Phosphorescence of Aromatic Compounds

In contrast to polyenes the aromatic molecules exhibit not only the Ti — So absorption, but also the longlived T — So emission, which gives rise to phosphorescence phenomena of rigid solvents and crystals. This is another important field of applications of spin-orbit quadratic response theory. Such calculations refer to lifetimes, transition moments, oscillator strengths and polarization directions for the radiative decay of molecular triplet states. These quantities may either be averaged over the triplet levels or refer to specific triplet spin sublevels depending on the conditions for the relevant experimental measurements. 

Vibrational analysis of the benzene phosphorescence bands indicates that the radiative activity is induced predominantly by e2g vibrations [155, 156]. A weak but observable activity of b2g vibrations has also been found [156, 155, 157]. By introducing spin-orbit- and vibronic coupling through second order perturbation theory Albrecht [158] showed that the vibronic interaction within the triplet manifold is responsible for the larger part of the phosphorescence intensity. This also follows from comparison of the vibrational structure in phosphorescence and fluorescence spectra [159]. The benzene phosphorescence spectrum in rigid glasses [155] reveals a dominant vibronic activity of 

Main mechanisms of benzene phosphorescence. Triplet and Singlet manifolds of states. From ref. [83]. 

I-B Vibronic structure of benzene phosphorescence in the response formalism 

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Ellis DW (1979) Analysis of aromatic compounds in water using fluoresence and phosphorescence. PB report no. 212268. National Technical Information Service, Springfield, VA, USA... 

Substituents have considerable influence on emission characteristics of aromatic compounds. Heavy atom substituents tend to reduce the fluorescence quantum yield 4>f in favour of phosphorescence emission f. In halogen series the effect increases in the order F < Cl < Br < I. In Table 5.1 are recorded experimental data for halogen substituted Naphthalenes. 

Several years ago it was found that many aromatic carbonyl compounds are photoreduced by aliphatic and aromatic amines (Cohen et al., 1973 Davidson, 1975). It was suggested that these reactions proceed by electron transfer from the amine to the triplet carbonyl compound. Evidence in support of this mechanism was that the ability of amines to quench the phosphorescence of carbonyl compounds, and to react with them, correlated with the ionisation potential of the amines. On the basis of this evidence the reaction scheme in... 

Nevertheless, analytical problems can be found where such disadvantages can be outweighed by the convenient final resolution of the problem based on LTP data. On other occasions, both fluorescence and phosphorescence data can easily be obtained from a given sample, providing complementary information. One example is the use of phosphorescence measurements at low temperature to examine mixtures of aromatic compounds dissolved in -alkanes (Shpol skii spectrometry). The narrow-band fluorescence and phosphorescence spectra complement one another and have proved most useful in the analysis of complex samples such as multicomponent mixtures of polycyclic aromatic hydrocarbons (PAHs) in fuel materials. 

Bronislaw Znatowicz (1851-1917), chemist, professor at Warsaw University, co-founder and first editor-in-chief of the journal Chemik Polski, author and translator of chemical textbooks and monographs. He investigated the reduction and nitration of aromatic compounds and the production of dyes from tar, and studied the phenomenon of phosphorescence and electrolysis of organic compounds. 

It is well established that o grgen quenches both excited singlet and triplet states of aromatic compounds. Thus, indole derivatives in solution at room temperature display a ratio for the rate constant Icnk/I qp of approximately two (48, 49). However, the quenching of alkaline phosphatase phosphorescence by oxygen proceeds at a rate kqp = 1.2 x lO s (47). The magnitude of this rate... 

Electron transfer rates between adrenaline and related benzene diols and complexes of iron(III) with some substituted 1,10-phenanthrolines have been reported [67] in surfactant systems. In cationic systems the reactions take place in the aqueous phase and reaction rates are lower than they are in simple aqueous systems, but in anionic surfactant systems the reaction rates are enhanced, reactions probably taking place at the micellar interface. The rates of exit and entrance of aromatic compounds from and into micelles have recently been studied using phosphorescence decay measurements [68] exit rate constants of aromatic hydrocarbons are of the order of 10 to 10 s " S whereas values of 10 to 10 (moll ) s have been reported for intramicellar energy transfer processes. Release of aromatic phosphorescence probes from micelles followed by their deactivation in the aqueous phase is hence expected to be an important mode of deactivation of the triplet state [69]. Kinetic schemes for triplets that are partitioned between aqueous and micellar phases are considered for the cases of single occupancy and double occupancy of the micellar units. 

