Intramolecular Photoinduced Electron Transfer Process Research Articles

Thio and Seleno- Psoralens as Efficient Triplet Harvesting Photosensitizers For Photodynamic Therapy.

The Psoralen (Pso) molecule finds extensive applications in photo-chemotherapy, courtesy of its triplet state forming ability. Sulfur and selenium replacement of exocyclic carbonyl oxygen of organic chromophores foster efficient triplet harvesting with near unity triplet quantum yield. These triplet-forming photosensitizers are useful in Photodynamic Therapy (PDT) applications for selective apoptosis of cancer cells. In this work, we have critically assessed the effect of the sulfur and selenium substitution at the exocyclic carbonyl (TPso and SePso, respectively) and endocyclic oxygen positions of Psoralen. It resulted in a significant redshifted absorption spectrum to access the PDT therapeutic window with increased oscillator strength. The reduction in singlet-triplet energy gap and enhancement in the spin-orbit coupling values increase the number of intersystem crossing (ISC) pathways to the triplet manifold, which shortens the ISC lifetime from 10-5 s for Pso to 10-8 s for TPso and 10-9 s for SePso. The intramolecular photo-induced electron transfer process, a competitive pathway to ISC, is also considerably curbed by these exocyclic functionalizations. In addition, a maximum of 115 GM of two-photon absorption (2PA) with IR absorption (660-1050 nm) confirms that the Psoralen skeleton can be effectively tweaked via single chalcogen atom replacement to design a suitable PDT photosensitizer.

Ligand-regulated photoinduced electron transfer within metal–organic frameworks for efficient photocatalysis

By judiciously controlling the twisted conjugation between ligands and coordinated carboxylates to inhibit intramolecular photoinduced electron transfer process, an acridone-based metal–organic framework was obtained for efficient photocatalysis.

Target-Induced In Situ Formation of Organic Photosensitizer: A New Strategy for Photoelectrochemical Sensing.

Small-molecule photosensitizers have great application prospects in photoelectrochemical (PEC) sensing due to their defined composition, diversified structure, and adjustable photophysical properties. Herein, we propose a new strategy for PEC analysis based on the target-induced in situ formation of the organic photosensitizer. Taking thiophenol (PhSH) as a model analyte, we designed and synthesized a 2,4-dinitrophenyl (DNP)-caged coumarin precursor (Dye-PhSH), which was then covalently coupled onto the TiO2 nanoarray substrate to obtain the working photoanode. Due to the intramolecular photoinduced electron transfer process, Dye-PhSH has only a very weak photoelectric response. Upon reacting with the target, Dye-PhSH undergoes a tandem reaction of the detachment of the DNP moiety and the intramolecular cyclization process, which leads to a coumarin dye with a pronounced photoelectric effect, thus achieving a highly selective turn-on PEC response to PhSH. For the first time, this study was to construct a PEC sensor by exploiting specific organic reactions for the in situ generation of small molecule-based photoactive material. It can be anticipated that the proposed strategy will expand the paradigm of PEC sensing and holds great potential for detecting various other analytes.

In Situ Lipid Membrane Formation Triggered by Intramolecular Photoinduced Electron Transfer.

A major goal of synthetic biology is the development of rational methodologies to construct self-assembling non-natural membranes, which could enable the efficient fabrication of artificial cellular systems from purely synthetic components. However, spatiotemporal control of artificial membrane formation remains both challenging and limited in scope. Here, we describe a new methodology to promote biomimetic phospholipid membrane formation by the photochemical activation of a catalyst-sensitizer dyad via an intramolecular photoinduced electron-transfer process. Our results offer future opportunities to exert spatiotemporal control over artificial cellular constructs.

Supramolecular bimetallic assemblies for photocatalytic hydrogen generation from water.

A series of supramolecular assemblies of the type [Ru(L-L)2(L'-L)MX2)](n+) are reported where L-L is 2,2'-bipyridine (bipy), 4,4'-di-tetra-butyl-bipyridine (tbbipy) or 4,4'-diethoxycarbonyl-2,2'-bipyridine (dceb), L-L' is tetrapyrido[3,2-a:2',3'-c:3'',2''-h:2''',3'''-j]phenazine (tpphz), 2,2':5',2''-terpyridine (2,5-bpp), 2,2':6',2''-terpyridine, (2,6-bpp), 2,5-di(pyridine-2-yl)pyrazine (2,5-dpp) or 2,3-di(pyridine-2-yl)pyrazine (2,3-dpp), and MX2 is PdCl2, PtCl2 or PtI2. The photocatalytic behaviour with respect to hydrogen generation of these compounds and their ultrafast photophysical properties are discussed as a function of the nature of the peripheral ligands, the bridging ligands and the catalytic centre. The results obtained show how differences in the chemical composition of the photocatalysts can affect intramolecular photoinduced electron transfer processes and the overall photocatalytic efficiency.

Taming C60 fullerene: tuning intramolecular photoinduced electron transfer process with subphthalocyanines.

