9-Borafluoren-9-yl and diphenylboron tetracoordinate complexes of F- and Cl-substituted 8-quinolinolato ligands: synthesis, molecular and electronic structures, fluorescence and application in OLED devices.

Six new four-coordinate tetrahedral boron complexes, containing 9-borafluoren-9-yl and diphenylboron cores attached to orthogonal fluorine- and chlorine-substituted 8-quinolinolato ligand chromophores, have been synthesised, characterised, and applied as emitters in organic light-emitting diodes (OLEDs). An extensive steady-state and time-resolved photophysical study, in solution and in the solid state, resulted in the first-time report of delayed fluorescence (DF) in solid films of 8-quinolinolato boron complexes. The DF intensity dependence on excitation dose suggests that this emission originates from triplet-triplet annihilation (TTA). Density functional theory (DFT) and time-dependent density functional theory (TDDFT) studies give insight into the ground and excited state geometries, electronic structures, absorption energies, and singlet-triplet gaps in these new organoboron luminophores. Finally, given their highly luminescent behaviour, organic light-emitting diode (OLED) devices were produced using the synthesised organoboron compounds as emissive fluorescent dopants. The best OLED displays green-blue (λmaxEL = 489 nm) electroluminescence with an external quantum efficiency (EQE) of 3.3% and a maximum luminance of 6300 cd m-2.

Luminescent halogen-substituted 2-(N-arylimino)pyrrolyl boron complexes: the internal heavy-atom effect.

A group of new boron complexes [BPh2{κ2N,N'-NC4H3-2-C(H)[double bond, length as m-dash]N-C6H4X}] (X = 4-Cl 4c, 4-Br 4d, 4-I 4e, 3-Br 4f, 2-Br 4g, 2-I 4h) containing different halogens as substituents in the N-aryl ring have been synthesized and characterized in terms of their molecular properties. Their photophysical characteristics have been thoroughly studied in order to understand whether these complexes exhibit an internal heavy-atom effect. Phosphorescence emission was found for some of the synthesized halogen-substituted boron molecules, particularly for 4g and 4h. DFT and TDDFT calculations showed that the lower energy absorption band resulted from the HOMO to LUMO (π-π*) transition, except for 2-I 4h, where the HOMO-1 to LUMO transition was also involved. The strong participation of iodine orbitals in HOMO-1 is reflected in the calculated absorption spectra of the iodine derivatives, especially 2-I 4h, when spin-orbit coupling (SOC) was included. Organic light-emitting diodes (OLEDs) based on these complexes, in the neat form or dispersed in a matrix, were also fabricated and tested. The devices based on films prepared by thermal vacuum deposition showed the best performance. When neat complexes were used, a maximum luminance (Lmax) of 1812 cd m-2 was obtained, with a maximum external quantum efficiency (EQEmax) of 0.15%. An EQEmax of ca. 1% along with a maximum luminance of 494 cd m-2 were obtained for a device fabricated by co-deposition of the boron complex and a host compound (1,3-bis(N-carbazolyl)benzene, mCP).

Ground and excited state properties of furanoflavylium derivatives.

The comparison of the ground-state reactivity of anthocyanins and aurone model compounds (i.e. with and without the furano bridge) has shown that the kinetic paradigm does not depend on the bridge but only on the hydroxyl substituent pattern, independently of the presence of the bridge: (i) bell shaped kinetics for those with two hydroxyl substituents in position 4' and 7, and (ii) four distinct kinetic steps for the mono substituted compounds with a hydroxyl in position 4'. The excited state proton transfer (ESPT) properties of these compounds were also investigated using steady-state and time-resolved spectroscopic techniques. It was found that the ESPT efficiency is significantly higher for the bridged compounds. Interestingly, pH-dependent steady-state fluorescence emission experiments show that in 4',7-dihydroxyfuranoflavylium the hydroxyl group in position 7 is the more acidic one in the excited state, while 1H NMR titration curves indicate a higher acidity constant in the ground state for the proton at the hydroxyl group in position 4'. Differently, the fluorescence emission spectrum of the quinoidal base deprotonated at position 7 is only observed upon excitation of the flavylium cation while the one from the base deprotonated at 4' is observed upon direct excitation.

