Electronic energy migration in solid versus liquid host matrices for concentrated perylenediimide dye solutions

In this paper, we continue our evaluation of Forster-type theories of exciton diffusion in disordered environments. The perylenediimide dye Lumogen Red is used as a donor molecule in two different liquids, CHCl(3) and dimethylformamide, and the energy transfer to the acceptor molecule Rhodamine 700 is measured using time-resolved fluorescence decays. The exciton motion is measured over Lumogen Red concentrations ranging from 1 × 10(-4) to 5 × 10(-2) M, and the results are compared to previous results for exciton diffusion in a solid polymer. Depending on the theoretical approach used to analyze the data, we find that the energy migration in the liquids is a factor of 2-3 faster than in the solid polymer, even after taking molecular translation into account. Measurements for a Lumogen Red concentration of 10 mM in the different host environments yield diffusion constants ranging from 2.2 to 3.1 nm(2)/ns in the liquids, as compared to 1.1-1.2 nm(2)/ns in solid poly(methyl methacrylate) (PMMA). The results in the liquids are in good agreement with theoretical predictions and numerical simulations of previous workers, while the results in solid PMMA are 2-3 times slower. This discrepancy is discussed in the context of the rapid energetic averaging present in the liquid environments but absent in the solid matrix, where unfavorable configurations and low energy trapping sites are frozen in by the static disorder.     

Free energy gap laws for the pulse-induced and stationary fluorescence quenching by reversible charge transfer in polar solutions

The Stern-Volmer constants for either pulse-induced or stationary fluorescence being quenched by a contact charge transfer are calculated and their free energy dependencies (the free energy gap laws) are specified. The reversibility of charge transfer is taken into account as well as spin conversion in radical ion pairs, followed by their recombination in either singlet or triplet neutral products. The natural decay of triplets as well as their impurity quenching by ionization are accounted for when estimating the fluorescence quantum yield and its free energy dependence.     

Proton and electron transfer in the excited state quenching of phenosafranine by aliphatic amines

The quenching of the excited singlet and triplet states of phenosafranine by aliphatic amines was investigated in acetonitrile and methanol. The rate constants for the quenching of the excited singlet state depend on the one-electron redox potential of the amine suggesting a charge transfer process. However, for the triplet state, quenching dependence on the redox potential either is opposite to the expectation or there is not dependence at all. Moreover, in MeOH the first-order rate constant for the decay of the triplet state, k(obs) presents a downward curvature as a function of the amine concentration. This behavior was interpreted in terms of the reversible formation of an intermediate excited complex, and from a kinetic analysis the equilibrium constant K(exc) could be extracted. The log K(exc) shows a linear relationship with the pKb of the amine. On the other hand, for the triplet state quenching in acetonitrile k(obs) varies linearly with the amine concentration. Nevertheless, the quenching rate constants correlate satisfactorily with pKb and not with the redox potential. The results were interpreted in terms of a proton transfer quenching, reversible in the case of MeOH and irreversible in MeCN. This was further confirmed by the transient absorption spectra obtained by laser flash photolysis. The transient absorption immediately after the triplet state quenching could be assigned to the unprotonated form of the dye. At later times the spectrum matches the semireduced form of the dye. The overall process corresponds to a one-electron reduction of the dye mediated by the deprotonated triplet state.     

Light-induced excited spin state trapping and charge transfer in trigonal bipyramidal cyanide-bridged complexes

Three members of the family of trigonal bipyramidal (TBP) complexes of general formula [M(tmphen)(2)](3)[M'(CN)(6)](2) (tmphen = 3,4,7,8-tetramethyl-1,10-phenanthroline) or [M(3)M'(2)], which are known to exhibit thermally induced spin crossover and charge transfer, have been investigated for optical and photomagnetic properties. The light-induced excited spin-state trapping (LIESST) effect found in classical spin crossover compounds, such as [Fe(phen)(2)(NCS)(2)], was explored for the [Fe(3)Fe(2)] and [Fe(3)Co(2)] compounds. Similarly, inspired by the light-induced charge-transfer properties of K(0.2)Co(1.4)[Fe(CN)(6)]·6.9H(2)O and related Prussian blue materials, the possibility of photo-induced magnetic changes was investigated for the [Co(3)Fe(2)] TBP complex. Optical reflectivity and magnetic susceptibility measurements were used to evaluate the photoactivity of these compounds. A comparison of these data before and after light irradiation demonstrates that (i) the spin crossover of the Fe(II) centers in the [Fe(3)Fe(2)] and [Fe(3)Co(2)] analogues and the (ii) charge transfer events in the [Co(3)Fe(2)] complex occur with temperature and irradiation. In addition, photomagnetic behavior is exhibited by all three compounds. The photo-conversion efficiency has been estimated at 20% of photo-induced high spin Fe(II) centers in [Fe(3)Co(2)], 30% of paramagnetic Co(II)-Fe(III) pairs in [Co(3)Fe(2)], and less than 2% of photo-induced high spin Fe(II) centers in [Fe(3)Fe(2)].     

