Absorption and fluorescence of 9,10-diiodoanthracene in undegassed solutions

A procedure for producing 9,10-diiodoanthracene that is an alternative to the known procedure is described. The spectra of absorption and fluorescence in hexane, carbon tetrachloride, and polymethylmethacrylate are measured at 22°C. The quantum yield of undegassed liquid solutions in hexane and carbon tetrachloride in excitation at the maximum of the 0←0 band is practically the same and is equal to 0.018. In excitation in shorter-wave vibrational bands, it is different for different solvents and decreases noticeably as the frequency of the exciting light increases. The decrease in the yields is explained by the existence of a fast nonradiative process from hot (Franck-Condon) vibrational S₁-states capable of competing with vibrational relaxation. A possible scenario of this approach is discussed. Institute of Molecular Physics, National Academy of Sciences of Belarus, 70, F. Skorina Ave., Minsk, 220072, Belarus. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 66, No. 2, pp. 197–200, March–April, 1999.     

Dispersion interactions,electronic absorption spectra of anthracenes in polar glassy media at 77–300 K,and the change in polarizability upon excitation

The effect of dispersion interactions on the electronic spectra of organic compounds is analyzed within second-order perturbation theory. A formula is obtained which makes is possible to determine the change in the polarizability of the molecules upon excitation, Δα_eg, using data on the effect of the bulk polarizability P_n of the solvent on the position of the electronic spectrum. It is shown that when the solutions are cooled, the long-wavelength shift of the absorption spectra for the studied anthracenes in alcohols is mainly due to dispersion interactions and is caused by an increase in P_n. The possibility of using the new formula for Δα_eg is demonstrated for anthracenes as an example. It is found that for anthracene, 1-chloroanthracene, 9,10-dichloroanthracene, 9,10-dibromoanthracene, 9,10-diiodoanthracene Δα_eg is 16.5, 16.9, 17.2, 18.6, and 20.2 ų respectively. __________ Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 74, No. 2, pp. 162–168, March–April, 2007.     

Luminescence of biologically active 24-epicastasterone and a model compound

Biologically active brassinosteroid 24-epicastasterone, ring B of which contains a C=O group and has the nπ*-configuration for a low-lying electronic excited state, exhibits rapid fluorescence. The wavelengths of the fluorescence maxima of the steroid dissolved in hexane and acetonitrile are equal to 332 and 394 nm, respectively. The fluorescence lifetime of the steroid dissolved in acetonitrile is τ = 9.9 nsec. Solutions of 24-epibrassinolide do not luminesce. The long-wavelength electronic absorption band λ_max^abs = 340 nm in the absorption spectrum of an ethanol solution of model compound 2, ring D of which contains a C=O group π*-conjugated with the C=C double bond of ring C, like in the spectrum of the steroid, has a low extinction coefficient. An ethanol solution of 2 does not fluoresce. 24-Epicastasterone at 77 K in ethanol solution exhibits phosphorescence with λ_max^phos = 447 nm. The phosphorescence decay is exponential with τ = 0.79 msec. Compound 2 also phosphoresces. The phosphorescence spectrum of its ethanol solution has a maximum at 490 nm. The phosphorescence decay is nonexponential in the early stage. The phosphorescence lifetime is 25 msec in the exponential decay region. __________ Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 75, No. 2, pp. 182–186, March–April, 2008.     

Fluorescence spectroscopy of potential electroluminescent materials: Substituent effects on DSB and segmented PPV derivatives