Boron complexes of aromatic 5-substituted iminopyrrolyl ligands: synthesis, structure, and luminescence properties.

A group of new mononuclear boron chelate compounds [BPh2{κ2N,N'-5-R-NC4H2-2-C(H)[double bond, length as m-dash]N-Ar}] (R = Ar = C6H57; R = C6H5, Ar = 2,6-iPr2C6H38; R = Anthracen-9-yl (Anthr), Ar = C6H59; R = Anthr, Ar = 2,6-iPr2C6H310) were synthesized via the reaction of B(C6H5)3 with the corresponding 5-substituted 2-(N-arylformimino)pyrrole ligand precursors 3-6. These complexes were prepared in order to evaluate the luminescence potential derived from the substitution of the position 5 of the pyrrolyl ring with an aromatic group. Compounds 7-10 were photophysically characterized in solution and in the solid state. The 5-phenyl-2-iminopyrrolyl-BPh2 complexes 7 and 8 are blue emitters and have enhanced photoluminescence quantum yields in the solid state (ΦPL) up to 0.95, whereas the 5-anthracenyl derivatives 9 and 10 have green-bluish fluorescence and a ΦPL of 0.49 and 0.24, respectively. DFT and TDDFT studies were performed, considering the effect of solvent and dispersion, in order to show how the geometries of compounds 7-10 changed from the ground to the excited state, to assign electronic transitions, and to rationalize the observed luminescence. These materials were applied in organic light-emitting diodes (OLEDs), with various device structures, the best showing an external quantum efficiency of 2.75% together with a high luminance of 23 530 cd m-2.

Ground- and Excited-State Acidity of Analogs of Red Wine Pyranoanthocyanins.

Pyranoflavylium cations are synthetic analogs containing the same basic chromophore as the pyranoanthocyanins that form in red wine during maturation and are responsible for its final color. Determination of the ground- and excited-state acidities for a series of eight substituted hydroxy pyranoflavylium cations shows that they are weak acids in the ground state (pKa ranging from 3.4 to 4.4 in aqueous buffer solution), but substantially more acidic in the first excited singlet state (pKa * ranging from ca. 0.2 to 0.7 in 30% methanol-water). Unlike the ground-state acidities, which show no obvious trend with electronic effects of the substituents, the excited-state pKa * values correlate well with Hammett sigma parameters for the substituents on the pyranoflavylium chromophore. This difference in the transmission of electronic effects between ground and excited state is reflected in the localization of the HOMO of the cation and conjugate base in distinct regions of the chromophore as compared to delocalization of the LUMO over the entire molecule. The current results provide further support for the conclusion that excited-state proton transfer is the dominant deactivation pathway for the pyranoflavylium cation excited singlet state in aqueous or aqueous-organic media and presumably for pyranoanthocyanins as well.

Fluorescence Enhancement of a Cationic Fluorene-Phenylene Conjugated Polyelectrolyte Induced by Nonionic n-Alkyl Polyoxyethylene Surfactants.

