The Importance of Excited‐State Energy Alignment for Efficient Exciplex Systems Based on a Study of Phenylpyridinato Boron Derivatives

Understanding excited‐state dynamics is critical for improving the photoluminescence (PL) efficiency of exciplexes. A series of exciplexes based on conventional hole‐transporting materials as donor and newly developed phenylpyridinato boron derivatives as acceptor were investigated. High PL efficiencies were achieved in only some combinations, and a large difference in performance among combinations provided insight into nonradiative processes in exciplex systems. Furthermore, the triplet local excited states (³LE) of each donor and acceptor were found play an important role in triplet exciplex harvesting. Significant contributions from triplets were clearly observed when the charge‐transfer excited states (¹CT and ³CT) and ³LE were ideally aligned. We also demonstrated fine control of relative energy alignment via the concentration to improve the PL efficiency.     

Effect of Solvent Polarity on the Electronic Structure and Emission Frequencies of Exciplexes of Aromatic Hydrocarbons with Methoxybenzenes and Methylnaphthalenes

The dependence of exciplex emission spectra on the solvent polarity was studied for exciplexes with the Gibbs free energy of excited-state electron transfer, G ^* _et , exceeding –0.1 eV (for pyrene, fluoranthene, 1,12-benzoperylene, and 9-cyanoanthracene with methoxybenzenes or dimethylnaphthalene). These exciplexes showed stronger changes in the spectral shift of exciplex emission and the extent of charge transfer with increasing solvent polarity than the exciplexes having more negative G ^* _et values. The parameters (difference in energy of charge transfer (CT) and locally excited (LE) states in a vacuum, (H ⁰ ₂₂– H ⁰ ₁₁), and the matrix element for electronic coupling of CT and LE states H ₁₂and mrelated to the dipole moment of the CT state and the size of the exciplex) determining the extent of charge transfer, the spectral shift, and other properties of exciplexes were evaluated. The parameters H ₁₂and mfor the exciplexes examined fall in the interval 0.1–0.5 and 1.0–1.7 eV, respectively, and the difference (H ⁰ ₂₂– H ⁰ ₁₁) is proportional to G ^* _et .     

Lifetimes of partial charge transfer exciplexes of 9-cyanophenanthrene and 9-cyanoanthracene

The fluorescence decays of several exciplexes with partial charge transfer have been investigated in solvents of various polarity. The measured lifetimes are found to be in reasonable agreement with the activation enthalpy and entropy of exciplex decay obtained earlier from the temperature dependence of the exciplex emission quantum yields. For exciplexes with 9-cyanophenanthrene substantial contribution of the higher local excited state into the exciplex electronic structure is found and borrowed intensity effect enhances the exciplex emission rate constants.     

Electron-transfer fluorescence quenching of aromatic hydrocarbons by europium and ytterbium ions in acetonitrile

To make the effects of molecular size on photoinduced electron-transfer (ET) reactions clear, the ET fluorescence quenching of aromatic hydrocarbons by trivalent lanthanide ions M3+ (europium ion Eu3+ and ytterbium ion Yb3+) and the following ET reactions such as the geminate and free radical recombination were studied in acetonitrile. The rate constant k(q) of fluorescence quenching, the yields of free radical (phi(R)) and fluorescer triplet (phi(T)) in fluorescence quenching, and the rate constant k(rec) of free radical recombination were measured. Upon analysis of the free energy dependence of k(q), phi(R), phi(T), and k(rec), it was found that the switchover of the fluorescence quenching mechanism occurs at deltaG(fet) = -1.4 to -1.6 eV: When deltaG(fet) < -1.6 eV, the fluorescence quenching by M3+ is induced by a long-distance ET yielding the geminate radical ion pairs. When deltaG(fet) > -1.4 eV, it is induced by an exciplex formation. The exciplex dissociates rapidly to yield either the fluorescer triplet or the geminate radical ion pairs. The large shift of switchover deltaG(fet) from -0.5 eV for aromatic quenchers to -1.4 to -1.6 eV for lanthanide ions is almost attributed to the difference in the molecular size of the quenchers. Furthermore, it was substantiated that the free energy dependence of ET rates for the geminate and free radical recombination is satisfactorily interpreted within the limits of the Marcus theory.     