The absorption and fluorescence of substituted distyrylbenzene (DSB) derivatives and segmented poly(phenylene vinylene) (PPV) derivatives are characterized by long-wavelength absorption maxima and absorption coefficients of λ_a = 380–450 nm, ε = 20,000–60,000 M⁻¹ cm¹ and fluorescence maxima, quantum yields, and decay times of λ_r = 440–530 nm, Φ_f = 0.2–0.9, and Τ = 0.8–2.5 ns, respectively. Alkoxy substituents at the central phenylene ring of DSB groups increase the bathochromic shift in the spectra in comparison to DSB, without a significant decrease in the high DSB fluorescence quantum yield. Both phenyl and cyano substitutions at the vinylene bridge lead to a further bathochromic shift of the fluorescence and a decrease in the quantum yield to ca. 0.4. The DSB derivatives and the related segmented PPV derivatives show nearly the same absorption spectra, fluorescence spectra, and radiative rate constantsk _f= Φ_f/Τ, indicating the efficacy of the segmentation of the polymer chain. The radiative rate constants determined by the Φ_f and Τ values and by the Strickler/Berg formula are in reasonable agreement. This supports the possibility of interpreting the properties of the polymers in terms of their DSB units. The decrease in the emission anisotropy can be ascribed to multistep energy transfer processes between different oriented segments.     

Luminescence of oxotetrahydrobenzo[c]phenanthridine in polar and nonpolar solvents: Inversion of the energy levels of the Sππ* and Tnπ* states

The spectral-luminescent properties and electronic-level structure of a derivative of oxotetrahydrobenzo[c]phenanthridine—3,3-dimethyl-1,2,3,4-tetrahydrobenzo[c]phenanthridine-1-one (DTP)—in polar and nonpolar solutions are studied by the luminescence and picosecond spectroscopy techniques at 293 and 77 K. It is shown that DTP exhibits ππp* fluorescence only in polar proton-containing methanol, ethanol, and n-propanol and that this fluorescence is associated with a chromophore + alcohol complex. Solutions of DTP in n-propanol and the nonpolar mixture methylcyclohexane + toluene (6:1) at 77 K display ππp* phosphorescence with a quantum yield Φ_Ph=0.1, a lifetime τ_Ph=1.9 s, and a frequency of the 0-0 transition equal to 21550 cm⁻¹. The fact that DTP fluorescence is absent in aprotic solvents and exists in alcohols is explained by the inversion of the energy levels of the S _ππ* and T _n π* states, as a result of which the energy level of the fluorescing S _ππ* state in alcohol becomes the lowest. The picosecond spectra and kinetics of the inducedS _n (ππ*) ← S ₁(π π*) and T _k (ππ*) ← T ₁(ππ*) absorption made it possible to estimate the lifetime of the S ₁(ππ*) state of DTP in n-propanol at 293 K (20±10 ps) and the position of the triplet-triplet absorption peak (495 nm). __________ Translated from Optika i Spektroskopiya, Vol. 94, No. 6, 2003, pp. 993–998. Original Russian Text Copyright ? 2003 by Bondarev, Knyukshto, Tikhomirov, Pyrko.     

Spectral characteristics and fluorescence quenching of N,N′-bis(4-pyridyl)-3,4:9,10-perylenebis(dicarboximide) (BPPD)

The photophysical properties such as electronic absorption, molar absorptivity, emission spectra, fluorescence quantum yield and fluorescence lifetime of N,N′-bis(4-pyridyl)-3,4:9,10-perylene bis(dicarboximide) (BPPD) have been measured in different solvents. Both electronic absorption and fluorescence spectra are not sensitive to medium polarity, while the fluorescence quantum yield (?_f) is solvent dependent. The ground state geometry has been computed by using density functional theory (DFT), the transition from HOMO to LUMO from perylene core with maximum absorption at 512 nm and HOMO–LUMO energy difference equal 2.53 eV. BPPD dye undergoes molecular aggregation to dimmer or higher aggregates in dimethyl sulfoxide (DMSO). Crystalline solids of BPPD gives excimer-like emission at 676 nm. The fluorescence quenching of BPPD is also studied using hydrated ferric oxide nanoparticle (FeOOH), and the Stern–Volmer rate constants (K_sv) were calculated as 8×10⁶ and 9.2×10⁶ M^?1 in ethanol and ethylene glycol, respectively.     