The modulation of conjugated polyelectrolyte fluorescence response by nonionic surfactants is dependent on the structures of the surfactant and polymer, polymer average molecular weight, and polyelectrolyte-surfactant interactions. In this paper, we study the effect of nonionic n-alkyl polyoxyethylene surfactants (CiEj) with different alkyl chain lengths (CiE5 with i = 6, 8, 10, and 12) and number of oxyethylene groups (C12Ej with j = 5, 7, and 9) on the photophysics and ionic conductivity of poly{[9,9-bis(6'-N,N,N-trimethylammonium)-hexyl]-2,7-fluorene-alt-1,4-phenylene}bromide (HTMA-PFP) in dimethyl sulfoxide-water 4% (v/v). Molecular dynamics simulations show that HTMA-PFP chains tend to approach as the simulation evolves. However, the minimum distance between the polymer centers of mass increases upon addition of the surfactant and grows with both the surfactant alkyl chain length and the number of oxyethylene groups, although there are no specific polymer-surfactant interactions. A significant increase in the polymer emission intensity has been observed at surfactant concentrations around their critical micelle concentrations (cmcs), which is attributed to polymer aggregate disruption. However, an increase in the solution conductivity for concentrations above the C12E5 cmc has only been observed for the HTMA-PFP/C12E5 system. The enhancement of fluorescence emission intensity and conductivity upon surfactant addition increases with polymer average molecular weights and seems to be controlled by the polymer-surfactant proximity, which is maximum for C10E5 and C12E5.

Boron complexes of aromatic ring fused iminopyrrolyl ligands: synthesis, structure, and luminescence properties.

The condensation reactions of 2-formylindole (1) or 2-formylphenanthro[9,10-c]pyrrole (2) with various aromatic amines afforded the corresponding phenyl or phenanthrene ring fused mono-/bis-iminopyrrole ligand precursors 3-8, which, upon reaction with BPh3 in an appropriate molar ratio, led to the new mono- and diboron chelate compounds Ph2B[NC8H5C(H)[double bond, length as m-dash]N-2,6-Ar] (Ar = 2,6-iPr2C6H39; C6H510), Ph2B[(NC8H5C(H)[double bond, length as m-dash]N)2-1,4-C6H4]BPh211, Ph2B(NC16H9C(H)[double bond, length as m-dash]N-Ar) (Ar = 2,6-iPr2C6H312; C6H513), and Ph2B[(NC16H9C(H)[double bond, length as m-dash]N)2-1,4-C6H4]BPh214, respectively. Boron complexes 12-14, containing a phenanthrene fragment fused to the pyrrolyl C3-C4 bond, are highly fluorescent in solution, with quantum efficiencies of 37%, 61% and 58% (in THF), respectively, their emission colours ranging from blue to orange depending on the extension of π-conjugation. Complexes 9-11, containing a benzene fragment fused to the pyrrolyl C4-C5 bond, are much weaker emitters, exhibiting quantum efficiencies of 10%, 7% and 6%, respectively. DFT and TDDFT calculations showed that 2,6-iPr2C6H3N-substituents or, to a smaller extent, the indolyl group prevent a planar geometry of the ligand in the excited state and reveal the existence of a low energy weak band in all the indolyl complexes, which is responsible for the different optical properties. Non-doped single-layer light-emitting diodes (OLEDs) were fabricated with complexes 9-14, deposited by spin coating, that of complex 13 revealing a maximum luminance of 198 cd m-2.

Earliest events in α-synuclein fibrillation probed with the fluorescence of intrinsic tyrosines.

The fluorescence of the four tyrosines of α-synuclein (Syn) was used for probing the earliest events preceding the fibrillation of Syn, during the onset of the so-called lag-time of fibrillation. Steady-state fluorescence experiments revealed an increase in the fluorescence intensity (FI) for Syn solutions at pH values 3 and 2, in comparison with pH7, and fluorescence decays indicated that the FI increase did not result from suppression of excited-state proton transfer from the tyrosines to aspartates and glutamates, exposure of tyrosines to more hydrophobic environments, or reduction of homo-energy transfer. Instead, the FI increase was due to changes in the population of the tyrosine rotamers at low pH values. Stopped-flow experiments (pH-jumps) showed that the FI enhancement involves two processes: a fast (sub-7 ms) intramolecular (concentration-independent) process, which we assign to the protein collapse at low pH, and a slower intermolecular (concentration-dependent) process of protein dimerization/oligomerization, starting at 4-10s after acidification. To the best of our knowledge, this is the first work on the experimental detection of these earliest processes in the fibrillation of Syn.

