Search

3-Pentadecylphenol (PDP) belongs to a natural group of amphiphilic phenols and plays a lot of different interesting functions. Effects on the phase behavior of bilayer mixtures of DPPC (dipalmitoyl phosphatidylcholine) or MPPC (l-myristoyl-2-palmitoyl-phosphatidylcholine) and PDP has been investigated using electron microscopy, differential scanning calorimetry (DSC) and fluorescence spectroscopy. Addition of 3-pentadecylphenol increases the phase transition temperature T m . Above 20 mole % PDP in DPPC the phase behavior becomes increasingly complex. Coexistence of various phases within the bilayer with different composition and structural properties is suggested and is responsible for the changes of the physico-chemical properties of the lipid bilayer. Excellent analogy between fluorescence and calorimetric data demonstrate that phase effects are primarily caused by the abundance of the large phenolic headgroup. DPPC and MPPC bilayers are similarly affected but MPPC is more sensitive to added PDP.

Low temperature co-fired ceramic (LTCC) technology was originally developed for the realization of multilayer circuits of high reliability. It was recognized that LTCC-technology is a valuable development in thick film technology which launches new application areas as it becomes evident that complex three-dimensional structures can be easily realized. Aspects of the realization of a sophisticated biological monitoring module comprising a three-dimensional network of channels and cavities is demonstrated.

One of the most intriguing predictions of energy landscape models is the existence of non-exponential protein folding kinetics caused by hierarchical structures in the landscapes. Here we provide the strongest evidence so far of such hierarchy and determine the time constants and weights of the kinetic components of the suggested hierarchic energy landscape. To our knowledge, the idea of hierarchical folding energy barriers has never been tested over such a broad timescale. Refolding of yeast phosphoglycerate kinase was initiated from the guanidine-unfolded state by stopped-flow or manual mixing and monitored by tryptophan fluorescence from 1 ms to 15 min. The strategy to build a model that describes folding of yeast phosphoglycerate kinase was to start from the simplest paradigm and modify it stepwise to the necessary minimal extent after repeated comparisons with the experiments. We made no a priori assumptions about the folding landscape. The result was a hierarchic finite level landscape model that quantitatively describes the refolding of yeast phosphoglycerate kinase from 1 ms to 15 min. The early steps of the folding process happen in the upper region of the landscape, where the surface has a hierarchic structure. This leads to stretched kinetics in the early phase of the folding. The lower region of the energy landscape is dominated by a trap that reflects the accumulation of molten globule intermediate state. From this intermediate, the protein can reach the global energy minimum corresponding to the native state through a cross-barrier folding step.

Supramolecular host–guest complexes formed between dye molecules, which are absorbing and fluorescing in the visible range, and cyclodextrin molecules are studied with the aim to generate a self-assembled nanostructured layer on a surface. Studies in solution show that an aminocoumarin dye, i.e. coumarin-6, binds strongly to β-cyclodextrin and yields fluorescent structures. The complex formation is substantiated by 17 nm blue shift of the fluorescence spectrum. The binding of thiolated cyclodextrin, 6-monodeoxy-6-monothio-β-cyclodextrin, to gold surface is then studied by induced change in the second-harmonic signal intensity generated from the surface. The saturation times of surface adsorption, as a measure of binding kinetics, are estimated for thiolated cyclodextrin as well as hexadecanethiol. The results support formation of self-assembled layers of the host molecules, which can serve as basis for the formation of supramolecular host–guest surface layers.

