The fluctuations of orientation and induction interactions in solution and their impact on the broadening of absorption and fluorescence spectra are considered in terms of a bands-of-energy model. Also covered is the application of principles of thermodynamics and self-organization of systems for calculation of solvatochromic shift, among them a component owing to the work on electronic polarization of solvent at the instant of electronic transition in the solute. The findings on solvatochromic shift and spectral broadening open the way to the calculation of solvent effects on the rate constant of nonradiative transitions. As demonstrated herein for 15 fluorophores, the novel theory of nonradiative decay of the intramolecular charge-transfer excited states is carried out for dyes and organic compounds of different nature, both for polar and nonpolar media.

The locations of the 0 0 0 -bands for S 1 ← S 0 and S 1 → S 0 transitions have been found for C 60 solutions in hexane. It is shown that the profile of the S 1 ← S 0 band is mainly shaped by h u (4), t 1u (4)-and h g (1), a g (2)-modes that are active in absorption. Bands involving the h u (4)-and t 1u (4)-modes in the emission process have also been identified in the fluorescence spectrum. The appearance of the 0 0 0 -band in the forbidden 1 1 T 1g ← 1 1 A g transition is explained by symmetry reduction in the C 60 +environment system due to the interaction of electrons with local phonons. The temperature coefficients of the red shift for the 256.3-and 328.3-nm bands of allowed 1 T 1u ← 1 1 A g transitions for C 60 in hexane are equal to -1.45 and -0.46 cm -1 ⋅K -1 , respectively. The peak and half-width values of the 337.2-nm band for C 60 in polystyrene remain unchanged on cooling to 77 K. Absorption in the 700-800-nm region for concentrated hexane solutions of fullerene at 292 K results from the production of (C 60 ) n -clusters.Keywords: vibronic spectra, broadening, temperature effect, dispersion interactions, 0-0 band, solvatochromic shift, (C60)n-clusters.Introduction. The spectral properties of fullerene C 60 are interesting because of the broad application of materials based on carbon nanoparticles in science, industry, and medicine [1][2][3][4][5][6][7]. The solution of several astrophysical problems was also directly related to the study of the spectral properties of fullerenes [8,9]. Polymers that can be used as a basis for creating optical emission limiters, photo-conducting materials, and materials suitable for transforming solar energy into electrical are being sought [3, 10-12]. For example, the ability of C 60 bound covalently to phthalocyanine to transform long-lived (0.2 ms) photoinduced states with charge separation [11] was used to fabricate a solar cell in which the polymer matrix acted as an external light-collecting antenna and effected charge transfer into the reaction center, the phthalocyanine-fullerene dyad [12].Fullerene C 60 is a spherical molecule with point-group symmetry I h . The unique structure is responsible for its unusual photophysical properties that differ substantially from those of heterocyclic compounds. The vibronic spectra of C 60 exhibit both allowed and symmetry-forbidden transitions. Extensive theoretical and experimental investigations enabled bands and lines in spectra of fullerene C 60 to be identified [9,[13][14][15][16][17][18][19][20][21][22][23][24][25][26] and general trends of the shift upon changing the nature of the medium to be explained [13,14]. As a rule, n-alkanes were used as the solvents [13,14]. The bands and symmetry assignment of transitions for C 60 have been described in detail [16]. Measurements at low temperatures were made in glassy solvents such as toluene [15], 3-methylpentane, and hexane [16,17] at 77 K, in inert-gas matrices at 4.2 K and lower [9,18,19,26], and in the gas phase (in molecular beams cooled by expansion) [21][22][23][24]....

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The effect of dispersion interactions on the electronic spectra of organic compounds is analyzed within second-order perturbation theory. A formula is obtained which makes is possible to determine the change in the polarizability of the molecules upon excitation, ∆α eg , using data on the effect of the bulk polarizability P n of the solvent on the position of the electronic spectrum. It is shown that when the solutions are cooled, the long-wavelength shift of the absorption spectra for the studied anthracenes in alcohols is mainly due to dispersion interactions and is caused by an increase in P n . The possibility of using the new formula for ∆α eg is demonstrated for anthracenes as an example. It is found that for anthracene, 1-chloroanthracene, 9,10-dichloroanthracene, 9,10-dibromoanthracene, 9,10-diiodoanthracene ∆α eg is 16.5, 16.9, 17.2, 18.6, and 20.2 Å 3 respectively.

The theory demonstrating the role of medium at the fluorescence quenching of polar compounds in solutions is briefly presented. It has been shown, that the rate of S(1) --> X(n) nonradiative conversion between the intramolecular charge transfer states depends on the permanent dipole moments in the ground (S(0)) and excited (S(1), X(n)) states as well as on solvent polarity. A relation for the rate of nonradiative excited-state energy conversion has been obtained and employed to test the known literature data for solvent effect on the S(1)-state lifetime of some biologically significant carotenoids and dyes (phthalimides). For phthalimides, the solvent isotope effect on the S(1)-state energy conversion, when hydrogen is replaced by deuterium in the OH groups of alcohols and water, has been analyzed. Based on the data for fluorescence quenching in solvents of different polarity, the dipole moments in the intermolecular charge transfer S(1) state have been obtained for carotenoids (peridinin, fucoxanthin, uriolide acetate) and for hydrogen-bonding complexes, which are formed by 4-amino-, 4-methylamino-, and 4-dimethylamino-N-methylphthalimides in alcohols and water.

The solvent effect on the position and the shape of the absorption spectrum of peridinin for 12 protic and aprotic solvents as well as the temperature effect for methanol were studied using a solvatochromic theory based on the Onsager sphere cavity model. (Experimental data have been provided by T. Polivka and V. Sundström.) Solvatochromic calculations combined with estimations of orientation broadening of the absorption spectrum by convolution allowed the conclusion that the orientation (dipole-dipole), induction and dispersion solute-solvent interactions reasonably describes the position of the 0-0 frequency. The orientation interactions led to the blue solvatochromic shift, separating them from the induced and dispersion interactions, which produce a red shift. The FWHM of Gaussian of inhomogeneous broadening originated from the fluctuations of orientation interactions was demonstrated to be high (945 cm(-1)) even for such a nonpolar solvent as hexane. The value of |Δμ|/cos φ of -18.7 D has been found (Δμ = μ2 - μg, φ is the angle between Δμ and μg). By assigning peridinin to the idealized C2v point group, the large change of dipole moment |Δμ| of 18.7 D under S2←S0 transition is obtained for peridinin in gas phase. Moreover, the S2-excited state dipole moment μ2 has the opposite orientation relative to that at the ground S0 state μg. The determined gas-phase 0-0 energy of the S2←S0 transition, 22 910 cm(-1) (2.84 eV) is employed to calculate the polarizability change between the S0 and S2 states of 376 Å(3). The finding for the effective Onsager radius is of 9.4 Å. Obtained results for electrostatic properties of the S2 state are compared with those known from Stark spectroscopy and quantum-mechanical calculations.

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