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Absolute excited state dipole moments from solvatochromic shifts
Spectrochimdca Acta, Vol. 36A, pp. 971 to 974 Pergamon Press Ltd. 1980. Printed in Great Britain
0584-8539180/1101--0971502.0010
Absolute excited state dipole moments from solvatochromic shifts P A U L SUPPAN
Department of Chemistry, University of Southampton, Southampton, U.K. and CHRIS TSIAMIS*
Department of Chemistry, University of Thessaloniki, Greece
(Received 16 April 1980) Abstract--The theory of solvatochromic shifts in solvents of different dielectric constants and refractive indexes can be used to measure both the absolute value and the direction of dipole moment vectors in excited states. An example of such measurement is given for aromatic amino- and dimethylamino ketones.
INTRODUCTION
The observation of solvent-induced shifts of electronic absorption bands of molecules has been used extensively for the study of changes in electron distribution in excited states of solute molecules [1]. The most readily available quantity is the change in permanent dipole moment, A/x~_,, between the ground state and the excited state subscribed g and e, respectively. For a series of solvents of similar refractive index, n, but different static dielectric constant, e, the change in the permanent dipole moment is related to the observed energy shift, A E I _ 2 , between solvents 1 and 2 by A II,g-e
-AE1-2 = ltg a 3
Af(eh_2,
(1)
have been recognized to offer potentially the means for such absolute excited state dipole measurements. RESULTS A N D DISCUSSION
Basic theory and experimental verification In the usual solvent shift plots of AEI_2 against it is well known that solvents of high refractive index yield points which fall well outside the linear relationship. This is illustrated for the first absorption band of 4-aminobenzophenone in a number of solvents (Fig. 1). Clearly a series of 'non-polar' but polarizable solvents gives excess stabilization to the electronic transition. It has been suggested [2] that this can be accounted for by an
f(8)1_ 2
where p.g is the ground state dipole moment of the solute molecule, a is the radius of the (spherical) cavity occupied by the solute molecule in the solvent, and f(e) is the function f(e) = 2(e - 1)/(2e 4- 1). An important limitation of this widely used relationship is that it yields only that component of Altg_e which lies along the ttg axis. Therefore, it cannot be inferred that an absolute excited state dipole moment is obtained. Indeed, there are some case of molecules of related chemical structure where the change in dipole moment calculated from (1) gives surprisingly different results which are not easily explained by differences in the nature of their electronic states: one such example is found in the comparison of p-amino and pdimethylamino aromatic ketones and aldehydes which will be discussed in th~s paper. In order to obtain values of absolute excited state dipole moments, we have studied the extension of the theory of solvent shifts to solute permanent dipole-solvent induced dipole interactions, which * Present address: Department of Chemistry, Laughborough University of Technology, Loughborough, U.K.
\ % o
0 II
c'~l.,
33,000
32,000
31 , c o o
30,000
I
0.4
I
I
0.6
I
I
0.8
I
o
I
LO
Fig. 1. [(e) plot for solvent dependence of 1st absorption band of 4-aminobenzophenone. Abscissa: /(e); ordinate: wavenumber v/cm -1. 971
972
PAUL SUPPAN a n d CHRIS TSIAMIS
independent second term to AE]_2 as follows
2
4 (~'~3
2
Af(n2),_2,
(2)
where the first term is the permanent dipole interaction term, the second term represents the stabilization difference of the ground state dipole IXg and the excited state dipole Ix, by the induced dipoles in the solvent. It is assumed that these induced dipoles follow the solutes dipole field 'in-
stantaneously', unlike the orientation of the solvent permanent dipoles which cannot change significantly during the electronic transition (about 10 -15 s). In the case of the induced dipoles, therefore, the stabilization energy in the ground state is Idl,gRg, Rg being the reaction field set up by the solvents induced dipoles, Rg=(gl, g/tl3)f(rt2), and similarly the reaction field in the excited state is Re = (izda3)f(n2). For a series of 'non-polar' solvents of different refractive index we find that a linear relationship as suggested be equation (2) is obeyed (Fig. 2).
