Phosphorescence and its characteristics in pyridine vapor

The phosphorescence emission of pyridine-d₀ and -d₅ has been observed in the vapor phase by means of time-resolved spectroscopy. The results of the experiments, which are described in full detail in this paper, on the phosphorescence spectrum, the excitation spectrum, the phosphorescence decay and sensitization of biacetyl phosphorescence indicate that the emission concerned is the genuine phosphorescence of the pyridines. For pyridine-d₀ (-d₅), the wavelength of the phosphorescence maximum is 450 nm (440 nm), the phosphorescence quantum yield 1.5 × 10^?6) (1.7 × 10^?6) and the phosphorescence lifetime is 1.2 μs (2.1 μs), the values for pyridine-d₅ being given in parentheses. The phosphorescence characteristics of pyridine are compared with those of other related molecules. The nature of the phosphorescent triplet state of pyridine is discussed with particular regard to the exceptionally fast non-radiative decay from that state.     

Phosphorescence from pyrimidine vapor

The phosphorescence from pyrimidine vapor has been observed by a method of time-resolved laser spectroscopy. The phosphorescence spectrum commences at 350.5 nm and consists mainly of three totally symmetric vibrations in the ground state, v_6a, v₁₂ and v_9a as in the case of the zero-point vibrational level fluorescence of pyrimidine vapor. The phosphorescence quantum yield and lifetime are found to be about 1 × 10^?4 and 50 μs.     

Vibronic structure of the gas phase phosphorescence spectrum of p-benzoquinone

The phosphorescence spectrum of p-benzoquinone has been studied in the gas phase. The vibronic structure is analyzed in terms of frequencies of vibrations strong in the Raman spectrum. The most predominant progression vibrations are found to be two fundamentals of ν₃(CO) and ν₂(CC stretching) around 1680 cm⁻¹ in the Raman spectrum. These modes have not been observed separably in the phosphorescence spectrum of gaseous p-benzoquinone because of low resolution spectrographs employed in early work. Several vibrational frequencies in the excited ³A_u(nπ*) state are proposed on the basis of the analysis of several prominent sequence bands in the emission.     

Single crystal ODMR and phosphorescence studies of the 3nπ* state of aliphatic carbonyl: 2-indanone

We have made single crystal ODMR and phosphorescence studies of the T₁ state of 2-indanone. It was shown that the T₁ state of 2-indanone is very similar to that of the ³nπ^* state cyclopentanone. The well resolved phosphorescence spectrum indicates that the ³nπ^* 2-indanone has a distorted structure. A proposed decay scheme was confirmed from a single crystal ODMR study at low magnetic field.     

Multiphoton tea CO2 laser excitation of the triplet state of propynal

Multiple infrared photon excitation of propynal triplet molecules gives rise to a strongly perturbed phosphorescence. Following absorption of a few IR photons per molecule the phosphorescence spectrum extends to higher energy, the intensity increases, the decay — deviating from the original exponential decay — accelerates and the emission quantum yield drops dramatically. These findings are explained in terms of temperature sensitive radiative (T₁ → S₀) and radiationless (T₁ → S₀) processes with the vibrational temperature as the determining factor. During the perturbed triplet decay, the IR excitation initially confined to the vibrational degrees of freedom becomes distributed among all degrees of freedom which results in a decrease in the vibrational temperature and thus a complex phosphorescence decay.     

Triplet—triplet energy transfer from benzophenone to tris(2,2′-bipyridine)ruthenium(II) chloride in acetic acid: relevance of trivial absorption effects

Energy transfer from benzophenone to Ru(bipy)²⁺₃ has been studied in acetic acid by phosphorescence quenching of the donor and phosphorescence sensitization of the acceptor. A general method is described for the correction of trivial effects on the Stern—Volmer plots when both absorption and emission spectra of the donor overlap the absorption spectrum of the acceptor. The combination of intensity and lifetime data indicates that the quenching is not simply diffusional.     

