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Stilbene—amine exciplexes. Substituent effects
Journal of Photochemistry and Photobiology, A: Chemistry, 54 (1990) 299-309
Stilbene-amine Jing-Chen
exciplexes. Substituent effects
Mai, Yuan-Chuau
Department of Chem&y,
Lin and Tong-Ing
National Taiwan Universi@
Hot Taipei (Taiwan)
(Received January 9, 1990 in revised form April 20, 1990)
Abstract Fluorescence is observed for 24 exciplexes formed between excited para-substituted trarrrstilbene (X E H, CN, Cl, CH3, CH(CH& and CH,O) and four tertiary amines (triethykmine, diisopropylmethylamine, diisopropylethylamine and Nfl-dimethylaniline) in n-hexane at room temperature. The maximum fluorescence energies are well correlated with the Rehm and Weller theoretical values with the exception of the triethylamine systems. The systematic shift in exciplex energy also correlates we11 with the Hammett up constant; however, for the triethylamine-stilbene systems, the correlation is less linear. The thermodynamic stability of the exciplex determines the degree of linearity in the Hammett correlation. Investigations of the products, Stern-Volmer quenching, dipole moments and exciplex lifetimes were also carried out to determine the structure-reactivity relationship.
1. Iutroduction The photochemical interactions between singlet tmns-stilbene (TS) and tertiary amines have been extensively studied [l-9]. The mechanism of this reaction is known to occur through consecutive electron transfer and proton transfer to generate 1,2diphenylethyl radical and Z-aminoalkyl radical pair. The reactive intermediates identified in the TS-triethylamine system include singlet state TS, exciplex, radical anion and radical cation. Recently, 1,Zdiphenylethyl radical has been identif?ed by the spintrapping technique [lo]. Structural changes influence the oxidation potential of the electron donor or the reduction potential of the acceptor molecule and thus the formation stability of the exciplexes, which will then influence the photochemistry or photophysics of the reaction [ll-131. The substituent also has some effect on the basic@ of the generated radical anion of TS and thus influences the proton transfer reaction. We have studied the systematic variation in the quenching of TS by substituted amines and have observed the Hammett correlation between kg and al [14]. In this paper, we report the interaction between substituted TS (l-6) and tertiary amines diisopropylmethylamine (DIPMA), diisopropylethylamine (triethylamine (TEA), (DIPEA) and Nfl-dimethylaniline (DMA)).
=--Q-x
X=H (l), X-C%’ (2), X=Cl (3), x- CH (4) x= CH(cH& (S), x=&o (k)
+Author to whom correspondence
lOlO-6030/90/$3.50
should be addressed.
Q Elsevier Sequoia/Printed
in The Netherlands
300 2. Experimental
details
2. I. Matetials
All the para-substituted trans-stilbenes were prepared from the corresponding para-substituted benzaldehydes and benzyl chloride by the Wittig reaction [l5]. The solvents were of fluorescence spectroscopic grade (Merck) and were used as received. Triethylamine (TEA), diisopropylethylamine (DIPEA) and N,ZV-dimethylaniline (DMA) were of reagent grade (Merck) and were purified by vacuum distillation. Diisopropylmethylamine (DIPMA) was prepared from diisopropylamine according to ref. 16. 2.2. Methods
Fluorescence spectra were recorded on a Perkin-Elmer IS-5 luminescence specsolutions trometer. Fluorescence quenching data were obtained for nitrogen-purged of 0.0001 M TS and amines (O.l- 2 M). 2.3. Lifetime measurements An Nd-YAG laser (Quanta-Ray DCR-2) was used to excite the fluorophore (substituted TS) at 355 or 266 nm. The exciplex fluorescence at maximum emission was collected through a 0.5 m Jarell-Ash monochromator and an RCA lP28 photomultiplier. The fluorescence decay was recovered by a transient waveform recorder (Gould-Biomation 6500, 2 ns resolution) which was interfaced to a microcomputer. The sample was kept at 298 K in a thermostat to within 0.2 K. In general, data were averaged over 300 laser shots, and the measured first-order fluorescence rate constant (and lifetime) was obtained from the fluorescence decay curve by a deconvolution process. The reduction and oxidation potentials (in volts against silver/saturated silver chloride electrodes) were measured on a cyclic voltametre (BAS-100) in polar aprotic solvent (CH&N or N$!-dimethylformamide) with a platinum disc as the working electrode and tetraethylammonium perchlorate (0.1 M) as the supporting electrode. 2-4. Imadiation of 2 and DIPMA
