Determination of reorganization energy of fluorenone and 4-hydroxyfluorenone in neat and binary solvent mixtures

Spectrochimica Acta Part A 67 (2007) 444–449

Determination of reorganization energy of fluorenone and 4-hydroxyfluorenone in neat and binary solvent mixtures Marek J´ozefowicz ∗ Institute of Experimental Physics, University of Gda´nsk, ul. Wita Stwosza 57, 80-952 Gda´nsk, Poland Received 12 May 2006; received in revised form 18 July 2006; accepted 28 July 2006

Abstract Steady-state absorption and fluorescence measurements of fluorenone and 4-hydroxyfluorenone in neat and binary solvent mixtures were used to explore the reorganization energy in liquid system. The results of spectroscopic measurements were used to calculate, according to Marcus theory, the outer-sphere solvent reorganization energy, λ0 , and the internal molecular reorganization energy, λin . Preferential solvation of fluorenone and 4-hydroxyfluorenone in binary solvent mixtures has been studied by monitoring the outer-sphere solvent reorganization energy. In cyclohexane–tetrahydrofuran mixtures, the deviation from linearity in the λ0 versus the solution polarity is due to non-specific dipolar solvent–solute interactions. For cyclohexane–ethanol binary mixtures, both non-specific and specific (hydrogen bond) interactions contribute to the observed changes. © 2006 Elsevier B.V. All rights reserved. Keywords: Fluorenone; 4-Hydroxyfluorenone; Reorganization energy; Preferential solvation

1. Introduction Molecules are stabilized by their interaction with the solvent, and this induces a shift in their absorption and emission spectra as compared with that in the gas phase [1,2]. It is well-known that the solvent can influence the physico-chemical properties of a solute in a wide variety of chemical processes [3,4]. Effects of this nature are commonly interpreted as resulting from changes in the polarity, a general term that comprises the overall solvating capability of the medium. When a dipolar solute is introduced into a neat polar solvent, the solvent responds predominantly by vibrational motions to optimize the solute–solvent interactions. On the basis of Marcus formalism [5,6], reorganization energy can be divided into two parts: an internal (inner-sphere) component, λin , and an outer-sphere component, λ0 . The inner-sphere component encompasses changing bond lengths and geometries within a molecule, while the outer-sphere encompasses intermolecular electronic interactions such as alignment of a polar solvent’s dipole moments. In simple continuum theory, where a dipolar interaction is considered the only important interaction, the emission

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energy varies linearly with a parameter f(ε) that is defined as f(ε) = 2(ε − 1)/(2ε + 1), where ε is the static dielectric constant of the solvent [1,2]. The situation is different in binary solvent mixtures. The local composition of the solvent species around a dilute solute dissolved in a solvent mixture can be significantly different than the bulk composition [7–11]. In the case of a polar solute in a liquid mixture containing similar size polar and nonpolar components, one would expect the local composition of the more polar solvent around the solute to be higher than in the bulk [7–11]. This concept of preferential solvation has long been used qualitatively to rationalize measured solute properties that deviate from a linear dependence on the solvent composition. Two types of interactions have been shown to lead to preferential solvation. The first is dipole–dipole interaction between a polar fluorophore and the more polar component of the solvent mixture. Large dipole moments of both the polar solvent component and the solute cause dielectric enrichment in the solvation shell. The second-specific interactions occur with solvent molecules that are so close to the solute that intramolecular bonds, such as hydrogen bonds, can form. These interactions are usually directional and reflect a strong interaction between the solute and a single solvent molecule. The photophysical properties of fluorenone derivatives have recently attracted considerable attention, because they are excellent model compounds for the study of the effect of