30,000 (CH3)2N%0
k
29,500
0
~
c~CH3
N(CHs)2 ~7 29,500
29,400
i
o~
[2
29,000 -• I
I
3.8
A
I
I
4.2
4.6
,
(a)
, b),
% 33,000
%
,o, C~H
$
NH2
30,600
® &?.,O00
30,200 U
31,000 I
I
38
I
1
42
I
I
46
/I
I 3.8
4,2
(c) (d) Fig. 2. f(n 2) plot for various solute absorption bands in a series of non-polar solvents. (a) Michler's ketone 1st (CT) band; (b) 2-acetonaphthone L band; (c) 4-aminobenzophenone 1st (CT) band; (d) 2-naphthaldehyde L band. Abscissa: f(n2); ordinate: wavenumber v/cm -t. Legend: O cyclohexane; V cyclohexene; [] 1,4-cyclohexadiene; • 1,3-cyclohexadiene; Q Di-iso-propyl ether; 4> Tetrahydrofuran; • 1,4-Dioxane.
-I 4.6
Absolute excited state dipole moments from solvatochromic shifts Table 1. Observed and calculated solvent shifts for the first absorption band of 4-aminobenzophenone, using the following parameters. ~ g = 5 . 5 D , a = 0 . 3 5 n m , g.e=13.5D, angle o~(g.g, I~e) = 0 Solvent shifts A~/cm -1 Solvent C6H12 C6H10 C6H 6 1,4-C6H 8 THF DMF
Observed Reference 33170-32470 33170-31900 33170-32260 33170-31000 33170-30200
973
(tzg = 5, 6 D, AN ----8 D with a = 0.35 nm). In the case of 4 - a m i n o b e n z o p h e n o n e this first excited state has been described [3] as a C T (charge-transfer) state in view of its large dipole moment. The electronic structure suggested for this state is essentially
Calculated
O H2N
= 700 = 1270 = 910 = 2160 = 2970
387 1137 790 2150 3020
R
In the case of the first absorption band of 4a m i n o b e n z o p h e n o n e it is also found that the addition of the induced dipole term provides quantitative explanation for the deviations observed in the simple f(e) plot (Table 1). On this basis we feel reasonably confident that equation (2) is an accurate description of the overall solvatochromic shift for polar solute molecules (excluding effects of specific associations and some special solvent anomalies).
Excited state dipole moment of 4-(dimethyl)aminobenzaldehyde The uncorrected f(e) solvent shift plot for the first absorption band of 4-(dimethyl)aminobenzaldehyde is shown in Fig. 3. Restricting the solvents to those which differ only in f(e), but not in f(n2), and which are known not to give specific association effects such as hydrogen-bonding, we find Ap~z_, = 4 . 3 D along the Ixg axis, with the following parameters: wg = 6 D, a = 0.36 nm. This figure is remarkably small when compared with the related molecule 4 - a m i n o b e n z o p h e n o n e I
o
?
30,600
and such a charge transfer lies close to the ground state dipole vector. This is in a g r e e m e n t with the fact that both terms in equation (2) taken separately yield similar results for ~ = 1 2 . 8 D , A~ (7.6 D) + ~ (5, 6 D) = 13.2 D for f(e). W h e n the same calculation is m a d e for 4-(dimethyl)aminobenzaldehyde the two terms give widely different results: as stated above, the change of dipole mom e n t along the N~ axis is 4.3 D, but w h e n only the second t e r m is considered, the absolute value of Ixe comes out as 13 D. This figure agrees with the one for 4 - a m i n o b e n z o p h e n o n e , as expected. T h e implication is that the ground- and excited-state dipoles lie along quite different axes at an angle of about 50 ° . A similar situation is found for Michler's ketone (4,4'-bis(dimethyl)aminobenzophenone). Clearly, many excited state dipole moments obtained from the simple f(e) plots may require re-interpretation in view of these results. CONCLUSIONS
The f(n 2) plot of solvatochromatic shifts for polar solute molecules in a series of non-polar solvents can provide a m e a s u r e m e n t of the absolute excited state dipole moments. W h e n used in conjunction with the f(e) plot in a series of polar solvents of the same refractive index, information is also obtained about the direction of the excited state dipole vector relatively to the resultant ground state dipole vector. T a k e n together with the interpretation of hydrogen-bonding effects, such a detailed investigation of solvatochromic shifts may provide new insights into the electronic structure of excited molecules. EXPERIMENTAL
Solvents. Benzene, pronalys grade (May and Baker)
30,400
30,200
I
0.4
I
0.6
I
I
0.8
I
I
1.0
Fig. 3. f(e) plot for solvent-dependence of the first absorption band of 4-dimethylaminobenzaldehyde. Abscissa: f(e); ordinate: wavenumber u/cm-1.
