Ground and Excited State Dipole Moments of LAURDAN Determined from Solvatochromic and Thermochromic Shifts of Absorption and Fluorescence Spectra A

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Ground and Excited State Dipole Moments of LAURDAN Determined from Solvatochromic and Thermochromic Shifts of Absorption and Fluorescence Spectra A. Kawski, B. Kukliński, P. Bojarski, and H. Diehla Institute of Experimental Physics, University of Gdansk, ul. Wita Stwosza 57, 80-952 Gdansk, Poland a Biophysik, FBI, Universität Bremen, Postfach 330440, D-28334 Bremen, Germany Reprint requests to Prof. A. K., ul. Gen. W. Sikorskiego 11, PL-84-200 Wejherowo, Poland

Z. Naturforsch. 55a, 817-822 (2000); received July 17, 2000

The electric dipole moments in the ground /ig and excited state of the fluorescent probe LAUR- DAN are determined from solvatochromic and thermochromic shifts to be /ig = 3.46 D, /^ = 10.6 D and /ig = 3.76 D, /Je = 10.24 D, respectively. These values concern the free LAURDAN molecule. The values of the dipole moments of LAURDAN (= 6-decanoyl-2-dimethylamine-naphthalene) exceed dis- tinctly those of PRODAN (= 6-propionyl-2-dimethylamine-naphthalene).

Key words: Solvatochromic and Thermochromic Shifts; Dipole Moments in the Ground and Excited States; LAURDAN-Fluorescent Probe.

1. Introductions that they are comparable to those determined experimentally for PRODAN, which of course may be incorrect.

LAURDAN (6-dodecanoyl-2-dimethylamine-naph- The change in the dipole moment A/i = /^ - /i.g caused thalene), a fluorescent probe, is often used to study the for PRODAN by the optical excitation was studied polarity of lipid interfaces [1-6]. In highly polar solvents experimentally by several investigators [7-12] who LAURDAN shows a considerable red shift of its fluores- obtained very different values. The value of /^ = 20 D cence spectrum due to dipolar relaxation processes. This determined by Weber and Farris [7] for PRODAN is effect is attributed to the high dipole moment of the incorrect, as proved by Balter et al. [9]. In our previous LAURDAN molecule in its first single excited state. works [11, 12] both ^ and /^ have been simultaneous- Figure 1 shows the structural formula of LAURDAN ly determined for PRODAN by the "solvent perturba- (n = 10) and PRODAN (tt = 1). Due to the lack in the lit- tion" method [13,14] which makes use of solvatochrom- erature of the dipole moments of LAURDAN in the ic and thermochromic effects. It has been found that the ground, /ig, and excited, /^, state it is commonly assumed pJjMg ratio determined by the thermochromic method amounts to 2.7 and is smaller than that determined by the solvatochromic method, for which fijßg = 3.0. The ther- mochrmoic method is more accurate since the spread in CH3 I experimental data is smaller. This method strongly N restricts specific interactions between luminescent and solvent molecules as well as interactions between the sol- CH. vent molecules.

CH3-(CH2)N-C% In the present work both mentioned methods have been used to determine the dipole moments /ig and /^ of 0 LAURDAN.

n=1 PRODAN 2. Basic Equations for the Analysis n=10 LAURDAN of Electric Dipole Moments Fig. 1. Structural formula of 6-dodecanoyl-2-dimethylamine- naphthalene (LAURDAN) and 6-propionyl-2-dimethylamine- The following equations are based on the quantum- naphthalene (PRODAN). mechanical perturbation theory [13, 14] of the absorp-

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Dieses Werk wurde im Jahr 2013 vom Verlag Zeitschrift für Naturforschung This work has been digitalized and published in 2013 by Verlag Zeitschrift in Zusammenarbeit mit der Max-Planck-Gesellschaft zur Förderung der für Naturforschung in cooperation with the Max Planck Society for the Wissenschaften e.V. digitalisiert und unter folgender Lizenz veröffentlicht: Advancement of Science under a Creative Commons Attribution Creative Commons Namensnennung 4.0 Lizenz. 4.0 International License. 818 A. Kawski et al. • Ground and Excited State Dipole Moments of LAURDAN tion (vA) and fluorescence (ifc) band shifts (in wavenum- or bers) in solvents of different permittivities e and refrac- mi+m2 (m2>mi). B (11) tive indexes n for a spherical molecule [15]: m2 - mj

vK-\>p = mxf(£, n) + const, (1) Generally, the dipole moments /ig and p^ are not parallel

vA + Vp = -m2 [/(£, n) + 2 g («)] + const, (2) but make and angle (p. The use of (3) and (4) leads then to where Mi (3) hca3 2 2 Me =| Mg (12) _2 (/le-Mg) = 2(p +p -2pgpecos(p) hca3 hca3