Steady state and time-resolved fluorescence study of isoquinoline: reinvestigation of excited state proton transfer

In the present work we report some hitherto unnoticed features in the steady state and time-resolved measurements of isoquinoline in water and trifluoroethanol (TFE). Absorption spectra reveal that in water, neutrals as well cationic species are present. Emission spectrum shows structured features at shorter wavelengths accompanied with a broad band around 375 nm, which correspond to neutrals and cations respectively. However, time-resolved data indicate that protonation does not take place in the excited state in water. On the contrary, in stronger hydrogen bonding solvent TFE, distribution of decay components is observed and at longer wavelengths a small rise time is present. This is ascribed to neutral and cation-like species present in the ground as well as in the excited state. The difference in the results is explained in terms of different excited state potential energy surfaces for water and TFE; particularly, the presence of a rather small barrier for protonation in case of TFE.     

Dual excited state deactivation pathways in TPZ2: A centrosymmetric dye with both high fluorescence and triplet state quantum yield

Dual excited state deactivation pathways in TPZ2 leading to 50% fluorescence quantum yield and 50% triplet state generation yield, suggest TPZ2 is a molecule has potential application in fluorescence imaging and photodynamic therapy in the same time.     

Interaction between intramolecular charge transfer state of p-N,N-dimethylaminobenzonitrile and amines. Stereo-controlled fluorescence quenching

The first excited singlet state, an intramolecular charge transfer state, of p-N,N-dimethylaminobenzonitrile (DMABN) is quenched by tertiary amines. The quenching rate cannot be correlated with the oxidation potential of the amines, but is controlled by the size of the amine alkyl group. This unusual steric effect indicates a short-range interaction, which we attribute to three-electron bonding. Excited-state three-electron bonding interaction should be a general phenomenon and can lead to the formation of sigma-bonded exciplexes. The implication of this result on the origin of the anomalous dual fluorescence of DMABN is discussed.     

Directed,intramolecular electron and energy transfer in mixed-valence dimers

Laser flash photolysis on a series of unsymmetrical ruthenium dimers has provided evidence for directed, intramolecular excitation energy transfer by a one-electron pathway for mixed-valence, Ru^II-Ru^III, dimers and by simple energy transfer for Ru^II-Ru^II dimers.     

Properties of excited singlet states of N-arylurethanes : Photo-fries reactions,fluorescence, quenching and sensitization

The deactivation of the first excited S(ππ^*) states of N-arylurethanes (produced upon irradiation with UV light) by emission (fluorescence), chemical reaction (photo-Fries rearrangement and fragmentation), energy transfer to quenchers, and radiationless transitions to ground and triplet states is investigated. Arylurethanes exhibit fluorescence (λ_f ≈ 295–350 nm, φ_f ≈ 10^?2, τ_f ≈ 1–6 ns) and phosphorescencs (λ_p ≈ 370–410 nm). The variations of the quantum yields of the fluorescence and of the photo-Fries rearrangement of N-arylurethanes by substituents and solvents are essentially due to variations of the rate constants for the radiationless processes. Fluorescence and photo-Fries reactions can be quenched by diffusion-controlled energy transfer to aliphatic ketones. Quenching is accompanied by sensitization of the ketone fluorescence. The urethane fluorescence and photo reactions may be sensitized by aromatic hydrocarbons. The results of all the quenching and sensitization experiments demonstrate that the photo-Fries reactions of N-arylurethanes proceed via the first excited singlet states of the urethanes.     

Electronic polarization reversal and excited state intramolecular charge transfer in donor/acceptor ethynylpyrenes

An attempt to tune the electronic properties of pyrene (Py) by coupling it with a strong electron donor (-PhNMe2, DMA)/acceptor (anthronitrile, AN) through an ethynyl bridge has been undertaken. A moderate electron donor (iPrOPh-, IPP)/acceptor (2-quinolinyl, 2Q) has also been incorporated, and all four molecules were studied with reference to a neutral molecule, namely, 1-phenylethynylpyrene (PhEPy). All the arylethynylpyrenes (ArEPy's) have been thoroughly characterized, and their electronic properties were studied by absorption and emission spectral properties of these ArEPy's. The electrochemical characteristics were also studied for arriving at the electrochemical band gap which has been compared with the HOMO-LUMO energy gap derived from the photophysical measurements and theoretical calculations performed by density functional theory (DFT) using B3LYP/6-31G basis sets. The results obtained from experimental and theoretical studies are critically discussed.     