Spectral-luminescence properties of a norbornene-substituted tetraazachlorin and its metal complexes

The spectral-luminescence properties of a tetraazachlorin derivative with a norbornene fragment annelated to a reduced pyrrole ring and its complexes with zinc and palladium have been studied at 293 and 77 K. For the norbornene-substituted free base, differences in fluorescence from unsubstituted tetraazachlroin and its dibenzobarrelene-substituted analog are found. The fluorescence lifetime is observed to rise by ∼7 times for the free base and by ∼1.6 ties for the Zn complex on going from 293 to 77 K. An essential dependence of the photophysical parameters on the nature of the solvent is noted. The fluorescence polarization spectrum of the norbornene-substituted tetraazachlorin reveals in the Soret band region at least four electronic transitions. For the Pd complex, weak phosphorescence in the near IR region has been detected; the 0–0 band maximum is at 990 nm and the singlet–triplet interval amounts to 5800 cm^–1, which is larger by 400 cm^–1 than for Pd tetraazaporphine. The quantum yields of the photosensitized formation of singlet oxygen have been determined using a relative luminescence method.     

Quenching of fluorescence for fluoro derivatives of the laser dye DCM in polar solutions

We used fluorescence spectroscopy and nanosecond flash photolysis to study the photophysical properties of the laser dye 4-dicyanomethylene-2-methyl-6-(p-dimethylamino)styryl-4H-pyran (DCM) and its two fluoro derivatives: (E)-2-〈2-[2-(4-dimethylaminophenyl)ethenyl]-6-trifluoromethyl-4H-4-pyranylidene〉malononitrile (DCMF3) and (E)-2-〈2-[2-(4-dimethylaminophenyl)ethenyl]-6-n-heptafluoro-propyl-4H-4-pyranylidene〉 malononitrile (DCMF7), in nonpolar n-hexane, mixtures of low-polarity toluene and polar dimethylsulfoxide (DMSO), n-propanol at room temperature and at 77 K. The fluorescence quantum yield Φ_fl of the laser dye DCM increases linearly as the polarity of the binary solvent mixture (toluene+DMSO) increases, from 0.08 in toluene to 0.80 in DMSO. The dependence of Φ_fl on the polarity of the mixture toluene+DMSO for DCMF3 and DCMF7 reaches a maximum for a small amount (∼2 vol.%) of added polar DMSO; and with further increase in the DMSO concentration (≥50 vol.%), the fluorescence of the fluoro derivatives of DCM is practically completely quenched. The quantum yield for intersystem crossing Φ_ST for DCM, DCMF3, and DCMF7 is no greater than 0.01 in solutions of different polarities. We discuss the mechanisms for nonradiative deactivation of the electronic excitation energy for the fluoro derivatives compared with DCM. __________ Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 73, No. 5, pp. 606–612, September–October, 2006.     

The Influence of Temperature on Coumarin 153 Fluorescence Kinetics

The influence of temperature varied in the range 183 K–323 K on the fluorescence quantum yield, fluorescence lifetime, absorption and emission transition moments and non-radiative deactivation rate was determined for the well known and largely used dye Coumarin 153, dissolved in 1-chloropropane. The Kennard-Stepanov relation connecting the absorption and emission spectra was used to check for the presence of more than one absorbing/emitting species and to investigate whether intramolecular vibrational redistribution completes in the C153 excited S ₁ state before the emission takes place. The emission spectrum corresponding to S ₁→S ₀ transition, was fitted at each temperature to the model function including the information on the dye vibrational modes coupling. In this way the displacement in equilibrium distance for the most active vibrational mode was determined for C153 in S ₁ and in S ₀. Using the temperature dependence of the fluorescence decay time and quantum yield, the non-radiative deactivation rate was determined. Its temperature dependence was compared to that calculated using the theoretical model with the most active vibrational mode displacement values taken from steady-state spectra analysis. The somewhat surprising dependence of the fluorescence decay time and quantum yield on temperature was related to non-trivial coupling between low-frequency vibrational modes of C153 in the excited and ground states.     