A solution of 2 (0.18 g, 0.0625 M) and DIPMA (0.86 M) in 15 ml of acetonitrile was irradiated using a medium pressure mercury lamp (Pyrex filter) for 20 h. Removal followed by column of the volatile material, chromatography (silica gel; n-hexane-ethylacetate) of the residual oil, gave p-cyano-cis-stilbene (0.0308 g; 19.6%), N~-diisopropyl-Z@-cyano)phenyl-3-phenylpropylamine (Sa) (0.035 g; 8.68%) and N,ZVdiisopropyl-2-phenyl-3-(p-cyano)phenylpropylamine (9a) (0.0446 g; 18.17%) (last two as oils). Sa: proton nuclear magnetic resonance (IH NMR) (CDC13) 0.95 (12H, Zd), 2.5-3.5 (9H, m); mass spectrum, m/e (%) 305 (M+, O-2%), 116 (7H, m>, 7.02-7.2 l-9%), 114 (H2C=N+(CH(CH&)z, lOO%), 91 (C6H5-Cl&+, 14%, (NC-C&L+-CH2+, 72 (H2C=N+HCH(CH&, 18.4%). 9a: ‘H NMR (CDQ) 0.95-1.2 (12H, two d), 2.5-3.5 (7H, m), 7.0-7.2 (9H, m); mass spectrum, m/e (%) 305 (M+, O.l%), 116 (NC-CCs&-CHCH2+, 2.8%), 114 5.7%), 72 (H&=N+HCH(CH&, (H2C=N+(CH(CH&)z, lOO%), 91 (C~H~-CHZ+, 19.6%). 2.5. Irradiation of 6 and DIPhL4 A solution of 6 (0.2 g) and DIPMA (1.07 M) in 15 ml of acetonitrile was irradiated for 25 h. Removal of the volatile material, followed by column chromatography (silica
301
gel; n-hexane-ethylacetate (25:l)) of the residual oil, gave n-methoxy-cis-stilbene (0.0367 g; 18.3%), l-p-methoxyphenyl,2-phenyl,3-phenyl,4-p-metho~henylbut~e (lob)and its regioisomeric mixture (0.03 g; 7.9%), NJ+diisopropyl-2-(p-methoxy)phenyl-3-phenylpropylamine (Sb) and isomers (0.044 g; 14.3%). 1Ob (regioisomers): ‘H NMR (CDCL+) 2.3-3.2 (18H, m), 2.7 (3H, s) 3.72 (3H, s), 3.76 (3H, s), 6.7-7.3 (27H, m); mass spectrum, m/e (%) 422 (M*, 7.5%), 211 (C,EI,-CH+ -CHcHz-C&OCH3, lOO%), 121 (CH~O-C,&-CHcH2+, 77%), 91 (C,Hs---cH,+, 25%). Sb (regioisomers): ‘H NMR (CDCL) 0.95 (12H, 2d), 1.05 (12H, 2d), 2.5-3.4 (lOH, m), 3.68 (3H, s), 3.72 (3H, s), 6.7-7.3 (lSH, m); mass spectrum, m/e (%) 310 (M+, O.l%), 121 (CH,O-C,&-CH,+, II%), 114 (H&=N+(CH(CH&)z, lOO%), 91 2.2%), 72 (H$Z=N+HCH(CH&, 29%). (GHs--z+, 2.6. Irradiation of 4 and DIPMA A solution of 4 (0.2 g, 0.05 M) and DIPMA (5 ml) in 20 ml of acetonitrile was irradiated for 21 h. The non-volatile residual oil was subjected to chromatography (silica gel and n-hexane) to givep-methyl-cis-stilbene (0.007 g; 3.5%), l-(p-methyl)phenyl2, 3-diphenyl-4-(p-methyl)phenylbutane (10~)and its isomeric mixture (llc and 12~) (1:l:l ratio (from NMR); 0.023 g; 5.9%) and NJV-diisopropyl-2-(p-methyl)phenyl-3phenylpropylamine (SC) and its isomer (0,053 g; 17.6%). 10~ (mixture): ‘H NMR (CDCQ 2.20 (3H, s), 2.25 (3H, s), 2.28 (3H, s), 2.3-3.3 (9H, m), 6.6-7.3 (lSH, m); mass spectrum, m/e (%) 390 (M+; O-3%), 195 (CH,-Cc,H,-CH+-CHz--Cc6Hs, lOO%), 179 (20%), 105 (CH,-CC6H4-CH2+, 47%), 91 (C6H5-CHz*, 38.5%). SC (regioisomers): IH NMR (CDCI,) 0.95 (12H, two d), 1.0 (12H, two d), 2.25 (3H, s), 2.3 (3H, s), 2.5-3.2 (lOH, m), 6.8-7.3 (lSH, m); mass spectrum, m/e (%) 310 (MC, 2%), 178 (19%), 114 (lOO%), 105 (10.5%), 72 (32%). 2. Z Irradiation of 5 and DIPAA solution of 5 (0.15 g) and DIPMA (3.5 ml) in 12 ml of acetonitrile was irradiated for 20 h. Removal of the volatile material, followed by thin layer chromatography (silica gel, n-hexane), gave NJV-diisopropyl-2-@-isopropyl)phenyl-3-phenylpropyl~ne (Sd) (regioisomers) (0.041 g; 18.6%) and 1-(p-isopropyl)phenyl-3-phenyl-4-(p-isopropyl)phenylbutane (1Od) (isomers) (0.0196 g; 6.5%). Sd (regioisomers): ‘H NMR (CD&) 0.85 (12H, two d), 0.95 (12H, two d), 1.05 (12H, two d), 1.1 (12H, two d), 2.5-3.4 (lOH, m), 6.8-7.3 (18H, m). 1Od (mixture): mass spectrum, m/e (%) 446 (M+, 0.2%), 223 (lOO%), 181 (62.5%), 167 (23%), 133 (45%), 105 (42%). 2.8. Irradiation of 3 and DIPill A solution of 3 (0.2 g, 0.052. M) and DIPMA (3 ml) (1.08 M) in 15 ml of acetonitrile was irradiated for 25 h. Removal of the volatile material, followed by Rash column chromatography, gave I-phenyl-2-(p-chloro)phenylethane (13e) (0.0083 g; 0.42%), p-chloroks-stilbene (0.038 g; 19.1%) and N,Wdiisopropyl-2-(pchloro)phenylpropylamine (Se) (mixture) (0.05 g; 16.4%). 13e: mass spectrum m/e (%) 216 (M+, 20%), 125 (Cl-C&L+-CH,+, 69.5%), 91 (C6H5-CHz+, 100%). Se (mixture): ‘H NMR (CDC13) 0.7-l (12H, two d), 2.5-3.3 (7H, m), 6.9-7.3 (9H, m); mass spectrum, m/e (%) 316 (M+, 0%), 125 (Cl-C&L,-CH,+, 36.5%), 114 (H&=N+(CH(CH,),),, lOO%), 91 (&H,-CH,+, 4.5%), 72 (H$Z=N’HCH(CH,), 32%).
