Solvent Effects on the Electronic Spectra

Prepare approximately 10% solution of each solid in a range of solvents. Run the absorption spectra in the range of 200-900 nm. Locate the wavelengths of maximum absorption. In case of very intense bands, dilute to obtain absorbances within the chart range. Comment on any relation between these bands and the solvent. Er parameter or Donor and Acceptor numbers of die solvlents as defined in the two references. 

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Broo A, Pearl G, Zemer MC (1997) Development of a hybrid quantum chemical and molecular mechanics method with application to solvent effects on the electronic spectra of uracil and uracil derivatives. Journal of Physical Chemistry A 101 2478—2488. 

Understanding solvent effects on the electronic spectra allows qualitative prediction of relaxation dynamics of pNA in various solvents. A scheme showing possible mechanisms of relaxation in water and dioxane is presented in Fig. 5.7. Generally, upon excitation to the l Ai singlet state the energy may relax via 1C and/or ISC mechanisms. 

The papers cited contain spectral data as well as a discussion of the solvent effect on the electronic spectra. 

The solvent effects on the absorption spectra of ion pairs were studied by many authors and the direction of the observed shift depends on the change (increase or decrease) of dipole moment upon the electronic transition [25]. Generally a bathochromic shift is observed with an increase of solvent polarity. When going from a polar solvent to a less polar one, the association in the ground state increases more strongly than in the excited state this may be understood if the ion pair switches progressively from SSIP to CIP status. Observations of this type were often made, together with cation effects, as for instance in the case of alkali phenolates and enolates [7], fluorenyl and other carbanion salts [22] or even for aromatic radical anions [26, 27],... 

Semenov, S. G., and N. V. Khodureva. 1992. Quantum-chemical estimate of nonspecific solvent effect on the electronic structure and spectra of molecules modeling nucleophilic fragments of lignin. Opt. Spektrosk. 73(2) 280-290. 

A scheme for the treatment of the solvent effects on the electronic absorption spectra in solution had been proposed in the framework of the electrostatic SCRF model and quantum chemical configuration interaction (Cl) method. Within this approach, the absorption of the light by chromophoric molecules was considered as an instantaneous process. Tliere-fore, during the photon absorption no change in the solvent orientational polarization was expected. Only the electronic polarization of solvent would respond to the changed electron density of the solute molecule in its excited (Franck-Condon) state. Consequently, the solvent orientation for the excited state remains the same as it was for the ground state, the solvent electronic polarization, however, must reflect the excited state dipole and other electric moments of the molecule. Considering the SCRF Hamiltonian... 

In an attempt to evaluate the relative contribution of the aromatic dipolar structure of 1,2-diphenyltriafulvenes (V), in the ground state and in the first excited electronic state, we have investigated the solvent effects on their electronic spectra. 

Figure 3.3 A schematic of the solvent effect on the electronic transition of N -diethyl-4-nitroaniline (left) and how its UV-spectra in different solvents translates to the n scale of dipolarity/polarisability (right).

Abstract It is well known that solvents can modify the frequency and intensity of the solute spectral bands, the thermodynamics and kinetics of chemical reactions, the strength of molecular interactions or the fate of solute excited states. The theoretical study of solvent effects is quite complicated since the presence of the solvent introduces additional difficulties with respect to the smdy of analogous problems in gas phase. The mean field approximation (MFA) is used for many of the most employed solvent effect theories as it permits to reduce the computational cost associated to the smdy of processes in solution. In this chapter we revise the performance of ASEP/MD, a quanmm mechanics/molecular mechanics method developed in our laboratory that makes use of this approximation. It permits to combine state of the art calculations of the solute electron distribution with a detailed, microscopic, description of the solvent. As examples of application of the method we smdy solvent effects on the absorption spectra of some molecules involved in photoisomerization processes of biological systems. 

J. J. Aaron, M. Maafi, C. Parkanyi, and C. Boniface, Spectrochimica Acta, Part A, 51(603) (1995). Quantitative Treatment of the Solvent Effects on the Electronic Absorption and Fluorescence-Spectra of Acridines and Phenazines - The Ground and First Excited Singlet-State Dipole-Moments. 

Based on the solvent and substituent effects on the electronic spectra, the quantum yields, and the product distribution, it was concluded that the azoxy rearrangement takes place from a KK CT (charge-transfer) state rather than the nJt state as suggested by Bunce. 