Many donor–acceptor substituted organic compounds show internal charge separation upon excitation. Within this class, the following compounds are studied as suitable candidates for molecules with high second-order nonlinear optical (NLO) response: 4- N , N -dimethylamino-benzonitrile (DMABN), 6-propanoyl-2-(dimethylamino)-naphthalene (PRODAN), and various 4-(4- N , N -dimethylamino-phenyl)-1,3-diphenyl-pyrazolo[3,4-b]quinolines (DMA-DPPQ). The configuration interaction method with single and double excitations (CISD) is used in combination with the sum over states (SOS) approach within the semiempirical AM1 parametrization to calculate the vector component of the hyperpolarizability along the dipole axis β μ . The influence of changing molecular geometry on the second-order nonlinear properties is calculated for various intramolecular relaxation coordinates. Solvent effects are explicitly studied for DMABN by a self-consistent reaction field (SCRF) model revealing an increasing NLO-effect with increasing solvent polarity. The hyperpolarizability is discussed in relationship to the charge transfer properties of the excited state. The calculated values are in good agreement with experimental data for both DMABN and PRODAN. DMA-DPPQ is found to be a good NLO chromophore only when increasing either the donor or acceptor strength. Its nonlinear response can be drastically increased by an additional strong electron acceptor group, e.g. a NO 2 -group, at the primary acceptor moiety.

Many enzymes occurring in nature like superoxide dismutase are systems rather too big to be accessible for vibrational and quantum chemical investigations. Thus, enzyme-mimetic model compounds consisting of a biological active metal centre surrounded by a macrocyclic ligand are used to shed light on binding properties of the active metal centre. Far- and mid-range IR spectroscopic investigations and a conformational analysis with the semi-empirical ZINDO/1 method of superoxide dismutase-mimetic complex Cu[TAAB] 2+ are performed (TAAB=[ b , f , j , n ][1,5,9,13]tetra-aza-cyclohexadecine (tetra-anhydroamino benzaldehyde)). A distorted tetrahedral copper(II) centre with slightly twisted phenyl subunits is determined as the most stable conformation. Calculated mid- and far-IR spectra are in good agreement with the experimental data and confirm the proposed structure. A harmonic normal-coordinate analysis is used to assign the vibrational modes of the observed spectra.

Acid–base properties of four 3,5-dimethyl-1,7-diphenyl-bis-pyrazolo-[3,4-b; 4′,3′-e]-pyridine derivatives with different substituents in position 4, i.e. N,N-dimethylaminophenyl, phenyl, nitrophenyl, and methoxyphenyl (DMA-DMPP, H-DMPP, NO2-DMPP, and CH3O-DMPP) were investigated in nonaqueous solutions. Absorption and stationary fluorescence as well as time-resolved spectroscopy were applied as methods. The acidity of the amino group in DMA-DMPP which is preferentially protonated in the ground state, does practically not change in the first excited state. The second protonation step of DMA-DMPP and the protonation of the other compounds occurs at the heterocyclic subunit, in the ground state most probably on the pyridine nitrogen. The basicity of this nitrogen atom increases remarkably in the first excited state. Experimental results were compared with those of semiempirical calculations. The amino group as the preferential site of protonation before the pyridine nitrogen in the ground state was confirmed. According to quantum mechanical computations, one pyrazolo nitrogen is preferentially protonated in the first excited state. The protonation is accompanied by a geometrical relaxation leading to a planarized conformer with a lower total energy.

Fluorescence properties of 1-heptanoylpyrene in homogeneous solutions and incorporated into liposome membranes have been measured. The fluorescence intensity of the alkanoylpyrene is small in neat hydrocarbon solutions (Qf = 0.006) but increases with increasing solvent polarity. On the contrary, Qf of the 1-alkylpyrene is larger by a factor of 10 in a hydrocarbon solvent and decreases slightly in polar solvents. In alcohols the fluorescence yield of 1-heptanoylpyrene becomes considerably larger in comparison to a nonhydroxylic environment and nearly identical to that of heptylpyrene in the same solvent. The fluorescence spectra shift bathochromically in that polar environment. The fluorescence properties of 1-heptanoylpyrene incorporated in a liposome membrane indicate that the probe molecules are well embedded within the hydrophobic core of the bilayer. Semiempirical calculations were performed for 1-ethanoylpyrene and 1-ethylpyrene as model compounds in order to analyze the influence of solvent polarity and of hydrogen bonding on excited state energies and fluorescence properties of 1-acyl derivatives of pyrene. Hydrogen-bonding increases the energy gap between the lowest singlets, which are nearly degenerated, and the 3π*-n triplet and reduced consequently the intersystem crossing efficiency. But also a change in the sequence of the excited 1π*-π states is concluded from experimental data and should contribute to the increase of the fluorescence yield upon hydrogen bonding.

