absorption. In order to obtain quantitative information on the ET reaction, several kinetic data analysis techniques were used, including a conventional multiexponential fitting and a maximum entropy method for emission decay analysis, as well as a distributed decay model based on the Poisson distribution of fullerenes in the hybrids. The latter gradually simplifies the interpretation of the transient absorption spectra and indicates that the spectra of QD cations are essentially similar to those of neutral QDs, differing only by a minor decrease in the intensity and broadening. Furthermore, only a minor decrease in the ET rate with the increasing QD size was observed, the time constants being in the range 100–200 ps for all studied QDs. The charge recombination is extended to 10 ns or longer for all hybrids

Effect of Hole Transporting Material on Charge Transfer Processes in Zinc Phthalocyanine Sensitized ZnO Nanorods

The photoinduced electron transfer processes were studied for hybrid systems consisting of self-assembled monolayer of zinc phthalocyanine (ZnPc) assembled on ZnO nanorods and a film of organic hole transporting material (HTM) atop. Polythiophene (P3HT) or Spiro-OMeTAD were used as HTM. The study was carried out by ultrafast transient absorption spectroscopy technique with selective excitation of ZnPc at 680 nm or P3HT at 500 nm. Data analysis revealed that photoexcitation of ZnPc in the structure ZnO|ZnPc|P3HT results in a fast (1.8 ps) electron transfer from ZnPc to ZnO, which is followed by a hole transfer from the ZnPc cation to P3HT roughly in 30 ps. However, in the case of ZnO|ZnPc|Spiro-OMeTAD structure, the primary reaction upon excitation of ZnPc is a fast (0.5 ps) hole transfer from ZnPc to Spiro-OMeTAD, and the second step is electron injection from the ZnPc anion to ZnO in roughly 120 ps. Thus, we demonstrate two structurally very similar hybrid architectures that implement two different mechanisms for photoinduced charge separation found in dye-sensitized or in organic solar cells

Energy and Electron Transfer in Multilayer Films Containing Porphyrin−Fullerene Dyad

Photoinduced interlayer energy and electron transfer from a thiophene derivative, PVT3, to a porphyrin−fullerene dyad, P-F, was demonstrated. The laser flash photolysis method was utilized to characterize photoinduced processes in layered thin films constructed by the Langmuir−Blodgett and spin-coating techniques. Poly(3-hexylthiophene), PHT, was used as an electron-donating layer to the dyad and PVT3. Electron transfer through a multilayer film with a PHT−PVT3−P-F layer sequence was shown, yielding the final charge-separated state where the positive charges are located in the PHT network and the electrons in the fullerene sublayer. The crucial role of excited P-F for the overall charge transfer efficiency of a bilayer film containing a phthalocyanine derivative, ZnPH4, as an electron-donating moiety to P-F was demonstrated. The lifetime of the electrical signal of the charge separation was shown to be prolonged compared to that of the optical signal

Photoinduced Energy and Charge Transfer in Layered Porphyrin-Gold Nanoparticle Thin Films

In thin films of porphyrin (H2P) and gold nanoparticles (AuNPs), photoexcitation of porphyrins leads to energy and charge transfer to the gold nanoparticles. Alternating layers of porphyrins and octanethiol protected gold nanoparticles (dcore ∼3 nm) were deposited on solid substrates via the Langmuir−Schäfer method, forming bilayer films denoted as H2P/AuNP. Photoinduced electron transfer from the gold nanoparticle layer to the porphyrin layer was observed as a distinct photovoltage response of the H2P/AuNP film when studied via the time-resolved Maxwell displacement charge (TRMDC) method. Time-resolved fluorescence and absorption measurements of the H2P/AuNP film demonstrated a significant reduction of the lifetime of the excited singlet state of porphyrin caused by the gold nanoparticles. Transients of the charge transfer reaction were not observed in the time-resolved absorption measurements, which indicates that the quantum yield of the charge transfer is low in the H2P/AuNP film. Energy transfer from the excited singlet state of porphyrin to the gold nanoparticles is the main deactivation path of excited porphyrins in the H2P/AuNP film. The critical distance of the energy transfer was estimated to be 6.4 nm, based on the dependence of fluorescence quenching on the distance between the porphyrin and gold nanoparticle layers

Laterally Bound Co Porphyrin on CdTe QD: A Long-Lived Charge-Separated Nanocomposite