Theoretical DFT study of phosphorescence from porphyrins

Geometrical structure of free-base porphin (H₂P) and Mg- and Zn-porphyrins together with their vibrational frequencies and vibronic intensities in phosphorescence are investigated by density functions theory (DFT) with the standard B3LYP functional. These molecules have a closed-shell singlet ground state (S₀) and low-lying triplet (T₁) excited states of ππ* type. The S₀–T₁ transition probability and radiative lifetime of phosphorescence (τ_p) of these molecules are calculated by time-dependent DFT utilizing quadratic response functions for account of spin–orbit coupling (SOC) and electric-dipole transition moments including displacements along active vibrational modes. The infrared and Raman spectra in the ground singlet and first excited triplet states are also studied for proper assignment of vibronic patterns. The long radiative lifetime of free-base porphin phosphorescence (τ_p ∼ 360 s at low temperature limit, 4.2 K) gets considerably shorter for the metalloporphyrins. An order of magnitude reduction of τ_p is predicted for Mg-porphyrin but no change of phosphorescence polarization is found. A forty times enhancement of the radiative phosphorescence rate constant is obtained for Zn-porphyrin in comparison with the H₂P molecule which is accompanied by a strong change of polarization and spin-sublevel radiative activity. A strong vibronic activity of free-base porphin phosphorescence is found for the b_2g mode at 430 cm⁻¹, while the 679 and 715 cm⁻¹ vibronic bands of b_3g symmetry are less active. These and other out-of-plane vibrations produce considerable changes in the radiative constants of different spin sublevels of the triplet state; they also promote the S₁ → T₁ intersystem crossing. Among the in-plane vibrations the a_g mode at 1614 cm⁻¹ is found very active; it produces a long progression in the phosphorescence spectrum. The time-dependent DFT calculations explain the effects of the transition metal atom on phosphorescence of porphyrins and reproduce differences in their phosphorescence and EPR spectra.     

The two-photon phosphorescence excitation spectrum of triphenylene

The two-proton excited phosphorescence of triphenylene in PMMA matrix at 77 K was measured using a tunable flashlamp-pumped rhodamine dye laser in the effect spectral region 32000–36000 cm^?1. The band origin of the S₀S₂ transition, which is uncertain in the one-photon absorption spectrum, was observed at 33200 cm^?1. The vibronic features in the one-photon spectrum were reinterpreted.     

Phosphorescence and energy assignment of acetaldehyde-d4(3A″) in the vapor phase

The phosphorescence spectrum of CD₃CDO(³A″) has been obtained by collisional sensitization with triplet state sensitizers. The energy of CD₃CDO(³A″) has been assigned at 27 400 ± 200 cm^?1.     

Conformation of biphenyl in a cyclohexane host at 10 K

The site-selected fluorescence, phosphorescence, and luminescence excitation spectra of biphenyl in a cyclohexane Shpol'skii matrix have been measured. There are three principal sites. The relative population of the sites depends on the freezing protocol. The S₁ ↔ S₀ 0&.sbnd;0 band, a forbidden gg transition in D_2h planar biphenyl, is weakly observed for one of the sites. For this site the zero-phonon lines in fluorescence and phosphorescence are accompanied by prominent sidebands but such sidebands are nearly completely absent in the excitation spectrum. The other two sites have negligibly weak 00 bands in fluorescence and minimal sidebands. We attribute the sidebands to translational and/or librational motion of biphenyl in the cyclohexane cage.     

Photoexcited States of UV Absorbers,Benzophenone Derivatives

The UV absorption, phosphorescence and phosphorescence‐excitation spectra of benzophenone (BP) derivatives used as organic UV absorbers have been observed in rigid solutions at 77 K. The triplet–triplet absorption spectra have been observed in acetonitrile at room temperature. The BP derivatives studied are 2,2′,4,4′‐tetrahydroxybenzophenone (BP‐2), 2‐hydroxy‐4‐methoxybenzophenone (BP‐3), 2,2′‐dihydroxy‐4,4′‐dimethoxybenzophenone (BP‐6), 5‐chloro‐2‐hydroxybenzophenone (BP‐7) and 2‐hydroxy‐4‐n‐octyloxybenzophenone (BP‐12). The energy levels and lifetimes of the lowest excited triplet (T₁) states of these BP derivatives were determined from the first peak of phosphorescence. The time‐resolved near‐IR emission spectrum of singlet oxygen generated by photosensitization with BP‐7 was observed in acetonitrile at room temperature. BP‐2, BP‐3, BP‐6 and BP‐12 show photoinduced phosphorescence enhancement in ethanol at 77 K. The possible mechanism of the observed phosphorescence enhancement is discussed. The T₁ states of 2‐hydroxy‐5‐methylbenzophenone, 4‐methoxybenzophenone and 2,4′‐dimethoxybenzophenone have been studied for comparison.     

