Fluorescence spectroscopy of aminobenzenes in solvent mixtures

Journal of Luminescence 33 (1985) 335—344 North-Holland, Amsterdam

335

FLUORESCENCE SPECTROSCOPY OF AMINOBENZENES IN SOLVENT MIXTURES P. SUPPAN and E. GUERRY-BUTTY Institute of Physical Chemistry, University of Fribourg, CH - 1 700 Fribourg. Switzerland

Received 18 June 1985

The shifts of the fluorescence spectra of aniline (A), N-methyl-A and N,N-dimethyl-A in mixtures of cyclohexane and tetrahydrofuran, 1-butanol or 1,4-dioxan differ only because of the molecular volumes of the amines and not because of their excited-state hydrogenbonding properties.

1. Introduction When the fluorescence spectrum of a molecule M is measured in mixtures of two solvents N and P, it is often found that the displacement of the emission frequency maximum ~m5x is not a linear function of the mole fractions XN, xi,. If very small quantities of P added to N produce considerable shifts of the M* M fluorescence, then this is often taken as evidence for the formation of excited-state solute/solvent complexes M* P [1]. Thus, the steep shift of the fluorescence band of aniline (A) in mixtures of 1,4-dioxan (DX) and cyclohexane (CH) is ascribed to hydrogen bonding FNH2 0C2H40 [21.The shift —~

of the fluorescence spectrum of N,N-dimethylaniline (DMA) in the same mixtures is nearly linear with the mole fraction, this behaviour being consistent with the absence of hydrogen bonding when the amine hydrogens are replaced by methyl groups. However, it is well established that the effective or local solvent polarity “seen” by a solute molecule is determined not only by specific association with one of the solvent mixture’s components, but by a much more general process of unspecific association between a dipolar solute molecule and the solvent of higher dielectric constant, a process described as “dielectric enrichment” of the solute’s solvation shell [3]. In this paper we present evidence about the solvatochromic shifts of the fluorescence bands of A, N-methylaniline (NMA) and DMA, which suggest that dielectric enrichment is the major process involved in the non-linearity of the fluorescence spectral shift in several solvent mixtures; the different behaviour of A, NMA and DMA being due to their different molecular volumes rather than to different hydrogen-bonding properties. 0022-231 3/85/$03.30 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)

336

P. Suppan, E. Guerry-Butty

/ Fluorescence spectroscopy of aminobenzenes

Results and discussion Solvatochromic shifts in single solvents Figure 1 shows the solvatochromic shifts of the first absorption band and of the fluorescence band of A, NMA and DMA in a series of single solvents, the frequency p of the maximum of absorption or emission being plotted against the function f(D) 2(D 1)/(2D + 1) of the static dielectric constant of the solvent, a function which will be called the solvent’s dielectric polarity. In the absorption spectra the well-known anomalous blue shift in protic solvents shows that for all three molecules the ground-state specific association is dominated by the hydrogen bond between the amino lone pair and the protic solvent [4]. In the fluorescence spectra this anomaly disappears, the hydrogen bond being broken as a result of the new electron distribution in the lowest singlet excited state S1 which is characterized by the charge transfer (—~ 17* from the =

—

amino group lone pair towards the aromatic ring [5]. The effect of DX on the fluorescence spectra is noteworthy In the context of the present investigation: it shows a marked red shift anomaly in the order A > NMA> DMA, the “apparent polarity” of DX being 0.78 with A, 0.65 with NMA and 0.55 with DMA, whereas its dielectric polarity is only 0.41 from macroscopic measurements. From the dielectric solvatochromic shifts E{f(D)} shown by the lines in fig. I the S1 state dipole moment can be calculated; in a series of solvents of similar refractive indices and free of specific association anomalies the S~1—~ S1 (absorption) and S~ S0 (fluorescence) energies are [4] —*

~E(S0~S1)=

—

~

(1)

AE(S1 ~S0)=

—

~e~(IL~e)~f(D)

(2)

if ,.i5 and l’e are the solute dipole moments in the S0 and S1 states respectively, a being the molecular radius. Since the value and direction of ~u&~ are known, l~e can be calculated from the specific shifts ~E/~f(D) from (1) of (2) or by the ratio method which does not require the value of a [6]. Table I lists the parameters and the p~ values obtained for A, NMA and DMA. The important result is the similarity of the excited state (S1) dipole moment of the three aromatic amines; the specific dielectric shifts ~E/~f(D) increase in the order DMA
___________

.5

a

f(D)

F

A

29~

(

33.5.