M. J´ozefowicz / Spectrochimica Acta Part A 67 (2007) 444–449

microenvironment. The influence of the solvent polarity on the configuration and energy of the lowest singlet excited state (S1 ) has been established by steady-state and time-resolved fluorescence studies [11–18]. It is important to note that the carbonyl group in these molecules contains a free electron pair capable of interacting with H-bond donor (protic) solvents. Taking into account the hydrogen bond donor solvent tendency to interact with the oxygen from carbonyl group, as well as hydrogen bond donation ability of the hydroxyl group, these are the likely sites for intermolecular hydrogen bondings in protic compounds. This paper describes part of a study that aims to increase understanding of solvent reorganization energy in liquid systems by systematically varying the environment around a molecule. We measured absorption and fluorescence spectra of fluorenone (9Fl) and 4-hydroxyfluorenone (4HOFl) in neat solvents and in cyclohexane–tetrahydrofuran (CH–THF) and cyclohexane–ethanol (CH–EtOH) mixtures. The spectral data are analyzed using a model based on Marcus theory, providing important new information on the outer-sphere and internal molecular reorganization energies of 9Fl and 4HOFl in neat and binary solvent mixtures. In order to explore the role of specific solute–solvent interactions, the inner- and outer-sphere solvent reorganization energy in CH–EtOH binary mixtures has been determined as a function solvent composition. 2. Experimental details Fluorenone and 4-hydroxyfluorenone were purchased from Aldrich Chemical Co. and purified by recrystallization from toluene. Their purities were checked chromatographically. All of the solvents used were of the highest grade commercially available: ethanol (99.9%) was reagent grade; all others solvents were Aldrich spectral or HPLC grade. CH was distilled before use from a sodium–potassium amalgam, to ensure that was anhydrous. Additionally all solvents were checked in both steady-state and time-resolved fluorescence apparatuses for lack of fluorescent impurities in the wavelength ranges of interest. Absorption, excitation and fluorescence spectra were recorded using respectively a Shimadzu UV-2401 PC spectrophotometer and a Shimadzu RF-5301 spectrofluorometer with 5.0 nm band-widths in both excitation and emission. The fluorescence emitted was observed perpendicularly to the direction of the exciting beam. Samples for fluorescence measurements were prepared using a 0.2 cm high sensitivity micro quartz cell. The concentration of the solutions studied was ca. 5 × 10−4 M. The luminescence spectra reported have been corrected for the spectral response of the photomultiplier (Hamamatsu R-928) and monochromator pass, but not for reabsorption, which was negligible in these samples. 3. Theoretical background The absorption and fluorescence transition energies, ν˜ a and ν˜ f (in cm−1 ) are related to the free energy, G◦ , of the transition, the outer-sphere solvent reorganization energy, λ0 , and the internal molecular reorganization energy, λin , of the solute as

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follows [5,6]: ν˜ a = G◦ + λ0 + λin ,

(1)

ν˜ f = G◦ − λ0 − λin .

(2)

The total internal molecular reorganization energy (λin ) is the sum of the intramolecular reorganization energy (λ∗in ) associated with vibrations for which hνi < kT, and the intramolecular reorganization energy (λ∗∗ in ) associated with vibrations for which hνi > kT. Eqs. (1) and (2) are usually applied to broad, structureless spectra. Treating the solvent as a dielectric continuum, Brunschwig et al. [6] have shown that the full-width at halfF ) of the emission band can be related to the maximum (˜ν1/2 coupling of the solvation and molecular vibrations (inclusive torsions) to the electronic transitions: F hc ˜ν1/2

8 ln 2

= 2λ0 kT + 2λ∗in kT + λ∗∗ in hνi .

(3)

The deficiencies of continuum theory are well-known. It is based on a point dipole approximation, and higher order multipolar interactions, dispersion forces, and changes in polarizability on excitation are neglected, and the predicted solvent shifts depend strongly on the poorly defined cavity radius a. Nevertheless, the theory has been applied successfully to explain trends in the solvent-dependent absorption (emission) spectra of a number of solvatochromic dyes [5,6]. The solvent reorganization energy in wavenumbers is estimated using the following equation:   (μ)2 ε − 1 n2 − 1 λ0 = − , (4) 4πε0 a3 2ε + 1 2n2 + 1 where μ = μe − μg is the difference in the ground and excited state dipole moments, a the cavity radius, ε the static dielectric constant, and n is the refractive index. Eq. (4) describes the change in energy of the solute–solvent system resulting from relaxation of the solvent dipoles following a change in the solute’s dipole moment (due to an electronic transition). The total internal molecular reorganization energy in different solvents is given by:   λin hνi (˜ν1/2 hc)2 (μ)2 ε − 1 n2 − 1 λ∗in + = − − . 2kT 16kT ln 2 4πε0 a3 2ε + 1 2n2 + 1 (5) Most theories [19,20] of the solvent effect on the location of the absorption, ν˜ a , and fluorescence, ν˜ f , bands of the solute in different solvents lead, in spite of different assumptions, to similar expression for the difference (˜νa − ν˜ f ) and sum (˜νa + ν˜ f ): ν˜ a − ν˜ f = m1 f (ε, n) + const.,