Cobalt porphyrin (CoP) derivatives are potential compounds for photocatalytic CO2 reduction which must be activated by photoinduced electron transfer from a suitable electron donor. Herein, we have prepared and studied the photophysics of CdTe quantum dots (CQD) coupled with CoP derivatives where CQDs act as the light antenna and the electron donor and CoP acts as the electron acceptor. To facilitate the nanocomposite formation of CoP with CQD, CoP has been equipped with a −COOH anchoring group which leads to strong complexation between CQD and CoP as observed in the absorption spectra by a gradual shift in the Soret absorption band. This is attributed to the lateral binding geometry of CoP through the −COOH anchoring group and Co-center coordination to CQD, which helps to bring CoP close to the CQD. Our DFT calculations have identified that this lateral geometry is more favorable than the upright orientation on the CdTe (110) surface. The redox levels have been determined from cyclic voltammetry which shows that the electron transfer (ET) from CQD to CoP is feasible. The strong luminescence quenching of CQD in the presence of CoP has also suggested quantitative CQD/CoP nanocomposite formation and pointed to the ET from QDs to CoP. The charge carrier dynamics have been monitored using femtosecond transient absorption (TA) spectroscopy. The TA spectral analysis has shown efficient ET in CQD/CoP which proves that our 4 nm CQD acts as an efficient electron donor for the CoP counterpart. The CQD excited state lifetime is shortened along with delayed Soret band bleaching of CoP in this nanocomposite. From the global fitting of TA data, the estimated average ET time constant from CQD to a CoP molecule is approximately 70 ps, and the charge recombination time is ≫5 ns. Also, differences in the TA spectra after ET have been observed which can be associated with the changes in the binding geometry of CoP on the CQD surface, which is lateral in the case of the ground-state complex to the upright orientation after the ET process. Hence, the studied CQD/CoP nanocomposites are promising materials to initiate CO2 reduction through photoexcitation of the CQD that activates the CoP molecular catalyst through the ET

Effect of Anion Ligation on Electron Transfer of Double-Linked Zinc Porphyrin−Fullerene Dyad

The interaction between metalloporphyrins and their axial ligands plays an important role in the electron transfer (ET) processes in which the excited porphyrin participates. An efficient photoinduced ET reaction of a double-linked zinc(II) porphyrin-fullerene dyad was demonstrated in ionic environment. The chloride ion of tetrabutylammonium chloride (TBACl) electrolyte solution ligates the zinc porphyrin moiety in the dyad which results in a red shift of the absorption bands and lowers the energy of the charge-separated state by about 0.26 eV as compared to the nonligated dyad. Excitation of the porphyrin chromophore results in ET from porphyrin to fullerene in a moderately polar solvent, anisole. In nonionic and nonligating ionic environments, the ET reaction occurs through an intermediate state, an intramolecular exciplex, which has emission in the near-infrared region of the spectrum. This emission is not observed directly for the dyad in TBACl/anisole solution, but evidence of the exciplex intermediate was seen in the time-resolved measurements. The lower energy of the charge-separated state in the ligated environment explains the different ET reaction rates determined in the spectroscopic studies: the charge recombination process of the ligated dyad is about 5 times faster than that of the nonligated one

Driving Force Dependence of Photoinduced Electron Transfer Dynamics of Intercalated Molecules in DNA

A series of acridinium, quinolinium, and phenanthridinium ions (9-substituted-10-methylacridinium (AcrR+, R = H, Pri, and CH2Ph), 3-substituted-1-methylquinolinium (RQuH+, R = CN and Br), and 5-methylphenanthridinium (5-MePhen+) perchlorate salts) are shown to be intercalated into the DNA double helix from calf thymus. The one-electron reduction potentials (E0red) of these intercalators have been determined in the absence and presence of DNA by both cyclic voltammetry and second harmonic ac voltammetry. The E0red values of intercalators are shifted in a positive direction by intercalation into the DNA double helix. The one-electron oxidation potential (E0ox) of ethidium bromide, which is known to be intercalated into DNA, is also shifted in a positive direction by the intercalation. The wide range of E0red values of intercalators thus determined in the presence of DNA allows us to examine the exact driving force dependence of the rates of photoinduced electron transfer from the singlet excited state of ethidium bromide to the intercalators in DNA for the first time. The resulting data were evaluated in light of the Marcus theory of electron transfer to determine the reorganization energy and the electron coupling matrix element in DNA