345

E

___________

b

ED

~I

~-

f(D)

F

A

33-

C

(~)_N(CH)

____________

()

E

~—

_~_—®

f(D)

Fig. 1. Solvatochromic shifts of the first absorption (Lh) band and of the fluorescence band of A, NMA and DMA in single solvents at 20°C.Abcissa: solvent dielectric polarity f( D) = 2( D —1 )/(2 D + 1). Ordinate: transition energy at A ,,,~, in iO~ cm ~‘. Reference numbers for solvents: (1) cyclohexane; (2) 1,4-dioxane; (3) di-isopropylether; (4) THF; (5) dichloromethane; (6) acetonitrile; (7) 1-butanol; (8) ethanol; (9) dimethylformamide; (10) methanol.

30

(1 32

34

35

E

NH,

F

A

—1

338

P. Suppan, E. Guerry-Buttv

/

Fluorescence spectroscopy of aminobenzenes

Table I Excited-state (~~)dipole moments ~ (in Debye units) of A. NMA and DMA from absorption/fluorescence solvatochromic ratios, andcomparison with molecular volumes (a = cavity radius in A) ~XE/~F(D)

ABS

A NMA DMA

0.95 0.85 0.70

Relative

a

FLU

~E/~f(D)

3.8 3.2 2.9

5.0 4.7 5.1

3.3 3.5 3.7

1 0.85 0.74

ABS

FLU

t 0.89 0.73

0.84 0.76

The ground-state dipole moments are taken as 1.57 Debye for all three molecules, the ~s

5/p~ angle as 38°. ~E/~f(D) are the specific solvatochromic shifts of the first absorption band (ABS) and fluorescence band (FLU) respectively, in iO~cm

Solvatochromic shifts in solvent mixtures

Turning now to the solvatochromic shifts of the fluorescence spectra in solvent mixtures, fig. 2 shows the displacement of the emission maxima in mixtures of CH/DX, CH/THF, CH/BU as a function of the “polar” solvent mole fraction. It is clear that in all three mixtures the deviation from linearity increases in the order DMA < NMA
(3)

2a3f(D). E== -~p.

In a mixture of two solvents N, P of bulk mole ratio X=xP/xN this stabilization therefore increases if the solvation shell of M is enriched in the °

TI-IF

=

tetrahydrofuran, BU

=

1-butanol.

P. Suppan, E. Guerry-Butty

/

339

Fluorescence spectroscopy of aminobenzenes

more polar solvent P; the thermodynamic equilibrium is reached when the increase in electrostatic stabilization energy is balanced by the decrease in the entropy of mixing of the solvents of local mole ratio Y=y~/y~,and this leads to the dielectric enrichment condition (4)

Y=Xexp[_zr_6F(8)]

for a point of coordinates r, 0 from the solute dipole centre. The index of preferential solvation z describes the importance of the solute dipole/solvent (dielectric) stabilization: 2Af(D)/8iu3RT, (5) z 9Mp. =

where M and ~ are the mean molecular weights and densities of the solvents ~f( D) being the difference in the solvents’ dielectric polarities; R is the gas constant and T the absolute temperature. F(0) is the angular dependence of the solute—solvent interaction energy. equal to cos20 + k sin20 for pure rigid dipole—dipole interactions. The total solute—solvent stabilization energy can be obtained from the integration of the differential stabilization energy dEr 0 over all space; more simply it is given by eq. (6) in the single-shell approximation, according to which the first solvation shell of depth 2b (b being the solvents’ mean 6) molecular only needs be considered on account of the very fast (r decrease ofradius) the interaction energy with distance. E= --~(f(DN)+~f(D)Np[1

+Xe~]’}.