302
3. Results and discussion The emission spectra of the substituted TS were measured in n-hexane (Fig. 1) and the emission maxima and absorption maxima are shown in Table 1. In the presence of tertiary amines (for example, diisopropylethylamine; Fig. l), the intensity of the TS fluorescence is reduced and, simultaneously, new, structureless, red-shifted emission bands of the exciplexes are observed. At a sufficiently high concentration of amines, the TS fluorescence is almost completely quenched and the exciplex emission is symmetrical about the peak maximum. The observed energies El at the exciplex fluorescence maximum are listed in Table 2 and can be compared with the theoretical energies E2 according to Rehm and Weller [17] eV
Ez=Ed-E,-0.15j~O.l
(1) where Ed and E, represent the oxidation potential of the donor and the reduction potential of the acceptor respectively. For most exciplexes the correlation between El and E2 is fairly good with the exception of those formed with TEA. Electron-donating substituents shift the emission to higher energy and electron-accepting substituents shift the emission to lower energy for all the amines studied. The systematic shift in the exciplex maximum as a result of the effect of the substituent on TS can be seen from the plot of El vs. the Hammett up constants
r
MAX
= 100
00
2 m k 5
s
5 ::
z!
,o
z
350
400
500
450 Wavelength
TS and diisopropylethylamine
exciplexes in
1
Absorption
and emission maxima of para-substituted
p-X--l-S
*
1
350 380 356 356 357 373
2 3 4 5 6
600
(nm)
Fig. 1. Fluorescence spectra of pam-substituted n-hexane at room temperature. TABLE
550
@ml
TS and singlet excitation energies
GZ &ml
Go (ev)
299 303 299 298 297 303
3.79 3.63 3.85 3.79 3.79 3.67
303 TABLE
2
Comparison of El (observed energy at according to Rehm and Weller)
exciplex emission maximum) with E2 (theoretical energy
p-X-TS
Amine
DIPEA
DIPMA
DMA
2
3
1
4
5
6
A (nm> El (eV Ez (ev)
486 2.55 2.50
446 2.78 2.70
432 2.87 2.97
429 2.89
42s 2.91 3.01
421 2.94 3.05
A (m@ El (ev) EZ (ev)
482 2.57 2.53
446 2.78 2.73
435 2.85 3.00
429 3.02
425 2.91 3.04
411 3.02 3.07
A tnm) El (eV EZ (ev)
444 2.79 2.63
441 2.81 2.83
435 2.85 3.10
431 2.87 3.12
428 2.90 3.14
408 3.04 3.18
A (nm) G (eV -5 (eV
480 2.58 2.42
444 2.79 2.62
439 2.82 2.89
437 2.84 2.91
434 2.86 2.93
426 2.91 2.97
2.99
2.39
250 DIPEA . DI PEA A DMA
.’ l 24-%.
22--
2 l--
20 I -0.30
-0.10
o-10
0.30
0.50
C
UP
Fig. 2. Correlation between the exciplex fluorescence maxima El and the Hammett Q~ constants.
(Fig. 2) for three amines (DIPEA, DIPMA and DMA). The correlation is fairly good; the slopes p and the correlation coefficients T are as follows: p= - 3.29, r= 0.987 for DIPEA, p = - 3.39, r= 0.98 for DIP% p= - 2.5, r==0.963 for DMA. For TEA, the correlation is not as good with p= - 1.63 and r= 0.79. The correlation between E1 and ap is linear because charge transfer interaction plays an important role in the formation of exciplexes. Thus it is not surprising that there is a linear correlation between the reduction potential of the substituted TS and q, (Fig. 3 with r=0.987).
304
z
7
C ._
-1.70
+-
--
ol ? 2 v, -1.90-> m t-I v. h
-Z.lO--
;s Is 2;
L
-2.30, - 0.30
-0.10
0.10
0.30
0.50
0 ‘0
UP
Fig. 3. Correlation
TABLE
between
the reduction potential of the substituted TS and w+
3
Dissociation temperature Amirre
DIPEA DIPMA TEA DMA
enthalpies (eV) of p-X-TS-teti-amine exciplexes in n-hexane and the lifetimes (ns) in cyclohexane and n-hexane
solution
at room
p-X-TS 2
3
1
4
5
6
0.95 (14) 0.92 (14) 0.82 (4) 1.03
0.87 (22) 0.84 (7”) 0.74 (6”, 7) 0.95
0.67 (16) 0.64 0.54 0.75
0.62 (13) 0.59 (7”, 13) 0.49 (7) 0.70
0.60 (20) 0.57 (8*, 13) 0.47 (9) 0.68
0.51 (6) 0.48 (13) 0.38 0.59
Lifetimes in parentheses. “In n-hexane.
