Efficient Photoinduced Electron Transfer Research Articles

Exploiting singlet excited state conformation for rational design of highly efficient photoinduced electron transfer molecules.

In spite of the pivotal role of excited state electronic structures as regulation of photoinduced electron transfer (PET) process, the effect of excited state conformation on PET remains elusive. Here we exploit distinguishable emission characters of trans and cis singlet excited states of donor-acceptor-donor ensemble MTPAAZO to reveal that its PET efficiency and rate are closely depended on its singlet excited state conformation. The PET process occurs solely in cis conformation of MTPAAZO singlet excited states. Novel molecule (MTPA)2Ab as-designed with similar structure of MTPAAZO cis singlet excited states shows high PET efficacy and rate, leading to long-lived CS states. Our findings enable the rational design of the novel molecules with highly efficient PET process suitable for charge separation applications.

A dual responsive colorimetric/fluorescent turn-on sensor for highly selective, sensitive and fast detection of Fe3+ ions and its applications

A new dual responsive colorimetric and fluorescent turn-on sensor DN for Fe3+ based on Schiff base derivative was designed and synthesized by one-step reaction. The sensor DN exhibited highly selective, sensitive and fast recognition toward Fe3+ with distinct color change by naked eyes in DMSO/H2O (1:1, v/v) solution over other competitive metal ions. The interaction of DN with Fe3+ caused a significant enhancement in emission intensity due to the restricted C = N isomerization and the suppression of highly efficient photoinduced electron transfer (PET) process. The sensing complex DN-Fe3+ was well-characterized to be 1:2 by the Job’s plot, 1H NMR titration, FTIR, HR ESI-MS analysis and DFT studies. The detection limit of DN for Fe3+ was as low as 0.178 μM, which displayed high sensitivity towards Fe3+. The sensor DN-based filter paper strips can be used to conveniently and effectively detect Fe3+ ions by the naked eyes. Importantly, the sensor DN was further used for detecting Fe3+ in real water and food samples with acceptable results and good recoveries.

Влияние фотоиндуцированных процессов на поверхности квантовых точек на эффективность переноса электрона в структурах "Наночастицы TiO-=SUB=-2-=/SUB=-/Квантовые Точки"

The effect of photoinduced processes on the CdSe/ZnS quantum dots surface on the functionality of TiO2 nanoparticles/quantum dots hybrid structures has been studied. We demonstrate that pre-irradiation of QDs layers allows for a threefold increase in the photoinduced electron transfer efficiency in the structures. It was shown that photoinduced processes affect both the efficiency of nonradiative processes in QDs and electron transfer conditions in hybrid structures.

Методика оценки функциональности многослойных гибридных структур TiO-=SUB=-2-=/SUB=-/квантовая точка по генерации активных форм кислорода

A new approach for estimation of photoinduced electron transfer efficiency from CdSe/ZnS QDs to Titania NPs using ROS generation has been introduced. The 26% efficiency electron transfer has been realized in the prepared Titania NPs/QDs multilayered hybrid structures.

Natural Dyes and Their Derivatives Integrated into Organic Solar Cells.

Natural photosynthetic systems contain several dyes such as carotenoids or chlorophylls which are adequately arranged to produce efficient photoinduced charge separation and electron transfer. Several research groups have attempted integrating these natural dyes and photosynthetic systems into functional organic solar cells (OSCs) producing power conversion efficiencies (PCEs) up to 0.99%. The studies presented in this short review emphasize that functionalization of natural dyes can considerably improve their PCEs. For instance, chlorophyll derivatives can yield PCEs up to 2.1%, and copolymers produced with isoindigo as an electron-deficient unit generate high PCEs up to 8%, respectively, when combined with fullerene C70 based electron acceptors in the OSC active layers. An alternative approach for natural dye integration into OSC architectures is to place these light-harvesting antennas at the interface between the active layer and the charge collection layers in these low-cost photovoltaic devices. This strategy produces large PCE increases up to 35% with respect to OSCs prepared without the interlayer. When light-harvesting systems are combined with silver nanoprisms as interlayers, additional localized surface plasmon resonance effects result in high-performance OSCs that integrate natural photosynthetic systems and demonstrate a PCE over the milestone value of 10%.

Tuning Photoinduced Electron Transfer Efficiency of Fluorogenic BODIPY-α-Tocopherol Analogues.

