Nonradiative Decay Constants Research Articles

Gray-Tone Lithography Implementation of Drexhage’s Method for Calibrating Radiative and Nonradiative Decay Constants of Fluorophores

We present a straightforward method to realize non-planar dielectric structures with a controlled height profile for use in calibration of fluorophores. Calibration of fluorescence quantum efficiency and intrinsic radiative and nonradiative decay rates of emitters is possible by using changes in the local density of optical states, provided one can control the emitter-surface distance with nanometer accuracy. We realize a method that is accurate yet fast to implement. We fabricate PMMA wedges (4 mm x 4 mm x 2 \mu m) by gray-tone UV-lithography of Shipley S1813G2. Its applicability as dielectric spacer is demonstrated in Drexhage experiments for three different emitters in the visible and near-infrared wavelength regime. The decay-rate dependence of the fluorescent state of emitters on the distance to a silver mirror is observed and compared to calculations based on the local density of states. Quantitative values for (non)radiative decay rates and quantum efficiencies are extracted. Furthermore, we discuss how Drexhage experiments can help to scrutinize the validity of effective material parameters of metamaterials in the near field regime.

Conformationally dynamic π-conjugation: probing structure-property relationships of fluorescent tris(N-salicylideneaniline)s.

We recently reported the design and synthesis of a series of conformationally dynamic chromophores that are built on the C(3)-symmetric tris(N-salicylideneaniline) platform. This system utilizes cooperative structural folding-unfolding motions for fluorescence switching, which is driven by the assembly and disassembly of hydrogen bonds between the rigid core and rotatable peripheral part of the molecule. Here, we report detailed time-resolved spectroscopic studies to investigate the structure-property relationships of a series of functionalized tris(N-salicylideneaniline)s. Time-resolved fluorescence decay spectroscopy was applied to determine the main relaxation mechanisms of these π-extended fluorophores, and to address the effects of hydrogen bonding, steric constraints, and extension of the π-conjugation on their relaxation dynamics. Our results agree well with the conformational switching model that was previously suggested from steady-state experiments. Notably, extension of the π-conjugation from peripheral aryl groups resulted in the stabilization of the excited states, as evidenced by longer lifetimes and lower nonradiative decay constants. As a consequence, an increase in the fluorescence quantum yields was observed, which could be explained by the suppression of the torsional motions about the C-N bonds from an overall increase in the quinoid character of the excited states. A combination of time-resolved and steady-state techniques also revealed intermolecular interactions through π-π stacking at higher concentrations, which provide additional de-excitation pathways that become more pronounced in solid samples.

Photophysical properties of an asymmetrical 2,5-diarylidene-cyclopentanone dye possessing electron donor and acceptor substituents

Spectroscopic and photophysical properties have been measured in a variety of solvents for (2E,5E)-2-(p-cyanobenzylidene)-5-(p-dimethylaminobenzylidene)-cyclopentanone ( I), an asymmetrically substituted 2,5-diarylidene-cyclopentanone known to have potential for applications utilizing two-photon absorption (TPA). Compound I was synthesized by a simple procedure utilizing DIMCARB catalyst. Absorption and fluorescence maxima have been correlated with the E T(30) solvent polarity scale. Theoretical TD-DFT calculations and Lippert–Mataga analysis demonstrate the internal charge transfer (ICT) nature of the S 0 → S 1 excitation. Radiative and nonradiative decay constants have been determined from fluorescence quantum yields and lifetimes. Substantial variation with solvent in the rate of nonradiative decay is interpreted in terms of a competition between internal conversion and intersystem crossing.

Fluorescence properties of trans-ethyl- p-(dimethylamino) cinnamate in presence of bile acid host

The knowledge of the formation of bile acid micellar aggregates is of great importance because of the biological significance of these compounds and their pharmacological applications. The intramolecular charge transfer (ICT) fluorescence property of trans-ethyl- p-(dimethylamino) cinnamate is used to study the micelles formed by aggregation of three most important bile acids, viz. cholic acid, deoxycholic acid and chenodeoxycholic acid by steady state and picosecond time-resolved fluorescence spectroscopy. The ICT fluorescence band intensity was found to increase with concomitant blue shift with the addition of bile acids. The blue shift in ICT fluorescence maxima as well as decrease in nonradiative decay constants in presence of bile acids indicate the passage of the probe towards the micro domains formed from the aggregated bile acids. Binding constant of the probe with micelles as well as critical micelle concentration and average polarity parameter of the micellar environments were obtained from the variation of fluorescence intensity on increasing concentration of bile acids in the medium.

