Mechanism Of Exciplex Formation Research Articles

A colorimetric and fluorescent probe containing diketopyrrolopyrrole and 1,3-indanedione for cyanide detection based on exciplex signaling mechanism

A new fluorescent probe 1 comprising diketopyrrolopyrrole and indanedione-based Michael receptor, which recognized cyanide anion with high selectivity was designed and synthesized. Addition of CN− aqueous solution to 1 in THF resulted in a rapid color change from purple to yellow together with a large blue shift from 553 to 480nm, while other anions did not induce any significant color change. Furthermore, the Michael addition of cyanide to 1 elicited 3.11 and 6.13-fold PL enhancements at 484 and 572nm, respectively, which constituted the fluorescence signature for cyanide detection. The mechanism of exciplex formation was proposed for this interesting observation. The detection limits were 0.36μM using the fluorescence spectra changes, which was far lower than the WHO guideline of 1.9μM. The results indicated 1 can act as dual-channel, high selective, immediacy and sensitive optical probe for CN−.

A Fluorescence Turn-on Sensor for Cyanide Anion Based on Exciplex Signaling Mechanism

Abstract A new fluorescent chemosensor comprising anthracene and indanedione-based Michael receptor, which recognizes cyanide anion with high selectivity was designed and synthesized. The Michael addition of cyanide to the chemosensor elicits a fluorescence enhancement at a longer wavelength, which constituted the signature for cyanide detection. The mechanism of exciplex formation was proposed for the interesting observation.

Transient Exciplex Formation Electron Transfer Mechanism

Transient exciplex formation mechanism of excited‐state electron transfer reactions is analyzed in terms of experimental data on thermodynamics and kinetics of exciplex formation and decay. Experimental profiles of free energy, enthalpy, and entropy for transient exciplex formation and decay are considered for several electron transfer reactions in various solvents. Strong electronic coupling in contact pairs of reactants causes substantial decrease of activation energy relative to that for conventional long‐range ET mechanism, especially for endergonic reactions, and provides the possibility for medium reorganization concatenated to gradual charge shift in contrast to conventional preliminary medium and reactants reorganization. Experimental criteria for transient exciplex formation (concatenated) mechanism of excited‐state electron transfer are considered. Available experimental data show that this mechanism dominates for endergonic ET reactions and provides a natural explanation for a lot of known paradoxes of ET reactions.

Unified Interpretation of Exciplex Formation and Marcus Electron Transfer on the Basis of Two-Dimensional Free Energy Surfaces

The mechanism of exciplex formation proposed in a previous paper has been refined to show how exciplex formation and Marcus electron transfer (ET) in fluorescence quenching are related to each other. This was done by making simple calculations of the free energies of the initial (DA*) and final (D+A-) states of ET. First it was shown that the decrease in D-A distance can induce intermolecular ET even in nonpolar solvents where solvent orientational polarization is absent, and that it leads to exciplex formation. This is consistent with experimental results that exciplex is most often observed in nonpolar solvents. The calculation was then extended to ET in polar solvents where the free energies are functions of both D-A distance and solvent orientational polarization. This enabled us to discuss both exciplex formation and Marcus ET in the same D-A pair and solvent on the basis of 2-dimensional free energy surfaces. The surfaces contain more information about the rates of these reactions, the mechanism of fluorescence quenching by ET, etc., than simple reaction schemes. By changing the parameters such as the free energy change of reaction, solvent dielectric constants, etc., one can construct the free energy surfaces for various systems. The effects of free energy change of reaction and of solvent polarity on the mechanism and relative importance of exciplex formation and Marcus ET in fluorescence quenching can be well explained. The free energy surface will also be useful for discussion of other phenomena related to ET reactions.

Photophysical and photoconductive properties of a carbazolyl derivative with a mesogen group of alkoxycyanobiphenyl in mesophases

4-Cyano-4′-(1-( N-carbazolyl)- n-octyloxy)biphenyl (8OCBCz), where the hydrogen atom of a well-known mesogen 4-cyano-4′- n-octyloxybiphenyl (8OCB) is substituted with one carbazolyl (Cz) chromophore, was synthesized, and the photophysical and photoconductive properties of its neat form and its binary mixtures with 4-cyano-4′- n-decyloxybiphenyl (10OCB) were investigated. The 8OCBCz exhibited intermolecular exciplex fluorescence in concentrated solutions. When 8OCBCz was quenched from its molten state, it formed an amorphous state, which exhibited exciplex fluorescence and transient photocurrent signals of both the positive and negative charge carriers. However, their mobilities could not be determined because of the strongly dispersive signals. The mixtures exhibited smectic A (Sm A) and nematic (N) mesophases. These mesophases and the isotropic phase exhibited exciplex as well as monomer fluorescence. The exciplex formation mechanism in the Sm A mesophase differed from those in the N and isotropic phases. The transient photocurrent signals for both the positive and negative charge carriers were also observed for the binary mixtures. Ionic conduction was suggested for the N and isotropic phases, whereas electronic conduction was suggested for the Sm A mesophase. The ionic carrier species were proposed.