The
calculation &=-%,o-
effect of structure on the stability of the exciplexes of the exciplex dissociation on enthalpies E3 [18] (Ed-E,)
- 0.13 eV
can be seen from
the (2)
where Eo, o is the singlet excitation energy of the substituted TS. The exciplex dissociation enthalpies (Table 3) indicate that TEA generally forms weakly bound exciplexes, whereas DIPEA, DIPMA and DMA form strongly bound exciplexes. In addition, electron-donating groups produce exciplexes that are less strongly bound. The most strongly bound exciplexes are those with strong electron-withdrawing substituents on the TS. For the TEA-TS exciplex system, the small exciplex dissociation enthalpies lead to the formation of loosely bound exciplexes, reversible formation of the exciplexes and efficient redissociation giving shorter exciplex lifetimes (Table 3). Evidence for reversible exciplex formation is also provided by the temperature dependence of the fluorescence quenching data (Table 4) [l]. It is evident that the thermodynamic stability determines whether the Hammett correlation will be observed. This observation is
TABLE 4 of temperature on the Stem-Volmer quenching constants of the para-substituted TS by amines in n-hexane (excitation: 313 mu)
Effect
Amine
Temptratwe
p-x-l-s
(“Cl 3
DIPEA
5 15 25 35 45
7.83 7.65 7.46 6.20 6.11
5
18.79 16.64 12.07 11.71 10.53
DIPMA
15
25 35 45
L;
.-: E
5
6
4.53 4.38 4.15 4.25 3.37
4.43 3.69 3.16 2.89 2.73
3.61 3.24 3.79 2.59 2.46
3.78 4.40 4.31 3.52 2.70
0.89 0.69 0.41 0.29 0.31
23
E
.-E
4
0 tsopropy l Chloro A Methyl
I
2t
W Y
Li
0.20
0.50
o-40
0.30
2 -_e-l 2e+l
Fig. 4. Frequency of p-X-TS-DIPEA
0.60
n2-1 2n2+1
exciplex fluorescence maxima W. solvent polarizability.
consistent with the I-methylnaphthalene-triethylamine and naphthalene-triethylamine exciplex systems [13]. The substituent on the TS also influences the dipole moment of the exciplex. The dipole moment of the excipiex can be estimated from the variation in the exciplex * . emlssIon m-mum v,, with solvent polarity [I91 %nax=~o-
2P2 E-i
ha3
( -
2~+1
-
-
n2--l
4n2+2 1
(3)
where v. is the hypothetical gas phase emission frequency, p is the dipole moment, a is the solvent cavity radius (approximately 4.5 A), h is Planck’s constant and n is the refractive index. From the linear plot of the v_ data (Fig. 4), dipole moments
for TS-DIPEA systems are obtained (Table 5). The dipole moment increases with increasing reduction potential of the substituted TS with the exception of p-cyano-TS (6.56 debye). This abnormal value may be due to the delocalization of the transferred charge into the cyano triple bond. The photochemistry of para-substituted TS (X = CN, 0CH3, CH3, CH(CH&, Cl) with diisopropylmethylamine is shown in Scheme I. p-X-TS
+ DIPMA
---&
p-X-ck-stilbene
(7)
+ C6H5CH2CH@-X-C6H4)CH2N(CH(CH3)2)2
(8)
+ @-X-C,H,)CH,CH(C,H,)CH,N(CH(CH,),),
(9)
+ (p-X-CSH~)CH&H(CaH~)CH(C6H&H&-X-C&,)
(10)
+ (p-X-C,H,)CH,CH(CaEF,)CH(p-X-CJL,)CH,C,H,
(11)
+ CBHSCH2CH@-X-C6H4)CH@-X-C6H4)CH2C6H5 -t C,jH,CH,CH,@-X-C&L) Scheme
(12)
(13)
1.
The products include cis-stilbenes, regioisomers of the amine adduct (8 and 9), regioisomers of the tetraphenylbutanes (10, I1 and 12) and the reduction product 1,2diphenylethane (13). The ratio of 8 to 9 is about unity for most of the substituted TS. For p-cyano-TS, the two regioisomers are separated by chromatography. For most of the substituents (X = CH30, CH3, CH(CH& and Cl), regioisomeric 8 and 9 cannot be separated by chromatographic methods or any other physical method. The NMR spectra of the p-methyl-TS-amine and p-methoxy-TS-amine adducts indicate that the two regioisomers are present in a ratio of 1:1 (Fig. 5). For the p-methoxy-TS-amine adduct and the p-methyl-TS-amine adduct the two singlets (at 3.7 ppm and 2.2 ppm respectively) are of equal intensity; this indicates that the regioisomers are formed in equal amounts. This implies that the protonation from the aminium radical ion to the TS anion radical is not selective and therefore the structure of the TS anion radical is delocalized (III) rather than localized (I and II) (X=CN, OCH3, CH3, CH(CH&, Cl).
TABLE
5
Dipole moments substituted TS
of para-substituted
Substituent
Slope”
X=CN X=Cl X=H X=CH3 X = CI-I(CH& X = OCH,
2647 6430 4681 4455 4181 76.5
“Least-squares
TS-DIPEA
(cm - ‘)
exciplexes and reduction potentials
of para-
CL(debye)
E(A/A-)
6.56 10.23 8.73 8.51 8.25 3.53
0.95 0.87 0.67 0.62 0.60 0.51
slope of plot of f(e, n) vs. exciplex maximum emission (in cm-‘).