was treated as suggested by VOGEL [4], to remove thiophen and was re-distilled prior to its use. Tetrahydrofuran and di-iso-propyl ether (BDH) were also purified as described in the literature [5]. Cyclohexane, cyclohexene and 1,4-dioxane (BDH) were spectroscopic grade and were used without any further purification. Cyclohexadi1,3-ene and cyclohexadi-l,4-ene (Aldrich) although contained minor amounts of impurities detectable by gas chromatography, were also used without further purification. Solutes. Anthracene (BDH) scintillation grade was employed. 2'-Acetonaphthone (Kodak) was recrystallized twice from ETOH. 2-Acetyl phenanthrene and 4,4'-bis (dimethylamino) benzophenone (Aldrich) were purified as
974
PAUL SUPPAN and CHRIS TSIAMIS
described by SUPPAN [6]. 4-Dimethylaminobenzaldehyde and 4-aminobenzophenone (Koch-Light) were purified by dissolving the compound in aqueous acetic acid, filtering the solution and precipitating with ammonia. The process was repeated several times. Finally, the solid was recrystallized from ethanol. 9-Anthraldehyde (Fluka) was treated as above and was recrystallized twice from aqueous acetic acid. Reagents. Hydrochloric acid, glacial acetic acid, 35% ammonia solution (Fison's SLR grade) and spectroscopic grade ethanol were used for the purification of the solutes. Instruments. The spectra were recorded on a PyeUnicam SP8-200 u.v.-Vis spectrophotometer equipped with a thermostatted cell holder. The instrument operating at a fixed bandwidth of 0.5 nm and scanning speed of 0.5 nm/s. Solutions were made in an inert atmosphere and the concentration of the solute was such that the maximum absorbance of the band under investigation was in
the range 0.8-0.9,~. Spectrosil (TSL) teflon stoppered cells of 10 mm path length were employed. Spectra were measured at 20°C using freshly prepared solutions with pure solvent in the reference cell.
REFERENCES
[la] E. G. MCRAE, J. Phys. Chem. 61, 562 (1957). [lb] E. LIPPERT, Z. Electrochem. 61, 692 (1957). [2] P. SUPPAN, J. Chem. Soc. A, 3125 (1968). [3] P. SUPPAN, J. Mol. Spectroscopy, 1969, 30, 17. [4] A. VOGEL, Textbook o[ Practical Organic Chemistry, Fourth edition, p. 266. Longman, (1978). [5] A. VOGEL, Textbook of Practical Organic Chemistry, Fourth edition, pp. 273 and 274. Longman, London (1978). [6] P. SUPPAN, J. Chem. Soc. Faraday Trans. 71, 539 (1975).
0584-8539180/1101--0971502.0010
Absolute excited state dipole moments from solvatochromic shifts P A U L SUPPAN
Department of Chemistry, University of Southampton, Southampton, U.K. and CHRIS TSIAMIS*
Department of Chemistry, University of Thessaloniki, Greece
(Received 16 April 1980) Abstract--The theory of solvatochromic shifts in solvents of different dielectric constants and refractive indexes can be used to measure both the absolute value and the direction of dipole moment vectors in excited states. An example of such measurement is given for aromatic amino- and dimethylamino ketones.
INTRODUCTION
The observation of solvent-induced shifts of electronic absorption bands of molecules has been used extensively for the study of changes in electron distribution in excited states of solute molecules [1]. The most readily available quantity is the change in permanent dipole moment, A/x~_,, between the ground state and the excited state subscribed g and e, respectively. For a series of solvents of similar refractive index, n, but different static dielectric constant, e, the change in the permanent dipole moment is related to the observed energy shift, A E I _ 2 , between solvents 1 and 2 by A II,g-e
-AE1-2 = ltg a 3
Af(eh_2,
(1)
have been recognized to offer potentially the means for such absolute excited state dipole measurements. RESULTS A N D DISCUSSION
Basic theory and experimental verification In the usual solvent shift plots of AEI_2 against it is well known that solvents of high refractive index yield points which fall well outside the linear relationship. This is illustrated for the first absorption band of 4-aminobenzophenone in a number of solvents (Fig. 1). Clearly a series of 'non-polar' but polarizable solvents gives excess stabilization to the electronic transition. It has been suggested [2] that this can be accounted for by an
f(8)1_ 2
where p.g is the ground state dipole moment of the solute molecule, a is the radius of the (spherical) cavity occupied by the solute molecule in the solvent, and f(e) is the function f(e) = 2(e - 1)/(2e 4- 1). An important limitation of this widely used relationship is that it yields only that component of Altg_e which lies along the ttg axis. Therefore, it cannot be inferred that an absolute excited state dipole moment is obtained. Indeed, there are some case of molecules of related chemical structure where the change in dipole moment calculated from (1) gives surprisingly different results which are not easily explained by differences in the nature of their electronic states: one such example is found in the comparison of p-amino and pdimethylamino aromatic ketones and aldehydes which will be discussed in th~s paper. In order to obtain values of absolute excited state dipole moments, we have studied the extension of the theory of solvent shifts to solute permanent dipole-solvent induced dipole interactions, which * Present address: Department of Chemistry, Laughborough University of Technology, Loughborough, U.K.