(6)

ν˜ a + ν˜ f = m2 (f (ε, n) + 2g(n)) + const.,

(7)

m1 = m2 =

 g )2 2(μ e − μ , hca3 2(μ2e − μ2g ) hca3

,

(8) (9)

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M. J´ozefowicz / Spectrochimica Acta Part A 67 (2007) 444–449

Table 1 A and ˜ F ), the outer-sphere (λ ) and internal molecular reorganization (λ ) energies for 9Fl The absorption and fluorescence full-width at half-maximum (˜ν1/2 ν1/2 0 in and 4HOFl in neat solvents of different polarity Solvent

n-Pentane n-Hexane n-Heptane Toluene Diethyl ether Butyl acetate o-Dichlorobenzene Ethyl acetate THF Acetone Acetonitrile

f(ε, n)

0.032 0.032 0.033 0.049 0.195 0.204 0.222 0.233 0.243 0.317 0.337

9Fl

4HOFl

A (cm−1 ) ˜ν1/2

F (cm−1 ) ˜ν1/2

λ0 (×10−2 eV)

λin (eV)

A (cm−1 ) ˜ν1/2

F (cm−1 ) ˜ν1/2

λ0 (×10−2 eV)

λin (eV)

4775 4690 4725 4790 4680 4928 5010 4820 4750 4980 5100

3793 3739 3759 3720 3701 3627 3520 3600 3600 3590 3441

0.310 0.310 0.319 0.474 1.886 1.973 2.148 2.254 2.351 3.067 3.260

0.393 0.382 0.386 0.377 0.366 0.351 0.329 0.344 0.344 0.338 0.308

4827 4592 4720 4655 4541 4587 4620 4591 5010 4895 4973

3608 3547 3628 3252 3104 3047 3068 3129 3106 2950 2831

0.293 0.293 0.302 0.448 1.784 1.866 2.031 2.131 2.223 2.900 3.083

0.356 0.344 0.359 0.288 0.255 0.245 0.248 0.258 0.253 0.224 0.204

where μe and μg are the dipole moments in the excited and ground states, respectively, h is Planck’s constant, c is the velocity of light in vacuum, f(ε, n) and g(n) are the solvent polarity functions given by the following equations:   2n2 − 1 ε − 1 n2 − 1 f (ε, n) = 2 − 2 , (10) n +2 ε+2 n +2   3 n4 − 1 g(n) = . (11) 2 (n2 + 2)2

for all solvents, obtained by the use Eq. (5), equals 0.356 and 0.276 eV for 9Fl and 4HOFl, respectively. It is necessary to note that the internal molecular reorganization energy is dominant for both studied molecules in used solvents of different polarities. It is instructive to compare these results with the results obtained for 2-methylaminofluorenone (2MAFl) and 2-

The parameters m1 and m2 can be obtained from the absorption and fluorescence band shifts (Eqs. (6) and (7)). 4. Result and discussion 4.1. Neat solvent The absorption and fluorescence full-width at half-maximum A and ˜ F ), the outer-sphere and internal molecular (˜ν1/2 ν1/2 reorganization energies for 9Fl and 4HOFl in neat solvents of different polarity are given in Table 1. The dipole moment of 9Fl (4HOFl) has been reported to increase from 2.98D (2.51D) in the ground state to 5.34D (4.91D) in the excited state [21]. Assuming that the ground and excited state dipole moments are parallel (the angle between ground and excited state dipole moments: ϕ9Fl = 4◦ , ϕ4HOFl = 11◦ [21]) and using a cavity radius of a9Fl = 3.3A and a4HOFl = 3.4A, the outer-sphere solvent reorganization energy is increased from 0.0031 eV (n-pentane) to 0.0326 eV (acetonitrile) for 9Fl and from 0.0029 eV (n-pentane) to 0.0308 eV (acetonitrile) for 4HOFl. For non-polar solvents the optical dielectric constant (n2 ) is very close to the static dielectric constant (ε), and λ0 is only slightly higher than zero (see Table 1). Naturally, the obtained λ0 values show a linear increase with the increase of solvent polarity function (Fig. 1). It is seen from Fig. 1 that the total internal molecular reorganization energy (λin ) decreases slightly with the increase f(ε, n). As a first approximation, the value of λin in neat solvent may be than taken to be a constant, independent on local environment. The average internal molecular reorganization energy

Fig. 1. Dependence of the outer-sphere solvent reorganization energy (λ0 ) and the internal molecular reorganization energy (λin ) on the polarity function f(ε, n).

