External electric field effects on absorption and fluorescence spectra of a fullerene derivative and its mixture with zinc-tetraphenylporphyrin doped in a PMMA film

Electroabsorption and electrofluorescence spectra of a fullerene derivative, C60(C18)2, and its mixture with zinc-tetraphenylporphyrin (ZnTPP) have been measured by using electric field modulation spectroscopy. The change in dipole moment is significant in the electroabsorption spectra both of C60(C18)2 and of a complex composed of C60(C18)2 and ZnTPP, indicating that the excited states both of C60(C18)2 and of a complex between C60(C18)2 and ZnTPP have a large charge-transfer character. The fluorescence quantum yield of C60(C18)2 decreases in the presence of an electric field, which probably arises from the field-induced acceleration of the intramolecular nonradiative process of C60(C18)2 in the fluorescent state. In a mixture between ZnTPP and C60(C18)2, electrofluorescence spectra show the field-induced enhancement for the fluorescence of ZnTPP and the field-induced de-enhancement for the fluorescence both of C60(C18)2 and of the complex between ZnTPP and C60(C18)2. A theoretical analysis clearly shows that the field-induced enhancement of the ZnTPP fluorescence in a mixture results from the field-induced deceleration of the rate of the electron transfer from the excited ZnTPP to C60(C18)2. The standard free energy gap for the photoinduced electron-transfer process is estimated based on the theoretical simulation of the field-dependent fluorescence intensity.     

External electric field effects on fluorescence of pyrene butyric acid in a polymer film: concentration dependence and temperature dependence

Fluorescence spectra and electrofluorescence spectra (plots of the electric field-induced change in fluorescence intensity as a function of wavelength) have been measured at different temperatures for pyrene butyric acid (PBA) in a PMMA film at different concentrations. At a low concentration of 0.5 mol % where fluorescence emitted from the locally excited state of PBA (LE fluorescence) is dominant, LE fluorescence spectra show only the Stark shift in the presence of an electric field (F), which results from the difference in molecular polarizability between the ground and emitting states. At a high concentration of 10 mol % where the so-called sandwich-type excimer fluorescence (EX(1)) is dominant, both EX(1) and LE fluorescence are quenched by F. Another fluorescence assigned to a partially overlapped excimer (EX(2)) also exists at room temperature, and this emission is enhanced by F. As the temperature decreases, three fluorescence emissions whose electric field effects are different from each other become clear besides EX(1) and LE fluorescence, indicating that at least five fluorescence components exist at high concentrations at low temperatures. At a medium concentration of 5 mol % where EX(1) is comparable in intensity to the LE fluorescence, the intensity of EX(1) is not affected by F at any temperature, but LE fluorescence and EX(2) are markedly influenced by F at room temperature, and four fluorescence emissions are confirmed at low temperatures.     

Electric field effects on absorption and fluorescence spectra of trimethylsilyl- and trimethylsilylethynyl-substituted compounds of pyrene in a PMMA film

External electric field effects on absorption and fluorescence spectra of 1,3,6,8-tetrakis(trimethylsilyl)pyrene and 1,3,6,8-tetrakis(trimethylsilylethynyl)pyrene (TMSPy and TMS(E)Py, respectively) have been examined in a poly(methyl methacrylate) (PMMA) film at various concentrations at various temperatures. TMS(E)Py preferentially forms an aggregate in a PMMA film, as the concentration increases, indicating that the acetylenic groups enhance the pi-pi interactions between pyrene molecules. The change in molecular polarizability following excitation has been determined both for the monomer and for the aggregate, based on the electroabsorption spectra. The change in molecular polarizability following emission has also been determined in both compounds, based on the electrofluorescence spectra. TMSPy exhibits two excimer fluorescence emissions at high concentrations which are ascribed to the partially overlapping excimer and the sandwich-type excimer, respectively, besides the monomer fluorescence emitted from the locally excited state. The sandwich-type excimer fluorescence as well as monomer fluorescence is quenched by an electric field, whereas the fluorescence of the partially overlapping excimer is enhanced by an electric field. Excimer fluorescence of TMS(E)Py, which arises from the sandwich-type excimer, is quenched by an electric field at any temperature. Only one species of the partially overlapping excimer is confirmed in TMSPy, while no partially overlapping excimer is confirmed in TMS(E)Py.     