Picosecond transient absorption spectra of 4-(N,N-dimethylamino)-benzonitrile (DMABN), 6-cyanobenzquinuclidine (CBQ), and benzquinuclidine (BQ) in polar and nonpolar solvents were measured for the first time at wavelengths

The photophysical properties of 10-decyl- and 10-propenyl-9-acridones, important emitters in electro- and chemiluminescence, are investigated in dependence of the solvent polarity and its hydrogen bonding ability at room temperature as well as in frozen matrix at 77 K. Fluorescence is inefficient in non-polar solvents at room temperature due to fast intersystem crossing but increases with solvent polarity due to the inversion of the S 1 (ππ ∗ ) and the T (nπ ∗ ) states. In the case of 9-[10-(1-propenyl)]-acridone the dependencies of the fluorescence spectrum and quantum yield on the ET(30) solvent parameters indicate that intermolecular hydrogen bonded solute-solvent complexes are formed in the excited state, inducing efficient non-radiative deactivation.

Geometries, conformational energies and activation barriers for rotations about the single bonds connecting the various subunits of a new class of bulky electron donor–acceptor systems derived from the bis-pyrazolopyridine chromophore are characterized for several representatives by semiempirical methods (AM1 and PM3). For the dimethylaminophenyl derivative DMA–DMPP (4-(4′- N , N -dimethylaminophenyl)-3,5-dimethyl-1,7-diphenyl-bis-pyrazolo-[3,4-b;4′,3′e]pyridine) dual fluorescence was observed in polar, protic solvents and was discussed in the context of the TICT (twisted intramolecular charge transfer) hypothesis, first formulated to explain dual fluorescence of 4-( N , N -dimethylamino)benzonitrile (DMABN). Besides the parent H–DMPP molecule, derivatives substituted with different donor or acceptor groups were compared with respect to their optimum ground-state geometries.

4-( N , N -Dimethylamino)-benzonitrile (DMABN) is the classical model compound showing solvent-induced dual fluorescence, a phenomenon attributed to formation of an intramolecular charge-transfer state and concomitant rotational relaxation of the dimetylamino group about the amine–aryl single bond. This mechanism is supported by the fluorescence properties of various stereoselectively substituted derivatives of DMABN and, therefore, the ground-state properties of such a set of model compounds are studied as a basis for a detailed examination of the influence of substitution on excited-state relaxation pathways. The factors examined in this study are torsion about the single bond connecting the donor amino group to the aryl moiety, the amine wagging vibration and the bending of the cyano group (rehybridization). Ground-state geometries and electrostatic properties as well as the energetics of these modes are obtained from semiempirical AM1 calculations and are compared with Hartree–Fock ab initio results using 3-21G* and 6-21G** split valence basis sets and those obtained by the Becke3 density functional theory method with the Lee–Young–Parr correlation function and the D95** basis set.

Contbrmational energies of PRODAN (6-Propionyl-2-(dimethylamino)-naphthalene) have been calculated by Hartree-Fock ab initio methods as a function of the twisting and wagging motion of the amino and the rotational motion of the propionyl group. In agreement with experimental data, PRODAN is found planar in its optimum geometry. The CIS method is used to compute molecular orbital energies, excitation energies and dipole moments in dependence of the rotational isomerization of the dimethylamino group. The calculations allow the peculiar fluorescence properties of PRODAN to be explained by the TICT model.

The influence of solvation on α-helix formation is studied for a 14-residue homoalanine and a 16-residue alanine-based polypeptide, in aqueous solution. Four variants of Dynamic Monte Carlo (DMC) simulations, using a multiple annealing procedure are performed. The results obtained under in vacuo conditions are compared to simulations considering either contributions to the solvation energy of non-favourable solvent interactions by hydrophobic groups, or the total solvation energy, both calculated by a continuum approximation. The highest average helix lengths are found when the total solvation energy (hydrophilic and hydrophobic contributions) is considered, although solvation counteracts the formation of compact peptide structures. The influence of solvation on helix formation is discussed.