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Table 2 The outer-sphere (λ0 ) and internal molecular reorganization (λin ) energies for 9Fl and 4HOFl determined for different mole fraction of THF xp

a f (ε9Fl eff , n)

−2 λ9Fl eV) 0 (×10

λ9Fl in (eV)

f (ε4HOFl , n)a eff

λ4HOFl (×10−2 eV) 0

λ4HOFl (eV) in

0 0.015 0.030 0.073 0.135 0.238 0.612 0.882 0.938 1

0.026 0.027 0.035 0.058 0.098 0.140 0.177 0.191 0.192 0.198

0.309 0.318 0.413 0.683 1.154 1.649 2.085 2.251 2.262 2.351

0.383 0.384 0.367 0.389 0.370 0.366 0.351 0.362 0.351 0.344

0.026 0.037 0.092 0.107 0.135 0.146 0.178 0.193 0.195 0.198

0.292 0.303 0.393 0.652 1.100 1.573 1.988 2.145 2.156 2.223

0.344 0.319 0.312 0.300 0.292 0.287 0.269 0.262 0.256 0.256

a

εeff  are calculated using Bakhshiev model [11]. f(εeff , n) = [(εeff  − 1)/(2εeff  + 1)] − [(n2 − 1)/(2n2 + 1)], n = XCH nCH + XTHF nTHF .

dimethylaminofluorenone (2DMAFl) [17]. The increased flexibility of the fluorenone derivatives (2MAFl, 2DMAFl) leads to a bigger width of the fluorescence spectrum compared with the fluorenone molecule, suggesting a difference in the equilibrium geometry of the S0 and S1 state. It can be seen [17] that the internal molecular reorganization energy for these two compounds decreases along with the increasing solvent polarity, and for more polar solvents the outer-sphere reorganization energy becomes dominant. Furthermore, as expected the value of the λin for 9Fl and 4HOFl molecules is very small compared to the overall energy involved in the electronic transition [18]. 4.2. Binary solvent mixtures 9Fl and 4HOFl in the binary solvent mixtures clearly show non-linearities of solvatochromic shifts of the absorption and fluorescence spectra as a function of the mole fraction of the polar solvent [11,18]. The value of λ0 and λin can be immediately calculated with Eqs. (4) and (5). It is important to note here that the measurements of the dielectric constants of the solvent mixtures (ε) are not available for the system under investigation because the local composition of the solvent near the solute may be different from the bulk composition (preferential solvation). Therefore, the effective dielectric constants εeff  of the solvent shell of 9Fl (4HOFl) can be calculated using the spectral shift method—Bakhshiev’s theoretical model of preferential solvation [11]. Tables 2 and 3 comprises the composition-dependent polarity function (f(εeff , n)), the outer-sphere and intramolec-

ular reorganization energies for the 9Fl and 4HOFl in CH–THF and CH–EtOH mixtures as a function of the mole fraction of the polar component, at room temperature. Taking into account that the factor [(εeff  − 1)/(2εeff  + 1)] − [(n2 − 1)/(2n2 + 1)] decreases as the THF (EtOH) increases, the solvent reorganization energy is also expected to decrease as the amount of polar component increases. The intramolecular reorganization energy of 9Fl and 4HOFl in the mixed solvents show only minor changes with composition of the solution, as illustrated in Figs. 2 and 3. For CH–THF and CH–EtOH system, the λin remains practically constant beyond the initial steep fall. Figs. 2 and 3 also shows plot of λ0 versus XTHF(EtOH) for 9Fl and 4HOFl. Deviation from linearity, as obtained in the present case, may be explained in terms of the preferential solvation of the solute by one of the component solvents [7–11]. In the CH–EtOH solution there is a much larger initial increase in λ0 when a small amount of alcohol is added to non-polar solvent, compared to the CH–THF solution. When the volume fraction of alcohol is increased from 0 to 0.04, the λ0 increases from 0.31 × 10−2 (0.29 × 10−2 ) eV to 2.69 × 10−2 (1.82 × 10−2 ) eV for 9Fl (4HOFl). In contrast, as mole fraction of THF is increased from 0 to 0.03, the λ0 increases from 0.31 × 10−2 (0.29 × 10−2 ) eV to 0.41 × 10−2 (0.39 × 10−2 ) eV for 9Fl (4HOFl). In solutions containing more than about 0.1 mole fraction of the alcohol component, the graph of λ0 versus XEtOH is roughly linear as a result of changes in the bulk solution polarity. These results along with our previous studies [11,18] indicate that strong non-linear dependence of the