Ground- and Excited-State Dynamics of Aluminum and Gallium Corroles

The steady-state absorption and emission spectra and the temporal fluorescence decay profiles of two metallocorroles, Al(tpfc)(py)(n) and Ga(tpfc)(py)(n) (n = 1,2), have been measured in a noncoordinating solvent, benzene, in a coordinating solvent, pyridine, and in mixed benzene-pyridine solutions. The ground-state spectra reveal that an equilibrium between the pentacoordinate corrole (n = 1) and the hexacoordinate corrole (n = 2) is established in the mixed benzene-pyridine solutions. The ground-state equilibrium constants are 135 M(-1) and 1.0 M(-1) at 295 K for the Al and Ga species, respectively. The excited-state radiative and nonradiative decay constants of the pentacoordinate and the hexacoordinate species have been obtained from measurements of the fluorescence quantum yields and monoexponential fluorescence decay times in pure benzene and pure pyridine. Temporal fluorescence decays of the gallium system in a mixed benzene-pyridine solution are biexponential due to dissociation of the hexacoordinate species in the excited state leading to the establishment of a dissociation-association equilibrium. The rate constants for the pyridine association and dissociation processes for the gallium corrole in the excited state have been measured, k(a)* = 2.3 x 10(8) M(-1) s(-1) and k(d)* = 2.9 x 10(8) s(-1), respectively, leading to a value for the excited-state association equilibrium constant of K(a)* = 0.78 M(-1).

The environmental effect on the fluorescence intensity in solution. An analytical model

In this paper a mathematical model describing the non-specific interactions of the medium surrounding a fluorophore on its fluorescence intensity is proposed. The model, which has been developed for quantitative analytical applications, is based on the following general ideas: (1) the medium affects the fluorescence quantum yield across the non-radiative decay constant (k(nr)); (2) the k(nr) can be simplified to the singlet-to-triplet intersystem crossing (k(ISC)) constants; (3) k(ISC) follows the energy gap law and then depends on the singlet and triplet energy difference, and (4) the medium, due to solvation, changes the energy of both excited levels (singlet and triplet), then the constants and finally the fluorescence intensity. In our model, the strength of the fluorophore solvation by the solvent (represented by its refraction index, n, dielectric constant, epsilon, and electric charge) changes the singlet (excited)-to-fundamental and the singlet-to-triplet energy gaps, thus the k(ISC) and k(IC) (internal conversion constant) values and in consequence the fluorescence quantum yield. The final model relates the fluorescence intensity (F) with the solvent dielectric constant and refraction index. Finally, the model is particularized for the case of a medium composed of a solvent and a solute, obtaining an F-to-solute concentration relationship and enabling this fact to be used for analytical applications. The very first experimental data are shown demonstrating the fulfilment of this model.

Fluorescence behavior of intramolecular charge transfer probe in anionic, cationic, and nonionic micelles

The intramolecular charge transfer (ICT) property of trans-ethyl p-(dimethylamino) cinnamate is used to probe the anionic, cationic, and nonionic micelles by steady-state and picosecond time-resolved fluorescence spectroscopy. The ICT fluorescence band intensity was found to increase with concomitant blue shift with addition of surfactants. All the experimental results suggest that the probe molecule resides in the micelle–water interface rather than going into the core. However, the penetration is more toward the micellar core in nonionic surfactants when compared with ionic micelles. The decrease in nonradiative decay constants in micellar environments indicate restricted motion of the probe toward the formation of ICT state. Critical micelle concentrations were determined from the sharp change in fluorescence intensity and effective dielectric constants of the micelle–water interface were calculated from the correlation diagram of 0 , 0 transition energy with polarity of the medium.