Fluorogenic analogues of α-tocopherol developed by our group have been instrumental in monitoring reactive oxygen species (ROS) within lipid membranes. Prepared as two-segment trap-reporter (chromanol-BODIPY) probes, photoinduced electron transfer (PeT) was utilized to provide these probes with an off/on switch mechanism warranting the necessary sensitivity. Herein, we rationalize within the context of Marcus theory of electron transfer how substituents on the BODIPY core and linker length joining the trap and reporter segments, tune PeT efficiency. DFT and electrochemical studies were used to estimate the thermodynamic driving force of PeT in our constructs. By tuning the redox potential over a 400mV range, we observed over an order of magnitude increase in PeT efficiency. Increasing the linker length between the chromanol and BODIPY by 2.8 angstroms, in turn, decreased PeT efficiency 2.7-fold. Our results illustrate how substituent and linker choice enable "darkening" the off state of fluorogenic probes based on BODIPY fluorophores, by favoring PeT over radiative emission from the singlet excited state manifold. Ultimately, our work brings light to the sensitivity ceiling one may achieve in developing fluorogenic antioxidant analogues of α-tocopherol. The work provides general guidelines applicable to those developing fluorogenic probes based on PeT.

A Heterogeneous Hydrogen‐Evolution Catalyst Based on a Mesoporous Organosilica with a Diiron Catalytic Center Modelling [FeFe]‐Hydrogenase

AbstractA diiron [FeFe]‐hydrogenase model complex tethered with a maleimide group, FeFe1, was covalently grafted on the pore surface of a periodic mesoporous organosilica with thiol groups (SH‐PMO) to form an efficient heterogeneous hydrogen (H2)‐evolution catalyst FeFe1@PMO. The coordination structure of the FeFe1 complex and the ordered pore structure were almost completely preserved even after immobilization of FeFe1 on SH‐PMO. The FeFe1@PMO promoted photocatalysis for H2 evolution in water containing a photosensitizer [Ru(bpy)3]2+, with a turnover number (TON) of 310 over 120 min. The TON was greater than those of an analogous homogeneous FeFe1 catalyst (TON=180) and conventional diiron complexes immobilized on solid supports (TON=6–18). The increased TON for FeFe1@PMO compared to the homogeneous FeFe1 was attributed to the improvement in the stability of the FeFe1 complex by immobilization on the pore surface of SH‐PMO. A [Ru(bpy)3]2+ photosensitizer tethered with a maleimide (Ru1) was prepared and co‐immobilized on FeFe1@PMO to form an all‐solid‐state photocatalyst FeFe1‐Ru1@PMO. FeFe1‐Ru1@PMO evolved H2 without the additional [Ru(bpy)3]2+ photosensitizer, suggesting efficient photoinduced electron transfer from the immobilized Ru1 to the immobilized FeFe1.

A Simple and Rapid Fluorescent Probe for Detection of Cr3+ Based on a Coumarin Schiff Base in Aqueous Solution.

A new Cr3+ probe was synthesized using simple Schiff base reaction, which showed prominent fluorescence increasing switch before and after addition Cr3+. The probe proved to have excellent properties, based on both UV-vis absorption and fluorescence spectra. Those properties included high switching performance, good selectivity, and small interference with other metal ions. The fluorescent change mechanism of the probe was attributed to the combined action between the restricted C=N isomerization and the suppression of highly efficient photo-induced electron transfer (PET) process. Moreover, this fluorescence probe for Cr3+ detection also has great potential for bioimaging of cancer cells.

A Supramolecular [10]CPP Junction Enables Efficient Electron Transfer in Modular Porphyrin-[10]CPP⊃Fullerene Complexes.

Efficient photoinduced electron transfer was observed across a [10]cycloparaphenylene ([10]CPP) moiety that serves as a rigid non-covalent bridge between a zinc porphyrin and a range of fullerenes. The preparation of iodo-[10]CPP is the key to the synthesis of a porphyrin-[10]CPP conjugate, which binds C60 , C70 , (C60 )2 , and other fullerenes (KA >105 m-1 ). Fluorescence and pump-probe spectroscopy revealed intramolecular energy transfer between CPP and porphyrin and also efficient charge separation between porphyrin and fullerenes, affording up to 0.5 μs lifetime charge-separated states. The advantage of this approach towards electron donor-acceptor dyads is evident in the case of dumbbell-shaped (C60 )2 , which gave intricate charge-transfer behavior in 1:1 and 2:1 complexes. These results suggest that [10]CPP and its cross-coupled derivatives could act as supramolecular mediators of charge transport in organic electronic devices.