Simulated T ← S spectrum of 2,4,5-trimethylbenzaldehyde in durene

The T_1,2 ← S₀ phosphorescence excitation spectrum of 2,4,5-trimethylbenzaldehyde in durene has been simulated using forty-five zero-order Born-Oppenheimer product states of which thirty-two belong to T₁ (ππ*), the others to T₂ (nπ*). The spectrum is very complicated in the region 400–600 cm^?1 above the T₁ (ππ*) ←3 S₀ origin band at 24150 cm^?1. In this tangled region conventional vibrational analysis is not useful. Several comments on the physical properties of the excited triplet states of 2,4,5-trimethylbenzaldehyde are given.     

Time-resolved fluorescence spectra using time-correlated photon counting: Picosecond fluorescence in aqueous solutions of Cr(III) complexes at room temperature

A new procedure for measuring time-resolved emission spectra has been implemented. This technique has subnanosecond time resolution combined with the sensitivity and dynamic range needed to cope with extremely weak luminescence. Using this method the emissions of Cr(NH₃)₂ (NCS)₄^? and Cr(NCS)₆^3- in aqueous solution at room temperature have each been analyzed into two components. The fast component has a broad spectrum and is assigned to prompt fluorescence with lifetime below 100 ps. The slow component is dominated by phosphorescence but may include some delayed fluorescence. The phosphorescence lifetime is 5.5 ± 0.5 ns in Cr(NH₃)₂ (NCS)₄^? and 1.65 ± 0.1 ns in Cr(NCS)₆^3-. Order of magnitude estimates have been derived for other photophysical parameters.     

Preparation and luminescence properties of green-light-emitting afterglow phosphor Ca8Mg(SiO4)4Cl2:Eu2+

Green-light-emitting long-lasting phosphorescence phosphor, Eu²⁺ activated calcium magnesium chlorosilicate Ca₈Mg(SiO₄)₄Cl₂, has been prepared by a modified solid-state reaction method using Ca₂SiO₄:Eu²⁺ as a precursor. Its properties have been discussed and analyzed utilizing XRD, photoluminescence, excited-state decay curve and long-lasting phosphorescence decay curve. Upon UV light excitation, the emission spectrum of Ca₈Mg(SiO₄)₄Cl₂:Eu²⁺ phosphor is composed of two separate bands centered at 425 nm and 505 nm, respectively. Furthermore, after irradiation by a 320-nm UV light for 3 min, the 2% Eu²⁺-doped Ca₈Mg(SiO₄)₄Cl₂ phosphor emits intense green-light-emitting afterglow from the 4f⁶5d¹→4f⁷ transition of Eu²⁺, and its afterglow can be seen with the naked eye in the dark clearly for more than 3 h after removal of the excitation source. The disappearance of the high-energy 425 nm band in the afterglow emission spectrum is explained by its different crystallographic sites. The afterglow decay curve of the Eu²⁺-doped Ca₈Mg(SiO₄)₄Cl₂ phosphor contains a fast decay component and another slow decay one. The possible mechanism of this long-lasting phosphorescence phosphor is also discussed based on the experimental results.     

The reinterpretation of the zero-field double resonance spectra of the lowest excited 3T1u state of the F center in CaO in terms of random internal strain of Eg symmetry

The zero-field double resonance spectra of the relaxed excited ³T_1u state of the F center in CaO were determined and interpreted previously. In the present paper the phosphorescence microwave double resonance (PDMR) spectra are reinterpreted in terms of random internal strain of E_g symmetry. The strain variables of the subensemble of F centers responding to a particular microwave-induced transition are exclusively determined by the microwave and the phosphorescence frequency. Consequently, the level structure in this subensemble can be calculated. To each PMDR spectrum, two graphical representations describing the possible spread in the phosphorescence frequency and the vibronic level structures of the F centers contributing to the PMDR signal are related. From these graphical representations, the occurring radiationless transitions can be traced. It is shown that, at the low energy side, the bounds of the PMDR lines are determined by the random strain distribution whereas the upper bounds are determined by fast radiationless transitions or by the lack of a population difference between the spin levels involved in the microwave-induced transition.     