Photophysics of pyrene-labelled compounds of biophysical interest

The effects of the chemical constitution and structure of the substituent on the excited state dynamics of several model fluorescent pyrene-labelled molecules of biophysical interest have been examined. Nine new 1-substituted pyrenyl compounds, Py-NH-CO-C2H5, Py-NH-CO-Leu-Boc, Py-CH2-NH-CO-C2H5, Py-CH2-NH-CO-Leu-Boc, Py-CO-NH-C3H7, Py-CO-NH-Leu-OMe, Py-CH2-CO-NH-C3H7, Py-CH2-CO-NH-Leu-OMe and Py-C3H6-CO-NH-Leu-OMe, have been synthesized and their electronic spectra, fluorescence quantum yields and excited state lifetimes measured. These data have been used to calculate the radiative, kr, and non-radiative decay constants of their S1 states and the values of these constants correlated with the structures of the tethers. Non-radiative S1 decay rates (mainly intersystem crossing to T1) vary in parallel with the radiative rates so that the excited state lifetimes and radiative rate constants change considerably with the structure of the substituent whereas the quantum yields of fluorescence do not. An excellent correlation between [epsilon]max of the S1-S0 transition and either kr or the excited state lifetime is observed as long as no additional intermolecular or intramolecular excited state decay process of significant rate competes with the 'normal' radiative and non-radiative (ISC) decay processes of the pyrenyl chromophore. This correlation may have predictive value. Rates of bimolecular quenching of the S1 states of these molecules by molecular oxygen have been measured. The quenching process is diffusion-controlled with a spin statistical factor of 1, indicating that the S1-T1 electronic energy spacings of all the derivatives exceed the O2(1Deltag-3Sigmag-) electronic excitation energy of ca. 1 eV.

Photophysical study of a family of [Ru(phen)2(Mendpq)]2+ complexes in different solvents and DNA: a specific water effect promoted by methyl substitution

The synthesis and spectroscopic characterisation of a family of ruthenium complexes [Ru(phen)(2)(Me(n)dpq)](2+)(n= 0, 1, 2) are reported and the photophysical effect of the progressive methyl substitution on the dpq ligand ascertained in a range of solvents and in the presence of DNA: [Ru(phen)(2)(dpq)](2+)(1), [Ru(phen)(2)(Medpq)](2+)(2) and [Ru(phen)(2)(Me(2)dpq)](2+)(3)(where dpq is dipyrido[3,2-f:2',3'-h]-quinoxaline; Medpq is 2-methyldipyrido[3,2-f:2',3'-h]-quinoxaline; Me(2)dpq is 2,3-dimethyldipyrido[3,2-f:2',3'-h]-quinoxaline and phen is 1,10-phenanthroline). The increase in electron density following substitution renders the quinoxaline nitrogen atoms more basic in the ground state to yield pK(a) values of -1.9, -2.3, and -2.7 for 3, 2, and 1, respectively. The methyl groups have a pronounced effect on the excited state photophysics of 1-3. In organic solvents, the non-radiative decay constant correlates well with the solvent polarity parameter pi*, with the effect being more pronounced with increasing methyl substitution. On the contrary, in aqueous solution there is a ca. four-fold decrease in the non-radiative decay constant upon methyl substitution. This "specific water effect" may be explained in terms of deactivation of the excited state by hydrogen bonding interactions between water and the quinoxaline nitrogen atoms, which is reduced on moving from 1 to 3. The excited state lifetimes and luminescence intensities for substantially increase when bound to DNA in aerated aqueous solutions, with a six-fold increase, compared to a more moderate three-fold increase for 2 and 3.