吖啶橙-罗丹明6G共振能量转移荧光猝灭法测定尿中1-羟基芘

建立了吖啶橙(AO)-罗丹明6G(R6G)共振能量转移荧光猝灭法测定尿中1-羟基芘的新方法.在λex/λem=470/556nm,十二烷基苯磺酸钠(SDBS)存在下,AO-R6G能够发生有效的能量转移,使R6G的荧光大大增强;1-羟基芘(1-OHP)的加入使R6G的荧光猝灭.方法的线性范围是21.3～982 μg/L;检出限为6.4 μg/L;平行7次测定相对标准偏差为0.98%～2.0%;回收率为96.0%～104.4%.该方法用于锅炉工尿样中1-羟基芘的测定,结果与常规的高效液相色谱法一致.     

Distinguishing chromophore structures of photocycle intermediates of the photoreceptor PYP by transient fluorescence and energy transfer

The cinnamoyl chromophore is the light-activated switch of the photoreceptor photoactive yellow protein (PYP) and isomerizes during the functional cycle. The fluorescence of W119, the only tryptophan of PYP, is quenched by energy transfer to the chromophore. This depends on the chromophore's transition dipole moment orientation and spectrum, both of which change during the photocycle. The transient fluorescence of W119 thus serves as a sensitive kinetic monitor of the chromophore's structure and orientation and was used for the first time to investigate the photocycle kinetics. From these data and measurements of the ps-fluorescence decay with background illumination (470 nm) we determined the fluorescence lifetimes of W119 in the I(1) and I (1') intermediates. Two coexisting distinct chromophore structures were proposed for the I(1) photointermediate from time-resolved X-ray diffraction ( Ihee, H., et al. Proc. Natl. Acad. Sci. U.S.A., 2005, 102, 7145 ): one with two hydrogen bonds to E46 and Y42, and a second with only one H-bond to Y42 and a different orientation. Only for the first of these is the calculated fluorescence lifetime of 0.22 ns in good agreement with the observed one of 0.26 ns. The second structure has a predicted lifetime of 0.71 ns. Thus, we conclude that in solution only the first I(1) structure occurs. The high resolution structure of the I(1') intermediate, the decay product of I(1) at alkaline pH, is still unknown. We predict from the observed lifetime of 1.3 ns that the chromophore structure of I(1') is quite similar to that of the I(2) intermediate, and I(1') should thus be considered as the alkaline (deprotonated) form of I(2).     

Excited state intramolecular proton transfer in Schiff bases. Decay of the locally excited enol state observed by femtosecond resolved fluorescence

Although the late (t>1 ps) photoisomerization steps in Schiff bases have been described in good detail, some aspects of the ultrafast (sub-100 fs) proton transfer process, including the possible existence of an energy barrier, still require experimental assessment. In this contribution we present femtosecond fluorescence up-conversion studies to characterize the excited state enol to cis-keto tautomerization through measurements of the transient molecular emission. Salicylideneaniline and salicylidene-1-naphthylamine were examined in acetonitrile solutions. We have resolved sub-100 fs and sub-0.5 ps emission components which are attributed to the decay of the locally excited enol form and to vibrationally excited states as they transit to the relaxed cis-keto species in the first electronically excited state. From the early spectral evolution, the lack of a deuterium isotope effect, and the kinetics measured with different amounts of excess vibrational energy, it is concluded that the intramolecular proton transfer in the S1 surface occurs as a barrierless process where the initial wave packet evolves in a repulsive potential toward the cis-keto form in a time scale of about 50 fs. The absence of an energy barrier suggests the participation of normal modes which modulate the donor to acceptor distance, thus reducing the potential energy during the intramolecular proton transfer.     