Luminescent Di- and Trinuclear Boron Complexes Based on Aromatic Iminopyrrolyl Spacer Ligands: Synthesis, Characterization, and Application in OLEDs.

New bis- and tris(iminopyrrole)-functionalized linear (1,2-(HNC4 H3 -C(H)N)2 -C6 H4 (2), 1,3-(HNC4 H3 -C(H)N)2 -C6 H4 (3), 1,4-(HNC4 H3 -C(H)N)2 -C6 H4 (4), 4,4'-(HNC4 H3 -C(H)N)2 -(C6 H4 -C6 H4 ) (5), 1,5-(HNC4 H3 C-(H)N)2 -C10 H6 (6), 2,6-(HNC4 H3 C-(H)N)2 -C10 H6 (7), 2,6-(HNC4 H3 C-(H)N)2 -C14 H8 (8)) and star-shaped (1,3,5-(HNC4 H3 -C(H)N-1,4-C6 H4 )3 -C6 H3 (9)) π-conjugated molecules were synthesized by the condensation reactions of 2-formylpyrrole (1) with several aromatic di- and triamines. The corresponding linear diboron chelate complexes (Ph2 B[1,3-bis(iminopyrrolyl)-phenyl]BPh2 (10), Ph2 B[1,4-bis(iminopyrrolyl)-phenyl]BPh2 (11), Ph2 B[4,4'-bis(iminopyrrolyl)-biphenyl]BPh2 (12), Ph2 B[1,5-bis(iminopyrrolyl)-naphthyl]BPh2 (13), Ph2 B[2,6-bis(iminopyrrolyl)-naphthyl]BPh2 (14), Ph2 B[2,6-bis(iminopyrrolyl)-anthracenyl]BPh2 (15)) and the star-shaped triboron complex ([4',4'',4'''-tris(iminopyrrolyl)-1,3,5-triphenylbenzene](BPh2 )3 (16)) were obtained in moderate to good yields, by the treatment of 3-9 with B(C6 H5 )3 . The ligand precursors are non-emissive, whereas most of their boron complexes are highly fluorescent; their emission color depends on the π-conjugation length. The photophysical properties of the luminescent polyboron compounds were measured, showing good solution fluorescence quantum yields ranging from 0.15 to 0.69. DFT and time-dependent DFT calculations confirmed that molecules 10 and 16 are blue emitters, because only one of the iminopyrrolyl groups becomes planar in the singlet excited state, whereas the second (and third) keeps the same geometry. Compound 13, in which planarity is not achieved in any of the groups, is poorly emissive. In the other examples (11, 12, 14, and 15), the LUMO is stabilized, narrowing the gap between the highest occupied molecular orbital and the lowest unoccupied molecular orbital (HOMO-LUMO), and the two iminopyrrolyl groups become planar, extending the size of the π-system, to afford green to yellow emissions. Organic light-emitting diodes (OLEDs) were fabricated by using the new polyboron complexes and their luminance was found to be in the order of 2400 cd m(-2) , for single layer devices, increasing to 4400 cd m(-2) when a hole-transporting layer is used.

Effect of water content on the acid-base equilibrium of cyanidin-3-glucoside.

Laser Flash Photolysis was employed to measure the deprotonation and reprotonation rate constants of cyanidin 3-monoglucoside (kuromanin) in water/methanol mixtures. It was found that the deprotonation rate constant kd decreases with decreasing water content, reflecting the lack of free water molecules around kuromanin, which may accommodate and stabilize the outgoing protons. On the other hand, the reprotonation rate constant, kp, increases with the decrease in water concentration from a value of kp = 2 × 10(10) l mol(-1) s(-1) in water up to kp = 6 × 10(10) l mol(-1) s(-1) at 5.6M water concentration in the mixture. The higher value of kp at lower water concentrations reflects the fact that the proton is not freely escaping the solvation shell of the molecule. The deprotonation rate constant decreases with decreasing water content, reflecting the lack of free water molecules around kuromanin that can accommodate the outgoing protons. Overall, the acidity constant of the flavylium cation decreases with the decrease in water concentration from pKa values of 3.8 in water to approximately 4.8 in water-depleted media, thus shifting the equilibrium towards the red-coloured form, AH(+), at low water contents. The presence, or lack, of water, will affect the colour shade (red to blue) of kuromanin. This is relevant for its role as an intrinsic food component and as a food pigment additive (E163).

