Ground- and Excited-State Dipole Moments of 6-Propionyl-2-(dimethylamino)naphthalene Determined from Solvatochromic Shifts* A. Kawski Luminescence Research Group, Institute of Experimental Physics, University of Gdansk, ul. Wita Stwosza 57, 80-952 Gdansk, Poland Reprint requests to Prof. A. K„ Fax: +48 58 3413 175 Z. Naturforsch. 54a, 379-381 (1999); received April 30, 1999 The dipole moments in the ground- and excited-state of the fluorescence probe 6-propionyl-2-(dim- ethylamino)naphthalene (PRODAN) are determined from solvatochromic shifts to be = 2.1 D and tje = 6.4 D. These values concern the free molecule. In the first excited singlet state the dipole moment is only 3 times greater than in the ground state. 1. Introduction the increments in the dipole moments, A/J, determined for different fluorescent molecules, based on the Lippert- The hydrophobic fluorescent probe 6-propionyl-2- Mataga solvent polarity parameter are much overrated in (dimethylamino)naphthalene (PRODAN), is highly sen- comparison to the values /ue obtained by electrooptical sitive to the solvent polarity and can potentially reveal methods [7-10], the polarity of its immediate environment. The spectral In the present paper we determine the dipole moments properties of PRODAN (Figure 1) are of interest in bio- ,ug and yue of PRODAN by the "solvent perturbation chemistry and are described by Weber and Farris [1] and method" [8, 11], utilizing the absorption and fluores- by Lakowicz [2]. cence shifts in different solvents measured by Catalan et al. [4]. CH, I " N "CH. 2. Basic Equations of the Analysis jOOT of Dipole Moments CH3-CH2-C V The following equations are based on the quantum- mechanical perturbation theory [8, 11] of the absorp- PRODAN tion (vA) and fluorescence (vF) band shifts (in wave- Fig. 1. Structural formula of 6-propionyl-2-(dimethylamino) numbers) in different solvents, when the dipole mo- 3 naphthalene (PRODAN). ments /ug and are parallel and when a/a = 1/2 (a is the polarizability and a the Onsager interaction radius of the solute) [10]: The change of the electric dipole moment, A/U = /UE~IUG (where /je and /jg are the dipole moments in the excited- vA-vF = mx • f(e,n) + const, (1) and ground-state, respectively) of PRODAN has been v + v = -m [/(£,«) + 2g(n)] + const, (2) experimentally studied by several authors [1, 3, 4], us- A F 2 ing the Lippert-Mataga equation [5,6]. It was shown that where ftc , 2n~ + 1 £-1 * Address for correspondence: PL-84200 Wejherowo, ul. Gen. l z (3) W. Sikorskiego 11, Poland. n + 2 I £ + 2 n +2 0932-0784 / 99 / 0600-0379 $ 06.00 © Verlag der Zeitschrift für Naturforschung, Tübingen • www.znaturforsch.com 380 A. Kawski • Dipole Moments of PRODAN Determined from Solvatochromic Shifts Fig. 2. Plots of vA-vF versus /(£, n) (3) for PRODAN in different solvents: 1 - cyclohexane, 2 - triethylamine, 3 - an- f(€,n) + 2g(n) isole, 4 - chloroform, 5 - ethyl acetate, 6 - dichloromethane, 7 - benzonitrile, 8 - acetone, 9 - N,N-dimethylformamide, Fig. 3. Plots of vA+vF versus/(e, n) + 2g(n) for PRODAN in 10 - dimethyl sulfoxide, 11 - acetonitrile. the same solvents as in Figure 2. is the solvent polarity parameter [12] and Using standard methods of statistical analysis, Koutek [14] verified sixteen equations, based on the existing 4 , x 3 n — (4) theories of long range solute-solvent interactions that 2 (n2+ 2)2 describe the dependence of the absorption and fluores- cence band maxima shifts, vA and vF, respectively. Nine 2{/je-figy (5) selected luminescent compounds have been studied in m\ - 3 h c a view of characteristic functions of the electric permittiv- ity £ and the refractive index n. The values of /je thus ob- 2(/ie tained were compared to those determined independent- m2 (6) h c a3 ly by electrooptical measurements. The optimum results (mean relative error ±11.7 -s- 14.6%) were obtained for £ and n are the permittivity and the refractive index of the solvent polarity parameter/(£, n) expressed by (3), the solvent, respectively, h is the Planck constant, and c [14]. the velocity of light in vacuo. The parameters m j and m2 can be determined from the absorption and fluorescence band shifts (1) and (2), and 3. Results and Discussion the values of /jg and from (5) and (6) [10, 13]: l 3 A A In Figs. 2 and 3 the spectra shifts vA- vF and vA+vF _ m2 —mj h c a' (7) of PRODAN, observed by Catalan et al. [4], are plot- 2 ni\ / ted for eleven selected solvents versus the solvent po- larity