Effect of a d.c. electric field on the fluorescence spectra of cationic polymethine dyes in a polymer film

We have observed that in an external d.c. electric field, in the differential fluorescence spectra of both symmetric and asymmetric polymethine dyes in a polymer film, the intensity of the emission in the long-wavelength region of the band increases while the intensity in the short-wavelength region decreases. In symmetric polymethines, these effects are opposite to those in the differential absorption spectra, while in asymmetric polymethines they are symbatic. The spectral effects were interpreted by a change in the probability of the vibronic transitions due to redistribution of electron density in the chromophore, in the ground state and the excited state of the dye, in an external electric field. Translated from Teoreticheskaya i éksperimental'naya Khimiya, Vol. 44, No. 6, pp. 343–347, November–December, 2008.     

Vibronic absorption, fluorescence, and phosphorescence spectra of psoralen: a quantum chemical investigation

Excited state potential energy hypersurfaces of 7H-furo[3,2-g][1]benzopyran-7-one (psoralen) have been explored employing (time-dependent) Kohn-Sham density functional theory. At selected points, we have determined electronic excitation energies and electric dipole (transition) moments utilizing a combined density functional/multireference configuration interaction method. Spin-orbit coupling has been taken into account employing an efficient, non-empirical spin-orbit mean-field Hamiltonian. Franck-Condon factors have been computed for vibrational modes with large displacements in the respective Dushinsky transformations. The simulated band spectra closely resemble experimental band shapes and thus validate the theoretically determined nuclear structures at the S(0), S(1), and T(1) minima. In the S(1) (pi(HOMO)-->pi*(LUMO)) state, the lactone bond of the pyrone ring is significantly elongated. From excited vibrational levels of the S(1) state a conical intersection between a (pi-->sigma*) excited state and the electronic ground state may be energetically accessible. Fast non-radiative decay via this relaxation pathway could explain the low fluorescence quantum yield of psoralen. The T(1) (pi(HOMO-1)-->pi*(LUMO)) exhibits a diradicaloid electronic structure with a broken C(5)-C(6) double bond in the pyrone ring. A variational multireference spin-orbit configuration interaction procedure yields a phosphorescence lifetime of 3 s, in excellent agreement with experimental estimates.     

Time-dependent density functional theory investigation of the absorption, fluorescence, and phosphorescence spectra of solvated coumarins

Using time-dependent density functional theory (TD-DFT) and the polarizable continuum model, we have computed the electronic transitions of a large panel of coumarin dyes in their enol, keto, cationic, and anionic forms. Several processes have been studied: absorption, fluorescence, 0-0 phosphorescence, and triplet-triplet excitations. For each process, detailed comparison with experimental data has been carried out. Using the PBE06-31+G(d) scheme, it turns out that for a given electronic transition the experimental shifts resulting from the substitution of the coumarin core are nicely reproduced. Indeed, once a simple statistical correction is applied, the mean absolute errors on the absorption and fluorescence wavelengths are limited to 8 nm (0.09 eV) and 9 nm (0.07 eV), respectively. A valuable correlation between the experimental and theoretical phosphorescence auxochromic displacements has also been unravelled. The differences between the wavelengths of the various electronic processes of a given dye tend to be fairly predicted, especially for the fluorescence-phosphoresence shifts that are strongly overestimated by TD-DFT.     

The magnetic field effect in the presence of the electric field on fluorescence of a methylene-linked compound of pyrene and n,n-dimethylaniline doped in a polymer film

Recent experimental data by Mizoguchi and Ohta are analyzed under the assumption of fast equilibrium between the locally excited (LE), radical-ion-pair (RIP), and exciplex states. In the absence of the equilibrium, no magnetic field effect on the LE fluorescence would be observed. Owing to the equilibrium, the relative electric-field-induced changes of the quantum yields of the LE and exciplex emissions as functions of the magnetic field are linearly connected to each other. The electric field shifts the equilibrium from the LE state toward the RIP state. Predictions are made for the magnetic field effect on the fluorescence kinetics.     

