Spectra, emission and absorption

Generally the first thing to be done in preparation for the photochemical study of a compound is to determine the visible and ultraviolet absorption spectrum of the compound. Besides furnishing information concerning the nature of the excited state potentially involved in the photochemistry (see Section 1.4), the absorption spectrum furnishes information of a more applied nature as to the wavelength range in which the material absorbs and its molar absorptivity e. From this information it is possible to decide what type of light source to use for the irradiation, what solvents can be used to 

Since temperature, disorder, and other factors widen the experimental absorption and emission spectral lines, it is common and convenient to include an arbitrary widening factor F in the overlap between the initial and the propagated vibrational functions (f 4 t)). For instance, if only one breathing mode is taken into account, a common value of its vibrational frequency is used for the initial and final electronic states, and a harmonic approximation is assumed, the (f) 4 t)) reduces to [54] 

Eo is the difference between the minima of the upper and lower energy surfaces, and F is the mentioned arbitrary damping factor whose value determines the width of the vibrational lines. Values of F ranging between 10 cm and 200 cm are common. The full absorption or emission intensity profile is calculated as the superposition of the profiles of the individual electronic origins, with weight factors having die same ratios as the oscillator strengths of the individual absorptions. 

The results of some applications of the methods to the study of structural and optical properties of lanthanide ions in crystals are summarized here. They are organized to show their ability for giving insight and building a model of their electronic structure and interactions. We also focus on showing their capacity to predict optical properties, a very valuable characteristic on the line of search for new materials. 

The spectral distribution of the radiant flux from a source is called its emission spectrum. The thermal radiation discussed in Sect. 2.2 has a continuous spectral distribution described by its spectral energy density (2.13). Discrete emission spectra, where the radiant flux has distinct maxima at certain frequencies are generated by transitions of atoms or molecules between two bound states, a higher energy state Ek and a lower state Ei, with the relation 

Examples of discrete absorption lines are the Fraunhofer lines in the spectrum of the sun, which appear as dark lines in the bright continuous spectrum (Fig. 2.19). 

They are produced by atoms in the sun s atmosphere that absorb at their specific eigenfrequencies the continuous blackbody radiation from the sun s photosphere. 

Examples of discrete absorption lines are the Fraunhofer lines in the spectrum of the sun, which appear as dark lines in the bright continuous spectrum (Fig. 2.12). They are produced by atoms in the sun s atmosphere that absorb at their specific eigenfrequencies the continuous blackbody radiation from the sun s photosphere. A measure of the absorption strength is the absorption 

At thermal equilibrium the population follows a Boltzmann distribution. Inserting (2.18) yields the power absorbed within the volume AV — AAz out of an incident beam with the cross section A 

however, possible to pump molecules into higher energy states by various excitation mechanisms such as optical pumping or electron excitation. 

This allows the measurement of absorption spectra for transition from these states to even higher molecular levels (Sect. 10.3). 

If radiation with a continuous spectrum passes through a gaseous molecular sample, molecules in the lower state Ei may absorb radiant power at the eigenfrequencies = Ek Ei)/h, which is thus missing in the transmitted 

however, possible to pump molecules into higher energy states by various excitation mechanisms such as optical pumping or electron excitation. This allows the measurement of absorption spectra for transition from these states to even higher molecular levels (Vol. 2, Sect. 5.3). 

The OD at 280 nm is 0.063. Thus, the ratio of the ODs, OD26o/OD28o is 1.825, a value very close to 1.8, indicating that DNA is pure. 

In general, DNA contamination with protein can be calculated using the following equation  

Equation (12.2) is to be applied only when the ratio of the optical densities is lower than 1.8. 

Excitons in semiconductors have a finite lifetime due to a recombination of the photo-excited electron-hole pair. In QDs, the energy released upon exdton annflii- 

As is the case of organic fluorophores, the range of energies emitted from a colloidal dot sample after excitation is centered at a value that is smaller than that required to excite the sample (and which must be at least as large as its band gap). In other words, the wavelength of fluorescence is longer than that of the absorbed light. 