Organic electroluminescence from singlet and triplet exciplexes: Exciplex electrophosphorescent diode

We have observed emission from singlet and triplet exciplexes in electron donor (D)–acceptor (A) organic electroluminescent systems. The exciplex singlet-to-triplet fraction appears to be a function of electronic properties of the (D–A) complex molecular components and applied electric field. The fluorescence from exciplex singlets could be eliminated using a heavy atom (Gd)-substituted acceptor molecule allowing the efficient intersystem crossing of singlet into triplet exciplex states. By this a pure exciplex electrophosphorescent diode has been realized. These findings are of importance for the understanding of the exciplex formation mechanism and, thus, for material choice in device applications.

New Fluorescent Chemosensor Based on Exciplex Signaling Mechanism

[reaction: see text]. A novel fluorescent chemosensor (compound 1) containing aminonaphthol, which selectively recognizes fluoride anion with high sensitivity, was synthesized. The fluorescence of compound 1 was quenched rapidly by fluoride ion, and a new peak at a longer wavelength emerged concurrently, which constituted the signature for fluoride detection. The mechanism of exciplex formation was proposed for the interesting observation.

Charge Transfer Kinetics and Solvatochromism of 1-(9-Anthryl)-3-(4-dimethylaniline) Propane in 1,4-Dioxane: Nonideal Quadrupolar Charge Distribution and the Origin of the Dioxane Anomaly

The solvent 1,4-dioxane behaves like a polar solvent even though its dielectric constant is small. This excess polarity is referred to as the “dioxane anomaly”. Several explanations of this anomaly have been offered in the literature including hydrogen bonding, conformational polarity, and quadrupolar interactions. We have investigated the origins of this anomaly by employing the unique photochemical properties of the ADMA (1-(9-anthryl)-3-(4-dimethylaniline) propane) molecule. ADMA forms an intramolecular exciplex in the excited state. Solvent polarity governs the mechanism of exciplex formation. In nonpolar solvents the decay of the locally excited-state population is monoexponential, while in polar solvents (e > 5.4) the decay of the locally excited-state population is biexponential. Although dioxane has a dielectric constant of 2.2, ADMA dissolved in dioxane exhibits biexponential kinetics. From this result and from the solvatochromic shift of the ADMA exciplex peak in dioxane, we have concluded that ...

Electroplex emission from a layer of a mixture of a europium complex and tris(8-quinolinolato) aluminum

With a europium complex, tris(α-thenoyltrifluoroacetonato) bis(triphenylphosphine oxide) europium (Eu(TTA) 3(TPPO) 2), as the light-emitting layer, N, N′-diphenyl- N, N′-di( m-tolyl)-benzidine (TPD) as the hole transport layer and tris(8-quinolinolato) aluminum (ALQ) as the electron transport layer, the triple-layer electroluminescent (EL) device emits red light characteristic of Eu 3+ emission. However, as the mixture of Eu(TTA) 3(TPPO) 2 and ALQ is co-evaporated as the light-emitting layer to form a bilayer EL device, a new wide-banded emission peaked at c. 640 nm was obtained. This emission is neither from ALQ nor from the europium complex. The photoluminescence (PL) of the thin film on quartz substrate evaporated from one mixed solid powder of Eu(TTA) 3(TPPO) 2 and ALQ is composed of distinct PL emissions of Eu(TTA) 3(TPPO) 2 and ALQ, denying an exciplex formation mechanism. It is impossible to form a host–guest system. We propose that the EL emission peaked at c. 640 nm is from an electroplex route: a transition between the lowest unoccupied molecular orbital (LUMO) of Eu(TTA) 3(TPPO) 2 and the highest occupied molecular orbital (HOMO) of ALQ.