Photocatalytic Properties of Hybrid Nanostructures Based on Nanoparticles of TiO2 and Semiconductor Quantum Dots

Multilayer hybrid nanostructures based on nanoparticles of TiO2 and CdSe/ZnS semiconductor quantum dots were formed. Generation of active forms of oxygen by hybrid nanostructures under visible-light irradiation is demonstrated to take place, which is indicative of efficient photoinduced electron transfer from quantum dots to titanium-oxide nanoparticles contained in the hybrid nanostructures.

Long lived-charge separation of ultrafast bimolecular electron transfer at PCE10 and fullerene interfaces

A profound understanding of the ultrafast interfacial charge transfer (CT), charge separation (CS), and charge recombination (CR) are paramount in enhancing the photoconversion efficiency (PCE) of the solar cell devices. Here, a combination of steady-state and femtosecond transient absorption spectroscopies with broadband capabilities are used to explore and decipher the photoinduced charge carrier dynamics at the PCE10/C60 interfaces. Our experimental results verified the efficient and ultrafast (sub-picosecond) photoinduced electron transfer (ET) from the PCE10 polymer to fullerene acceptor C60. Notably, the slow CR and fast ET make the current PCE10/C60 system ideal for its potential use in solar cells devices.

Microcrystal induced emission enhancement of a small molecule probe and its use for highly efficient detection of 2,4,6-trinitrophenol in water

In this contribution, we reported a very simple and small molecule material, 2,5-dimethoxyterephthalaldehyde (DMA). It exhibited a relatively weak fluorescence in solution, while showed a steadily increased green fluorescence with typical aggregation- induced enhanced emission (AIE) effect for forming a cubic-like microcrystal structure in THF-H2O mixed solvent. The microcrystals presented significantly higher fluorescence than that of amorphous aggregates. The DMA microcrystals suspension showed a good response to 2,4,6-trinitrophenol (TNP) with a LOD of 1.2×10−7 M, which is the best result of TNP detection in aqueous solution. Quantum chemical calculation revealed that DMA is a donor (D)-receptor (A) type molecule with methoxy unit as donor and carbonyl moiety as receptor. Its emission arises from an intramolecular charge transfer (ICT) from methoxy units to carbonyl units. NMR indicated that there is a strong hydrogen bond interaction between DMA and TNP. Hydrogen bond interaction can effectively decrease the intermolecular distance of DMA and TNP, which will increase the efficiency of photoinduced electron transfer (PET) and fluorescence resonance energy transfer (FRET), and hence will be advantageous for its selectivity. The microcrystal induced enhanced emission could be generally used for kinds of target molecules analysis.

(Invited) Bodipy-Nanocarbon Hybrids for Mimicking Early Events of Natural Photosynthesis

Mimicking the early events of natural photosynthesis, and utilizing that information to build energy harvesting devices is considered one of the avenues to tap into the vast amount of solar energy. Research performed on this topic by scientists across the globe has been highly promising and has strengthened our fundamental understanding of natural and artificial photosynthesis. In the process of mimicking the early events of photosynthesis, excitation transfer and electron transfer in a sequential fashion, similar to what happens in the antenna and reaction centre of natural photosynthesis, are deemed to be of paramount interest. Accordingly, supramolecular donor-acceptor systems capable of performing these photo processes are considered very important. Although several donor-acceptor systems are known to undergo either energy transfer or electron transfer or simultaneous occurrence of these two photo processes in competition upon photoexcitation, there are not many systems where energy transfer and electron transfer in a sequential route have been reported. Nanocarbons such as fullerenes, single wall carbon nanotubes, graphene oxide, and graphene have been widely used in the construction of such multi-modular donor-acceptor systems. Porphyrins and phthalocyanines are class of widely used photosensitizers. In recent years, application BF2-chelated dipyrromethene (BODIPY) and BF2-chealted azadipyrromethene (azaBODIPYs), in building artificial photosynthetic models have witnessed a rapid growth. This is primarily due to the synthetic versatility of these sensitizers to couple with nanocarbons to modulate their photo- and redox properties to promote efficient photoinduced energy and electron transfer processes. In few instances, accomplishing the combined ‘antenna-reaction center’ functionality of photosynthesis has been possible. The present contribution summarizes recent progress made in our laboratory in this area of research.