Table 3 The outer-sphere (λ0 ) and internal molecular reorganization (λin ) energies for 9Fl and 4HOFl determined for different mole fraction of EtOH xp

a f (ε9Fl eff , n)

−2 λ9Fl eV) 0 (×10

λ9Fl in (eV)

f (ε4HOFl , n)a eff

λ4HOFl (×10−2 eV) 0

λ4HOFl (eV) in

0 0.021 0.042 0.098 0.179 0.303 0.685 0.916 0.956 1

0.026 0.215 0.232 0.239 0.257 0.265 0.277 0.285 0.287 0.288

0.309 2.496 2.694 2.775 2.984 3.077 3.216 3.309 3.332 3.344

0.383 0.381 0.388 0.391 0.368 0.287 0.283 0.263 0.257 0.257

0.026 0.210 0.230 0.238 0.255 0.263 0.279 0.285 0.286 0.288

0.292 1.661 1.820 1.883 2.017 2.081 2.207 2.255 2.263 2.278

0.344 0.329 0.329 0.348 0.338 0.327 0.316 0.264 0.270 0.275

a

εeff  are calculated using Bakhshiev model [11]. f(εeff , n) = [(εeff  − 1)/(2εeff  + 1)] − [(n2  − 1)/(2n2  + 1)], n = XCH nCH + XEtOH nEtOH .

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Fig. 2. Plot of λ0 and λin as a function of mole fraction XTHF for 9Fl and 4HOFl in CH–THF solvent mixtures.

outer-sphere solvent reorganization energy on the mole fraction of EtOH in CH–EtOH mixtures for molecules under study is attributed to formation of hydrogen-bond complexes between solute and solvent in addition to preferential solvation. It is well known that the hydrogen bonding breaks down at higher temperatures, therefore fluorescence spectra for molecules under study were measured at different temperatures. In previous works [11], we have shown that the fluorescence spectra of 9Fl and 4HOFl in CH–THF show only a decreased

Fig. 3. Plot of λ0 and λin as a function of mole fraction XEtOH for 9Fl and 4HOFl in CH–EtOH mixtures.

intensity of fluorescence without change in spectral distribution. However, in CH–EtOH the fluorescence maximum shifts from 494 (507) to 476 (488) nm for 9Fl (4HOFl). Fig. 4 shows the fluorescence spectra of 9Fl in binary mixtures of CH–THF and CH–EtOH determined at different temperatures. The temperature dependent fluorescence full-width at half-maximum F ) of 9Fl and 4HOFl in binary mixtures of CH–THF and (˜ν1/2 CH–EtOH (polar component mole fraction 0.04) determined over the temperature range from 293 to 373 K are shown in Fig. 5.

Fig. 4. Fluorescence spectra of 9Fl in CH–THF (A) and CH–EtOH (B) determined at different temperatures: 1, 293 K; 2, 303 K; 3, 313 K; 4, 323 K; 5, 333 K; 6, 350 K; 7, 363 K; 8, 373 K (polar component mole fraction 0.04).

M. J´ozefowicz / Spectrochimica Acta Part A 67 (2007) 444–449

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Fig. 5. Fluorescence full-width at half-maximum for 9Fl (A, B) and 4HOFl (C, D) molecules in CH–THF (A, C) and CH–EtOH (B, D) determined at different temperatures.