Exciplex fluorescence emission from simple organic intramolecular constructs in non-polar and highly polar media as model systems for DNA-assembled exciplex detectors

Organic intramolecular exciplexes, N-(4-dimethylaminobenzyl)-N-(1-pyrenemethyl)amine (1) and N'-4-dimethylaminonaphthyl-N-(1-pyrenemethyl)amine (2), were used as model systems to reveal major factors affecting their exciplex fluorescence, and thus lay the basis for developing emissive target-assembled exciplexes for DNA-mounted systems in solution. These models with an aromatic pyrenyl hydrocarbon moiety as an electron acceptor appropriately connected to an aromatic dimethylamino electron donor component (N,N-dimethylaminophenyl or N,N-dimethylaminonaphthyl) showed strong intramolecular exciplex emission in both non-polar and highly polar solvents. The effect of dielectric constant on the maximum wavelength for exciplex emission was studied, and emission was observed for 1 and 2 over the full range of solvent from non-polar hydrocarbons up to N-methylformamide with a dielectric constant of 182. Quantum yields were determined for these intramolecular exciplexes in a range of solvents relative to that for Hoechst 33,258. Conformational analysis of 1 was performed both computationally and via qualitative 2D NMR using (1)H-NOESY experiments. The results obtained indicated the contribution of pre-folded conformation(s) to the ground state of 1 conducive to exciplex emission. This research provides the initial background for design of self-assembled, DNA-mounted exciplexes and underpins further development of exciplex-based hybridisation bioassays.     

Steric effect on fluorescence quenching

In this communication we have reported the steric effect on the fluorescence quenching rate constants of the electron transfer (ET) process. We have done a comparative study using donor (D)-acceptor (A) systems with different exergonicity (-deltaG(f)). Different carbazole derivatives (CZ): 1,4-dicyanobenzene (DCB) systems (-deltaG(f) = 0.7-0.8 eV) were found to be among those limiting systems that show a clear-cut steric dominance in the process of fluorescence quenching. It is known that with increasing exergonicity the ET distance increases and hence steric dependence becomes insignificant. On the other hand, with decreasing exergonicity the ET distance decreases and a pronounced steric dominance should be observed. However, in the D-A systems having lower exergonicity compared to CZ-DCB systems, this steric dominance is observed only in polar medium. In non-polar medium due to exciplex formation the D-A distance effectively becomes much longer and therefore no steric dominance is observed.     

Exciplex intermediates in photoinduced electron transfer of porphyrin-fullerene dyads

The photoinduced electron transfer in differently linked zinc porphyrin-fullerene dyads and their free-base porphyrin analogues was studied in polar and nonpolar solvents with femto- to nanosecond absorption and emission spectroscopies. A new intermediate state, different from the locally excited (LE) chromophores and the complete charge-separated (CCS) state, was observed. It was identified as an exciplex. The exciplex preceded the CCS state in polar benzonitrile and the excited singlet state of fullerene in nonpolar toluene. The behavior of the dyads was modeled by using a common kinetic scheme involving equilibria between the exciplex and LE chromophores. The scheme is suitable for all the studied porphyrin-fullerene compounds. The rates of reaction steps depended on the type of linkage between the moieties. The scheme and Marcus theory were applied to calculate electronic couplings for sequential reactions, and consistent results were obtained.     

Formation Enthalpy of Exciplexes with Partial Charge Transfer as a Function of the Electron-Transfer Driving Force

The dependence of the formation enthalpy ()H _Ex* and the Gibbs energy (G _Ex*) of exciplexes with partial charge transfer on the Gibbs energy of electron transfer G _et*, the parameters of the electronic structure of an exciplex (the difference in the energies of the charge transfer (CT) state and a locally excited state (LE) in a vacuum ( H ₂₂ ⁰– H ₁₁ ⁰), the matrix element of the electronic coupling of the CT and LE states H ₁₂, the dipole moment of the CT state, and the repulsion energy in an exciplex a"), and the polarity of the medium was analyzed. The consideration of the repulsion energy in the exciplex is necessary for correlation of the experimental values of H _Ex* and the spectral shift of the exciplex emission with respect to the LE state. All of these parameters depend on the particular nature of the exciplex, which is the reason for the lack of the general dependence of H _Ex* and G _Ex* on G _et* for exciplexes with partial charge transfer.     

Bonded exciplex formation: electronic and stereoelectronic effects

As recently proposed, the singlet-excited states of several cyanoaromatics react with pyridine via bonded-exciplex formation, a novel concept in photochemical charge transfer reactions. Presented here are electronic and steric effects on the quenching rate constants, which provide valuable support for the model. Additionally, excited-state quenching in poly(vinylpyridine) is strongly inhibited both relative to that in neat pyridine and also to conventional exciplex formation in polymers, consistent with a restrictive orientational requirement for the formation of bonded exciplexes. Examples of competing reactions to form both conventional and bonded exciplexes are presented, which illustrate the delicate balance between these two processes when their reaction energetics are similar. Experimental and computational evidence is provided for the formation of a bonded exciplex in the reaction of the singlet excited state of 2,6,9,10-tetracyanoanthracene (TCA) with an oxygen-substituted donor, dioxane, thus expanding the scope of bonded exciplexes.     