(eV>
307 (A)
9 ppm
6
7
6
5
4
3
2
1
0
‘C%
Fig. 5. Proton NMR spectra for the amine adducts of p-methyl-TS with diisopropylmethylamine.
(A) and p-methoxy-TS
(B)
308 TABLE
6
Quantum yields of amine adducts at various amine concentrations
P’IPM-4 (Ml
0.13 0.26 0.39 0.52 0.65 0.78 1.30
in acetonitrile
p-X-TS 4
5
6
0.019 0.053 0.087 0.113 0.132 0.143 0.226
0.010 0.031 0.075 0.086 0.099 0.135 0.162
0.0014 0.0070 0.0200 0.0266 0.0403 0.0587 0.0615
1
0.240
2
3
0.154
0.023 0.041 0.083 0.102 0.113 0.116 0.125
This
is contrary to the selective deprotonation of the aminium radicals [5]. Quantum yields @ of total amine adduct formation at several amine concentrations in acetonitrile are given in Table 6. The double reciprocal plots of @-’ vs. [amine]-’ are linear with correlation coefficients greater than 0.96 (r=0.99 for p-methyl-TS, r = 0.96 for p-methoxy-TS, I= 0.98 for p-isopropyl-TS, r = 0.99 for p-chloro-TS). This is in agreement with the previously proposed mechanism of reversible exciplex formation [I]. There is no obvious substituent effect on the quantum yields of the amine adduct.
4. Conclusions The exciplex fluorescence maxima are well correlated with the Rehm and Weller theoretical calculations for substituted TS with DIPEA, DIPMA and DMA. For the TEA-TS system, the correlation is less linear because of the smaller exciplex dissociation enthalpies and weakly bound exciplexes. Thus the thermodynamic stability of the exciplexes determines the degree of linear correlation. A linear correlation is also observed between exciplex energy and Hammett ap constants for DIPEA-TS, DIPMA-TS and DMA-TS systems. For the TEA-TS system, the correlation is less pronounced.
Acknowledgments The authors are grateful for the financial of ROC {Taiwan). T.-I. Ho wishes to express of Professor T. M. Su and Mr. Sen Hsu.
support of the National Science Council his gratitude for the technical assistance
309 References
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D. Lewis and T.-I. Ho, 3. Am. Chem. Sot., 99 (1977) 7991. D. Lewis, Act. Chem. Res., 12 (1979) 152. D. Lewis and T.-I. Ho, J. Am. Chem. Sot-, 102 (1980) 1751. D. Lewis and J. T. Simpson, 3. Phys. Chem., 83 (1979) 2015. D. Lewis, T.-I. Ho and J. T. Simpson, J. Org. Chem., 46 (1981) 1077. D. Lewis and J. T. Simpson, J. Am. Chem. Sot., 102 (1980) 7593. D. Lewis, T.-I. Ho and J. T. Simpson, 3. Am. Chem. Sot., 104 (1982) 1924. Hub, S. Schneider, F. Dorr, J. T. Simpson, J. D. Oxman and F. D. Lewis, J. Am
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10 11 12 13 14
15 16 17 18 19
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F. Dorr, J. D. Oxman and F. D. Lewis, J. Am. Chem Sot., 106 (1984) 701. T.-I. Ho, K Nozaki, A. Naito, S. Okazaki and H. Hatano, J. C/zem. Sot., Chem Commu)L, (1989) 206. S.-P. Van and G. S. Hammond, J. Am. Chem. Sot., 100 (1978) 3895. F. D. Lewis and R. E. Dykstra, J. Photochem. PhotobioL, A: Chem., 49 (1989) 109. L. Biczok, T. Berces and F. Marta, J. Photo&em. PhorobioL, A: Chem, 48 (1989) 265. T.-I. Ho, J. Phorochem., 36 (1987) 185. T.-I. Ho, C. M. Chang, S. R. Wang and C. P. Cheng, J. Chem Sot., Dalton Trans., (1988) 123. H. T. Clarke, H. B. Gillespie and S. Z. Weisshans, J. Am. Chem. Sot., 55 (1933) 4571. D. Rehm and A. Weller, 2. Phys. Chem. N. F., 69 (1970) 183. H. Kuibbe, D. Rehm and A. Weller, Bet: Bumenges. Phys. Chem, 73 (1969) 839. N. Mataga, Y. Torihashi and K. Ezumi, meor. Chim. Acta, 2 (1964) 158.
Stilbene-amine Jing-Chen
exciplexes. Substituent effects
Mai, Yuan-Chuau
Department of Chem&y,
Lin and Tong-Ing
National Taiwan Universi@
Hot Taipei (Taiwan)
(Received January 9, 1990 in revised form April 20, 1990)
Abstract Fluorescence is observed for 24 exciplexes formed between excited para-substituted trarrrstilbene (X E H, CN, Cl, CH3, CH(CH& and CH,O) and four tertiary amines (triethykmine, diisopropylmethylamine, diisopropylethylamine and Nfl-dimethylaniline) in n-hexane at room temperature. The maximum fluorescence energies are well correlated with the Rehm and Weller theoretical values with the exception of the triethylamine systems. The systematic shift in exciplex energy also correlates we11 with the Hammett up constant; however, for the triethylamine-stilbene systems, the correlation is less linear. The thermodynamic stability of the exciplex determines the degree of linearity in the Hammett correlation. Investigations of the products, Stern-Volmer quenching, dipole moments and exciplex lifetimes were also carried out to determine the structure-reactivity relationship.