Unveiling the Eigen-Weller ion pair from the excited state proton transfer kinetics of 3-chloro-4-methyl-7-hydroxycoumarin.

The prototropic reactions of the first excited singlet state of 3-chloro-4-methylumbelliferone (3Cl4MU), in dioxane:water mixtures (Dx:H2O), were revisited using ps-time-resolved fluorescence techniques. The data response to the dielectric constant of the mixtures revealed the presence of an additional fourth kinetic species, kinetically coupled to the neutral (N*), the tautomeric (T*), and anionic (A(-)*) forms of 3Cl4MU, which is assigned to the elusive geminate (A(-)*···H(+)) ion pair. From the data analysis, all rate constants of the prototropic and diffusion processes involved were separately evaluated. The results showed that, whenever the geminate ionic pair is not kinetically detected, the evaluated values for deprotonation and protonation rate constants can substantially deviate from the real ones, depending on the efficiencies of pair recombination and dissociation. Finally, the results provide convincing kinetic evidence for the Eigen-Weller mechanism (intermediacy of the geminate ionic pair) in a quasi-aqueous medium, which to our knowledge had not yet been given.

Tunable fluorophores based on 2-(N-arylimino)pyrrolyl chelates of diphenylboron: synthesis, structure, photophysical characterization, and application in OLEDs.

Reactions of 2-(N-arylimino)pyrroles (HNC4H3C(H)=N-Ar) with triphenylboron (BPh3) in boiling toluene afford the respective highly emissive N,N'-boron chelate complexes, [BPh2 {κ(2)N,N'-NC4H3C(H)=N-Ar}] (Ar=C6H5 (12), 2,6-Me2-C6H3 (13), 2,6-iPr2-C6H3 (14), 4-OMe-C6H4 (15), 3,4-Me2-C6 H3 (16), 4-F-C6H4 (17), 4-NO2-C6H4 (18), 4-CN-C6H4 (19), 3,4,5-F3-C6H2 (20), and C6F5 (21)) in moderate to high yields. The photophysical properties of these new boron complexes largely depend on the substituents present on the aryl rings of their N-arylimino moieties. The complexes bearing electron-withdrawing aniline substituents 17-20 show more intense (e.g., ϕf =0.71 for Ar=4-CN-C6H4 (19) in THF), higher-energy (blue) fluorescent emission compared to those bearing electron-donating substituents, for which the emission is redshifted at the expense of lower quantum yields (ϕf=0.13 and 0.14 for Ar=4-OMe-C6H4 (15) and 3,4-Me2-C6H3 (16), respectively, in THF). The presence of substituents bulkier than a hydrogen atom at the 2,6-positions of the aryl groups strongly restricts rotation of this moiety towards coplanarity with the iminopyrrolyl ligand framework, inducing a shift in the emission to the violet region (λmax =410-465 nm) and a significant decrease in quantum yield (ϕf=0.005, 0.023, and 0.20 for Ar=2,6-Me2-C6H3 (13), 2,6-iPr2-C6H3 (14), and C6F5 (21), respectively, in THF), even when electron-withdrawing groups are also present. Density functional theory (DFT) and time-dependent DFT (TD-DFT) calculations have indicated that the excited singlet state has a planar aryliminopyrrolyl ligand, except when prevented by steric hindrance (ortho substituents). Calculated absorption maxima reproduce the experimental values, but the error is higher for the emission wavelengths. Organic light-emitting diodes (OLEDs) have been fabricated with the new boron complexes, with luminances of the order of 3000 cd m(-2) being achieved for a green-emitting device.