(6)

In the single-shell approximation the preferential solvation index Z in eq. (6) is simply Z= Cp.2r~6~f(D),

where C can be considered as a numerical constant for one particular solvent mixture at one specific temperature. In the present case C has been taken as 7.0 X 102 cgs if p. is in Debye units and r is in A, respectively. The solvents’ molecular volumes are sufficiently close (15.3 ±1.7 A3) to include M and ~ in the constant without introducing a meaningful error. The temperature is taken as 300 K and the angular dependence factor can be reduced to an average value F(0) 0.5 in the single shell approximation. Then C (9MF(0))/(817öRT) 700 cgs. The effective cavity radii r are taken as 3.7, 3.9 and 4.1 A, respectively, for A, NMA and DMA. Note that if there is no preferential solvation (Z 0, e.g. for p. 0 or =

=

=

=

[f(D 1,) —f(DN)] 0) then the stabilization energy follows the linear function of the mole fractions =

E1~~= -~[xNf(DN)+xpf(Dp)]

=

-~f(D)1~~.

(8)

340

P. Suppan, E. Guerry-Butty

/ Fluorescence spectroscopy of aminobenzenes

Q

0.

a I LI

I

LI

/

I

I

I — C,

I / U/U,•• 0

~

;J I 0

“, 0 C.,

0

a

0.

x I LI

CO

w

I

I

I 0) CO

0 C.)

U. I

x a

I

ILI

CD

0

•)

S

i

SI

LI

a. LI

a a 0 a 2

a.

U

-l~ C’)

I

a C’)

— C.)

a C’)