Extra-liganding effects in Zn-octaethylporphyrin solutions in a temperature interval of 300-77 K

It is established experimentally for the first time that Zn-octaethylporphyrin in a solution of dried methyl cyclohexane is in unliganded form in the temperature interval of 293-77 K. This form is characterized by the shortest-wave position of absorption and luminescence spectra, the independence of the degrees of fluorescence and phosphorescence polarizations from the excitation wavelength, and high phosphorescence time (τ_T=126 μsec) and quantum yield (φp=0.085) at 77 K. Adding electrodonor components to the methyl cyclohexane or going over to alcohol-containing polar media leads to the formation of a monoliganded form of ZnOEP, for which we observe a bathochromic shift of ≈150–350 cm⁻¹ for Q(0,0) absorption and luminescence bands, a splitting of ≈50–100 cm⁻² for the Q(0, 0) absorption band, and phosphorescence quenching (τ_T=60 μsec, φp=0.05-0.03 at 77 K). The effects revealed are explained by a decrease in the symmetry of a ZnOEP molecule in extra liganding and by an increase in the probability of nonradiative activation for a T₁-state due to the extra ligand being involved in the exchange of electron-excitation energy and to the enhancement of a spin-orbital coupling as a result of the disturbance of the planarity, of a macrocycle. Institute of Molecular and Atomic Physics, National Academy of Sciences of Belarus, 70, F. Skorina Ave., Minsk, 220072, Belarus. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 65, No. 6, pp. 900–907, November–December, 1998.     

Triplet-Triplet fluorescence of 9,10-dibromanthracene in hexane

The temperature dependence of the triplet-triplet fluorescence (TTF) spectrum of 9,10-dibromanthracene in hexane is studied in the near infrared region between 800 and 1050 nm in the temperature range from 200 to 320 K. The TTF spectrum exhibits two distinct bands at 842 and 949 nm at 293 K. Upon cooling from 293 to 200 K, the intensity of the 842-nm band decreases and that of the 949-nm band increases. The different behavior of the band intensities is explained by the overlap of the emission at 950 nm, which can be related to a photoproduct. The 842-nm band is assigned to the 0-0 T ₂(³ B _1g →T ₁(³ B _2u transition. The quantum yield of TTF at 293 K measured within the 842-nm band is 8.3×10^?7 (±20%)), and for emission in the region from 800 to 1050 nm it is equal to 1.4×10^?6 (after subtraction of the contribution from the S ₁→S ₀ singlet-singlet fluorescence). The effect of the intersystem crossing and internal conversion on the temperature dependence of TTF is discussed.     

On the decay of KCl:Cu+ fluorescence

Summary The fluorescence of Cu⁺ in KCl was investigated in the range 19–300 K, in particular by performing accurate measurements of lifetime and quantum yield. The data were found irrespective of the doping method. It was evidenced that, among the models proposed in the literature, the Pedrini model gives the most reasonable explanation for the Cu⁺ de-excitation mechanism: two excited states in thermal equilibrium are involved in the emission process, one of which is metastable and lying just below the emitting level.     

A Molecular Thermometer Based on the Delayed Fluorescence of C70 Dispersed in a Polystyrene Film

A new optical molecular thermometer, based on the thermally activated delayed fluorescence of C₇₀ dispersed in a polystyrene film, was developed. In the presence of oxygen, the fluorescence intensity of the C₇₀ film is essentially temperature independent in a wide range. In the absence of oxygen, however, the fluorescence intensity markedly increases with temperature. At room temperature (25°C), and after degassing the sample, the fluorescence intensity of C₇₀ increases 22 times, while at 100°C the fluorescence intensity is increased by 70 times. With our system, the very weak fluorescence of C₇₀ (Φ_F ≅ 5 × 10⁻⁴, in toluene) can be increased up to 91 times (up to an estimated maximum value Φ_F = 0.046). The estimate value of the singlet-triplet gap (29 kJ mol⁻¹) and the fluorescence lifetime (0.63 ns) of the C₇₀ in film are in agreement with the values reported in the literature for C₇₀ in solution. The values of the phosphorescence lifetime at room temperature (23 ms) and the quantum yield of triplet formation (0.989) were also determined. The system is completely reversible with respect to heating-cooling cycles.     