The photophysical properties of 4-(4′-N,N′-dimethylaminophenyl)-1,7-diphenyl-3,5-dimethyl-bis-pyrazolo-[3,4-b;4′,3′-e]pyridine (DMA-DMPP), highly emissive in non-polar solvents and composed from a dimethylanilino electron donor and a bulky electron acceptor, have been investigated. The position of the fluorescence maximum as well as the fluorescence quantum yield and the radiative decay rate are strongly solvent polarity dependent; the magnitude of this effect becomes dramatically larger as the solvent polarity exceeds that of 1-chlorobutane. In protic solvents dual fluorescence is observed. In accordance with theoretical calculations the results are explained in terms of a polarity induced change in the energetic ordering of two emissive states of largely different charge transfer character.

Spectral and time resolved fluorescence studies were performed for 4-(N,N-dimethylamino)-benzonitrile (DMABNZ) and some derivatives which were designed to model twisted intramolecular charge transfer (TICT) states, in hexane binary solvent mixtures with ethanol. It is attempted to elucidate the mechanism of local interactions between solute and polar solvent molecules and their capacity to induce intramolecular charge transfer. It is shown that crossing to the TICT state occurs in these mixed solvents at low alcohol concentrations but the mechanism of the intermolecular interaction responsible for this effects depends strongly on the structural and photophysical features of the solute. Whereas 2-MODMABN shows effective ground state solvation because of its short fluorescence lifetime, 3-MDMABN is the best example for the appearance of successive excited state association with single alcohol molecules which leads to different solvate cluster sizes and finally to TICT emission. As excited DMABN reacts much slower with ethanol monomers than the sterically hindered derivative, complexation with cyclic alcohol multimers which become the most important structural feature at higher ethanol concentrations is the predominant mechanism in that case. It leads to efficient charge transfer in these solute clusters with several alcohol molecules, and bulk polarity effects on the resulting TICT fluorescence describe the spectral shift in the high alcohol concentration regime.

Cyclodextrins (CDx) and their complexes are good model systems to study the structural complexity of macro-molecular systems in general and, in particular, unspecific molecular recognition processes in host-guest systems. In this paper the conformational space of α-CDx, β-CDx and of a 2-naphthol-β-CDx complex are studied by molecular mechanics methods and are discussed in adapted symmetry coordinates. In contrast to highly symmetrical α-CDx, symmetry breaking due to a strong relative tilt of the glucose subunits and to an elliptical squeeze of the macrocycle is demonstrated for β-CDx. The formation of symmetrical fragments within globally unsymmetrical β-CDx is shown. The conformational landscapes of β-CDx and the β-CDx-2-naphthol complex are then converted into correlational maps between individual geometry variables to discuss dynamic aspects of the macromolecules. The severe impact of complexation on the conformational space of the host is demonstrated as correlational maps simplify drastically and the geometrical variables defining the complex geometry become highly correlated. The impact of complexation on β-CDx symmetry is demonstrated, as two distinct types of complex structure can be characterized.

A new push-pull stilbene was synthesized, and its photophysical properties investigated by spectroscopic and theoretical methods. Solvatochromic shifts in the fluorescence were in accordance with Onsager's continuum model, whereas shifts in the absorption spectra only correlated with the polarizability of the solvents. Excited state dipole moments were determined and compared to semi-empirically calculated values. With increasing solvent polarity the fluorescence quantum yield increases steeply as the solvent dielectric permittivity exceeds 8. It is concluded that a strongly luminescent and highly dipolar π, π ∗ state becomes the lowest excited state in polar solvents, and radiative deactivation therefore competes effectively with non-radiative pathways. Preventing internal rotation of molecular subunits is thus no prerequisite for obtaining highly fluorescent push-pull stilbenes.

