Process photophysical

Birks [3] defines a photophysical process as a physical process (i.e. one which does not involve a chemical change) resulting from the 

Absorption of light from the ultraviolet or visible part of the spectrum results in electronic transitions which produce excited states. Two types of excited states are to be considered singlet and triplet. In a singlet state all spins are paired this is almost always the case for the ground state for which the notation S0 is commonly used. Excited singlet states are thus produced if the electronic excitation occurs with complete retention of electron spin. Triplet states are formed when excitation occurs with resultant spin inversion in this case two electrons are unpaired. 

Because of quantum mechanical restrictions, only transitions from the ground state S0 to excited singlet states S] are allowed and occur with an appreciable probability. As direct excitation to a triplet state is forbidden by quantum mechanics, the absorption coefficient of such a transition is 

Since there is a symmetrical relationship between the transition probabilities for the absorption and for the emission of radiation, if the absorption process is forbidden so also to an equal extent, is the emission process. Prohibition of emission results in a longer lifetime of the excited triplet state which varies between 10-4 and 10 s. Radiative deactivation 

Another alternative for the decay of excited states is the transfer of electronic energy to suitable acceptors. Such a process can be represented by 

Electronically excited states have only a short lifetime. In general, several processes are responsible for the dissipation of the excess energy of an excited state. These will be discussed in the following sections. For this purpose it is useful to distinguish between photophysical and photochemical pathways of deactivation, although such a distinction is not always unequivocal. (Cf. the formation of excimers. Section 5.4.2.) The present chapter deals with photophysical processes, which lead to alternative states of the same species such that at the end the chemical identity of the molecule is preserved. Photochemical processes that convert the molecule into another chemical species will be dealt with in later chapters. 

Once a molecule is excited into an electronically excited state by absorption of a photon, it can undergo a number of different primary processes. Photochemical processes are those in which the excited species dissociates, isomerizes, rearranges, or reacts with another molecule. Photophysical processes include radiative transitions in which the excited molecule emits light in the form of fluorescence or phosphorescence and returns to the ground state and nonradiative transitions in which some or all of the energy of the absorbed photon is ultimately converted to heat. 

Fluorescence is defined as the emission of light due to a transition between states of like multiplicity, for example, S, - S + hv. This is an allowed transition, and hence the lifetime of the upper state with respect to fluorescence is usually short, typically 10 6-10 9 s. For example, the fluorescence lifetime of OH in the electronically excited A2X+ state is 0.7 p.s (McDer- 

Intersystem crossing (ISC) is the intramolecular crossing from one state to another of different multiplicity without the emission of radiation. In Fig. 3.9 (ISC), shows the transfer from the first excited singlet state S, to the first excited triplet state T,. Since the process is horizontal, the total energy remains the same and the molecule initially is produced in upper vibrational and rotational levels of T, from which it is deactivated as shown by the vertical wavy line. Similarly, (ISC)2 shows the intersystem crossing from T, to upper vibrational and rotational states of the ground state S(l, from which vibrational deactivation to v = 0 then occurs. 

In most photochemical systems, the triplet state is populated via a radiationless transition from the lowest excited singlet state following initial excitation. This transition is also known as intersystem crossing in photochemistry. Very little is known experimentally about the exact nature of this process, but the rate or 

Kottis and Lefebvre (322) have suggested that if polarized light is used to excite randomly oriented molecules to the triplet state, observation of the changes in the AMg = +1 ESR spectrum can reveal the correlation of the polarization properties of the excitation with the principal axis system of the triplet zero-field tensor. Such photoselection experiments have been carried out successfully by Lhotse and coworkers (323) and El-Sayed and Siegel (324) on a number of aromatic systems. Piette and collaborators (325) have studied the effect of metal complexation on the zero-field parameters and lifetimes of the phosphorescent triplet of aromatic-metal complexes with similar photoselection technique. The changes in 

Clarke (326) has studied the optical electron spin polarization in triplet anthracene and has observed ESR emission at 1.5°K which was attributed to a non-Boltzman distribution over the triplet spin levels at low temperature. The dynamics of optical spin polarization in triplet naphthalene at 1.6°K was also reported by Sixl and Schwoerer (327a) and van der Waals et al. (327b). have used a general method to study dynamics of populating and depopulating triplet spin levels by microwave-induced delayed phosphorescence. These experiments enable measurements of the lifetimes of each triplet spin state and thus can provide important information about intramolecular decay processes and intermolecular triplet energy transfer. 