Model for conformational relaxation of flexible conjugated polymers: application to p-phenylenevinylene trimers in nonpolar solvents.

Photoexcitation of flexible conjugated polymers is invariably followed by a fast conformational/torsional relaxation towards a configuration favouring coplanarity of the conjugated segments. In general, the experimental relaxation rate constant (k(CR)) depends on the solvent viscosity (η) and temperature (T), and is not proportional to T/η. A theory capable of explaining the observed dependence of k(CR) on T and η over a wide range of these variables is not available. This gap is filled here by presenting a stochastic model that includes the participation of the oligomer side chain in storing and dissipating the stresses induced by photoexcitation. The model is able to account for the softening of solute-solvent interactions and its predictions are found to be in excellent agreement with the observed relaxation rate constants of a series of substituted p-phenylenevinylene trimers [ChemPhysChem 2009, 10, 448-454] on T, η and the size of the side-chains.

Photophysical properties of iminopyrrolyl boron complexes: a DFT interpretation.

The three compounds [BPh(2)(κ(2)N,N'-NC(4)H(3)C(H)=N-C(6)H(5))] (A), [BPh(2)(κ(2)N,N'-NC(4)H(3)C(H)=N-1,4-C(6)H(4)-N=H)C-H(3)C(4)N-N,N'κ(2))BPh(2)] (B) and [BPh(2)(κ(2)N,N'-NC(4)H(3)C(H)=N-4,4'-C(6)H(4)-C(6)H(4)-N=(H)C-H(3)C(4)N-N,N'κ(2))BPh(2)] (C) are blue to green light emitters with average to excellent quantum yields. DFT and TD-DFT calculations were performed in order to understand their behaviour. The geometry of their ground and singlet excited state was optimised, and their absorption patterns and emission were calculated. The large dihedral angles between the planes of the N-aryl and 2-formiminopyrrolyl moieties of the chelating ligand drop considerably, leading to an almost planar geometry in the singlet state. In the three species, the lowest energy absorption occurs at 383, 428, and 419 nm, being calculated at 377, 502, and 529 nm in the gas-phase (slightly shifted in the right direction in THF). It results from a HOMO to LUMO excitation, and was assigned to a transition between π and π* orbitals (ILCT) of the iminopyrrolyl ligand, despite small differences in the exact composition of the HOMO and LUMO in A, B, and C. Introduction of the LB94 functional did not improve the calculated absorption wavelengths, but PBE0 with an all electron basis set within the SOPERT approximation led to better agreement with the experimental absorption maxima and a good reproduction of the excited state lifetimes. The estimated emission wavelengths reproduce the experimental trends.

Syntheses and photophysical properties of new iminopyrrolyl boron complexes and their application in efficient single-layer non-doped OLEDs prepared by spin coating.

Efficient non-doped OLEDs have been achieved using new binuclear tetracoordinate organoboron complexes containing 2-(N-aryl)formiminopyrrolyl ligands.

Photoprotection and the photophysics of acylated anthocyanins.