\ % o
0 II
c'~l.,
33,000
32,000
31 , c o o
30,000
I
0.4
I
I
0.6
I
I
0.8
I
o
I
LO
Fig. 1. [(e) plot for solvent dependence of 1st absorption band of 4-aminobenzophenone. Abscissa: /(e); ordinate: wavenumber v/cm -1. 971
972
PAUL SUPPAN a n d CHRIS TSIAMIS
independent second term to AE]_2 as follows
2
4 (~'~3
2
Af(n2),_2,
(2)
where the first term is the permanent dipole interaction term, the second term represents the stabilization difference of the ground state dipole IXg and the excited state dipole Ix, by the induced dipoles in the solvent. It is assumed that these induced dipoles follow the solutes dipole field 'in-
stantaneously', unlike the orientation of the solvent permanent dipoles which cannot change significantly during the electronic transition (about 10 -15 s). In the case of the induced dipoles, therefore, the stabilization energy in the ground state is Idl,gRg, Rg being the reaction field set up by the solvents induced dipoles, Rg=(gl, g/tl3)f(rt2), and similarly the reaction field in the excited state is Re = (izda3)f(n2). For a series of 'non-polar' solvents of different refractive index we find that a linear relationship as suggested be equation (2) is obeyed (Fig. 2).
30,000 (CH3)2N%0
k
29,500
0
~
c~CH3
N(CHs)2 ~7 29,500
29,400
i
o~
[2
29,000 -• I
I
3.8
A
I
I
4.2
4.6
,
(a)
, b),
% 33,000
%
,o, C~H
$
NH2
30,600
® &?.,O00
30,200 U
31,000 I
I
38
I
1
42
I
I
46
/I
I 3.8
4,2
(c) (d) Fig. 2. f(n 2) plot for various solute absorption bands in a series of non-polar solvents. (a) Michler's ketone 1st (CT) band; (b) 2-acetonaphthone L band; (c) 4-aminobenzophenone 1st (CT) band; (d) 2-naphthaldehyde L band. Abscissa: f(n2); ordinate: wavenumber v/cm -t. Legend: O cyclohexane; V cyclohexene; [] 1,4-cyclohexadiene; • 1,3-cyclohexadiene; Q Di-iso-propyl ether; 4> Tetrahydrofuran; • 1,4-Dioxane.
-I 4.6
Absolute excited state dipole moments from solvatochromic shifts Table 1. Observed and calculated solvent shifts for the first absorption band of 4-aminobenzophenone, using the following parameters. ~ g = 5 . 5 D , a = 0 . 3 5 n m , g.e=13.5D, angle o~(g.g, I~e) = 0 Solvent shifts A~/cm -1 Solvent C6H12 C6H10 C6H 6 1,4-C6H 8 THF DMF
Observed Reference 33170-32470 33170-31900 33170-32260 33170-31000 33170-30200
973
(tzg = 5, 6 D, AN ----8 D with a = 0.35 nm). In the case of 4 - a m i n o b e n z o p h e n o n e this first excited state has been described [3] as a C T (charge-transfer) state in view of its large dipole moment. The electronic structure suggested for this state is essentially
Calculated
O H2N
= 700 = 1270 = 910 = 2160 = 2970
387 1137 790 2150 3020
R
In the case of the first absorption band of 4a m i n o b e n z o p h e n o n e it is also found that the addition of the induced dipole term provides quantitative explanation for the deviations observed in the simple f(e) plot (Table 1). On this basis we feel reasonably confident that equation (2) is an accurate description of the overall solvatochromic shift for polar solute molecules (excluding effects of specific associations and some special solvent anomalies).
Excited state dipole moment of 4-(dimethyl)aminobenzaldehyde The uncorrected f(e) solvent shift plot for the first absorption band of 4-(dimethyl)aminobenzaldehyde is shown in Fig. 3. Restricting the solvents to those which differ only in f(e), but not in f(n2), and which are known not to give specific association effects such as hydrogen-bonding, we find Ap~z_, = 4 . 3 D along the Ixg axis, with the following parameters: wg = 6 D, a = 0.36 nm. This figure is remarkably small when compared with the related molecule 4 - a m i n o b e n z o p h e n o n e I
o
?