Dual probe solubilisation in two distinct regions of pure and mixed micelles: a pico-second time resolved fluorescence study

Photophysical parameters of a newly synthesised fluorescence probe, 2-[3-( N-methyl- N-phenyl amino)-2-propenylidene] indanone, have been studied in three different micellar media formed by the surfactants sodium dodecyl sulphate (SDS), cetyl trimethyl ammonium bromide (CTAB) and triton X-100 (TX100) and their binary mixtures (SDS–TX100, CTAB–TX100). Our study indicates that the probe molecules are distributed between the core and Stern/ palisade layer of the micelle. The extent of distribution depends on the nature of the micelle. The radiative decay constant ( k r) for the excited S 1 state has been found to be independent of the nature of micellar media. However, the non-radiative decay constant ( k nr) exhibits significant medium dependence in the order SDS > CTAB ≈ TX100. Non-linear variation of k nr with component surfactant composition in the mixed micelle has been observed and has been explained in terms of synergistic interaction between the surfactant molecules.

Steady-state fluorescence and photophysical properties of a ketocyanine dye in binary surfactant and polymer–surfactant mixture

Steady-state and time-resolved fluorescence studies have been done using a ketocyanine dye in heterogeneous media containing binary mixtures of sodium dodecyl sulphate (SDS) and triton X-100 (TX-100), and SDS+polyvinyl pyrrolidone (PVP). The dye molecules are found to distribute between core and interfacial region of the micelles. The band corresponding to the dye molecule in the core region exhibits red edge excitation shift. Study of lifetime in picosecond domain reveals that the de-excitation of the excited state of the dye via radiative pathway ( k r) is independent of environment. The non-radiative decay constant ( k nr) on the other hand has been found to depend on the nature of environment, the core region being characterised by a faster decay rate. The slower decay rate in the interfacial region has been explained as due to hydrogen bonding interaction of the carbonyl group of the dye with protic solvents. Interesting variation of k nr in the core region with surfactant composition has been observed which is explained in terms of surfactant–surfactant interaction in the micelles. Microviscosities at the micelle–water interface has been determined. For binary mixture of surfactants, the value of microviscosity has been found to deviate from ideality. An index of deviation from ideality has been proposed.

Luminescence study of lanthanide(III) ions in non-aqueous solutions containing azide ions

Luminescence lifetime and intensity measurements of lanthanide(III) ions, Ln3+ (Eu, Gd and Tb), in non-aqueous solutions containing azide ions (N3−) have been performed to study the quenching effect of N3− using time-resolved laser-induced luminescence spectroscopy and conventional luminescence spectroscopy. The luminescence Stern–Volmer quenching constants KsvΦ and azide non-radiative decay constants kN3 of the excited Eu(III) ion in the presence of N3− were measured in N,N-dimethylformamide (DMF), dimethyl sulfoxide (DMSO), N-methylformamide (NMF), formamide (FA) and methanol (MeOH). The obtained quenching effect of the Ln3+ luminescence by azide in the non-aqueous solvents shows the order: Gd>Eu>Tb, which does not obey the energy gap law, ΔE, between the emitting and the ground state of the Ln3+ ion (Gd>Tb>Eu). The anomaly in the quenching pattern observed in the case of Eu3+ can be attributed to its partial reduction to Eu2+, additionally to the energy transfer from excited state of Eu3+ to N3−. The values of quenching rate constants kN3measured for Eu3+ in various solutions depend on the acceptor number (AN) of the solvents used.

Luminescence properties of lanthanide(III) ions in concentrated carbonate solution

Luminescence properties of lanthanide(III) ions (Ln = Nd, Sm, Eu, Gd, Tb, Dy and Tm) were investigated by measuring the excitation and emission spectra, and emission lifetimes in H2O and D2O solutions of 3 moll−1 K2CO3, where anionic tetra-carbonate complexes, [Ln(CO3)4]5- were the predominant species. Electronic transitions of the carbonato complex corresponding to both the excitation and emission spectra were assigned from the energy level diagrams of Ln(III) and compared with those of the aqua ion. Enhancement of emission intensity of the complex was observed at particular excitation transitions of Eu(III), Gd(III) and Tb(III), and at particular emission transitions of Sm(III), Eu(III), Dy(III) and Tm(III). The enhancement at the emission transition was estimated quantitatively as a branching ratio from the lowest emitting state of Ln(III), and discussed in terms of hypersensitivity. Emission lifetimes of the carbonato complexes were all longer than those of aqua ions in H2O solution, while the lifetimes of the complexes for Eu(III) and Tb(III) shorter than those in D2O solution. The difference in non-radiative decay constants for the excited complex in H2O and D2O solutions was found to be proportional to an exponential of the energy gap of Ln(III). The lifetime ratio between the H2O and D2O solutions showed the order of Sm > Dy > Eu > Tb, corresponding to the opposite order of the energy gap. These were discussed in terms of an energy gap law, i.e. a relationship between the energy gap of Ln(III) and vibration energies of the ligand or water molecules.

