Lived Charge-separated State Research Articles

Synthesis and Photophysical Properties of Bipolar Compound: Triphenylamine/Diazafluorene-Carbazole.

Carbazole and triphenylamine, are two well-known hole transporting units that are attached to electron transporting unit 4,5-diazafluorene in a fascinating way to bring out non-planar configuration of a molecule. The synthesized compound exhibits good thermal stability (Td > 515°C) and high glass transition temperature (Tg, 191°C). Optical bandgap (Egopt) obtained from solid state absorption spectra was calculated to be 2.93eV. Solid state photoluminescence spectra displays the emission maxima at 473nm. The emission characteristics of the compound observed in solvents of different polarity confirms the existence of intramolecular charge transfer in their excited state. Density functional theory studies reveal that HOMO and HOMO-1 localized on triphenylamine is spatially separated from LUMO of 4,5-diazafluorene, which manifest its bipolar character. The realization of long lived charge separated state upon photo-excitation from time resolved photoluminescence studies ascertains the charge transfer from triphenylamine to 4,5-diazafluorene. The experimental and theoretical analysis of the compound proved it to be a promising candidate for the fabrication of OLED devices.

Comparative Charge Transfer Studies of Porphyrin-Fullerene Dyads: Substituents Effect.

The present study deals with the study of charge transfer and photophysical properties of synthesized non-metallated fullerene-porphyrin dyad-III (H2P3-C60) via Prato reaction. The porphyrin has been substituted with electron donating groups (3,5-di-tert-butylbenzene) at meso positions facilitating the effective charge transfer for the formation of long lived charge-separated states in dyad molecule. The photophysical and thermal activated conducting properties of dyad was comprehensively studied to establish the effect of substituents and also by comparing with our reported dyad II having only meso-phenyl groups. The charge separation efficiency in dyad III has been calculated in solution as well as in films showing 62% and ˜99% quenching efficiency respectively suggesting faster photoinduced charge transfer from porphyrin to fullerene moiety. Transient absorption spectroscopy (TAS) showed formation of longer lived charge separated states in solution as well as in films compared to dyad-II. The thermal activated electrical conductivity measurement of dyad-III showed conductivity enhancement on increasing temperature. The activation energy for conduction was also found lower than dyad-II suggesting a substantial effect of electron-donating groups present on porphyrin ring in conducting behavior of dyad-III. Through this study with a very simple dyad structure we established that the presence of electron donating groups in dayds, highly influence photophysical and electronic properties. This special characteristic further makes them potential material to be used in various opto-electronic applications.

Fluorene–Thiophene Copolymer Wire on TiO2: Mechanism Achieving Long Charge Separated State Lifetimes

Insertion of interfacial molecules in bulk heterojunction and dye sensitized solar cells is effective to retard charge recombination reactions, and thus to improve solar cell performance. So far, to extend charge separated state lifetime, the molecule was designed to increase distance between an n-type and a p-type semiconductors to reduce their electronic coupling. Here we investigated a series of thiophene-fluorene molecular wires on the TiO2 nanoporous surface, and propose a model to explain a long lived charge separated state. The polymer wire acts as a sensitizer aligned in parallel to the TiO2 surface, and injects an electron into the TiO2 with electron injection efficiency of >80 %. Time-resolved microwave conductivity measurements suggest that a generated hole can be mobile, and we found with DFT calculation that a hole appears to be localized at the thiophene units which are not directly attached to the TiO2 surface. Charge recombination between the mobile electron in the TiO2 and the hole at the...

Long lived charge separated states in vinylbenzonitrile substituted derivatives of pyrene and anthracene

Abstract The present study investigates the photophysical properties of two highly conjugated pyrene and anthracene derivatives viz ( E )-4-(2-(pyren-1-yl) vinyl) benzonitrile (PyCN) and ( E )-4-(2-(anthracen-9-yl) vinyl) benzonitrile (AnCN) in the presence and absence of electron donors such as N , N -diethylaniline (DEA) and N , N -diphenylamine (DPA). The observed rates of back electron transfer (BET) in the case of PyCN-DEA and AnCN-DEA are 1.3 × 10 5 and 4.8 × 10 4 s −1 , respectively, which are 3–4 orders of magnitude lower compared to the well-known pyrene–amine and anthracene–amine systems. Mechanistic investigations indicate that the donor-acceptor systems undergo different pathways in presence of tertiary and the secondary amine. The results indicate that the design strategy is highly efficient in reducing the rate of back electron transfer, which results in long lived charge-separated excited states.

N- versus p-doping of graphite: what drives its wet-chemical exfoliation?

We have performed the syntheses of a novel pyrene-porphyrazine conjugate (ZnPzPy) and a reference porphyrazine (ZnPz) to promote the wet-chemical exfoliation of graphite based on the synergetic use of ultrasonication, centrifugation, and doping. ZnPzPy features, on one hand, a hydrophobic pyrene to anchor onto the basal plane of graphene, and, on the other hand, an amphoteric porphyrazine to either p- or n-dope graphene. To this end, we have characterized individual building blocks, that is, ZnPzPy and exfoliated graphite, and the resulting electron donor-acceptor nanohybrid, that is, ZnPzPy/graphene (ZnPzPy-G), by means of an arsenal of microscopic and spectroscopic techniques. From a full-fledged characterization we conclude that ZnPzPy facilitates the exfoliation of graphite affording suspensions featuring 9.5% of single- or few-layered ZnPzPy-G with a mean average size of 200 ± 140 nm. Importantly, a notable shift of charge density from graphene to ZnPzPy in the ground state of ZnPzPy-G corroborates the preference of exfoliated graphite to undergo p-doping rather than n-doping. As an immediate consequence, a full charge separation leads in the excited state to a 750 ± 150 ps lived charge separated state.