30,600
and such a charge transfer lies close to the ground state dipole vector. This is in a g r e e m e n t with the fact that both terms in equation (2) taken separately yield similar results for ~ = 1 2 . 8 D , A~ (7.6 D) + ~ (5, 6 D) = 13.2 D for f(e). W h e n the same calculation is m a d e for 4-(dimethyl)aminobenzaldehyde the two terms give widely different results: as stated above, the change of dipole mom e n t along the N~ axis is 4.3 D, but w h e n only the second t e r m is considered, the absolute value of Ixe comes out as 13 D. This figure agrees with the one for 4 - a m i n o b e n z o p h e n o n e , as expected. T h e implication is that the ground- and excited-state dipoles lie along quite different axes at an angle of about 50 ° . A similar situation is found for Michler's ketone (4,4'-bis(dimethyl)aminobenzophenone). Clearly, many excited state dipole moments obtained from the simple f(e) plots may require re-interpretation in view of these results. CONCLUSIONS
The f(n 2) plot of solvatochromatic shifts for polar solute molecules in a series of non-polar solvents can provide a m e a s u r e m e n t of the absolute excited state dipole moments. W h e n used in conjunction with the f(e) plot in a series of polar solvents of the same refractive index, information is also obtained about the direction of the excited state dipole vector relatively to the resultant ground state dipole vector. T a k e n together with the interpretation of hydrogen-bonding effects, such a detailed investigation of solvatochromic shifts may provide new insights into the electronic structure of excited molecules. EXPERIMENTAL
Solvents. Benzene, pronalys grade (May and Baker)
30,400
30,200
I
0.4
I
0.6
I
I
0.8
I
I
1.0
Fig. 3. f(e) plot for solvent-dependence of the first absorption band of 4-dimethylaminobenzaldehyde. Abscissa: f(e); ordinate: wavenumber u/cm-1.
was treated as suggested by VOGEL [4], to remove thiophen and was re-distilled prior to its use. Tetrahydrofuran and di-iso-propyl ether (BDH) were also purified as described in the literature [5]. Cyclohexane, cyclohexene and 1,4-dioxane (BDH) were spectroscopic grade and were used without any further purification. Cyclohexadi1,3-ene and cyclohexadi-l,4-ene (Aldrich) although contained minor amounts of impurities detectable by gas chromatography, were also used without further purification. Solutes. Anthracene (BDH) scintillation grade was employed. 2'-Acetonaphthone (Kodak) was recrystallized twice from ETOH. 2-Acetyl phenanthrene and 4,4'-bis (dimethylamino) benzophenone (Aldrich) were purified as
974
PAUL SUPPAN and CHRIS TSIAMIS
described by SUPPAN [6]. 4-Dimethylaminobenzaldehyde and 4-aminobenzophenone (Koch-Light) were purified by dissolving the compound in aqueous acetic acid, filtering the solution and precipitating with ammonia. The process was repeated several times. Finally, the solid was recrystallized from ethanol. 9-Anthraldehyde (Fluka) was treated as above and was recrystallized twice from aqueous acetic acid. Reagents. Hydrochloric acid, glacial acetic acid, 35% ammonia solution (Fison's SLR grade) and spectroscopic grade ethanol were used for the purification of the solutes. Instruments. The spectra were recorded on a PyeUnicam SP8-200 u.v.-Vis spectrophotometer equipped with a thermostatted cell holder. The instrument operating at a fixed bandwidth of 0.5 nm and scanning speed of 0.5 nm/s. Solutions were made in an inert atmosphere and the concentration of the solute was such that the maximum absorbance of the band under investigation was in
the range 0.8-0.9,~. Spectrosil (TSL) teflon stoppered cells of 10 mm path length were employed. Spectra were measured at 20°C using freshly prepared solutions with pure solvent in the reference cell.
REFERENCES
[la] E. G. MCRAE, J. Phys. Chem. 61, 562 (1957). [lb] E. LIPPERT, Z. Electrochem. 61, 692 (1957). [2] P. SUPPAN, J. Chem. Soc. A, 3125 (1968). [3] P. SUPPAN, J. Mol. Spectroscopy, 1969, 30, 17. [4] A. VOGEL, Textbook o[ Practical Organic Chemistry, Fourth edition, p. 266. Longman, (1978). [5] A. VOGEL, Textbook of Practical Organic Chemistry, Fourth edition, pp. 273 and 274. Longman, London (1978). [6] P. SUPPAN, J. Chem. Soc. Faraday Trans. 71, 539 (1975).