First singlet state

The absorption spectra of the silicon(IV) naphthalocyanines follows the pattern already seen with the analogous zinc(II) (Section 9.22.12.1.4) and aluminum(III) (Section 9.22.13.2) derivatives 354 the red band is shifted about 100 nm further to the red, with intensification. Bis(trihex-ylsiloxy)silicon(IV) naphthalocyanine has Amax(PhH) = 776 nm (e = 650,000 M-1 cm-1) and <1>A 0.35 (oxygen-saturated benzene).389 The solution is fluorescent (main emission at 780 nm, Tf 2.85 ns) the triplet energy (ca. 22 k cal mol-1) is a little less than the energy of the first singlet state of dioxygen, and the process (Equation (9)) is reversible (cf. compounds (40) and (41), Section 9.22.10.2). 

The condition for observing induced emission is that the population of the first singlet state Si is larger than that of So, which is far from the case at room temperature because of the Boltzmann distribution (see above). An inversion of population (i.e. NSi > Nso) is thus required. For a four-level system inversion can be achieved using optical pumping by an intense light source (flash lamps or lasers) dye lasers work in this way. Alternatively, electrical discharge in a gas (gas lasers, copper vapor lasers) can be used. 

Most chromophores absorb light along a preferred direction1 (see Chapter 2 for the definition of absorption transition moment, and for examples of transition moments of some fluorophores, see Figure 2.3), depending on the electronic state. In contrast, the emission transition moment is the same whatever the excited state reached by the molecule upon excitation, because of internal conversion towards the first singlet state (Figure 5.2). 

This situation occurs when excitation brings the fluorophores to an excited state other than the first singlet state from which fluorescence is emitted. Let a be the angle between the absorption and emission transition moments. The aim is to calculate cos dE and then to deduce r by means of Eq. (5.20). 

Electron-transfer catalytic cycles with oxygen were also discovered in photochemical reactions with participation of an excited sensibilizer (9,10-dicyanoanthracene [DCNA]) and stilbene. The sensitizer assists an electron transfer from the substrate to oxygen. Oxygen transforms into the superoxide ion. Stilbene turns into benzaldehyde. In the absence of the sensitizer, this reaction does not take place even on photoirradiation (when oxygen exists in the first singlet state). In the singlet state,... 

It is easy then to write down the oscillation condition for a dye laser. In its simplest form a dye laser consists of a cuvette of length L [cm], with dye solution of concentration m [cm-3], and of two parallel end windows carrying a reflective layer, each of reflectivity R, which form the laser resonator. With mi molecules/ cm3 excited to the first singlet state, the dye laser will start oscillating at a wavelength A if the overall gain is equal to or greater than one ... 

The constant S on the left-hand side of (35) contains only parameters of the resonator, i.e., the active length L, and reflectivity R. Other types of losses, like scattering, diffraction, etc., may be accounted for by an effective reflectivity, Ren The value (A) is the minimum fraction of the molecules that must be raised to the first singlet state to reach the threshold of oscillation. One may then calculate the function (A) from the absorption and fluorescence spectra for any concentration m of the dye and value 5 of the cavity. In this way one finds the frequency for the minimum of this function. 

Fig. 1. Rise of the perylene cation absorption, which reflects the electron injection dynamics, after excitation of the first singlet state with a 15 fs pump pulse. Variation of the electronic coupling via a change in the anchor group as well as the insertion of one or two -CH2- groups leads to a systematic change in the time scale of both electron injection and charge recombination.

These reactions ((2) — (5)) are apparently ionic processes and do not proceed via the triplet state. The first singlet state is said to be responsible for these nucleophilic substitutions. 

A much more interesting example of the intermediate case is encountered when the density of states is rather small but the vibronic coupling elements are large (due to favorable Franck-Condon vibrational overlap factors). The consequences of this type of intramolecular vibronic coupling are seen in the anomalously long radiative lifetimes of the first singlet states of N02, S02, and CS2 86-99 and in the many extra unexpected lines in the spectra of these molecules. 

Delocalization due to Energy Transfer to the First Singlet State hiu0l =8.4eV and to the Plasmon State h(or = 21.4 eV in Water by an Electron with Energy Et... 

Figure 1.2. Excitonic dispersion curves for the first singlet state of the anthracene crystal. These curves are calculated in the point-dipole approximation, the transitions to upper states being accounted for by a constant dielectric permittivity.13,37...

This leads, with D 2500 cm 1 for the first singlet state, to... 

In the case of electronic spectroscopy in molecular crystals, the first singlet state of the isotopic mixed crystal of anthracene has been investigated in reflectivity and shown to be of amalgamation type.129 Actually, while the gap of resonances A = va — vg) is about 70 cm-1, the excitonic bandwidth is estimated at 500 cm-1. Very few mixed crystals have been investigated in the whole range of concentrations,120 whereas examples of low concentrations (of impurities, for instance) are very numerous The anthracene crystal contains j8-methylanthracene naturally and shows an impurity level below the excitonic band.120 (This does not suffice, however, to predict that a mixed anthracene-j8-methylanthracene crystal will be of the persistence type.)... 

Sensitization by charge transfer requires that the LUMO of the diacetylene monomer be below the excited singlet level of the dye. The latter is at Is — Eg where I, and Eji are ionization potential and energy of the first singlet state of the dye, respectively. On the basis of polarographic data I, — E j —2.7. .. —3.3 eV is estimated for cyanine dyes The lowest unoccupied orbital of a previously neutral diacetylene... 

The first singlet state is S A with an H —C—H angle of about 102 Thell /l, state can be identified by the transition B Z7, - a /l, in the region 5000 to 9tXX) A [Herzberg and Johns (465)]. Recently, emission bands due to the transition B B, have been reported by Masanet and Vermeil... 

Stilbene is a molecule which exists in two isomeric states because of the large energy barrier associated with rotation around the central double bond (see fig. 7.13). It has been known for some time [22] that when stilbene is excited to its first singlet state by absorption of UV light, it rapidly fluoresces to the ground state but with some molecules in the other isomeric form. Thus, if c -stilbene is irradiated with ultraviolet light, one obtains approximately 50% tran -stilbene and... 

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