functions/(£, n) and/(£, n) + 2g(n), respective- ni] + m he a 2 (8) ly. A linear regression was carried out and a fit to these 2 m\ data was obtained. The measured points satisfied well (1) and (2) except for chloroform (point 4) and dichlo- or romethane (point 6) (see Figure 3). Points 4 and 6 indi- = (m2>m1). (9) cate less polar solvents, as one could expect based on m2 - W] the high permittivity. These polar molecules are very Thus, for a given Onsager radius, the ground- and excit- small and have a notable dipole moment. The pairing ed-state dipole moments can simultaneously be deter- of dipoles can form non-dipolar dimers (self-associat- mined by the spectroscopic method. The solvent polarity ing liquid) and reduce the microscopic polarity of the function, /(e,/?), (3), different from Lippert-Mataga solvent [15]. function, depends most strongly on £ over the interval The slopes of the fitted lines presented in Figs. 2 and 2<£< 10, and to a lesser degree on n. 3 were found to be /?;, = 2500 cm"1 and m-, = 5000 cm"1, 381 A. Kawski • Dipole Moments of PRODAN Determined from Solvatochromic Shifts respectively. Assuming the Onsager interaction radius of 2 D, the value approximated by Macgregor and Weber the solute, a = 4.2 Ä, adopted from crystallographic data [17]. However, ~ 20 D in the fluorescent state is in- by Weber and Farris [ 1 ], we obtain from (7) and (8) /ug = correct, as was shown by Balter et al. [3 |. 2.14 D (7.14- 1CT30 Cm) and = 6.43 D (21.45 • 1(T30 Assuming a = 4.6 Ä, the calculated Onsager cavity ra- Cm). The dipole moment difference A/j = Pe-/Jg between dius of a sphere with the volume being the sum of the the excited and ground states is about 4.3 D. From (9) volumes of all atoms [18], we obtain now for mx = 1 1 one can determine/je if /jg is known from dielectric meas- 2500 cm" and m2 = 5000 cm" from (7) and (8) /jg = urements. Thus the absolute /je value can be derived in- 2.46 D and /ue = 7.37 D. These values are only somewhat dependently of any assumption on a. Based on (9), for greater than the /jg and /ut values for a = 4.2 Ä. /jg = 2.14 D we have /ue = 6.42 D. It is necessary to mention that one determines the di- The solvent polarity function/(£, n), which is related pole moment of a free molecule on the ground state by to the non-specific interactions, can be correlated with our theory [8]. In order to eliminate the specific solute- the empirical parameter n* [16]. As was shown by Kou- solvent association as a result of the formation of hydro- tek [14], the equation gen bonds, and the formation of solvation shells around the solute molecule in two component solvents (non-po- TT* = 1.04/(£, n) + 0.014 (10) lar/polar) it is useful to apply the thermochromic shifts between 7r* and/(£, n) is fullfilled. method [10, 19-23]. The shifts of vA-vF and vA+vF of the studied com- pound in the same solvents plotted versus the 7t* values are linear (except for points 4 and 6), and from the slopes 4. Conclusions 1 -1 we obtain m, = 2450 cm" and m2 = 4900 cm . In this case the dipole moments fjg and (from (7) and (8)) are a) The dipole moments in the ground and excited states 2.15 D and 6.45 D, respectively. The difference between determined by the "solvent perturbation method" are the dipole moments determined by the use of the empir- /ug = 2.\ D and fje = 6.4 D for a = 4.2 Ä, and /jg = 2.5 D ical parameter 7t* (10) and the polarity function/(£, n) and fje = 7.4 D for a = 4.6 Ä. (3) is very small. Meanwhile, the dipole moments of b) These dipole moments concern the free PRODAN PRODAN determined by Balter et al. [3] and Catalan et al. molecule. [4^ are greater, as one should expect from the solvent c) The dipole moment in the first excited state is only £ — 1 n2 — 1 3 times greater than in the ground state. 2 A detailed study of the thermochromic shifts of polarity parameter in the form A/ = o £ +1 ~ 2~~ —1 PRODAN in one selected solvent is in progress. The presently received volume /jg = 2.14 D is close to [1] G. Weber and F. J. Farris, Biochem. 18, 3075 (1979). [13] A. Kawski, Naturwiss. 51, 82 (1964). [2] J. R. Lakowicz, Principles of Fluorescence Spectroscopy, [14] B. Koutek, Coll. Czech. Chem. Comm. 43, 2368 (1978). Plenum Press, New York, 1983. [15] C. J. F. Böttcher, Theory of Electric Polarization, Else- [3] A. Balter, W. Nowak, W. Pawelkiewicz and A. Kowal- vier Publ. Company, Amsterdam, 1952. czyk, Chem. Phys. Lett. 143, 565 (1988). [16] M.J. Kamlet, J. L. Abboud and R. W. Taft, J. Amer. Chem. [4] J.