Polyhydrogenated Graphene: Excited State Dynamics in Photo- and Electroactive Two-Dimensional Domains

Understanding the phenomenon of intense photoluminescence in carbon materials such as hydrogenated graphene, graphene nanoribbons, and so forth is at the forefront of investigations. In this study, six different types of hydrogenated graphene (phG) produced from different starting materials were fully characterized in terms of structure and optical spectroscopy. Comprehensive photoluminescence lifetime analyses of phGs were conducted by combining time-correlated single-photon counting with steady-state fluorescence spectroscopy and femtosecond transient absorption spectroscopy. The conclusion drawn from these assays is that graphene islands with diameters in the range from 1.1 to 1.75 nm reveal band gap photoluminescence between 450 and 800 nm. As a complement, phGs were implemented in hybrids with water-soluble electron accepting perylenediimides (PDIs). By virtue of mutual π-stacking and charge transfer interactions with graphene islands, PDIs assisted in stabilizing aqueous dispersion of phG. Implicit in these ground state interactions is the formation of 300 ps lived charge separated states once photoexcited.

Tetrathiafulvalene‐Fused Porphyrins via Quinoxaline Linkers: Symmetric and Asymmetric Donor–Acceptor Systems

A tetrathiafulvalene (TTF) donor is annulated to porphyrins (P) via quinoxaline linkers to form novel symmetric P-TTF-P triads 1 a-c and asymmetric P-TTF dyads 2 a,b in good yields. These planar and extended π-conjugated molecules absorb light over a wide region of the UV/Vis spectrum as a result of additional charge-transfer excitations within the donor-acceptor assemblies. Quantum-chemical calculations elucidate the nature of the electronically excited states. The compounds are electrochemically amphoteric and primarily exhibit low oxidation potentials. Cyclic voltammetric and spectroelectrochemical studies allow differentiation between the TTF and porphyrin sites with respect to the multiple redox processes occurring within these molecular assemblies. Transient absorption measurements give insight into the excited-state events and deliver corresponding kinetic data. Femtosecond transient absorption spectra in benzonitrile may suggest the occurrence of fast charge separation from TTF to porphyrin in dyads 2 a,b but not in triads 1 a-c. Clear evidence for a photoinduced and relatively long lived charge-separated state (385 ps lifetime) is obtained for a supramolecular coordination compound built from the ZnP-TTF dyad and a pyridine-functionalized C(60) acceptor unit. This specific excited state results in a (ZnP-TTF)(⋅+) ⋅⋅⋅(C(60) py)(⋅-) state. The binding constant of Zn(II) ⋅⋅⋅py is evaluated by constructing a Benesi-Hildebrand plot based on fluorescence data. This plot yields a binding constant K of 7.20×10(4) M(-1), which is remarkably high for bonding of pyridine to ZnP.

Water Stable Zr–Benzenedicarboxylate Metal–Organic Frameworks as Photocatalysts for Hydrogen Generation

The Zr-containing metal-organic frameworks (MOFs) formed by terephthalate (UiO-66) and 2-aminoterephthalate ligands [UiO-66(NH(2))] are two notably water-resistant MOFs that exhibit photocatalytic activity for hydrogen generation in methanol or water/methanol upon irradiation at wavelength longer than 300 nm. The apparent quantum yield for H(2) generation using monochromatic light at 370 nm in water/methanol 3:1 was of 3.5% for UiO-66(NH(2)). Laser-flash photolysis has allowed detecting for UiO-66 and UiO-66(NH(2)) the photochemical generation of a long lived charge separated state whose decay is not complete 300 μs after the laser flash. Our finding and particularly the influence of the amino group producing a bathochromic shift in the optical spectrum without altering the photochemistry shows promises for the development of more efficient MOFs for water splitting.

Long Lived Charge Separated States Induced by trans-Stilbene Incorporation in the Pores of Brønsted Acidic HZSM-5 Zeolites: Effect of Gallium on the Spontaneous Ionization Process

In situ CW-EPR, diffuse reflectance UV−visible spectroscopy and Raman scattering were used to monitor the spontaneous incorporation of trans-stilbene (t-St, C14H12) in the medium pore H2.2-GaZSM-5 zeolites [H2.2(GaO2)2.2(SiO2)93.8] by direct exposure under dry and inert atmosphere of solid t-St to dehydrated porous material without any solvent. The sorption of t-St with relatively low ionization potential (7.65 eV) occurs in Bronsted acidic H2.2-GaZSM-5 zeolites according to a complex and slow reaction sequence. First, charge separation occurs and t-St•+@H2.2-GaZSM-5•− radical pair is created, while long-lived t-St@H2.2GaZSM-5•−•+ electron−hole pair is formed through hole transfer. The analysis of the DRUVv spectra set recorded during the t-St sorption course shows the respective concentrations of all transient species as a function of time. In particular, note that system reorganization is observed through a second type of electron−hole pair. The broad and strong bands observed in the near-IR regions ove...

Synthesis and photophysical characterization of a titanium(IV) phthalocyanine–C 60 supramolecular dyad

An axially substituted titanium(IV) phthalocyanine–fullerene donor–acceptor supramolecular dyad has been prepared by two different approaches, one of them representing a convenient convergent strategy. The dyad system exhibits photoinduced electron transfer upon irradiation with visible light to produce a microsecond lived charge separated state.