Magnetic field effect on fluorescence in a mixture of N-ethylcarbazole and dimethyl terephthalate in a polymer film in the presence of electric fields

Magnetic field effects on the fluorescence spectrum and on the electrofluorescence spectrum (plots of the electric field-induced change in fluorescence intensity as a function of wavelength) have been examined in electron donor and acceptor pairs of N-ethylcarbazole (ECZ) and dimethyl terephthalate (DMTP) in polymer films at different ratios of donor/acceptor concentration. In the mixture having a high concentration of ECZ, electric field-induced quenching of the exciplex fluorescence originating from the photoinduced electron transfer becomes less efficient in the presence of a magnetic field. In the mixture having a low concentration of ECZ, on the other hand, no magnetic field effect was observed in the electrofluorescence spectrum, indicating that the hole carrier plays an important role in synergy effects of magnetic and electric field effects on exciplex fluorescence. In the absence of the applied electric field, the magnetic field does not affect either exciplex fluorescence with a peak at 450 nm or LE fluorescence emitted from the locally excited state of ECZ but enhances the broad emission with a peak at approximately 380 nm, probably assigned to the fluorescence of another type of exciplex between ECZ and DMTP. Thus, two kinds of magnetic field effects on fluorescence have been observed in a mixture of ECZ and DMTP in a polymer film.     

Electric-field-induced changes in absorption and emission spectra of CdS nanoparticles doped in a polymer film

Electric-field-induced changes in absorption and emission spectra of colloidal CdS nanoparticles ranging in size from 1.0 to 5.0 nm in diameter have been measured by using electric field modulation spectroscopy. The analysis of the electroabsorption spectra indicates that the dipole moment in the first exciton state becomes larger with increasing particle size. The presence of the large dipole moment following photoexcitation into the first exciton band suggests that the CdS nanoparticles have large CT character in the first exciton state. The quantum yields both of the exciton emission and of the trap emission are markedly reduced by application of an electric field. On the basis of the direct measurements of the field-induced change in emission decay profile, it is suggested that the field-induced de-enhancement of these emission yields results from the field-induced decreases both in lifetime and in initial population of each emitting state. It is also found that the emission intensity of CdS nanoparticles increases under the UV light irradiation in air and decreases in a vacuum condition and that fluorescence lifetime in the former case is longer than that in the latter. This enhancement and de-enhancement process in emission intensity is almost reversible at least in several cycles.     

The absorption,fluorescence and phosphorescence spectra of α and β-F,Cl, Br-naphthalenes in crystalline matrixes at 77 K

The paper studies the absorption, fluorescence and phosphorescence spectra of α and β-F, Cl, Br-naphtalenes (α, β-F, Cl,BrN) in different matrixes at 77 K and different concentrations. From these spectra one obtaines the vibrational frequences.     

External electric field effects on state energy and photoexcitation dynamics of diphenylpolyenes

External electric field effects on state energy and photoexcitation dynamics have been examined for para-substituted and unsubstituted all-trans-diphenylpolyenes doped in a film, based on the steady-state and picosecond time-resolved measurements of the field effects on absorption and fluorescence. The substitution dependence of the electroabsorption spectra shows that the dipole moment of the substituted stilbene in the Franck-Condon excited state becomes larger with increasing difference between the Hammet constants of the substituents. Fluorescence quantum yields of 4-(dimethylamino)-4'-nitrostilbene and 4-(dimethylamino)-4'-nitrodiphenylbutadiene are markedly reduced by an electric field, suggesting that the rates of the intramolecular charge transfer (CT) from the fluorescent state to the nonradiative CT state are accelerated by an external electric field. The magnitude of the field-induced decrease in fluorescence lifetime has been evaluated. The isomerization of the unsubstituted all-trans-diphenylpolyenes to the cis forms is shown to be a significant nonradiative pathway even in a film. Field-induced quenching of their fluorescence as well as field-induced decrease in fluorescence lifetime suggests that the trans to cis photoisomerization is enhanced by an electric field.     

Solvent effects on the steady-state absorption and fluorescence spectra of uracil, thymine and 5-fluorouracil

We report a comparison of the steady-state absorption and fluorescence spectra of three representative uracil derivatives (uracil, thymine and 5-fluorouracil) in alcoholic solutions. The present results are compared with those from our previous experimental and computational studies of the same compounds in water and acetonitrile. The effects of solvent polarity and hydrogen bonding on the spectra are discussed in the light of theoretical predictions. This comparative analysis provides a more complete picture of the solvent effects on the absorption and fluorescence properties of pyrimidine nucleobases, with special emphasis on the mechanism of the excited state deactivation.     