Spectral-luminescent properties of condensation products of 8-aminoquinoline,aromatic aldehydes,and dimedone

Hydrobenzophenanthrolinone derivatives were synthesized by three-component condensation of 8-aminoquinoline, aromatic aldehydes, and dimedone. The structures of the obtained substances were confirmed by NMR and IR spectroscopy and mass spectrometry. Spectral-luminescent investigations of the synthesized compounds showed that they were characterized by high oscillator strengths for allowed electronic transitions S _n ← S ₀ (n = 1–3). Low fluorescence quantum yields in EtOH (Φ_fl ~ 10^–4–10^–3) and an increase of the Φfl values in toluene (~10^–2) at room temperature and with lowering the temperature to 77 K (Φ_fl ~ 10^–1) for a number of the compounds under study were satisfactorily explained within the framework of Marcus theory.     

Effect of the polymethine chain length, the polarity and temperature of the medium on the spectroscopic properties of merocyanine dyes

We have studied the luminescent spectral properties and calculated the quadratic polarizability of dimethine, tetramethine, and hexamethine merocyanines based on 1,3-dihydro-1,3,3-trimethyl-(2H)-indol-2-ylidene and malonodinitrile in solutions and polymer films at room temperature and 77 K. We have shown that increasing the polarity of the solvent and lengthening the polymethine chain in the studied merocyanines lead to an increase in fluorescence quantum yields and narrowing of the fluorescence bands. This is explained by the decrease in bond length alternation and weakening of vibronic interactions in the singlet-excited fluorescent state. The mirror symmetry in the structured absorption and fluorescence spectra of merocyanine dyes in ethanol at 77 K, observed here for the first time, is due to enhancement of electrostatic interactions of the merocyanine and solvent molecules. Increased stiffness of the medium considerably decreases the probability of nonradiative processes, and the major channel for deactivation of the electronic excitation energy at 77 K is fluorescence. The higher values of the quadratic polarizability (6.0·10⁻²⁸ esu in ethanol and 3.3·10⁻²⁸ esu in chloroform) for hexamethine merocyanine, estimated by the solvatochromic method, lets us consider this compound as a promising material for designing nonlinear optical converters for harmonics of laser radiation. __________ Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 73, No. 1, pp. 26–35, January–February, 2006.     

Determination of fluorescence and non-radiative de-excitation rates of excited 3-pentanone at low pressures

3-Pentanone photophysics measurements and subsequent fluorescence quantum yield (FQY) model development are presented. A heated, flowing optical cell and laser excitation at 248, 266, 277, and 308 nm were utilized, allowing investigation of FQY and absorption cross-section values for 3-pentanone vapor from 298 to 690 K and 10 to 30 mbar. Measurements of FQY were also made for 20 mbar of 3-pentanone at 1.3 bar total pressure in nitrogen from 298 to 530 K and in air from 298 to 487 K. Absolute FQY was determined by calibration to Rayleigh scattering of nitrogen gas. Based on these FQY measurements and fluorescence lifetime data from previous work, the fluorescence rate k _f was determined to be 3.70×10⁵ s⁻¹. The current work extends knowledge of the non-radiative rate k _nr to vibrational energies of 15000 cm⁻¹, and the expression for k _nr was optimized to include a fit to these new data points. Finally, variation of FQY with 3-pentanone vapor pressure was used to optimize the vibrational relaxation cascade parameter α _3p . The updated FQY model for 3-pentanone vapor shows agreement within 8% to the current FQY data across the investigated range of temperatures and pressures.     