Laser photolysis study of the spectral-kinetic characteristics of short-lived triplet exciplexes and the mechanism of atomic hydrogen transfer in the course of 2,6-diphenyl-1,4-benzoquinone triplet quenching with aromatic amines

The spectral-kinetic characteristics of short-lived triplet exciplexes arising in the quenching of 2,6-diphenyl-1,4-benzoquinone triplet with aromatic amines: N,N,N′,N′-tetramethyl-p-phenylenediamine, triphenylamine and diphenylamine have been studied by means of the nanosecond laser photolysis technique. The absorption spectra of triplet exciplexes exhibit distinct maxima characteristic of the absorption spectra of corresponding amine radical cations. The state with complete charge transfer gives the basic contribution to the exciplex structures. A detailed analysis is presented of the kinetic and thermodynamic deactivation characteristics of triplet exciplexes in low-polar solvents.     

Bimolecular electron transfers that follow a Sandros-Boltzmann dependence on free energy

Rate constants (k) for exergonic and endergonic electron-transfer reactions of equilibrating radical cations (A(?+) + B ? A + B(?+)) in acetonitrile could be fit well by a simple Sandros-Boltzmann (SB) function of the reaction free energy (ΔG) having a plateau with a limiting rate constant k(lim) in the exergonic region, followed, near the thermoneutral point, by a steep drop in log k vs ΔG with a slope of 1/RT. Similar behavior was observed for another charge shift reaction, the electron-transfer quenching of excited pyrylium cations (P(+)*) by neutral donors (P(+)* + D → P(?) + D(?+)). In this case, SB dependence was observed when the logarithm of the quenching constant (log k(q)) was plotted vs ΔG + s, where the shift term, s, equals +0.08 eV and ΔG is the free energy change for the net reaction (E(redox) - E(excit)). The shift term is attributed to partial desolvation of the radical cation in the product encounter pair (P(?)/D(?+)), which raises its free energy relative to the free species. Remarkably, electron-transfer quenching of neutral reactants (A* + D → A(?-) + D(?+)) using excited cyanoaromatic acceptors and aromatic hydrocarbon donors was also found to follow an SB dependence of log k(q) on ΔG, with a positive s, +0.06 eV. This positive shift contrasts with the long-accepted prediction of a negative value, -0.06 eV, for the free energy of an A(?-)/D(?+) encounter pair relative to the free radical ions. That prediction incorporated only a Coulombic stabilization of the A(?-)/D(?+) encounter pair relative to the free radical ions. In contrast, the results presented here show that the positive value of s indicates a decrease in solvent stabilization of the A(?-)/D(?+) encounter pair, which outweighs Coulombic stabilization in acetonitrile. These quenching reactions are proposed to proceed via rapidly interconverting encounter pairs with an exciplex as intermediate, A*/D ? exciplex ? A(?-)/D(?+). Weak exciplex fluorescence was observed in each case. For several reactions in the endergonic region, rate constants for the reversible formation and decay of the exciplexes were determined using time-correlated single-photon counting. The quenching constants derived from the transient kinetics agreed well with those from the conventional Stern-Volmer plots. For excited-state electron-transfer processes, caution is required in correlating quenching constants vs reaction free energies when ΔG exceeds ～+0.1 eV. Beyond this point, additional exciplex deactivation pathways-fluorescence, intersystem crossing, and nonradiative decay-are likely to dominate, resulting in a change in mechanism.     

Activation Parameters for the Formation and Decay of Partial-Charge-Transfer Exciplexes of 9-Cyanoanthracene, 1,12-Benzperylene, and Pyrene

The factors affecting the rate of formation and decay of exciplexes with partial charge transfer, which form in the kinetic region of photoinduced electron transfer (G ^* _et > –0.2 eV), were studied. The rate of formation of exciplexes is controlled mainly by the diffusion of reactants and the low steric factor (0.15–1.0). The activation enthalpy and entropy for the exciplex formation (9–13 kJ mol^–1 and –(12–28) J mol^–1 K^–1) are close to the activation enthalpy and entropy of diffusion, respectively. Charge transfer in an exciplex and polarization of the medium generally occur after passing the transition state. In contrast, the activation enthalpy of exciplex decay (its conversion into the reaction products) is close to zero (±6 kJ mol^–1) and the activation entropy is strongly negative –(80–130) J mol^–1 K^–1.     