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One interesting new field in the area of optical spectroscopy is near-field scaiming optical microscopy, a teclmique that allows for the imaging of surfaces down to sub-micron resolution and for the detection and characterization of single molecules [, M]- Wlien applied to the study of surfaces, this approach is capable of identifying individual adsorbates, as in the case of oxazine molecules dispersed on a polymer film, illustrated in figure Bl.22,11 [82], Absorption and emission spectra of individual molecules can be obtamed with this teclmique as well, and time-dependent measurements can be used to follow the dynamics of surface processes. 

Fig. 1. Absorption and emission spectra in solution of a compound of stmcture (1).

The characteristic lines observed in the absorption (and emission) spectra of nearly isolated atoms and ions due to transitions between quantum levels are extremely sharp. As a result, their wavelengths (photon energies) can be determined with great accuracy. The lines are characteristic of a particular atom or ion and can be used for identification purposes. Molecular spectra, while usually less sharp than atomic spectra, are also relatively sharp. Positions of spectral lines can be determined with sufficient accuracy to verify the electronic structure of the molecules. 

Absorption and emission spectra of six 2-substituted imidazo[4,5-/]quinolines (R = H, Me, CH2Ph, Ph, 2-Py, R = H CH2Ph, R = Ph) were studied in various solvents. These studies revealed a solvent-independent, substituent-dependent character of the title compounds. They also exhibited bathochromic shifts in acidic and basic solutions. The phenyl group in the 2-position is in complete conjugation with the imidazoquinoline moiety. The fluorescence spectra of the compounds exhibited a solvent dependency, and, on changing to polar solvents, bathochromic shifts occur. Anomalous bathochromic shifts in water, acidic solution, and a new emission band in methanol are attributed to the protonated imidazoquinoline in the excited state. Basic solutions quench fluorescence (87IJC187). 

A xylylene-fc/.v-phosphonium salt 11 gave films of PPV 1 upon clectropolymer-ization. The absorption and emission spectra of the resultant material were blue-shifted with respect to PPV produced by other routes, suggesting that the electro-polymerized material has a shorter effective conjugation length, possibly because of incomplete elimination of phosphonium groups [22]. 

A comparison of the absorption and emission spectra of Ooct-OPV5 with those of the fully conjugated, similarly substituted polymer Ooct-PPV shows that the absorption and luminescence maxima of the five-ring model compound are only slightly blue-shifted relative to those of the polymer (see Fig. 16-11). Hence, the... 

Figure 4-9. INDO/SCI-simulalcd absorption and emission spectra of two slilbene molecules with a huge interchain distance (solid lines) and those of a cofacial dimer formed by two slilbene chains separated by 4 A (dolled lines).

There is unusually strong justification for the historical approach in presenting the important facts about x-rays to analytical chemists the information these chemists need in their work is largely that discovered in the early researches. This is particularly clear in connection with absorption and emission spectra, in which more refined investigations with more powerful equipment later revealed important complexities that the analytical chemist may ignore. Several of these complexities will be recorded below. 

Emission Spectra. The excited states of quinoxaline and several derivatives have been studied by means of their UV absorption and emission spectra. ... 

Each element has unique absorption and emission spectra. That is, each element has its own set of characteristic frequencies of light that it can absorb or emit. Also, Figures 7-10 and 7-11 show only the visible portions of absorption and emission spectra. Electron transitions also take place in regions of the electromagnetic spectrum that the human eye cannot detect. Instmments allow scientists to see into these regions. 

A helium +1 cation, like a hydrogen atom, has just one electron. Absorption and emission spectra show that He" has energy levels that depend on u, just like the hydrogen atom. Nevertheless, Figure 8 2 shows that the emission spectra of He and H differ, which means that these two species must have different energy levels. We conclude that something besides U influences orbital energy. 

Absorption and emission spectra of atoms and ions yield information about energy differences between orbitals, but they do not give an orbital s absolute energy. The most direct measurements of orbital energies come from a technique called photoelectron spectroscopy. 