Mechanism of singlet-oxygen induced delayed fluorescence of bacteriopheophytin in laser excitation

The relative intensity of photosensitized phosphorescence of singlet oxygen (¹O₂) at 1270 nm (L₁₂₇₀) and¹O₂-induced delayed fluorescence (L_df) of bacteriopheophytin a (BPh) (770 nm) in air-saturated solutions of BPh in hexafluorobenzene in excitation by 337-nm pulses of a nitrogen laso is investigated. It is established that L_df≪L₁₂₇₀. The ratio of the initial intensity of delayed fluorescence and phosphorescence of¹O₂(L_df)₀/(L₁₂₇₀)₀ changed from 0.02 to 0.30 as a function of the energies of laser pulses (2.5–5.0 mJ/cm¹) and the BPh concentration (6–18 μM). As the index of quantum efficiency of the delayed fluorescence, the authors used the coefficient

Organic Vapour Fluorescence Following Excitation into High Electronic Triplet States

Abstract

The fluorescence of anthracene vapors and its derivatives initiated by triplet-triplet excitation is observed. The quantum yield of inverce intersystem crossing of anthracene molecules is estimated (10^?2). The fluorescence initiated by triplet excitation is quenched by foreign pentane gas, thus permitting an estimation of the lifetime of highly excited triplet molecules.     

Fluorescence Properties of Phenol-Modified Zinc Phthalocyanine that Tuned by Photoinduced Intra-Molecular Electron Transfer and pH Values

Tetra[α-(4-hydroxyphenoxy)] zinc phthalocyanine, ZnPc(α-OPhOH)₄, was synthesized and its photophysics was found to be sharply pH dependent. Dual fluorescence emission around 700 nm was observed when it is dissolved in basic solution. The fluorescence of the phthalocyanine can be sharply switched off at pH 9.1 due to the intramolecular photoinduced electron transfer (PET) in ZnPc(α-OPhONa)₄, formed by the deprotonation of ZnPc(α-OPhOH)₄. The photophysics of both ZnPc(α-OPhOH)₄ and ZnPc(α-OPhONa)₄ were studied in detail by UV-vis absorption, steady state and time-resolved fluorescence and transient absorption (TA) to reveal the fluorescence quenching mechanism. Intra-molecular PET in ZnPc(α-OPhONa)₄ from the donor, PhONa subunits, to the acceptor, ZnPc moiety, was characterized by the much smaller fluorescence quantum yield (0.003) and lifetime (<0.20 ns). PET was further evidenced by the occurrence of charge separation state (CSS) in TA spectra, i.e. the bands due to anion radical of ZnPc and phenol radical. The lifetime of the charge separation state is ca. 3 ns, the efficiency of PET is ca. 99% and the rate constant of PET is 2.3 × 10¹⁰ s⁻¹.     

Creation of excitons and defects in a CsI crystal during pulsed electron irradiation

Data presented on the influence of the temperature in the range 80–650 K on the spectral kinetics of the luminescence and transient absorption of unactivated CsI crystals under irradiation by pulsed electron beams (〈E〉=0.25 MeV, t _1/2=15 ns, j=20 A/cm²). The structure of the short-wavelength part of the transient absorption spectra at T=80–350 K exhibits features, suggesting that the nuclear subsystem of self-trapped excitons (STE’s) transforms repeatedly during their lifetime until their radiative annihilation at T⩾80 K, alternately occupying di-and trihalide ionic configurations. It is established that a temperature-induced increase in the yield of radiation defects, as well as F and H color centers, and quenching of the UV luminescence in CsI occur in the same temperature region (above 350 K) and are characterized by identical thermal activation energies (∼0.22 eV). It is postulated that the STE’s in a CsI crystal can have a trihalide ionic core with either an on-center or off-center configuration; the high-temperature luminescence of CsI crystals is associated with the radiative annihilation of an off-center STE with the structure (I⁻(I⁰I⁻ e ⁻))*. Fiz. Tverd. Tela (St. Petersburg) 40, 640–644 (April 1998)