Effect of the electronic structure of heteroaromatic compounds on their phototransformations in polar media

The effect of the electronic structure of aromatic (anthracene) and heteroaromatic (phenazine, acridine, 9-methyl-1,2-benzacridine) compounds on the mechanism of the formation of exciplexes with N,N-dimethylaniline and their further transformations in media with various polarities was investigated by the quenching of the fluorescence of the exciplex. It was shown that for heteroaromatic compounds characterized by a substantial contribution from the n^* electronic configuration to the lowest excited singlet state, in contrast to aromatic compounds, the exciplex is formed in two stages, i.e., nonequilibrium Franck-Condon and equilibrium stages. The photodissociation processes for these systems in polar media also take place from a non-equilibrium state. The important role of the steric configuration of the components of the exciplex in its relaxation process was also noticed.Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 26, No. 4, pp. 448–454, July–August, 1990.     

Activation Enthalpy and Entropy for the Formation of 9-Cyanophenanthrene Exciplexes

Exciplexes of 9-cyanophenanthrene with a series of weak electron donors with the Gibbs energy of electron transfer G _et * varying in the range –(0.02–0.09) eV were studied. The exciplexes exhibited fairly intense emission both in nonpolar and aprotic polar solvents. The kinetics of the exciplex formation was found to be controlled mainly by diffusion and reactant orientation. This is clearly manifested in the low-temperature region in which the activation enthalpy of exciplex formation is very close to the activation enthalpy of diffusion, and the activation entropy of exciplex formation does not exceed 18 J mol^–1 K^–1 in absolute value.     

Temperature effect on the vapor-phase exciplexes of cyano-substituted anthracenes

The emission spectra and lifetimes of the vapor-phase exciplexes of four cyano-substituted anthracenes with N,N-dimethylaniline (DMA) as a donor are examined over a wide temperature range. The activation energies associated with the exciplex dissociation are calculated to be of the order of 10 kcal/mol. The entropy change in forming the exciplexes is discussed in relation to the efficiency of the exciplex formation. For various donors other than DMA, preliminary results on how they interact with excited 9,10-dicyanoanthracene are also given.     

Exciplexes of pyrene with indole and diethylaniline in liposomes and biomembranes

The formation of exciplexes of pyrene with indole and diethylaniline incorporated into the lipid bilayer was observed in phospholipid liposomes and membranes of sarcoplasmic reticulum from rabbit muscles. The intensity and lifetime of pyrene luminescence were found to decrease and the structureless emission of the exciplexes appeared. Exciplex emission in the membrane has a low quantum yield (compared with exciplexes formed in hexane solutions). This is probably due to the presence of polar groups in the membranes. It was shown that the formation of exciplexes is markedly dependent on the physical state of membranes. It is suggested also that pyrene can form an exciplex with tryptophan residues of membrane proteins.     

Magnetic field effect on indole exciplexes: a comparative study

A comparative magnetic field effect (MFE) study was done on indole exciplexes with various acceptors, anthracene, pyrene, all-s-trans-1,4-diphenylbuta-1,3-diene and 9-cyanophenanthrene. A surprisingly low magnetic field effect was detected for the 9-cyanophenanthrene exciplexes and was correlated with exciplex geometry. The wavelength dependence of magnetic field effect confirms the presence of single charge-transfer complex for all the exciplexes with 1,2-dimethylindole.     

Laser photolysis study of chloranil triplet exciplexes with stilbene

Absorption spectra and decay kinetics of the polar triplet exciplexes (contact radical-ion pairs) formed during quenching of the chloranil triplet state by trans- or cis-stilbenes in benzene with added acetonitrile and methanol, have been studied by laser flash photolysis. The exciplexes include cation-radicals of stilbene dimers, which are deactivated by reverse electron transfer within 10–50 nsec. The dynamics of the intercombination electron transfer and the exciplex dissociation into ion-radicals were determined. The isomerization of stilbene via triplet exciplex formation was not observed.N. N. Semenov Institute of Chemical Physics, Russian Academy of Sciences, Moscow 117977. Translated from Izvestiya Akademii Nauk, Seriya Khimicheskaya, No. 3, pp. 572–576, March, 1992.     

Radiative and radationless decay of exciplexes of 1,12-benzperylene

Many molecules in their excited states react with other species having suitable electron donor or acceptor properties to form complexes (exciplexes) stable only in the excited state. This letter reports a study of the modes of decay of a series of exciplexes in which the donor molecule (1,2-benzperylene) formed exciplexes with a series of dimethylaniline derivatives. By measuring the flourescence and intersystem crossing quantum yields, together with the fluorescence lifetimes of the exciplexes, it was possible to derive the rate constants for fluorescence, radationless decay to the ground state, and intersystem crossing. The first two decay processes were found to show a marked sensitivity to the exciplex energy, while the intersystem crossing rate constant was affected only by the presence of heavy atoms.