The mean lifetime of phosphorescence has been correlated with the triplet-state energy in aromatic hydro- 

Effects of Metal Ions on ESR Parameters of Triplet-State Molecules3 

An authoritative compilation of definitions and concepts is available in the Glossary of Terms Used in Photochemistry, 3rd edition,22 which can be downloaded from the IUPAC webpages or from those of the photochemical societies (http //pages.unibas.ch/epa or http //www.i-aps.org/). 

The terms used in this sentence are usually pronounced as an n to n state and a n to n state. As noted on page 790, the electronic transition leading to these states are usually written with arrows asn — tz and n tz transitions, respectively. 

Themultiplicity (m)of astateisgivenby m = 2S + 1, where Sis the total spin quantum number of the state. A species in which all the electrons are paired is called a singlet state (m = 2 x 0 + 1 = 1), a radical with one unpaired electron is termed a doublet state m = 2 x 5 + 1 = 2), and a biradical or excited state with two unpaired electrons is called a triplet (m = 2 x 1 + 1 =3). For a discussion of the research of G. N. Lewis and the association of the triplet state with phosphorescence of organic compounds, see Kasha, M. /. Chem. Educ. 1984, 61, 204. 

Absorption. A ground state molecule (So) may absorb a photon of UV-vis radiation, thus becoming an excited singlet state. The most commonly seen transitions are Sq Si or So S2, but So to higher excited singlet state transitions are also possible. 

Vibrational Relaxation. The absorption from So to S involves an energy change from the 0th vibrational level of So to some vibrational level of the excited state. The t/ = 0 vibrational level is the level most populated at room temperature for the ground electronic state of a molecule, and i/ = 0 also is the vibrational level of the excited electronic state that is most likely to be populated at equilibrium in condensed phases (i.e., solids or liquids). Unless the molecule dissociates before equilibrium can be obtained, rapid vibrational relaxation (with a rate constant of about 10 s ) converts the higher vibrational level of the excited state to its 0th vibrational level.  

Radiationless Decay. This is a process by which electronically excited states are returned to ground states (typically from Si to So) without the emission of radiation. Radiationless decay often has a relatively slow rate constant (ca. 10 s ) because the energy gap between Si and So is usually greater than that between S2 and Si or between other pairs of excited states. 

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Much use has been made of micellar systems in the study of photophysical processes, such as in excited-state quenching by energy transfer or electron transfer (see Refs. 214-218 for examples). In the latter case, ions are involved, and their selective exclusion from the Stem and electrical double layer of charged micelles (see Ref. 219) can have dramatic effects, and ones of potential imfKntance in solar energy conversion systems. 

Laser-based pump strategies are generally necessary to study reactions taking place on time scales faster tlian microseconds. Lasers can be used to produce L-jumps on time scales faster tlian microseconds or to initiate reactions tlirough rapid photochemical or photophysical processes. Lasers can also initiate ultrarapid mixing via a wide variety... 

Figure 7-12. Configuration coordinate diagram of the four essential states showing the photophysical processes. Also shown is the calculated PA spectrum based on level energies from EA spectroscopy.

Dendrimers with a polyphenyl core around a central biphenyl unit decorated at the rim with peryleneimide chromophores have been investigated both in bulk and at the single-molecule level in order to understand their time and space-resolved behavior [28]. The results obtained have shown that the conformational distribution plays an important role in the dynamics of the photophysical processes. Energy transfer in a series of shape-persistent polyphenylene dendrimers substituted with peryleneimide and terryleneimide chro-mophoric units (4-7) has been investigated in toluene solution [29]. 

Mechanism of Coloration of Spiropyran Generated by Photophysical Process... 

For photochemical reactions and photophysical processes the efficiency is determined by the quantum yield d>, which is defined as the number of molecules undergoing a particular process divided by the number of quanta of light absorbed ... 

The various intramolecular processes initiated by light absorption are illustrated schematically in Figure 1.1. Such a schematic representation of the energy levels and photophysical processes which can occur in the excited... 

Photophysical Processes in PET and Model Compounds. The photophysical processes in many polymer, copolymer, and polymer-additive mixtures have been studied (17. 18. 19). However, until recently, few investigations have been made concerning the photo-physical processes available to the aromatic esters in either monomeric or polymeric form. 

We (fl) have reported the photophysical processes of a series of model esters of PET, and tentatively assigned the fluorescence and phosphorescence of the aromatic esters as (n, tt ) transitions, respectively. We (9) also performed an extensive study of the photophysical processes available to dimethyl terephthalate (DMT) in order to relate this monomeric species to the PET polymer. In 1,1,1,3,3,3-hexafluoro-2-propanol (HFIP) (Table I), DMT has three major,absorptions which are according to Platt, s notation 191 nm, A- B, e = 40,620 1 mole" cm"1 244 nm, A-dLaT e = 23,880 1 mole-) cm" 289 nm, A U, e = 1780 1 mole")cm. ... 