In addition to the yellow emission ascribed to the J aggregates of 1,1′-diethyl-2,2′-cyanine, this compound also exhibits a high-energy emission around 400 nm in dilute aqueous solutions (c

Solvent polarity effects on the photophysical properties and spectral shifts of some 7-aminocoumarins were investigated in a combined experimental and theoretical study. Solvatochromic shifts were found in accordance with Onsager's coatinuum model and excited state dipole moments were estimated and compared to semiempirically calculated values. In solvents of a dielectric constant D < 10 the fluorescence quantum yield is generally unity with the only exception of the primary 7-aminocoumarins C120 and C151 in alkane solution. Ground and/or excited state complexation with polar solvent molecules enhances fluorescence. The onset of a nonradiative decay pathway for flexible 7-aminocoumarins with an acceptor substituent on the α-pyrone ring at a solvent D ≈ 10 is in good agreement with a rotatory decay mechanism to a twisted intramolecular charge transfer (TICT) state. This internal quenching is temperature activated, the activation barrier and pre-exponential factor increasing with polarity. TICT formation is discussed on the basis of dielectric solvation effects on the energy-hypersurface describing the twist of the amino group.

Steady-state and dynamic photophysical characteristics of a number of 4- and 5-amino-2-phenylindan-1,3-dione derivatives have been studied at 293 and 77 K in solvents of widely different polatiry and hydrogen-bonding ability. The observed changes of the fluorescence Franck-Condon transition energy, and of the fluorescence quantum yields and the singlet lifetimes, as well as the solvent deuterium-isotope effect, indicate the formation of intermolecular hydrogen bonds in the excited state with protic solvents for the 5-amino compounds at room temperature.

The photochemistry of some α-azohydroperoxides (phenyl(phenylazo)methyl hydroperoxide and its halogenated derivatives) was investigated in various solvents by microsecond and nanosecond flash photolysis methods and steady state product analysis. Photodecomposition of the molecule, demonstrated by the formation of benzaldehyde, is a major, but not the only, photochemical pathway. Investigation of the influence of solvent and substitution on the photoinduced transients provides strong evidence for alternative photoreactions. One of the species produced by flash photolysis was studied in detail and assigned to a transient zwitterion formed from the α-azohydroperoxide by proton transfer via an excited state pathway. Semiempirical calculations add further support to the proposed mechanism.

Intersystem crossing rates and triplet state properties of TICT (twisted intramolecular charge transfer) model compounds (dimethylaminobenzonitrile — DMABN — and its methyl substituted derivatives, 3-cyanoindoline, 3-cyanobenzquinuclidine — CBQ — and benzonitrile) were studied by various experimental and theoretical methods (emission spectroscopy, transient absorption techniques, EPR spectroscopy and semiempirical quantum chemical calculations) with the aim to obtain a quantitative deactivation pattern of this class of compounds in solvents of different polarity. The lowest triplet state, populated after excitation, is generally, a non-charge-transfer state, i.e. the 3 A 1 of planar DMABN, irrespective of solvent polarity and compound flexibility, with the only exception of the rigidly perpendicular CBQ, which relaxes to the 3 A 1 state of the benzonitrile moiety. In non-polar solvents intersystem crossing increases with increasing charge transfer character of the excited singlet; in ethanol, the intersystem crossing rate is about the same for all compounds studied, with the exception of benzonitrile, and in general significantly lower than in a non-polar environment. From the mechanistic point of view, intersystem crossing from 1 TICT can be described as a thermal charge recombination process in the normal Marcus region. With increasing solvent polarity, the decreasing energy gap between these two states then entails a decreasing intersystem crossing rate and an increasing 1 TICT lifetime, as experimentally observed. In hydroxylic solvents, internal conversion to the ground state is the second dominant deactivation pathway of 1 TICT states; it shows a significant deuterium isotope effect.

Cyclodextrins, when used as molecular receptors, can induce significant modifications of the photophysical and photochemical deactivation pathways of aromatic molecules. On the basis of examples, taken from the authors' work, the factors determining the behaviour of a photoexcited guest molecule in a cyclodextrin cavity are discussed with particular attention to the role of cydodextrin as constraint to molecular degrees of freedom and as tool for controlling the evolution of reaction intermediates to final photoproducts.

The appearance of wavelength dependent photochemistry (H-abstraction) and fluorescence in some typical polar aromatics (i.e. phenols, anilines, indoles) is correlated to the energetic position of Rydberg type excited states. This behaviour is also demonstrated for carbazole as an example of a larger aromatic molecular system. In bichromophoric molecules, interrelation between excited states of valence and Rydberg type is demonstrated. The significance of higher excited state photochemistry for primary radiation chemical processes is discussed.