0

LI C.’ cC C, ?2. -

E

I .5

~~~—NH(CH~)

I 1

A

X

d

P

29

30

29.5

29.75

0

29

\

U

\U

U

U U

5

Fig. 2. Solvatochromatic shifts of the fluorescence bands of A, NMA and DMA in various solvent added to CH. Ordinate: transition energy at A,,~,.in I0~cm~.

0

30 ~CH/BU

30.5

31

E

30

i

342

P. Suppan, E. Guerry-Butty

f(D)

/ Fluorescence spectroscopy of aminobenzenes

f(D)ef

Fig. 3. Definition of the effective f(D)~ff and linear f(D)

110 polarity functions of a solvent mixture. The f(D)~ff curve is the observed solvatochromic shift as a function of polar solvent mole fraction Xp, and f(D)~, f(D)N are the polarity functions of the neat polar and non-polar solvents, respectively.

The solvent mixture’s linear dielectric polarity f(D)~~and its effective dielectric polarity f( D)~1~ are then simply defined as the dielectric polarity f( D) of a single solvent of static dielectric constant D which would give the same stabilization energy (the same solvatochromic shift) as the mixture. A simple quantitative assessment of the extent of deviation from linearity can be obtained from the non-linearity ratio R defined as R

=

[fO1[f(D)efl

_f(D)1~0]~dx]/~f(D).

This is the ratio of the area (\\\) between the f(D)eff and f(D)15 functions to the area of the triangle (///) which would correspond to the above integral if the index of preferential solvation Z tends toward infinity. R is therefore a dimensionless quantity between 0 and 1, 0 corresponding to the linear function f(D)Cff =f(D)11~and 1 corresponding to “infinite” deviation from linearity. Table 2 provides a comparison between the experimental and calculated non-linearity ratios for the fluorescence of A, NMA and DMA in the three solvent mixtures CH/THF, CH/BU and CH/DX. The calculated values from eq. (6) are in good agreement for the CH/THF and CH/BU mixtures and show clearly that the differences in the non-linear solvatofluoric shifts follow from the molecular volumes of the solutes and not from any major difference in hydrogen bonding behaviour. Nor should such a difference be expected since it is clear that the ArR2N. .S hydrogen bond is broken in the S1 fluorescent state. The other possible hydrogen bond involving the amino protons (the solvent acting as a base) has little effect on the fluorescence spectra unless the solvent is a very much stronger hydrogen-bonding base than .

P. Suppan, E. Guerry-Butty

/ Fluorescence spectroscopy of arninobenzenes

343

Table 2 Experimental (Re) and calculated (Re) non-linearity ratios of the fluorescence spectra of A, NMA and DMA in mixtures of CH/THF, CH/BU, CH/DX. Z is the index of preferential salvation used in the calculation (eq. (6)) and ~f(D) the difference in the solvents’ dielectric polarity in Z

A NMA DMA A NMA DMA A NMA DMA

CH/THF CH/THF CH/THF CH/BU CH/BU CH/BU CH/DX CH/DX CHDX

R~

R~

Z

0.73 0.61 0.42 0.81 0.64 0.60 0.65

0.73 0.59 0.46 0.82 0.69 0.55 0.67 0.40 0.20

2.80 2.04 1.50 3.44 2.50 2.80) 2.42 1.25 0.60

0.55

0.40

~f(D) 0.410

0.505 0.355 0.245 0.145

THF, BU and DX; this was found also in the solvatochromic shifts of absorption spectra [9]. CH/DX mixtures

Several explanations have been proposed for the apparent high dielectric polarity of DX in many solvatochromic shift measurements. Of these, hydrogen bonding can be ruled out as a general mechanism and two other mechanisms at least can be considered: (i) the conformation polarization of DX from its nonpolar chair form to a highly dipolar boat form of higher energy [8]; (ii) the non-cancellation of the electric fields of the two opposite dipoles of the DX molecule at short distances [9]. The mechanism of conformation polarization leads to the prediction that the DX chair ~ boat equilibrium should depend on the strength of the electric field of the solute, p./a3. The DX apparent polarities shown in fig. 1 agree qualitatively with this prediction, since the field strengths increase in the order DMA (1.2 x iO~esu) < NMA(1.4 x iO~esu) < A(1.6 X iO~esu) following the decreasing molecular radii. It can be seen from table 2 that the calculated non-linearity ratios for A, NMA and DMA differ even more than the experimental values. Here the ~f( D) in eq. (6) has been taken from the experimental effective dielectric polarity of DX (fig. 1) as 0.15 for DMA, 0.25 for NMA and 0.38 for A. Clearly the large difference observed in the non-linearity of the fluorescence spectral shifts of the three amines in CH/DX result from the cumulative effects of the increasing molecular radii and of the decreasing effective dielectric polarity of DX in the order A > NMA> DMA. The calculated values are rather uncertain

344

P. Suppan, E. Guerry-Butty

/

Fluorescence spectroscopy of aminobenzenes

because of this effective polarity of DX but the qualitative trend is explained quite well within the model of dielectric enrichment. There is therefore no need to invoke any fundamental difference in hydrogen-bonding effect of DX on the fluorescence spectra of A, NMA and DMA. This is not to say that hydrogen bonds of the type S...H

I~N Ar R do not exist; it is to say that the effect of such bonds (if they exist) is negligible on the fluorescence spectra. —

Conclusions In the series of simple aromatic amines A, NMA and DMA the solvent effects on the absorption and fluorescence spectra are essentially similar; even apparently large differences such as those observed in CH/DX mixtures result only from the different molecular radii. We have deliberately excluded from this work the effect of solvents of extreme protic (e.g. H20) or basic character (e.g. aliphatic amines) which show different specific association effects with excited A, NMA and DMA; these effects will be the subject of a further communication.

Acknowledgements This work is part of project No. 2.219—0.84 of the Schweizerischer Nationalfonds zur Forderung der wissenschaftlichen Forschung.

References [1] E.A. Chandross, in: The Exciplex, M. Gordon and W.R. Ware, eds. (Acad. Press, New York, 1975). [2] S. Nagakura and H. Baba, J. Am. Chem. Soc. 74 (1952) 5693; G. Perichet, R. Chapelon and B. Pouyet, J. Photochem. 13 (1980) 67. [3] J. Midwinter and P. Suppan, Spectrochim. Acta 25A (1969) 953; K.-S. Nitsche and P. Suppan, Chimia 36 (1982) 346. [4] P. Suppan, J. Chem. Soc. A (1968) 3125. [5] J.N. Murrell, Tetrahedron 19 Suppl. 2 (1963) 277. [6] P. Suppan, Chem. Phys. Lett. 94 (1983) 272. [7) P. Suppan, Spectrochim. Acta 30 A (1974) 1939. [8] MB. Ledger and P. Suppan, Spectrochim. Acta A 23 (1967) 3007. [9] S.P. Van and G.S. Hammond, J. Am. Chem. Soc. 100 (1978) 3895.