Poly(aryl ether) branches of generation 1 to 3 have been appended to a pho-totautomerizable quinoHne core to investigate the effect of dendritic architecture on the excited state intramolecular proton transfer [45]. The changes observed in the absorption and emission spectra on increasing dendrimer generation indicate that the dendritic branches affect the planarity of the core and therefore the efficiency of the excited state intramolecular proton transfer and of the related fluorescence processes. 

Atomic spectra are much simpler than the corresponding molecular spectra, because there are no vibrational and rotational states. Moreover, spectral transitions in absorption or emission are not possible between all the numerous energy levels of an atom, but only according to selection rules. As a result, emission spectra are rather simple, with up to a few hundred lines. For example, absorption and emission spectra for sodium consist of some 40 peaks for elements with several outer electrons, absorption spectra may be much more complex and consist of hundreds of peaks. 

Absorption and Emission Spectra. The excitation-emission spectrum of 1 (bottom half of Fig. 1) shows that the relatively narrow emission band is nearly independent of the excitation wavelength and that the excitation spectrum is not only nearly independent of the wavelength at which the emission is monitored, but is also very similar to the absorption spectrum, both being somewhat broader than the emission band. This leaves no doubt that the observed emission is due to the polysilane, and its shape, location and the mirror image relation to the absorption permit its assignment as fluorescence. 

Mennucci B, Toniolo A, Tomasi J (2001) Theoretical study of guanine from gas phase to aqueous solution role of tautomerism and its implications in absorption and emission spectra. J Phys Chem A 105 7126... 

The electronic absorption and emission spectra and emission lifetimes of [Ir(/x-L)(CO)2]2 (L = pz, mpz and dmpz) have been determined.529 The intense low-energy absorption band around 400 nm is assigned to a d/2 > pz electronic transition. The three complexes all emit around 740 nm at 300 K and 670 nm at 77 K. The dimer excited states are stabilized relative to monomer levels by strong metal-metal bonding. 

A porphyrin compound with a 2,9-dimethyl- 1,10-phenanthroline functionality fused at the beta-pyrrole positions is a phthalocyanine analog, and formed a complex with zinc in the cavity and a further zinc binding to the phenanthroline group. The absorption and emission spectra of the compound with and without the external zinc demonstrated the strong effects of the second metal binding on the porphyrin 7r-system.840... 

We have seen in Chapter 1 that absorption and emission spectra are controlled at least in part by the Franck-Condon principle. However, this is only one of three major factors that must be considered. 

This method is perfectly suitable for low concentrations of fluorescent materials. However, in order to study factors which affect the fluorescence quantum yield, such as molecular association or photochemical reactions, much higher concentrations than can be used in the right-angle fluorescence method are required. This follows from the fact that the 0 - 0 vibrational bands in the absorption and emission spectra often overlap. Therefore at relatively high concentrations light emitted at these overlapping wavelengths will be reabsorbed. 

Fig. 1 Absorption and emission spectra of Cy3, CyS, Cy7, oxo-squaraine 13b (part 3 of this chapter) and dicyanomethylene squaraine 41j (part 4 of this chapter) in water (pH 7.4)...

A series of symmetrical and unsymmetrical, hydrophobic and hydrophilic squaraine probes such as 41 (Table 1) with substituted squaraine ring oxygen was developed and compared to conventional oxo-squaraines 10a,b and 13b [18, 50, 98, 107]. The substituent on the squaraine ring have a strong influence on the spectral properties. Substitution of the squaraine oxygen by S, C(CN)2> C(CN)COOR, N(CN), N(OH), C(CN)[PO(OEt)2], indanedione, barbituric, and thiobarbituric acid causes red-shifted absorption and emission spectra [50]. 

The solvatochromic behavior of these dyes in solution can be explained by the comparison of their permanent dipole moments. If the excited state exhibits a larger dipole moment (pii) than the ground state (/i0), it is preferentially stabilized by the more polar solvent, and the energy between these two states decreases, that is, the absorption and emission spectra both shift to the red region. 

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