Photophysical Processes and Photodegradation of Poly(ethylene terephthalate-co-2,6-naphtha1enedi carboxyl ate) Copolymers. We have recently reported the photophysical processes and the photo-degradative behavior of Doly(ethylene terephthalate-co-2,6-naphthalenedicarboxyl ate), PET-2,6-ND, copolymer yarns containing 0.5 - 4.0 mole percent 2,6-naphthalenedicarboxyl ate, 2,6-ND (1) and the parent naphthalenedicarboxyl ate monomer, Figure 3 and 4. 

Photophysical Processes in Dimethyl 4,4 -Biphenyldicarboxy-late (4,4I-BPDC). The ultraviolet absorption spectrum of dimethyl 4,4 -biphenyldicarboxyl ate was examined in both HFIP and 95% ethanol. In each case two distinct absorption maxima were recorded, an intense absorption near 200 nm and a slightly less intense absorption near 280 nm. The corrected fluorescence excitation and emission spectra of 4,4 -BPDC in HFIP at 298°K shows a single broad excitation band centered at 280 nm with a corresponding broad structureless emission band centered at 340 nm. At 77°K, the uncorrected phosphorescence spectra shows a single broad structureless excitation band centered at 298 nm, and a structured emission band having maxima at 472 and 505 nm with a lifetime, t, equal to 1.2 seconds. 

Photophysical Processes in Pol,y(ethy1eneterephthalate-co-4,4 -biphenyldicarboxyl ate) (PET-co-4,4 -BPDC). The absorption and luminescence properties of PET are summarized above. At room temperature the absorption spectrum of PET-co-4,4 -BPDC copolymers, with concentrations of 4,4 -BPDC ranging from 0.5 -5.0 mole percent, showed UV absorption spectra similar to that of PET in HFIP. The corrected fluorescence spectra of the copolymers in HFIP exhibited excitation maxima at 255 and 290 nm. The emission spectrum displayed emission from the terephthalate portion of the polymer, when excited by 255 nm radiation, and emission from the 4,4 -biphenyldicarboxylate portion of the polymer when excited with 290 nm radiation. 

Photophysical Processes in Pi butyl 4,4 -Sulfonyldibenzoate (4,4 -SD). The UV absorption spectra of dibutyl 4,4 -sulfonyl-dibenzoate (4,4 -SD) in both HFIP and 95% ethanol showed similar absorptions. The corrected excitation and emission fluorescence spectra of 4,4 -SD in HFIP at 298°K showed a structured excitation with band maxima at 236, 286, and 294 nm and a structured emission exhibiting band maxima at 322, 372, and 388 nm. The uncorrected excitation and phosphorescence spectra of 4,4 -SD in a 95% ethanol glass at 77°K displayed excitation band maxima at 268, 282, and 292 nm with strong phosphorescence emission with band maxima at 382, 398, and 408 nm with a mean lifetime (t) of 1.2 sec. 

Photophysical Processes in PET-4,4 -SD Copolymers. PET-4,4 -SD copolymers have UV absorption spectra similar to that of PET homopolymer in HFIP solution. Band maxima were exhibited at about 290, 245, and 191 nm in all the polymers. 

Figure 1 Jablonskii diagram illustrating the energy levels and photophysical processes for an aromatic chromophore (ISC = inter-system crossing IC = internal conversion).

Intramolecular and Intermolecular Photophysical Processes Two laws form the basis of interaction of light and substance ... 

The general scheme of photophysical processes followed by the photon absorption by the molecule induces the below-mentioned elementary stages [205-209] ... 

Resonance ionization spectroscopy is a photophysical process in which one electron can be removed from each of the atoms of a selected type. Since the saturated RIS process can be carried out with a pulsed laser beam, the method has both time and space resolution along with excellent (spectroscopic) selectivity. In a recent article [2] we showed, for example, that all of the elements except helium, neon, argon, and fluorine can be detected with the RIS technique. However, with commercial lasers, improved in the last year, argon and fluorine can be added to the RIS periodic table (see figure 2). 

Because of the great importance of PF as a class of conjugated polymers with excellent optical and electronic properties, several theoretical studies were performed to better understand the electronic structure and the photophysical processes, which occur in these materials [260-265],... 

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