Potential energy functions for an intramolecular proton transfer reaction in the ground and excited state

We describe the development of empirical potential functions for the study of the excited state intramolecular proton transfer reaction in 1-(trifuloroacetylamino)-naphtaquinone (TFNQ). The potential is a combination of the standard CHARMM27 force field for the backbone structure of TFNQ and an empirical valence bond formalism for the proton transfer reaction. The latter is parameterized to reproduce the potential energies both in the ground and the excited state, determined at the CASPT2 level of theory. Parameters describing intermolecular interactions are fitted to reproduce molecular dipole moments computed at the CASSCF level of theory and to reproduce ab initio hydrogen bonding energies and geometries for TFNQ-water bimolecular complexes. The utility of this potential energy function was examined by computing the potentials of mean force for the proton transfer reactions in the gas phase and in water, in both electronic states. The ground state PMF exhibits little solvent effects, whereas computed potential of mean force shows a solvent stabilization of 2.5 kcal mol⁻¹ in the product state region, suggesting proton transfer is more pronounced in polar solvents, consistent with experimental findings. Electronic Supplementary Material The online version of this article (doi:) contains supplementary material, which is available to authorized users. Contribution to the Fernando Bernardi Memorial Issue.     

Raman excitation profiles,fluorescence spectra and excited state proton transfer of solid 1.8 dihydroxyanthraquinone at 8 K

The single crystal fluorescence spectra as well as the Raman spectra and excitation profiles of solid 1.8 dihydroxyanthraquinone have been measured. Four electronic transitions in the visible range were found: a π—π* active in absorption, a Franck—Condon absorption emission active transition and two emissions due to excited state proton and Hatom transferred configurations.     

Proton, electron and energy transfer processes in excited phenol-olefin dyads

Dyads containing phenol and olefin subunits are versatile models for the investigation of proton, electron and energy transfer processes. As they are readily accessible, a number of analogues (allylphenols, cinnamylphenols and derivatives) have been prepared with a wide range of photophysical and photochemical properties. By means of appropriate structural modification of a very simple initial structure, it is possible to reproduce, at will, different types of behaviour. In addition to providing valuable fluorescence emission data, these systems are chemically productive, giving rise to irreversible photoreactions that constitute a fingerprint for the mechanism involved. They include photocyclisation to 5- and 6-membered ring cyclic ethers, Z/E isomerisation, di-pi-methane rearrangement, formation of ortho-quinone methides, photohydration and photodehalogenation. This rich photochemistry is highly sensitive to the microenvironment experienced, as indicated by the dramatic modifications observed within cyclodextrin cavities. Intramolecular OH...pi interactions, both in their ground and excited states, play a key role in the interesting properties of 2-allylphenol derivatives. This is supported by experimental data and also by theoretical calculations.     

Photoinduced electron transfer competitive with energy transfer of the excited triplet state of [60]fullerene to ferrocene derivatives revealed by combination of transient absorption and thermal lens measurements

The quenching processes of the exited triplet state of fullerene (3C60) by ferrocene (Fc) derivatives have been observed by the transient absorption spectroscopy and thermal lens methods. Although 3C60 was efficiently quenched by Fc in the rate close to the diffusion controlled limit, the quantum yields (phi(et)) for the generation of the radical anion of C60 (C60*-) via 3C60 were quite low even in polar solvents; nevertheless, the free-energy changes (deltaG(et)) of electron transfer from Fc to 3C60 are sufficiently negative. In benzonitrile (BN), the phi(et) value for unsubstitued Fc was less than 0.1. The thermal lens method indicates that energy transfer from 3C60 to Fc takes place efficiently, suggesting that the excited triplet energy level of Fc was lower than that of 3C60. Therefore, energy transfer from 3C60 to ferrocene decreases the electron-transfer process from ferrocene to 3C60. To increase the participation of electron transfer, introduction of electron-donor substituents to Fc (phi(et) = 0.46 for decamethylferrocene in BN) and an increase in solvent polarity (phi(et) = 0.58 in BN:DMF (1:2) for decamethylferrocene) were effective.     

Solid-emissive rhodamine: hydrogen bonding-assisted efficient intermolecular fluorescence resonance energy transfer in the solid state

Solid-emissive rhodamine complexes are obtained by mixing commercial rhodamine B (RhB) with the recently developed solid-emissive boron 2-(2′-pyridyl)imidazole (BOPIM) derivatives. The formation of intermolecular hydrogen bonds between RhB and BOPIM dyes plays a key role in the emission of RhB in the solid state. The disappearance of emissions from BOPIM dyes indicates the occurrence of efficient intermolecular fluorescence resonance energy transfer (FRET). The hydrogen bond also helps prevent the intermolecular interaction between the carboxyl moieties on RhB to alleviate concentration-induced fluorescence quenching because the emission of the complexes can be directly lightened by excitation at the RhB absorption (510 nm). Our results indicate that intermolecular FRET assisted by non-covalent interactions can be an efficient tool for constructing red or near-infrared solid emitters.