In this article, we will present an update about current status of inactivated poliovirus vaccine (IPV) and we will also discuss general concerns about inactivated polio vaccine (IPV) which are under discussion in scientific community about various aspects of IPV and at the end of this article, we will give our conclusions about possible universal use of IPV.

The emission yields of 1-(N,N-dimethylamino)-2-phenylethane are followed as a function of the excitation energy in solutions in n-hexane, 1,2-dichloroethane, and in ethanol at room temperature. In n-hexane intramolecular exciplex formation is slightly more favoured from the excited amino than the aryl moiety. The conversion from the excited amine to the exciplex is partly quenched by dichloroethane. In ethanolic solutions excited amine does not convert to the lowest aryl π, π∗, but exciplex formation is observed following excitation of both chromophores.

The quantum yield O(e aq ) of photoejected electrons from aromatic carboxylate anions in aqueous solutions (pH 9.5 – 12.3) appreciably increases with increasing excitation energy; for example, O(e aq ) for benzoate is 0.0025 for the S 1 state (excitation at 254 nm) and 0.0048 for the S 2 state (excitation at 214 nm). The O(e aq ) value also increases with the number of CH 2 groups separating the phenyl and carboxylate groups; for example, for phenylbutyrate O(e aq ) is 0.05 for the S 1 state and 0.010 for the S 2 state. Furthermore, increases of O(e aq ) are linked with decreases in the quantum yield of fluorescence. In order to evaluate a possible consumption of the photoejected electrons by the ground state carboxylates, the rate constants k (e aq + substrate) for the reaction of solvated electrons with the corresponding substrates were determined.

In order to probe solute—solvent interactions in the intramolecular charge transfer (TICT) state of 2,6, N , N -tetramethylcyanoaniline (TMCA) fluorescence spectra, quantum yields and lifetimes of this compound were measured in binary mixtures of n -hexane with chlorinated hydrocarbons. An isoemissive point in the fluorescence spectra was observed as the composition of the solvent was varied. Evidence for both clustering of polar solvent molecules around the excited solute and for non-specific long-range interactions with the polar bulk of the solution was found. The properties of the solvent affect primarily the efficiency of non-radiative deactivation. Fluorescence quenching by saturated amines gives good evidence that TMCA emission arises from a highly polar state in both polar and non-polar solutions.

Exciplex formation between excited triethylamine and ethanol in hydrocarbon solution is demonstrated by steady-state and time-resolved fluorescence spectroscopy. At least two types of excited complexes can be resolved. The 1:1 complex shows only small spectral shift versus the monomer and a binding energy of 12 kJ/mol is obtained.

Exciplex formation of triethylamine with ethanol and with deuterated ethanol at low alcohol concentrations in hydrocarbon solutions is shown by steady state and time-resolved fluorescence spectroscopy. The observed spectra and the decay kinetics show the appearance of three exciplex structures which can be discriminated by temperature and deuterium isotope effects. The spectrum of the primarily formed 1:1 heterodimer overlaps significantly with the monomer spectrum (spectral shift is 800 cm −1 ) and equivalent associations are found with tetrahydrofuran. The photophysical parameters change only slightly on complexation. Two further exciplex emissions with large spectral shifts compared with the monomer (4500 cm −1 and 9000 cm −1 ) arise at ethanol concentrations above 0.02 M, together with a diminution of the quantum yield. Complexation with non-deuterated ethanol leads primarily to high energy emissive structures and equilibrium is attained, as they decay with a common lifetime. Deuteration of ethanol results in a stabilization of the lowest energy exciplex, which is populated irreversibly. Exciplex structures are discussed, with reference to ab inition model calculations.