1. Iutroduction The photochemical interactions between singlet tmns-stilbene (TS) and tertiary amines have been extensively studied [l-9]. The mechanism of this reaction is known to occur through consecutive electron transfer and proton transfer to generate 1,2diphenylethyl radical and Z-aminoalkyl radical pair. The reactive intermediates identified in the TS-triethylamine system include singlet state TS, exciplex, radical anion and radical cation. Recently, 1,Zdiphenylethyl radical has been identif?ed by the spintrapping technique [lo]. Structural changes influence the oxidation potential of the electron donor or the reduction potential of the acceptor molecule and thus the formation stability of the exciplexes, which will then influence the photochemistry or photophysics of the reaction [ll-131. The substituent also has some effect on the basic@ of the generated radical anion of TS and thus influences the proton transfer reaction. We have studied the systematic variation in the quenching of TS by substituted amines and have observed the Hammett correlation between kg and al [14]. In this paper, we report the interaction between substituted TS (l-6) and tertiary amines diisopropylmethylamine (DIPMA), diisopropylethylamine (triethylamine (TEA), (DIPEA) and Nfl-dimethylaniline (DMA)).
=--Q-x
X=H (l), X-C%’ (2), X=Cl (3), x- CH (4) x= CH(cH& (S), x=&o (k)
+Author to whom correspondence
lOlO-6030/90/$3.50
should be addressed.
Q Elsevier Sequoia/Printed
in The Netherlands
300 2. Experimental
details
2. I. Matetials
All the para-substituted trans-stilbenes were prepared from the corresponding para-substituted benzaldehydes and benzyl chloride by the Wittig reaction [l5]. The solvents were of fluorescence spectroscopic grade (Merck) and were used as received. Triethylamine (TEA), diisopropylethylamine (DIPEA) and N,ZV-dimethylaniline (DMA) were of reagent grade (Merck) and were purified by vacuum distillation. Diisopropylmethylamine (DIPMA) was prepared from diisopropylamine according to ref. 16. 2.2. Methods
Fluorescence spectra were recorded on a Perkin-Elmer IS-5 luminescence specsolutions trometer. Fluorescence quenching data were obtained for nitrogen-purged of 0.0001 M TS and amines (O.l- 2 M). 2.3. Lifetime measurements An Nd-YAG laser (Quanta-Ray DCR-2) was used to excite the fluorophore (substituted TS) at 355 or 266 nm. The exciplex fluorescence at maximum emission was collected through a 0.5 m Jarell-Ash monochromator and an RCA lP28 photomultiplier. The fluorescence decay was recovered by a transient waveform recorder (Gould-Biomation 6500, 2 ns resolution) which was interfaced to a microcomputer. The sample was kept at 298 K in a thermostat to within 0.2 K. In general, data were averaged over 300 laser shots, and the measured first-order fluorescence rate constant (and lifetime) was obtained from the fluorescence decay curve by a deconvolution process. The reduction and oxidation potentials (in volts against silver/saturated silver chloride electrodes) were measured on a cyclic voltametre (BAS-100) in polar aprotic solvent (CH&N or N$!-dimethylformamide) with a platinum disc as the working electrode and tetraethylammonium perchlorate (0.1 M) as the supporting electrode. 2-4. Imadiation of 2 and DIPMA