Photophysical characteristics of coumarin 485 dye doped poly(methyl methacrylate) modified with various additives

The influence of medium on the photophysical properties of a fluorinated coumarin (coumarin 485, C485) is studied in detail. The absorption and fluorescence characteristics of coumarin 485 dye doped poly(methyl methacrylate) modified with various additives (1,4-dioxane, chloroform, n-butyl acetate, acetonitrile and ethanol) are studied and the results are compared with the respective liquid mixtures. The fluorescence lifetime, fluorescence quantum yield, radiative and the non-radiative decay constants of C485 in polymer and corresponding monomer compositions are obtained. The gain characteristics of C485 in various modified poly(methyl methacrylate) (MPMMA) matrices are studied at various nitrogen laser powers in a transverse pumping configuration and the results are compared with the gain characteristics in the corresponding liquid mixtures. It is found that the gain of C485 in MPMMA increases with the increase in the dielectric constant of the medium while in the liquid medium, the reverse effect is observed. It is also found that the gain coefficient of C485 in a solid matrix shows 90% reduction when compared to the liquid medium. The photobleaching effect of this dye is also studied for a fixed nitrogen laser power and it is found that the photobleaching effect decreases with the increase in the dielectric constant of the medium. A significant improvement in the photostability was achieved from coumarin 485 doped PMMA modified with ethanol. The number of pumping pulses needed for 80% reduction of the initial output is 13 600 pulses for the input power of 10 kW and the repetition rate of 2 Hz.

Twisted Intramolecular Charge Transfer Emission of 2-(4′-N,N-dimethylaminophenyl)benzimidazole in Micelles

Spectral characteristics of 2-(4′N,N-dimethylaminophenyl)benzimidazole have been studied as a function of surfactant concentration and as a function of acid concentration in three surfactants. Dual fluorescence is observed in all the micelles. Fluorescence intensities of the local emission (B band) and twisted intramolecular charge transfer (A band, TICT) increase by an 17–30% and 38 to 64% respectively. When dissolved in micelles lifetimes of both the states also increase in the presence of micelles. The increase in the fluorescence intensities is attributed to the decrease in the non-radiative decay constant. cmc of the surfactants can be determined from the variation in the fluorescence intensity and the lifetime data. The equilibrium constants are determined for the prototropic reactions of the fluorophore in all the micelles inS0andS1states and the values obtained are discussed.

Prototropic equilibrium of 2-aminofluorene in non-ionic micelles: Tweens

Spectral characteristics of 2-aminofluorene (2-AF) have been studied as a function of the concentration of four Tweens at a given pH, and as a function of pH at a given concentration of the Tweens. The effective dielectric constant (εeff) at a site of neutral 2-AF and the site where a prototropic reaction is occurring has been evaluated using the Stokes shifts of neutral 2-AF and the pKa values of the monocation–neutral equilibrium, respectively. These results, along with the single exponential decay of the fluorescence intensity of 2-AF, have clearly established that 2-AF is present at the interface of the micelles and water, and is bound to the oxygen atom of the polyoxy ethylene group through hydrogen bonding. The amino group of 2-AF is oriented towards the polar region and the fluorene ring towards the non-polar region of the micelles. The slight disagreement between the values of εeff determined by both the methods is due to a small percentage (5%) of the prototropic reaction occurring in the aqueous phase. In both cases, the solvent systems used have been obtained by employing different compositions of dioxane–water mixtures. The values of the binding constants, non-radiative decay constants and εeff determined by the above methods confirm that the hydrophobic character of the Tweens increases with increase of Tween number.