As seen in Fig. 4, the fluorescence maximum is blue-shifted in increasing the temperature of the CH–EtOH mixture, a result of thermally induced hydrogen-bond breaking. The intensity of the fluorescence spectra of 9Fl and 4HOFl in CH–THF decreases linearity with increasing temperature as a result of temperature quenching. F for It is seen (Fig. 5) that in CH–THF mixtures the ˜ν1/2 both molecules under study evolves continuously with decreasF has an exponential ing temperature. As can be seen, the ˜ν1/2 dependence on temperature. For CH–EtOH system the plot F versus temperature) can be drawn not as a single straight (˜ν1/2 line but as two straight lines. Between 320 (330) and 330 (350) K for 9Fl (4HOFl), the fluorescence full-width at half-maximum is discontinuously varied. A double linear correlation indicates that the nature of the emitting state is significantly different in the two ranges of temperature. These findings are understandable in that specific (hydrogen-bond) polar solvent-solute associations appear in CH–EtOH, but not in CH–THF, mixtures [11,16–18].

5. Conclusion It can be concluded that spectroscopic characteristics of 9Fl and 4HOFl molecules are determined by both non-specific and specific hydrogen-bonding solute–solvent interactions. The outer-sphere solvent reorganization energy of the 9Fl and 4HOFl serves as a good parameter for studying preferential solvation characteristics in mixed binary solvents. On the basis of the fluorescence band position, the band shape, and full-width at half-maximum, we conclude that a relatively large fraction of the dye molecules is hydrogen-bonded to an ethanol molecule, even at a relatively low ethanol mole fraction. The hydrogen-bond complexes of 9Fl and 4HOFl break up at higher temperatures.

Acknowledgements I express my gratitude to Prof. J´ozef Heldt for scientific discussion. This work was partially supported by the research grant of the University of Gdansk. BW 5200-5-0209-6. References [1] J.R. Lakowicz, Principle of Fluorescence Spectroscopy, Plenum Press, New York, 1983. [2] N. Mataga, T. Kubota, Molecular Interactions and Electronic Spectra, Marcel Dekker, New York, 1970. [3] Y. Marcus, The Properties of Solvents, John Wiley & Sons, New York, 1998. [4] Y. Marcus, Solvent Mixtures. Properties and Selective Solvation, Marcel Dekker Inc., New York, 2002. [5] R.A. Marcus, J. Phys. Chem. 94 (1989) 3078. [6] B.S. Brunschwig, S. Ehrenson, N. Sutin, J. Phys. Chem. 91 (1987) 4714. [7] P. Suppan, J. Photochem. Photobiol. A: Chem. 50 (1990) 293. [8] P. Suppan, J. Chem. Soc., Faraday. Trans. I 88 (1992) 963. [9] T. Molotsky, D. Huppert, J. Phys. Chem. A 107 (2003) 8449. [10] M. J´ozefowicz, K.A. Kozyra, J.R. Heldt, J. Heldt, Chem. Phys. 320 (2005) 45. [11] M. J´ozefowicz, J.R. Heldt, Chem. Phys. 294 (2003) 105. [12] L. Biczok, T. Berces, H. Linschitz, J. Am. Chem. Soc. 119 (1997) 11071. [13] R.S. Moog, N.A. Burozski, M.M. Desai, W.R. Good, C.D. Silvers, P.A. Thompson, J.D. Simon, J. Phys. Chem. 95 (1991) 8466. [14] L. Bicz´ok, T. B´erces, J. Phys. Chem. 92 (1998) 3842. [15] T. Yatsuhashi, Y. Nakajima, T. Shimada, H. Inoue, J. Phys. Chem. A 102 (1998) 3018. [16] M. J´ozefowicz, J.R. Heldt, J. Karolczak, J. Heldt, Z. Naturforsch. 58a (2003) 144. [17] M. J´ozefowicz, J.R. Heldt, J. Heldt, Z. Naturforsch. 59a (2004) 105. [18] M. J´ozefowicz, J.R. Heldt, J. Heldt, Chem. Phys. 323 (2006) 617. [19] L. Bilot, A. Kawski, Z. Naturforsch. 17a (1962) 621. [20] A. Kawski, in: J.F. Rabek (Ed.), Progress in Photochemistry and Photophysics, vol. 5, CRC Press, Boca Raton, 1992, p. 1. [21] M. J´ozefowicz, J.R. Heldt, Spectrochim. Acta A 67 (2007) 316–320.