Two subphthalocyanine-C60 conjugates have been prepared by means of the 1,3-dipolar cycloaddition reaction of (perfluoro) or hexa(pentylsulfonyl) electron deficient subphthalocyanines to C60. Comprehensive assays regarding the electronic features - in the ground and excited state - of the resulting conjugates revealed energy and electron transfer processes upon photoexcitation. Most important is the unambiguous evidence - in terms of time-resolved spectroscopy - of an ultrafast oxidative electron transfer evolving from C60 to the photoexcited subphthalocyanines. This is, to the best of our knowledge, the first case of an intramolecular oxidation of C60 within electron donor-acceptor conjugates by means of only photoexcitation.

Thermodynamic, kinetic and electronic structure aspects of a charge-transfer active bichromophoric organofullerene

Our recent work on charge transfer in the electronically push–pull dimethylaminoazobenzene–fullerene C60 donor–bridge–acceptor dyad through orbital picture revealed charge displacement from the n(N = N) (non-bonding) and π (N = N) type orbitals centred on the donor part to the purely fullerene centred LUMOs and (LUMO+n) orbitals, delocalized over the entire molecule. Consequently, this investigation centres around the kinetic and thermodynamic parameters involved in the solvent polarity dependent intramolecular photo-induced electron transfer processes in the dyad, indispensable for artificial photosynthetic systems. A quasi-reversible electron transfer pathway was elucidated with electrode-specific heterogeneous electron transfer rate constants. Solvent polarity controlled kinetic and thermodynamic parameters associated with intra-molecular photo-induced electron transfer processes in an electronically push–pull type dimethylaminoazobenzene–fullerene C60 donor–bridge–acceptor dyad are reported. A quasi-reversible electron transfer pathway is elucidated with electrode-specific heterogeneous electron transfer rate constants for the trans-dyad across the electrode/electrolyte interface.

Intramolecular interactions and photoinduced electron transfer in isoalloxazine-naphthalene bichromophores

The synthesis, the structural characterization and electrochemical and photophysical properties of new linear ( OFn) and macrocyclic ( MFn) bichromophoric systems containing one flavin and one naphthalene moiety, separated by (CH 2) n alkyl chains ( n = 4 or 6), are reported. The electrochemical properties of OF4, OF6, MF4, MF6 were investigated by cyclic voltammetry (CV) in ultra-dry THF and compared with the model compounds containing either the flavin unit ( FL6) or the monomethoxy- ( mona) and dimethoxy naphthalene ( dmona) derivatives, respectively. While only minor ground-state interactions were evidenced between the chromophores in the voltammetric curves and absorption spectra of bichromophores, luminescence data evidenced intramolecular photoinduced electron transfer processes as main deactivation routes of flavin-centered excited states. The rates of the intramolecular photoinduced electron transfer, involving the oxynaphthalene moieties as electron donors, were finally determined.

Porphyrin‐Appended Europium(III) Bis(phthalocyaninato) Complexes: Synthesis, Characterization, and Photophysical Properties

Mixed cyclization of 3-mono-, 4-mono-, or 4,5-di(porphyrinated) phthalonitrile compounds 2, 3, or 6 and unsubstituted phthalonitrile with the half-sandwich complex [EuIII(acac)(Pc)] (Pc=phthalocyaninate, acac=acetylacetonate) as the template in the presence of 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) in n-pentanol afforded novel porphyrin-appended europium(III) bis(phthalocyaninato) complexes 7-9 in 30-40% yield. These mixed tetrapyrrole triads and tetrad were spectroscopically and electrochemically characterized and their photophysical properties were also investigated with steady-state and transient spectroscopic methods. It has been found that the fluorescence of the porphyrin moiety is quenched effectively by the double-decker unit through an intramolecular photoinduced electron-transfer process, which takes place in several hundred femtoseconds, while the recombination of the charge-separated state occurs in several picoseconds. By using different phthalocyanines containing different numbers of porphyrin substituents at the peripheral or nonperipheral position(s) of the ligand, while the other unsubstituted phthalocyanine remains unchanged in these double-deckers, the effects of the number and the position of the porphyrin substituents on these photophysical processes were also examined.

Proton controlled intramolecular photoinduced electron transfer (PET) in podand linked squaraine–aniline dyads

Proton controlled intramolecular PET from an aniline moiety to a squaraine chromophore, attached to podand chains is reported, which to our knowledge is the first example where a squaraine dye is involved as an electron acceptor in an intramolecular PET process, and has implication in the design of PET based sensors.

Inter- and Intramolecular Photoinduced Electron-Transfer Processes between C60 and Diphenylaminofluorene in Solutions

Intramolecular photoinduced charge-separation and charge-recombination processes of a covalently bonded buckminsterfullerene−diphenylaminofluorene (C60−DPAF) dyad, in which the diphenylaminofluorene moiety is connected to C60 via a ketone group, have been investigated. Quenching of the fluorescence intensities and the observed short fluorescence lifetimes of the C60 moiety of the dyad in benzonitrile (PhCN) and DMF indicated that charge separation takes place via the singlet excited state of the C60 moiety at a fast rate with high efficiency. On the basis of the nanosecond transient absorption spectra, formation of the radical ion pair C60•--DPAF•+ was confirmed in DMF; the radical ion pair decays with a lifetime of 150 ns. From the temperature dependence of the charge-recombination rate constants, the reorganization energy was evaluated to be 0.81 eV in DMF, which is reasonably small, characteristic of fullerene derivatives. On the other hand, a mixture of DPAF and C60 showed intermolecular electron transfer via the triplet state of C60 in polar solvents such as PhCN.