In this respect, the solvatochromic approach developed by Kamlet, Taft and coworkers38 which defines four parameters n. a, ji and <5 (with the addition of others when the need arose), to evaluate the different solvent effects, was highly successful in describing the solvent effects on the rates of reactions, as well as in NMR chemical shifts, IR, UV and fluorescence spectra, sol vent-water partition coefficients etc.38. In addition to the polarity/polarizability of the solvent, measured by the solvatochromic parameter ir, the aptitude to donate a hydrogen atom to form a hydrogen bond, measured by a, or its tendency to provide a pair of electrons to such a bond, /, and the cavity effect (or Hildebrand solubility parameter), S, are integrated in a multi-parametric equation to rationalize the solvent effects. 

The observations of the effect of hydrogen bond formation on the electronic spectra of organic molecules represent experimental evidence for the polarization, i.e. the electron-shifts in the ground state, arising from true hydrogen bond formation. The difficulty to separate this effect from that due to the normal dielectric properties of solvents can be overcome in the case of phenolic substances by a comparison with the corresponding methyl ethers. 

Here, we summarize a recent study we have done on the effect of the environment on the electronic absorption and emission of 6-Propionyl-2-(A,A-dimethyl)aminona-phthalene (PRODAN) [8]. This system has widely been used as a fluorescence probe since it was introduced by Weber and Farris [31], The effect of polar solvents on the absorption and more effectively, on the fluorescence spectra of PRODAN is large,... 

In contrast to these nonpolar compounds, very dramatic solvent effects on UV/ Vis spectra have been observed for dipolar meropolymethine dyes, especially mero-cyanines, due mainly to the change in their dipole moments on electronic transition. An example is the following negatively solvatochromic pyridinium V-phenolate betaine, which exhibits one of the largest solvatochromic effects ever observed cf. Fig. 6-2 [10, 29]). 

Solvatochromic fluorescent probe molecules have also been used to establish solvent polarity scales. The solvent-dependent fluorescence maximum of 4-amino-V-methylphthalimide was used by Zelinskii et al. to establish a universal scale for the effect of solvents on the electronic spectra of organic compounds [80, 213], More recently, a comprehensive Py scale of solvent polarity including 95 solvents has been proposed by Winnik et al. [222]. This is based on the relative band intensities of the vibronic bands I and III of the % - n emission spectrum of monomeric pyrene cf. Section 6.2.4. A significant enhancement is observed in the 0 0 vibronic band intensity h relative to the 0 2 vibronic band intensity /m with increasing solvent polarity. The ratio of emission intensities for bands I and III serves as an empirical measure of solvent polarity Py = /i/Zm [222]. However, there seems to be some difficulty in determining precise Py values, as shown by the varying Py values from different laboratories the reasons for these deviations have been investigated [223]. 

Another example of intramolecular CT complex formation is provided by trans-4-dimethvlamino-4 -(1-oxobutvl)stilbene Solvent effects on the spectrum give a value of 22D for the excited state dipole moment. The effect of electric field on the fluorescence of 4-(9-anthry1)-N.N.-2.3,5,G-hexamethy1-aniline shows this compound forms an excited state whose dipole moment does not change with solvent . Chiral discrimination in exciplex formation between 1-dipyrenylamine and chiral amines is very weak . In the probe molecule PRODAN (6-propionyl)-2-(dimethylamino)—naphthalene the initially formed excited state converts to a lower CT state as directly evidenced by time-resolved spectra in n-butanol. Rate constants for intramolecular electron transfer have been measured in both singlet and triplet states of covalently porphyrin-amide-quinone molecules . Intramolecular excimer formation occurs during the lifetime of the excited state of bis-(naphthalene)hydrazides which are used as photochemical deactivators of metals in polyethylene . ... 

It is also encouraging to find that the effect of polar solvents on the electronic spectra of 1-naphthol appears to be qualitatively similar to the effect of polar substituents. This raises hope that the paradigm of the photoacidity of 1-naphthol could be potentially resolved in a general way. 

The electron donor-acceptor molecular complexes between iodine and thiazole, benzothiazole, and some derivatives have been studied in several organic solvents by UV spectroscopy <87CJC468>. In all cases, the presence of the thiazole ring produces a displacement of the Amax iodine band at 512 nm towards shorter wavelengths and a decrease of its absorbance. Moreover, a sharp isosbestic point near 470 nm was observed for all iodine-thiazole complexes. 2-Aryl and 2-hetarylbenzothiazoles showed fluorescence, the maxima of emission being between 350 nm and 395 nm. Both substituent and solvent effects on the spectra were observed <93MI 306-02>. The photophysical properties of bis(benzothiazolylidene)squaraine dyes have also been studied <93JPC13625>. 

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