A solution of 2 (0.18 g, 0.0625 M) and DIPMA (0.86 M) in 15 ml of acetonitrile was irradiated using a medium pressure mercury lamp (Pyrex filter) for 20 h. Removal followed by column of the volatile material, chromatography (silica gel; n-hexane-ethylacetate) of the residual oil, gave p-cyano-cis-stilbene (0.0308 g; 19.6%), N~-diisopropyl-Z@-cyano)phenyl-3-phenylpropylamine (Sa) (0.035 g; 8.68%) and N,ZVdiisopropyl-2-phenyl-3-(p-cyano)phenylpropylamine (9a) (0.0446 g; 18.17%) (last two as oils). Sa: proton nuclear magnetic resonance (IH NMR) (CDC13) 0.95 (12H, Zd), 2.5-3.5 (9H, m); mass spectrum, m/e (%) 305 (M+, O-2%), 116 (7H, m>, 7.02-7.2 l-9%), 114 (H2C=N+(CH(CH&)z, lOO%), 91 (C6H5-Cl&+, 14%, (NC-C&L+-CH2+, 72 (H2C=N+HCH(CH&, 18.4%). 9a: ‘H NMR (CDQ) 0.95-1.2 (12H, two d), 2.5-3.5 (7H, m), 7.0-7.2 (9H, m); mass spectrum, m/e (%) 305 (M+, O.l%), 116 (NC-CCs&-CHCH2+, 2.8%), 114 5.7%), 72 (H&=N+HCH(CH&, (H2C=N+(CH(CH&)z, lOO%), 91 (C~H~-CHZ+, 19.6%). 2.5. Irradiation of 6 and DIPhL4 A solution of 6 (0.2 g) and DIPMA (1.07 M) in 15 ml of acetonitrile was irradiated for 25 h. Removal of the volatile material, followed by column chromatography (silica
301
gel; n-hexane-ethylacetate (25:l)) of the residual oil, gave n-methoxy-cis-stilbene (0.0367 g; 18.3%), l-p-methoxyphenyl,2-phenyl,3-phenyl,4-p-metho~henylbut~e (lob)and its regioisomeric mixture (0.03 g; 7.9%), NJ+diisopropyl-2-(p-methoxy)phenyl-3-phenylpropylamine (Sb) and isomers (0.044 g; 14.3%). 1Ob (regioisomers): ‘H NMR (CDCL+) 2.3-3.2 (18H, m), 2.7 (3H, s) 3.72 (3H, s), 3.76 (3H, s), 6.7-7.3 (27H, m); mass spectrum, m/e (%) 422 (M*, 7.5%), 211 (C,EI,-CH+ -CHcHz-C&OCH3, lOO%), 121 (CH~O-C,&-CHcH2+, 77%), 91 (C,Hs---cH,+, 25%). Sb (regioisomers): ‘H NMR (CDCL) 0.95 (12H, 2d), 1.05 (12H, 2d), 2.5-3.4 (lOH, m), 3.68 (3H, s), 3.72 (3H, s), 6.7-7.3 (lSH, m); mass spectrum, m/e (%) 310 (M+, O.l%), 121 (CH,O-C,&-CH,+, II%), 114 (H&=N+(CH(CH&)z, lOO%), 91 2.2%), 72 (H$Z=N+HCH(CH&, 29%). (GHs--z+, 2.6. Irradiation of 4 and DIPMA A solution of 4 (0.2 g, 0.05 M) and DIPMA (5 ml) in 20 ml of acetonitrile was irradiated for 21 h. The non-volatile residual oil was subjected to chromatography (silica gel and n-hexane) to givep-methyl-cis-stilbene (0.007 g; 3.5%), l-(p-methyl)phenyl2, 3-diphenyl-4-(p-methyl)phenylbutane (10~)and its isomeric mixture (llc and 12~) (1:l:l ratio (from NMR); 0.023 g; 5.9%) and NJV-diisopropyl-2-(p-methyl)phenyl-3phenylpropylamine (SC) and its isomer (0,053 g; 17.6%). 10~ (mixture): ‘H NMR (CDCQ 2.20 (3H, s), 2.25 (3H, s), 2.28 (3H, s), 2.3-3.3 (9H, m), 6.6-7.3 (lSH, m); mass spectrum, m/e (%) 390 (M+; O-3%), 195 (CH,-Cc,H,-CH+-CHz--Cc6Hs, lOO%), 179 (20%), 105 (CH,-CC6H4-CH2+, 47%), 91 (C6H5-CHz*, 38.5%). SC (regioisomers): IH NMR (CDCI,) 0.95 (12H, two d), 1.0 (12H, two d), 2.25 (3H, s), 2.3 (3H, s), 2.5-3.2 (lOH, m), 6.8-7.3 (lSH, m); mass spectrum, m/e (%) 310 (MC, 2%), 178 (19%), 114 (lOO%), 105 (10.5%), 72 (32%). 2. Z Irradiation of 5 and DIPAA solution of 5 (0.15 g) and DIPMA (3.5 ml) in 12 ml of acetonitrile was irradiated for 20 h. Removal of the volatile material, followed by thin layer chromatography (silica gel, n-hexane), gave NJV-diisopropyl-2-@-isopropyl)phenyl-3-phenylpropyl~ne (Sd) (regioisomers) (0.041 g; 18.6%) and 1-(p-isopropyl)phenyl-3-phenyl-4-(p-isopropyl)phenylbutane (1Od) (isomers) (0.0196 g; 6.5%). Sd (regioisomers): ‘H NMR (CD&) 0.85 (12H, two d), 0.95 (12H, two d), 1.05 (12H, two d), 1.1 (12H, two d), 2.5-3.4 (lOH, m), 6.8-7.3 (18H, m). 1Od (mixture): mass spectrum, m/e (%) 446 (M+, 0.2%), 223 (lOO%), 181 (62.5%), 167 (23%), 133 (45%), 105 (42%). 2.8. Irradiation of 3 and DIPill A solution of 3 (0.2 g, 0.052. M) and DIPMA (3 ml) (1.08 M) in 15 ml of acetonitrile was irradiated for 25 h. Removal of the volatile material, followed by Rash column chromatography, gave I-phenyl-2-(p-chloro)phenylethane (13e) (0.0083 g; 0.42%), p-chloroks-stilbene (0.038 g; 19.1%) and N,Wdiisopropyl-2-(pchloro)phenylpropylamine (Se) (mixture) (0.05 g; 16.4%). 13e: mass spectrum m/e (%) 216 (M+, 20%), 125 (Cl-C&L+-CH,+, 69.5%), 91 (C6H5-CHz+, 100%). Se (mixture): ‘H NMR (CDC13) 0.7-l (12H, two d), 2.5-3.3 (7H, m), 6.9-7.3 (9H, m); mass spectrum, m/e (%) 316 (M+, 0%), 125 (Cl-C&L,-CH,+, 36.5%), 114 (H&=N+(CH(CH,),),, lOO%), 91 (&H,-CH,+, 4.5%), 72 (H$Z=N’HCH(CH,), 32%).