(Invited) Chiral-at-Metal Fullerene Hybrids for Catalysis

Fullerenes have found their main application in organic photovoltaic devices thanks to their ability to give rise a high efficient photo-induced electron transfer process and to their behaviour as n-type charge carriers.[1] On the other hand, in face of the great interest for carbon based materials in catalysis, the use of fullerenes in this field has been completely neglected as probably due to the difficult to endow fullerene cages with suitable active sites. In this regards, as a first step to address this issue, we have reported recently a wide range of asymmetric metal mediated or organocatalytic methodologies for a precise stereocontrol in the fullerenes functionalizations.[2]Herein, we present the enantioselective catalytic synthesis of fullerene hybrids where C60 is endowed with metal atoms such us iridium, rhodium and ruthenium with high enantiomeric excesses. A complete control on the stereochemistry of all the asymmetric centers was achieved, even on the stereogenic metal center.[3]The catalytic and photo-electrocatalytic properties of these new fullerene derivatives will be discussed together with their ability to transfer chirality in hydrogen transfer processes.[4][1] (a) B. C. Thompson, J. M. J. Fréchet, Angew. Chem. Int. Ed. 2008, 47, 58; (b) J. L. Delgado, P.-A. Bouit, S. Filippone, M. A. Herranz, N. Martin, Chem.Commun. 2010, 46, 4853. [2] (a) Maroto, E. E.; Izquierdo, M.; Reboredo, S.; Marco-Martínez, J.; Filippone, S.; Martín, N. Acc. Chem. Res. 2014, 47, 2660. (b) Filippone, S.; Maroto, E. E.; Martín-Domenech, A.; Suárez, M.; Martín, N. Nat. Chem. 2009, 1, 578. (c) Maroto, E. E.; Mateos, J.; Garcia-Borràs, M.; Osuna, S.; Filippone, S.; Herranz, M. Á.; Murata, Y.; Solà, M.; Martín, N. J. Am. Chem. Soc. 2015, 137, 1190. (d) Marco-Martínez, J.; Reboredo, S.; Izquierdo, M.; Marcos, V.; López, J. L.; Filippone, S.; Martín, N. J. Am. Chem. Soc. 2014, 136, 2897. (e) Marco-Martínez, J.; Marcos, V.; Reboredo, S.; Filippone, S.; Martín, N. Angew. Chem. Int. Ed. 2013, 52, 5115. [3] J. Marco-Martínez, S. Vidal, I. Fernández, S. Filippone and N. Martín, Angew. Chem. Int. Ed., 2017, 56, 2136. [4] R. Maria Giron, J. Marco-Martinez, S. Bellani, A. Insuasty, H. Comas Rojas, G. Tullii, M. R. Antognazza, S. Filippone and N. Martin, J. Mater. Chem. A, 2016, 4, 14284. Figure 1

Smart PET based organic scaffold exhibiting bright “Turn–On” green fluorescence to detect Fe3+ ion: Live cell imaging and logic implication

An efficient and simple photoinduced electron transfer (PET) based fluorescence turn–On probe 7 has been designed and synthesized by bridging naphthalimide and thiophene moieties through a piperazine unit. The photophysical behavior of probe and its affinity toward different metal ions have been investigated in partial aqueous medium, protein medium and live cells. The probe exhibited fluorescence “turn-On” response for Fe3+ ion with high sensitivity (limit of detection 0.373 μM) and selectively along with naked-eye sensitive visible green color in the medium. The significant change in photophysical behavior of the probe, upon a complexation between probe and Fe3+ is attributed to restriction in PET process. The probe 7 showed excellent biocompatibility and has been utilized in cellular imaging experiment to detect Fe3+ in MCF-7 live cells. Also, at molecular level probe 7 mimics the functions of a sequential logic circuit, corresponding to a memory device in which the two inputs (Fe3+ and AcO−) based sequential logic operations mimic the function of a memory element.

1,7‐Difluorophore‐Substituted AzaBODIPYs

Four different 1,7‐disubstituted azaBODIPYs containing fluorophores such as naphthalene, anthracene, and pyrene were synthesized over multiple steps involving aldol condensation, Michael addition, dipyrromethene formation, and BF2 complexation. All of these steps were optimized, and the target 1,7‐difluoro‐substituted azaBODIPYs along with other intermediate compounds were thoroughly characterized by HRMS and 1D and 2D NMR spectroscopy. Absorption and electrochemical studies indicated a strong interaction between the fluorophores and the azaBODIPY moieties in compounds 1–4. Fluorescence studies revealed that 1,7‐disubstituted azaBODIPYs with naphthalene and pyrene fluorophores were fluorescent and that singlet–singlet energy transfer from the fluorophores to the azaBODIPY moiety was possible, whereas 1,7‐dianthracenyl‐substituted azaBODIPY was completely nonfluorescent owing to efficient photoinduced electron transfer between the anthracene and azaBODIPY moieties.

Concentration-dependent photophysical switching in mixed self-assembled monolayers of pentacene and perylenediimide on gold nanoclusters.