Mechanism of exciplex formation between Cu-porphyrin and calf-thymus DNA as revealed by saturation resonance Raman spectroscopy

The excited-state complex (exciplex) formation that results from the photoinduced interaction of water-soluble cationic copper(II) 5,10,15,20-tetrakis[4-(N-methylpyridyl)]porphyrin [Cu(TMpy-P4)] with calf-thymus DNA has been studied in detail by resonance Raman (RR) spectroscopy using both ∼10 ns and ∼50 ps laser pulses. The obtained Raman saturation dependences were simulated using the rate equations approach on the basis of two different models. The first model was taken from the work of Strahan et al. and is based on the assumption of the preferential intercalation of Cu(TMpy-P4) at GC sites and includes the process of fast translocation of metalloporphyrin from GC sites into outside-bound AT sites under the action of a laser pulse. Another model was based on the assumption of binding of Cu(TMpy-P4) to both GC and AT sites of DNA and exciplex formation involving copper porphyrin initially located at AT sites. Our results show that: (i) the exciplex is formed during the action of a laser pulse, with both ∼10 ns and ∼50 ps duration; (ii) photoinduced accumulation of free Cu(TMpy-P4) molecules in a buffer solution and changes in the stationary distribution over metalloporphyrin binding modes do not take place under high-power irradiation; (iii) the mechanism of Cu(TMpy-P4) translocation is not sufficient for the explanation of the process of exciplex formation; (iv) further time-resolved studies are required to suggest a more elaborate mechanism for this process. Copyright © 1999 John Wiley & Sons, Ltd.

Coulomb‐induced intramolecular exciplex formation in semiflexible donor‐bridge‐acceptor compounds in nonpolar solvents as a function of temperature

AbstractThe mechanism of exciplex formation of two donor‐bridge–acceptor compounds, both containing a strong donor ‐ acceptor pair interconnected by a piperidine ring, was studied in nonpolar solvents as a function of temperature. Fluorescence spectra at low temperature in methylcyclohexane show that in both compounds the exciplex is formed via the harpooning mechanism, i.e. long‐range charge separation leading to an extended charge‐transfer state, followed by Coulomb‐induced folding to the exciplex state. The kinetic link between these two emissive states could be established by studying their transient fluorescence curves. Furthermore, the activation energy of the folding process was estimated from the decay rates of the extended charge‐transfer state in various solvents. An interpretation of the observed activation energies in terms of Kramers theory is presented, which gives an indication of the relative importance of internal steric effects and solvent viscosity on the dynamics of the folding process.

A mechanistic study of the physical quenching of Mg* by C 2H 2. Comparison of adiabatic and diabatic potential energy surfaces

The mechanism of exciplex formation, along the low-energy potential energy surfaces (PESs) of the Mg* + C 2H 2 system has been explored in the optimal C 2v geometry. Quasi-diabatic PESs were obtained from the MO-CI PESs, through calculation of the matrix elements of the ∂/∂ R operator by a finite difference method. Although of qualitative grade these PESs clearly show the fundamental role of the charge-transfer PES which crosses, at short distance of interaction, the repulsive neutral (covalent) ones, asymptotically correlated with the excited metal + neutral acetylene.

The photocycloaddition reaction of 8-methoxypsoralen with olefins

The photocycloaddition reactions of 8-methoxypsoralen (8-MOP) to the π-electron-rich olefin 2,3-dimethyl-2-butene (DMB) and the π-electron-deficient olefins fumaronitrile (FN) and dimethylfumarate (DF) were studied as a model for the photosensitizing 8-MOPpyrimidine base photocycloaddition reaction. The direct and sensitized photoreactions were carried out in a liquid methanol solution and a frozen benzene solution. The major photoadducts 8-MOP<⪢DMB, 8-MOP<⪢FN and 8-MOP<⪢DF were monitored by silica gel thin layer chromatography (TLC) and were isolated by preparative TLC and silica gel column chromatography. The structures of the photoadducts were determined by physical methods to be 1:1 C 4 cyclo-adducts formed through the pyrone double bond of excited 8-MOP. The same photoadducts were obtained on irradiation of the frozen solution. The fluorescence of 8-MOP was quenched by DMB, FN, DF and tetracyano-ethylenr. Linear Stern-Volmer plots with k q values of 7.27 × 10 8 1 mol −1 s −1 2.36 × 10 9 1 mol −1 s −1, 8.65 × 10 8 1 mol −1 and 2.91 × 10 9 1 mol −1 s −1 respectively were obtained. A singlet exciplex formation mechanism is proposed for the photocycloaddition reaction of 8-MOP to olefins. However, the triplet mechanism cannot be ruled out, particularly for 8-MOP<⪢DMB.

PHOTOCYCLOADDITION OF 5,7-DIMETHOXYCOUMARIN TO TETRAMETHYLETHYLENE

Abstract The photocycloaddition reaction of DMC to TME is studied as a model for skin-sensitizing DMC-thymine C4-cycloaddition reaction. The DMC-TME 1:1 C4-cycloadduct is obtained with a quantum yield of 0.16. The fluorescence of DMC is quenched by TME giving a linear Stern–Volmer plot and kq of 4 × 109 l mole−1 sec−1 and singlet exciplex formation mechanism is proposed for the cycloaddition of DMC to TME.