COS (p = • (13) 2(fil -pi) 6 m B m 2ßo Me 2 i - . r - (4) hca The parameters ml and m2 can be determined from the absorption and fluorescence band shifts given by (1) and £-\ n (2). From the discussion of (13) it follows that for m /m 2e + l 2 n2 + 1 l 2 f(e,n) = (5) = 1/2 and pjp^ = 3 is tp = 0, and the dipole moments /ig 2a e-\ V —— and /ie are parallel. 1- 3 3 2 a 2e + l v a 2n + xy

n -1 a n -1 3. Experimental 1- 2 n2 +1 a3 2n2 +1 (6) g(n) = 3.1 Methods 2 a -1 1- r 2n2 +1 The absorption and fluorescence spectra of LAUR- DAN in ethyl acetate between 246 and 383 K were p^ and //e are the dipole moments in the ground measured with the apparatus described in [12] using and excited state, respectively, a is the polarizability home constructed high pressure cells. The determina- and a the Onsager interaction radius of the solute, h the tion of permittivities £ and refractive indexes n at Planck constant, and c is the velocity of light in the different temperatures were determined from the vacuum. empirical formulas and are listed for ethyl acetate in In the case of isotropic polarizability of molecules, the [12, 15, 17]. condition cd a3 - 1/2 is frequently satisfied and (5) and (6) are then considerably simplified: 3.2 Solvatochromic Shifts of Absorption z 2 2n +l e-\ n -1 and Fluorescence Spectra of LAURDAN 2 (7) n + 2U + 2 n2 + 2 J' in Different Solvents

, , 3 rc4 -1 Figure 2 shows selected spectra of LAURDAN in dif- (8) ferent solvents. In unpolar n-hexane, the absorption and 1 fluorescence maxima are vA =29142 cm" and% = where/(e, n) is the solvent polarity parameter. 25610 cm"1, respectively. However, in the strongly polar 1 1 If the ground and excited state dipole moments are par- acetonitrile vA =28239 cm" and % = 22025 cm" . Pass- allel, based on (3) and (4) the following equations are ing from the unpolar rc-hexane (/(e, n) = 0.0043) to the obtained [13, 16]: strongly polar acetonitrile (/(£, n) = 0.8636) the fluorescence spectrum shifts very strongly (Av = 3585 cm-1) 3 / 2 F m2 -mi ( hca 1 (9) compared to the absorption spectrum (Ava = 903 cm" ). ^ 2m\ Figures 3 and 4 show the spectra shifts vA - Vp and vA + Vp of LAURDAN plotted for 14 solvents versus the 3n1/2 solvent polarity functions /(e, n) and /(e, n) + 2g (n), m] + m2 f hca (10) 2 tri] respectively. A linear regression was carried out and a fit 819 A. Kawski et al. • Ground and Excited State Dipole Moments of LAURDAN

od) c CO -Q 0,5 S

Di-n-butyl ether Ethyl acetate Acetonitrile

1 • i—• A'^r- • i i i • i i 18000 20000 22000 24000 26000 28000 30000 32000 34000 Fig. 2. Absorption and fluo- _ 4 rescence spectra of LAUR- v [Cm ] DAN in different solvents.

Table 1. Dipole moments determined from solvatochromic shifts and thermochromic shifts for LAURDAN. The conversion fac- 30 18 tor for the dipole moment: [^]SI/Cm = 3.33564 x 10" [/t]cgs/D, where D is the symbol of Debye and 1D = 10~ esu cm.

Onsager Solvatochromic shifts Thermochromic shifts radius a [A] mi m2 Pk Ajj pjpg mx m2 p% p^ Ap pjpg

[cm-1] in Debye [D] [cm-1] in Debye [D]

5.5 2640 5410 3.46 10.06 6.6 2.91 2540 5490 3.76 10.24 6.48 2.72

to these data was obtained *. The slopes of the fitted lines increase in temperature. With increase in temperature the presented in Figs. 3 and 4 were found to be mx = 2640 dielectric permittivity, e decreases [12, 17], which leads -1 -1 cm and m2 = 5410 cm , repectively, and they are giv- to the decrease of solvent polarity parameter /(e, n). en in Table 1. Since for LAURDAN, similarly as for PRODAN, a strong fluorescence quenching takes place with increase in temperature (Fig. 6), it was not possible to exceed the 3.3 Thermochromic Shifts of Absorption and Fluores- temperature 373 K. cence Spectra of LAURDAN in Ethyl Acetate Figures 7 and 8 show the thermochromic spectra shifts Figure 5 illustrates the LAURDAN absorption and vA - and vA + Vp versus the solvent polarity functions fluorescence spectra in ethyl acetate between 246 and f(e, n) and/(e, n) + 2 g (n) as well as on temperature T 383 K. A blue shift of the spectra is observed with (according to (14) and (15) in our previous paper [12]), respectively. The slopes of the fitted lines presented in -1 Figs. 7 and 8 were found to be mx = 2540 cm and * It should be remarked that in this case the sequence of m = 5490 cm-1, respectively, and they are given in experimental points in Fig. 4 is not identical to that in Fig. 3, 2 which is caused by the function g(n), i.e., a large irregular- Table 1. ity of the refractive index n when passing from one to another solvent. 820 A. Kawski et al. • Ground and Excited State Dipole Moments of LAURDAN