The proposed role of anthocyanins in protecting plants against excess solar radiation is consistent with the occurrence of ultrafast (5-25 ps) excited-state proton transfer as the major de-excitation pathway of these molecules. However, because natural anthocyanins absorb mainly in the visible region of the spectra, with only a narrow absorption band in the UV-B region, this highly efficient deactivation mechanism would essentially only protect the plant from visible light. On the other hand, ground-state charge-transfer complexes of anthocyanins with naturally occurring electron-donor co-pigments, such as hydroxylated flavones, flavonoids, and hydroxycinnamic or benzoic acids, do exhibit high UV-B absorptivities that complement that of the anthocyanins. In this work, we report a comparative study of the photophysics of the naturally occurring anthocyanin cyanin, intermolecular cyanin-coumaric acid complexes, and an acylated anthocyanin, that is, cyanin with a pendant coumaric ester co-pigment. Both inter- and intramolecular anthocyanin-co-pigment complexes are shown to have ultrafast energy dissipation pathways comparable to those of model flavylium cation-co-pigment complexes. However, from the standpoint of photoprotection, the results indicate that the covalent attachment of co-pigment molecules to the anthocyanin represents a much more efficient strategy by providing the plant with significant UV-B absorption capacity and at the same time coupling this absorption to efficient energy dissipation pathways (ultrafast internal conversion of the complexed form and fast energy transfer from the excited co-pigment to the anthocyanin followed by adiabatic proton transfer) that avoid net photochemical damage.

Characterization of the singlet and triplet excited states of 3-chloro-4-methylumbelliferone.

An extensive photophysical characterization of 3-chloro-4-methylumbelliferone (3Cl4MU) in the ground-state, S(0), first excited singlet state, S(1), and lowest triplet state, T(1), was undertaken in water, neutral ethanol, acidified ethanol, and basified ethanol. Quantitative measurements of quantum yields (fluorescence, phosphorescence, intersystem crossing, internal conversion, and singlet oxygen formation) together with lifetimes were obtained at room and low temperature in water, dioxane/water mixtures, and alcohols. The different transient species were assigned and a general kinetic scheme is presented, summarizing the excited-state multiequilibria of 3Cl4MU. In water, the equilibrium is restricted to neutral (N*) and anionic (A*) species, both in the ground (pK(a) = 7.2) and first excited singlet states (pK(a)* = 0.5). In dioxane/water mixtures (pH ca. 6), substantial changes of the kinetics of the S(1) state were observed with the appearance of an additional tautomeric T* species. In low water content mixtures (mixture 9:1 v:v), only the neutral (N*) and tautomeric (T*) forms of 3Cl4MU are observed, whereas at higher water content mixtures (water mole fraction superior to 0.45), all three species N*, T*, and A* coexist in the excited state. In the triplet state, in the nonprotic and nonpolar solvent dioxane, the observed transient signals were assigned as the triplet-triplet transition of the neutral form, N*(T(1)) → N*(T(n)). In water, two transient species were observed and are assigned as the triplets of the neutral N*(T(1)) and the anionic form, A*(T(1)) (also obtained in basified ethanol). The phosphorescence spectra and decays of 3Cl4MU, in neutral, acidified, and basified solutions, demonstrate that only these two species N*(T(1)) and A*(T(1)) exist in the lowest lying triplet state, T(1). The radiative channel was found dominant for the deactivation of the anionic species, whereas with the neutral the S(1) ⇝ S(0) internal conversion competes with fluorescence. For both N* and A* the intersystem crossing yield represents a minor deactivation channel for S(1).

Viscosity dependence of intramolecular excimer formation with 1,5-bis(1-pyrenylcarboxy)pentane in alkane solvents as a function of temperature.

Intramolecular excimer formation with 1,5-bis(1-pyrenylcarboxy)pentane, (1PC(5)1PC) is studied as a function of temperature in a series of alkane solvents and in toluene, covering a wide range of solvent viscosities η, from 0.2 to 125 cP. The rate constant k(a) of the monomer → excimer reaction is determined from the effectively single exponential monomer fluorescence decays. For the viscosity dependence of k(a) in n-alkanes, the Stokes-Einstein relation k(a) ∼ η(-1.0) does not hold. Instead, k(a) is proportional to η(-α), with α increasing upon cooling, from 0.56 at 85 °C to 0.86 at -30 °C. The activation energy E(a) of excimer formation with 1PC(5)1PC, always larger than the activation energy E(T/η) of solvent viscous flow, decreases when the solvent viscosity becomes smaller, from 20.7 kJ/mol in n-hexadecane to 11.8 kJ/mol in n-butane, approaching a value of 11-12 kJ/mol for the low viscosity solvents. As the excimer formation process depends on the restricted diffusion of the 1PC end groups as well as on the C-O and C-C rotations in the -O(CH(2))(5)O- chain, the limiting barrier of 11-12 kJ/mol is attributed to the activation energy E(c) of the multiple bond rotations. This fractional viscosity dependence (α < 1.0) is caused by the multidimensional character of the barrier crossing in the excimer formation process. This multidimensional character should also be taken into account in investigations of polymers and biological media employing excimer formation.