Photophysical control and switching on organic-inorganic hybrid interfaces are of great interest in diverse fundamental and applicative research areas. 6,13-Bis(triisopropylsilylethynyl)pentacene (TP) is well-known to exhibit efficient singlet fission (SF) for generation of high-yield triplet excited states in aggregated forms, whereas perylenediimide (PDI) ensembles show the characteristic excimer formation. Additionally, a combination of pentacene (electron donor: D) and PDI (electron acceptor: A) is expected to undergo an efficient photoinduced electron transfer (PET), and absorption of two chromophores combined covers the entire visible region. Therefore, the concentration-dependent mixed self-assembled monolayers (SAMs) composed of two chromophores enable us to control and switch the photophysical processes on a surface. In this work, a series of mixed SAMs composed of TP and PDI units on gold nanoclusters (GNCs) were newly synthesized by changing the relative molecular concentration ratios. Structural control of mixed SAMs on a gold surface based on the concentration ratios was successfully achieved. Time-resolved femtosecond and nanosecond transient absorption measurements clearly demonstrate photophysical control and switching of the above competitive reactions such as SF, electron transfer (ET) and excimer formation. The maximum quantum yields of triplet states (ΦT = ∼170%) and electron transfer (ΦET = ∼95%) were quantitatively evaluated by changing the concentration ratios. The rate constants of SF and excimer processes are largely dependent on the concentration ratios, whereas the rate constants of ET processes approximately remain constant. These findings are also discussed based on the statistical framework of the assembly of chromophores on the gold surface.

Steroid-photosensitizer conjugates: Syntheses and applications

This review focuses on progress in the development of different approaches to the design of steroid ([Formula: see text] estrogens, androgens, cholesterol) conjugates with coordination assemblies of metalloporphyrins, phthalocyanines and related complexes. Porphyrins and phthalocyanines have received considerable attention due to their novel composition, intriguing spectroscopic, photophysical, and redox properties, and potential application in light-harvesting and optoelectronic devices. With the development of more efficient imaging and therapeutic applications, these bio-conjugates are evaluated as multimodality agents (PET, fluorescence imaging) to monitor the mechanism of action of biologically active components in living systems and as agents for molecular recognition, oxygen atom transfer and catalysis. The tetrapyrrole components, which can be coupled via covalent and various non-covalent linkages, may exhibit strong interactions through efficient photo-induced electron and/or energy transfer processes.

Cover Feature: Efficient Photoinduced Energy and Electron Transfer in ZnII–Porphyrin/Fullerene Dyads with Interchromophoric Distances up to 2.6 nm and No Wire‐like Connectivity (Chem. Eur. J. 57/2017)

Cover Feature: Efficient Photoinduced Energy and Electron Transfer in Zn^II–Porphyrin/Fullerene Dyads with Interchromophoric Distances up to 2.6 nm and No Wire‐like Connectivity (Chem. Eur. J. 57/2017)

Efficient Photoinduced Energy and Electron Transfer in ZnII -Porphyrin/Fullerene Dyads with Interchromophoric Distances up to 2.6 nm and No Wire-like Connectivity.

The dyads 1-3 made of an alkynylated ZnII -porphyrin and a bis-methanofullerene derivative connected through a copper-catalyzed azide-alkyne cycloaddition have been synthesized. The porphyrin and fullerene chromophores are separated through a bridge made of a bismethanofullerene tether linked to different spacers conjugated to the porphyrin moiety [i.e., m-phenylene (1), p-phenylene (2), di-p-phenylene-ethynylene (3)]. Compounds 1-3 exhibit relatively rigid structures with an interchromophoric separation of 1.7, 2.0, and 2.6 nm, respectively, and no face-to-face or direct through-bond conjugation. The photophysical properties of compounds 1-3 have been investigated in toluene and benzonitrile with steady-state and time-resolved techniques as well as model calculations on the Förster energy transfer. Excited-state interchromophoric electronic interactions are observed with a distinct solvent and distance dependence. The latter effect is evidenced in benzonitrile, where compounds 1 and 2 exhibit a photoinduced electron transfer in the Marcus-inverted region, with charge-separated (CS) states living for 0.44 and 0.59 μs, respectively, whereas compound 3 only undergoes energy transfer, as in apolar toluene. The quantum yield of the charge separation (φCS ) of compounds 1 and 2 in benzonitrile is ≥0.75. It is therefore demonstrated that photoinduced energy and electron transfers in porphyrin-fullerene systems with long interchromophoric distances may efficiently occur also when the bridge does not provide a wire-like conjugation and proceed through the triplet states of the chromophoric moieties.