Structural, spectroscopic and photophysical analyses of substituted terthiophenes and quinquethiophenes as well as their corresponding polyesters

In this study, we present spectroscopic and photophysical results on oligothiophenes (trimers and pentamers) substituted with alkyl side chains as well as on the respective molecules incorporated in polyesters. The same oligothiophenes having two different electron acceptor groups, namely the carbonyl chloride and the acid substituent, at each end of the molecules have also been studied. These molecules provide a better correlation with their corresponding polyesters. Absorption and fluorescence spectra in solution are used to discuss the effect of the substitution as well as the length of the thiophene chain on the molecular conformation. Semiempirical calculations (AM1 and PM3) have also been performed to obtain the torsional potentials of the oligomers. Consequences of the conformational changes on the spectral and photophysical properties (fluorescence quantum yields, lifetimes and decay constants) of the various oligothiophenes and polyesters are examined. It is found that insertion of two alkyl chains on the central thiophene ring creates an important molecular twisting in the molecules. The length of the thiophene chain as well as the presence of end-substituents do not significantly influence the ground state molecular conformation. In the excited state, all molecules relax to more planar conformations. The fluorescence quantum yields and lifetimes are smaller for the alkyl-substituted molecules (without end-substituents) giving rise to higher values of the nonradiative decay constants compared to those of the unsubstituted molecules. This behavior might involve a decrease of the singlet–triplet energy gap for the twisted molecules which could enhance the intersystem crossing process. The incorporation of terthiophene and quinquethiophene derivatives in polyesters does not significantly change their conformation in the ground and excited states as well as their optical and photophysical properties.

Characterization of exciplexes generated by excited tris-diimine—ruthenium(II) complexes and silver(I) ion of ground state

Luminescent bimolecular and termolecular exciplexes of *[Ru(bpy) 3] 2− and *[Ru(phen) 3] 2+ (bpy = 2,2'-bipyridine, phen = 1,10-phenanthroline) formed by silver ions have been characterized and compared. Formation constants, luminescence lifetime, radiative and non-radiative decay constants were determined at room temperature. Oxidative quenching measurements in the [Ru(bpy) 3] 2−—Ag + system using MV 2+ (MV 2+ = 1,1'-dimethyl-4,4'-bipyridinium) as an electron acceptor resulted in quenching rate constants, (3.1 ± 0.2) × 10 9, (3.1 ± 0.3) × 10 9, (1.6 ± 0.3) × 10 9 and cage escape efficiencies, 0.11 ± 0.01, 0.05 ± 0.01, 0.06 ± 0.01, for *[Ru(bpy) 3] 2+, *[Ru(bpy) 3]Ag 3+ and *[Ru(bpy) 3]Ag 2 4+, respectively. Temperature-dependent luminescence lifetime measurements in aqueous solutions revealed that the exciplex formation causes a considerable change in the deactivation pathways of the excited species. Exciplex formation results in a decrease in the efficiency of the thermally activated decay through MLCT å dd transition and in an increase of the efficiency of deactivation to the ground state via other non radiative processes. The thermodynamic parameters of the exciplex formation have been estimated.

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 S 0 and S 1 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 S 1 state in comparison to that in the S 0 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 S 0 and S 1 states. Results so obtained are compared and discussed.

Subpicosecond population decay time of the first excited singlet state of thioxanthione in fluid solution

The population decay time of the first excited singlet state of an aromatic thione, thioxanthione, has been measured for the first time in fluid solution at room temperature, using a laser pump-probe method. The average value of τ(S1) is 0.53±0.12 ps in three solvents, acetonitrile, acetone and 1-octanol giving a nonradiative S1 decay constant of (1.9 ± 0.4) × 1012 s−1. Using these data and a calculated value of the radiative lifetime of S1, a quantum yield of S1 → S0 fluorescence, φf < 5 × 10−8, is estimated, a value which is consistent with the failure to observe prompt fluorescence in these compounds. The short S1 lifetime of thioxanthione is attributed to fast S1(1A2)−Tn(3A1) intersystem crossing, which can occur by a first order spin-orbit coupling mechanism because Tn(3A1) (n = 1 or 2) has a lower energy than S1(1A2) in the polar solvents employed.