Thermal unfolding kinetics of ubiquitin in the microsecond-to-second time range probed by Tyr-59 fluorescence.

Thermal folding/unfolding kinetics of wild-type ubiquitin (wt-UBQ) was studied in a wide time range, from microseconds to seconds, by combining rapid-mixing T-jump and laser T-jump with fluorescence detection (MTJ-F and LTJ-F, respectively) to monitor the fluorescence changes of Tyr-59 located on the 310-helix. The kinetics occurs exclusively in the millisecond to second time range, and the decays are strictly single exponential. From global analysis of folding and unfolding decays, the kf and ku values were determined, without use of the equilibrium constant Ku. The activation enthalpy of folding is negative (DeltaHf(#)(Tm) = -10.8 kcal/mol), but the free energy of the transition state is substantially larger than that of the unfolded state (DeltaGf(#)(Tm) = 7.6 kcal/mol RTm). Thus, wt-UBQ behaves as a two-state folder, when folding is monitored by the fluorescence of Tyr-59. The observation of kinetics on the microsecond time scale, when folding is monitored by the disruption of hydrogen bonds between beta-strands, using nonlinear infrared spectroscopy of the amide I vibrations (LTJ-DVE) [Chung, H. S.; Tokmakoff, A. Proteins: Struct., Funct., Bioinf. 2008, 72, 474-487], seems to result from the fact that MTJ-F monitors the effective unfolding (backbone exposure to water) of the thermally excited protein alone, while LTJ-DVE also monitors preliminary events (hydrogen-bond breaking) and thermal re-equilibration of the thermally excited protein.

How to change the aggregation in the DNA/surfactant/cationic conjugated polyelectrolyte system through the order of component addition: anionic versus neutral surfactants.

The competitive interaction has been studied between double-stranded DNA (dsDNA), the cationic conjugated polyelectrolyte (CPE) poly[9,9-bis(6-N,N,N-trimethylamonium)hexyl)-fluorene-phenylene)] bromide (HTMA-PFP) and anionic or neutral surfactants (sodium dodecyl sulfonate, SDSu, and n-dodecyl pentaoxyethylene glycol ether, C(12)E(5)) in 4% (v/v) dimethyl sulfoxide (DMSO)-water using UV/visible absorption and fluorescence spectroscopy. Dramatic changes are observed in the spectroscopic behavior of the system depending on the order of addition of the reagents, the surfactant charge, and concentration range. If the neutral C(12)E(5) is added to the HTMA-PFP/dsDNA complex, no significant spectroscopic changes are observed. However, if SDSu is added to the same complex, a dramatic increase of the absorbance and emission intensity is observed for surfactant concentrations above the critical micelle concentration (cmc). In contrast, if dsDNA is added to HTMA-PFP/surfactant systems (with surfactant concentrations above their cmc) no significant changes are observed with SDSu, while a dramatic quenching of polymer emission is observed with C(12)E(5), which can be explained quantitatively in terms of HTMA-PFP/surfactant/DNA complexation and the subsequent polymer aggregation upon charge neutralization. The results are compared with those for the binary systems (HTMA-PFP/DNA and HTMA-PFP/surfactants) and indicate the importance of electrostatic interactions between HTMA-PFP and oppositely charged species in the aggregation processes.