49 ' 1 1 1 1 * 1 1 1— 0,4 0,6 0,8 1,0 1,2 1,4 f(e,n) f(e,n) + 2g(n)

Fig. 3. Plots of vA - Vp versus/(e, n) for LAURDAN in differ- Fig. 4. Plots vA + % versus/(£, n) + 2 g (n) for LAURDAN in ent solvents: 1 = «-heptane, 2 = rc-hexane, 3 = triethylamine, the same solvents as in Figure 3. 4 = trichloroethylene, 5 = di-n-butyl ether, 6 = anisole, 7 = di- ethylether, 8 = ethyl acetate, 9 = tetrahydrofuran, 10 = 1,2- dichloromethane, 11 = 1,2-dichloroethane, 12 = acetone, 13 = N,N-dimethylformamide, 14 = acetonitrile.

<1) O C _0Q3

O -Q CO <>D TO CD

0,0 0,0 18000 20000 22000 24000 26000 28000 30000 32000 34000

v [cm1] Fig. 5. Absorption and fluorescence spectra shifts for LAURDAN in ethyl acetate at different temperaures T. A. Kawski et al. • Ground and Excited State Dipole Moments of LAURDAN 821

T[K] -246 •253 -263 273 •283 -296 —m—-30 5

T— -313 ...... 323 -333 343 - -v - •353 —0— -363 —O" -373 —•— -383

0,0 Fig. 6. Influence of temperature on 18000 20000 22000 24000 26000 the fluorescence intensity for LAUR- _ 1 DAN in ethyl acetate from 246 to v[cm J 383 K.

0,79201 -0,00102 *T 1,46121 - 0,00158 »T 0,92 0,96 1,00 1,04 1,08 0,40 0,44 0,48 0,52 0,56 373K

246K

5,2 1 ' " ' 1 *- -1 0,88 0,92 0,96 1,00 1,04 1,08 0,40 0,44 0,48 0,52 0,56 f(e,n) f(e,n) + 2g(n)

Fig. 7. Plots of vA - Vp versus/(£, n) for LAURDAN in ethyl Fig. 8. Plots vA + Vp versus/(£, n) + 2 g(n) for LAURDAN in acetate at different temperatures T. ethyl acetate at different temperaures T.

4. Ground and Excited State Dipole Moments same as calculated for MANA** (Fig. 9) by Sändez et al. of LAURDAN [18] with the help of Discover software from Biosym, which allows molecules to be constructed and optimized According to (3) and (4) or (9) and (10) the values of by molecular dynamics, and to calculate interatomic dis-

the dipole moments /ig and n^ depend not only on the tances. They used an a value of one-half the average

parameters mx and m2 but also on the Onsager interaction radius a. For LAURDAN one can assume a = 5.5 Ä, the ** eo[6-(N-methyl-N-alkylamine)naphthoyl]alcanoic acid. 822 A. Kawski et al. • Ground and Excited State Dipole Moments of LAURDAN

CH3 ratio pjpg = 2.91 determined from the solvatochromic

method is somewhat bigger than that iuJ/Lig = 2.72 N obtained from the thermochromic method. Asimilar beha- ^CH viour was observed for PRODAN [12]. The ratio nJfi^ is 3 for both molecules (LAURDAN, PRODAN) independent COOH — (CH ) —C" " "" of the Onsage radius, which is in accordance with (11).

^g This indicates that /ig and fle are parallel to one to another.

MANA 5. Concluding Remarks

Fig. 9. Stuctural formula of co-[6-(N-methyl-N-alkylamine)- The electric dipole moments of LAURDAN = naphthoyl]alcanoic acid (MANA). 3.76 D, jUe = 10.24 D for a = 5.5 Ä) are much bigger than those of PRODAN (/^ = 2.45 D, ^ = 6.65 D for a = 4.2 Ä). These values concern free molecules. The molecular size in the three dimensions. Comparing the thermochromic method allows for more accurate deter- structural formulae (Figs. 1 and 9) it is seen that both mination of the above quantities. chemical compounds LAURDAN and MANA differ only in the substituents CH and COOH, respectively. 3 Acknowledgement The values of fig and ^ for LAURDAN determined using both methods are given in Table 1. It is seen that the Dr. P. B. thanks the DAAD for the scholarship.

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