Fluorescence spectra, quantum yields and lifetimes are comparatively studied for aniline, N,N -dimethylaniline (DMA) and some further alkylsubstituted and rigid aniline derivatives in various solvents. A good correlation of Stokes shift with macroscopic solvent parameters is found for DMA but not for aniline. General and specific solvent effects are distinguished by studying the functional dependence of various spectroscopic and photophysical parameters on the polar co-solvent concentration in binary solvent mixtures. Excited state complexation of primary and secondary amines with alcohols is demonstrated by the double-exponential decay of the monomer fluorescence and the grow-in of exciplex emission found in aniline—ethanol— n -hexane ternary systems. Exciplex formation is attributed to solvent—donor hydrogen bonding and primarily causes enhanced intersystem crossing compared with that in the free molecule, an effect also evoked by N -methylation. Non-specific long-range interactions give rise to a general reduction in the non-radiative deactivation rate, with the exception of o,o′ -dimethyl-substituted anilines, for which an increase in this rate was found as the bulk becomes polar.

The influence of various solvents (water, methanol, diethyl ether, n -hexane, cyclohexane, as well as of mixtures) and of the excitation energy (λ exc ) on the fluorescence quantum yield ( Q F ) of phenol has been investigated in O 2 - free solutions. At excitation energies above 5.1 eV (λ exc ⩽ 240 nm) a strong decrease of Q F was observed in apolar solvents and in water. Addition of polar solvents reduces the effect. Various possibilities are discussed in order to explain this behaviour, especially the influence of H-bonds between the phenol molecules and the solvent on the occurrence of radiationless transitions.

The effects of the excitation wavelength on the quantum yields Q(eaq−) and QF of electron ejection and fluorescence respectively were measured for 4-cyano-N,N-dimethylaniline (CDMA) and related compounds in aqueous solutions. In the case of CDMA no electron ejection was found for excitation of the long-wavelength (1La, 1Lb) band system. At shorter wavelengths (λexc

Exciplex formation between triethylamine and some alcohols in hexane solution has been followed by nanosecond time- and energy-resolved fluorescence spectroscopy. The experimental results have been discussed on the basis of a successive association model and rate constants have been obtained from an iterative adjustment of the model parameters to the measured concentration dependence of the decay times. Fluorescence quenching, increasing in efficiency with decreasing alcohol chain length, may be attributed to a charge transfer reaction of the amine—alcohol exciplexes with alcohol clusters formed in low concentration hydrocarbon solutions. The deuterium isotope effect observed in these systems can be explained by hydrogen migration in triethylamine—alcohol exciplexes.

Fluorescence spectra, quantum yields and lifetimes of p-cyano-N,N-dimethylaniline (CDMA) were studied in various binary mixtures of n -hexane with chlorinated hydrocarbons. Solvent effects on the equilibrium between normal emission and fluorescence originating from the highly dipolar twisted internal charge transfer (TICT) state arise predominantly from non-specific solute-solvent interactions. The non-radiative deactivation rate of the TICT state is affected primarily by the solvent composition and slows down with the stabilization of the TICT state in environments of high polarity.

Two primary photoreactions were observed for derivatives of 2-phenylindian-1,3-dione in various solvents: (1) cleavage of the hydrogen atom in the α position to both keto groups resulting in the formation of 2-arylindan-1,3-dion-2-yl radicals; (2) photoisomerization to benzylidenephthalides as a result of bond rupture via a Norrish type I mechanism. Both processes are possible with different quantum yields from the diketo form (in apolar solvents), from the enol form (in alcohols) and from the enolate form (in alkaline water).

For the purpose of modelling medium effects on the lowest Rydberg transitions of amines, ab initio calculations on excited clusters of ammonia and helium were performed. As the excited state surface is strongly repulsive, shifts of the transitions to higher energies are reproduced. As charge is accumulated around the perturber atom, the spatial charge distribution is considerably altered. The diffuse character of the Rydberg orbitals is not principally changed after clustering.

The yields of fluorescence at 295 and 77 K and of phosphorescence at 77 K of phenol and anisole were determined in solvents of different polarities (hydrocarbons, ether, ethanol). Also intersystem crossing yields are reported for phenol and anisole in cyclohexane solution at 295 K. The data show that a non-radiative process, different from intersystem crossing, competes with the decay of S 1 phenol in liquid hydrocarbons. The nature of this deactivation pathway is discussed.