Stabilisers are usually determined by a time-consuming extraction from the polymer, followed by an IR or UV spectrophotometric measurement on the extract. Most stabilisers are complex aromatic compounds which exhibit intense UV absorption and therefore should show luminescence in many cases. The fluorescence emission spectra of Irgafos 168 and its phosphate degradation product, recorded in hexane at an excitation wavelength of 270 nm, are not spectrally distinct. However, the fluorescence quantum yield of the phosphate greatly exceeds that of the phosphite and this difference may enable quantitation of the phosphate concentration [150]. The application of emission spectroscopy to additive analysis was illustrated for Nonox Cl (/V./V -di-/i-naphthyl-p-phcnylene-diamine) [149] with fluorescence ex/em peaks at 392/490 nm and phosphorescence ex/em at 382/516 nm. Parker and Barnes [151] have reported the use of fluorescence for the determination of V-phenyl-l-naphthylamine and N-phenyl-2-naphthylamine in extracted vulcanised rubber. While pine tar and other additives in the rubber seriously interfered with the absorption spectrophotometric method this was not the case with the fluoromet-ric method. 

Spectroscopic data of nitroaromatics have been reviewed D in addition, several papers on luminescence of nitroaromatic compounds have appeared recently. The phosphorescence polarization of several aromatic nitro compounds has been studied and recent triplet-triplet absorption data on 1- and 2-nitro-naphthalene have become available ). 

Response theory describes the S-T transition probabilities in unsaturated hydrocarbons quite well more than 99 % of the So - Xi transition intensity is out-of-plane polarized in agreement with experiment for aromatics in ethylene, butadiene and naphthalene the y spin-sublevel of the T state is the most active one, where y is the long in-plane axis of the molecules [134,132]. The main difference between the triplet states of aromatic and aliphatic compounds is the lack of phosphorescence for the latter. We have related this to the fact that polyenes also lack fluorescence (or have very weak fluorescence). This have been explained from the effective quenching of singlet excited (tr r ) states, which is an inherent property for the short polyenes. Our results suggest that this situation also prevails for the lowest triplet states. 

Experimental Zero-Field Splittings of Some Nitrogen-Containing Aromatic Compounds in Their Phosphorescent Triplet State3... 

A review of chemiluminescent and bioluminescent methods in analytical chemistry has been given by Kricka and Thorpe. A two-phase flow cell for chemiluminescence and bioluminescencc has been designed by Mullin and Seitz. The chemiluminescence mechanisms of cyclic hydrazides, such as luminol, have been extensively analysed. " Fluorescence quantum yields of some phenyl and phenylethynyl aromatic compounds in peroxylate systems have been determined in benzene. Excited triplet states from dismutation of geminate alkoxyl radical pairs are involved in chemiluminescence from hyponitrite esters. Ruorophor-labelled compounds can be determined by a method based on peroxyoxalate-induced chemiluminescence. Fluorescence and phosphorescence spectra of firefly have been used to identify the multiplicity of the emitting species. " The chemiluminescence and e.s.r. of plasma-irradiated saccharides and the relationship between lyoluminescence and radical reaction rate constants have also been investigated. Electroluminescence from poly(vinylcarbazole) films has been reported in a series of four... 

While luminescence in vapor-deposited matrices accordingly should be a powerful technique for detection and quantitation of subnanogram quantities of PAH in complex samples, it suffers from two major limitations. First, it is obviously limited to the detection of molecules which fluoresce or phosphoresce, and a number of important constituents of liquid fuels (especially nitrogen heterocyclics) luminesce weakly, if at all. Second, the identification of a specific sample constituent by fluorescence (or phosphorescence) spectrometry is strictly an exercise in empirical peak matching of the unknown spectrum against standard fluorescence spectra of pure compounds in a hbrary. It is virtually impossible to assign a structure to an unknown species a priori from its fluorescence spectrum qualitative analysis by fluorometry depends upon the availabihty of a standard spectrum of every possible sample constituent of interest. Inasmuch as this latter condition cannot be satisfied (particularly in view of the paucity of standard samples of many important PAH), it is apparent that fluorescence spectrometry can seldom, if ever, provide a complete characterization of the polycyclic aromatic content of a complex sample. 

The fluorescence efficiencies of aromatic molecules are reduced by heavy atom substituents such as bromine and iodine and by certain other groups such as aldehyde and keto as well as nitro groups. However, in many cases the substituents that decrease the intensity of fluorescence enhance the intensity of phosphorescence. Consequently, aromatic nitro compounds, bromo- and iodo-derivatives, aldehydes, ketones, and some A-heterocyclics tend to fluoresce very weakly or not at all. However, most of them phosphoresce quite intensely. On the other hand, many substituents that are electron donors such as amino, hydroxy, and methoxy often tend to increase the quantum yields of fluorescence of molecules to which they are attached. 

Aromatic azides are stable photosensitive compounds and some of them mixed in cycllzed rubber are practically used as photosensitive resines.(j ) Such photosensitive polymers as poly(vinyl p-azidobenzo-ate)(2 ) and poly(vlnyl p-azldoclnnamate)(3) are also Investigated. These photosensitive resines are known to be spectrally sensitized by triplet sensitlzers(l,2,3) But the energies E(Ti) of the lowest triplet states Ti of aromatic azides have not been known, because these compounds emit no phosphorescence( ) and the trials on the measurement of their Sq- Ti absorptions are failed(4,3) ... 

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