Observation of the phosphorescence spectra from the spin sublevels of low emissivity: quinoxaline

A method to observe the phosphorescence emissions from the spin sublevels of low emissivity is proposed, and its application to quinoxaline is attempted. The newly observed phosphorescence spectra from the T_x and T_y sublevels differ considerably in vibrational structures from the usually observed phosphorescence from the T_z sublevel.     

Synthesis of main chain polymeric benzophenone photoinitiator via thiol‐ene click chemistry and Its use in free radical polymerization

Main chain polymeric benzophenone photoinitiator (PBP) was synthesized by using “Thiol‐ene Click Chemistry” and characterized with ¹H NMR, FTIR, UV, and phosphorescence spectroscopies. PBP as a polymeric photoinitiator presented excellent absorption properties (ε₂₉₄ = 28,300 mol^?1L^?1cm^?1) compared to the molecular initiator BP (ε₂₅₂ = 16,600 mol^?1L^?1cm^?1). The triplet energy of PBP was obtained from the phosphorescence measurement in 2‐methyl tetrahydrofurane at 77 K as 298.3 kJ/mol and according to phosphorescence lifetime, the lowest triplet state of PBP has an n‐π* nature. Triplet–triplet absorption spectrum of PBP at 550 nm following laser excitation (355 nm) were recorded and triplet lifetime of PBP was found as 250 ns. The photoinitiation efficiency of PBP was determined for the polymerization of Hexanedioldiacrylate (HDDA) with PBP and BP in the presence of a coinitiator namely, N‐methyldiethanolamine (MDEA) by Photo‐DSC. The initiation efficiency of PBP for polymerization of HDDA is much higher than for the formulation consisting of BP. © 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2010     

MULTI-SITE PHOSPHORESCENCE OF α, β-ENONES:LEVEL INVERSION IN 6β, 19-EPOXYCHOLEST-4-EN-3-ONE BY SOLVATION AND AGGREGATION

Millisecond time-resolved emission spectroscopy was used to probe the phosphorescence kinetics of the α-β-enone 6β, 19-epoxycholest-4-en-3-one (1) as a function of concentration in several paraffinic and hydroxylic glasses at 77 K. Only in methylcyclohexane/methylcyclopentane glass at low concentration (10^?4M) does the phosphorescence decay exponentially. It is interpreted as emission from the ³n* state. Upon increasing the concentration a second emission grows which is characterized by a longer lifetime, a decreased fine structure and a hypsochromically shifted S₀→¹nπ* excitation spectrum. This phosphorescence is ascribed to ³ππ* emission of aggregates of 1. In hydroxylic glasses the phosphorescence decay is multiexponential, even at 10^?4M concentration; from emission band shapes and lifetimes it follows that both ³nπ* and ³ππ* type emissions are present, the latter increasing with the alcohol concentration in the solvent. The two types of phosphorescence have different excitation spectra: that of the structureless and long-lived ³ππ* emission is shifted to the blue in the S₀→¹nπ* region and to the red in the S⁰→¹ππ* region. This emission is ascribed to complexes of 1 with the alcoholic solvent. The results of time-resolved measurements of the circular polarization of the luminescence are consistent with the assignments given above and indicate that in the H-bonded and possibly also in the free species ³ππ* and ³nπ* states are intermixed to a considerable extent.     

External spin—orbit coupling. The T1 state of naphthalene in a dibromobenzene host cyrstal

The origin of the T₁ state of naphthalene in a p-dibromobenzene host crystal is shown to be the strongest line in the spectrum at 20720.9 cm ⁻¹. The vibronic activity of the naphthalene phosphorescence indicates that a spin—orbit coupling mechanism different from the one vibronically induced in an isotopic mixed naphthalene crystal is active in a chemically mixed crystal with a halogen-containing host.