pH-Responsive amorphous room-temperature phosphorescence polymer featuring delayed fluorescence based on fluorescein

Numerous researchers have paid attention to achieve metal-free phosphorescence by exploring new structures or new mechanisms. Herein, a facile way is introduced to endow a common fluorescence dye, tetrabromofluorescein (4Br-Flu), some fabulous optical characteristics such as dual emission including thermally activated delayed fluorescence, room-temperature phosphorescence (RTP), and the excellent pH-sensitivity. Shortly, 4Br-Flu with good light-emitting properties is composed into the polymer system. The multiple bromine atoms promote the spin-orbit coupling effect and facilitate triplet excitation. Especially, the hydrogen bonding network of the polymer restricts the molecular motion of 4Br-Flu so that the system can emit long-wavelength RTP when 4Br-Flu is doped into polyvinyl alcohol or co-polymerized with acrylamide. Due to the reversible transformation of protonation and deprotonation, the 4Br-Flu based polymer responded to acid and alkali like a phosphorescent switch which makes it an excellent hydrogen chloride/ammonia gas leak detector in dry environment.     

Temperature and solvent viscosity effects on the absorption and fluorescence spectra of a series of pyrylium salts

The excitation and fluorescence spectra and the excited state lifetimes of pyrylium salts were studied in different polar solvents. An emission blue shift is observed when the temperature is lowered from 300 to 77 K. This phenomenon is believed to be due to solvent—solute interactions following changes in the electronic distribution in the excited state. At 77 K the excited state decay is faster than the solvent reorganization and the emission originates from the Franck—Condon state. At 300 K the solvent relaxation is now fast enough (about 50 ps) to allow the excited state to relax before emitting.     

Temperature dependence of fluorescence and phosphorescence from probe molecules in polymer substrates

The effect of temperature on the fluorescence and phosphorescence from a series of indole and coumaric acid derivatives incorporated in poly(methyl methacrylate), poly(styrene) and poly(vinyl alcohol) substrates has been investigated in the range 90–300 K. Activation energies for the temperature-dependent non-radiative decay of the triplet excited states showed abrupt changes at temperatures corresponding to the onset of local relaxation processes in the polymers. The values of activation energies and transition temperatures in any one polymer did not depend greatly on the probe, suggesting that the temperature-dependent triplet state deactivation mechanisms are mainly influenced by extrinsic polymer properties. However, the effect of temperature on the fluorescence intensity was found to depend markedly on the probe and state of ionization of any one probe. Possible explanations for these observations are presented.     

Noncovalent control of absorption and fluorescence spectra

Perylene dyes with N-tert-alkyl substituents were prepared in which noncovalent interactions of the crowded substituent cause a variation of the geometry of the core and induce hypsochromic shifts in absorption and fluorescence. The interpretation of the shifts was supported by means of DFT calculations and an X-ray crystal structure analysis.     

Solvatochromic effects in the absorption and fluorescence spectra of indazole and its amino derivatives

Absorption and fluorescence spectra of Indazole (In) in five solvents and those of 5-amino, 6-amino, and 7-aminoindazoles (5-AI 6-AI, 7-Al) in thirteen solvents have been studied. Using the multiparametric approach of Taft et al., spectral characteristics of these molecules have been analysed on the basis of electrostatic effects, hydrogen bond donor ability and hydrogen bond accepting ability of the solvents. In the excited singlet state, all three effects are nearly equally prominent, whereas in the ground state 5-AI acts as a better hydrogen bond acceptor than the other amines. Stokes shifts and the difference between the radiative decay constants determined experimentally and using Strickler and Berg's equation indicate different geometries of the amines in the S₀ and S₁ states. A decrease in the non-radiative decay constants of 5-AI and 6-AI with an increase in the polarity of the solvents predict better planar geometry in the S₁ state in comparison to that in the S₀ state. Semi-empirical quantum mechanical calculations have been used to find the nature of transitions, total atomic charges at the basic centres and dipole moments of all the aromatic amines in the S₀ and S₁ states. Results so obtained are compared and discussed.     

Stark effects on the phosphorescence spectra of p-benzoquinone

Stark effects on the phosphorescence spectra of p-benzoquinone have been studied in naphthalene crystals at temperatures between 1.7 and 30 K. The experimental data establish that the band splitting of 24 cm^?1 arises from a double minimum potential in the T₁ state.