302
3. Results and discussion The emission spectra of the substituted TS were measured in n-hexane (Fig. 1) and the emission maxima and absorption maxima are shown in Table 1. In the presence of tertiary amines (for example, diisopropylethylamine; Fig. l), the intensity of the TS fluorescence is reduced and, simultaneously, new, structureless, red-shifted emission bands of the exciplexes are observed. At a sufficiently high concentration of amines, the TS fluorescence is almost completely quenched and the exciplex emission is symmetrical about the peak maximum. The observed energies El at the exciplex fluorescence maximum are listed in Table 2 and can be compared with the theoretical energies E2 according to Rehm and Weller [17] eV
Ez=Ed-E,-0.15j~O.l
(1) where Ed and E, represent the oxidation potential of the donor and the reduction potential of the acceptor respectively. For most exciplexes the correlation between El and E2 is fairly good with the exception of those formed with TEA. Electron-donating substituents shift the emission to higher energy and electron-accepting substituents shift the emission to lower energy for all the amines studied. The systematic shift in the exciplex maximum as a result of the effect of the substituent on TS can be seen from the plot of El vs. the Hammett up constants
r
MAX
= 100
00
2 m k 5
s
5 ::
z!
,o
z
350
400
500
450 Wavelength
TS and diisopropylethylamine
exciplexes in
1
Absorption
and emission maxima of para-substituted
p-X--l-S
*
1
350 380 356 356 357 373
2 3 4 5 6
600
(nm)
Fig. 1. Fluorescence spectra of pam-substituted n-hexane at room temperature. TABLE
550
@ml
TS and singlet excitation energies
GZ &ml
Go (ev)
299 303 299 298 297 303
3.79 3.63 3.85 3.79 3.79 3.67
303 TABLE
2
Comparison of El (observed energy at according to Rehm and Weller)
exciplex emission maximum) with E2 (theoretical energy
p-X-TS
Amine
DIPEA
DIPMA
DMA
2
3
1
4
5
6
A (nm> El (eV Ez (ev)
486 2.55 2.50
446 2.78 2.70
432 2.87 2.97
429 2.89
42s 2.91 3.01
421 2.94 3.05
A (m@ El (ev) EZ (ev)
482 2.57 2.53
446 2.78 2.73
435 2.85 3.00
429 3.02
425 2.91 3.04
411 3.02 3.07
A tnm) El (eV EZ (ev)
444 2.79 2.63
441 2.81 2.83
435 2.85 3.10
431 2.87 3.12
428 2.90 3.14
408 3.04 3.18
A (nm) G (eV -5 (eV
480 2.58 2.42
444 2.79 2.62
439 2.82 2.89
437 2.84 2.91
434 2.86 2.93
426 2.91 2.97
2.99
2.39
250 DIPEA . DI PEA A DMA
.’ l 24-%.
22--
2 l--
20 I -0.30
-0.10
o-10
0.30
0.50
C
UP
Fig. 2. Correlation between the exciplex fluorescence maxima El and the Hammett Q~ constants.
(Fig. 2) for three amines (DIPEA, DIPMA and DMA). The correlation is fairly good; the slopes p and the correlation coefficients T are as follows: p= - 3.29, r= 0.987 for DIPEA, p = - 3.39, r= 0.98 for DIP% p= - 2.5, r==0.963 for DMA. For TEA, the correlation is not as good with p= - 1.63 and r= 0.79. The correlation between E1 and ap is linear because charge transfer interaction plays an important role in the formation of exciplexes. Thus it is not surprising that there is a linear correlation between the reduction potential of the substituted TS and q, (Fig. 3 with r=0.987).
304
z
7
C ._
-1.70
+-
--
ol ? 2 v, -1.90-> m t-I v. h
-Z.lO--
;s Is 2;
L
-2.30, - 0.30
-0.10
0.10
0.30
0.50
0 ‘0
UP
Fig. 3. Correlation
TABLE
between
the reduction potential of the substituted TS and w+
3
Dissociation temperature Amirre
DIPEA DIPMA TEA DMA
enthalpies (eV) of p-X-TS-teti-amine exciplexes in n-hexane and the lifetimes (ns) in cyclohexane and n-hexane
solution
at room
p-X-TS 2
3
1
4
5
6
0.95 (14) 0.92 (14) 0.82 (4) 1.03
0.87 (22) 0.84 (7”) 0.74 (6”, 7) 0.95
0.67 (16) 0.64 0.54 0.75
0.62 (13) 0.59 (7”, 13) 0.49 (7) 0.70
0.60 (20) 0.57 (8*, 13) 0.47 (9) 0.68
0.51 (6) 0.48 (13) 0.38 0.59
Lifetimes in parentheses. “In n-hexane.
The
calculation &=-%,o-
effect of structure on the stability of the exciplexes of the exciplex dissociation on enthalpies E3 [18] (Ed-E,)
- 0.13 eV
can be seen from
the (2)
where Eo, o is the singlet excitation energy of the substituted TS. The exciplex dissociation enthalpies (Table 3) indicate that TEA generally forms weakly bound exciplexes, whereas DIPEA, DIPMA and DMA form strongly bound exciplexes. In addition, electron-donating groups produce exciplexes that are less strongly bound. The most strongly bound exciplexes are those with strong electron-withdrawing substituents on the TS. For the TEA-TS exciplex system, the small exciplex dissociation enthalpies lead to the formation of loosely bound exciplexes, reversible formation of the exciplexes and efficient redissociation giving shorter exciplex lifetimes (Table 3). Evidence for reversible exciplex formation is also provided by the temperature dependence of the fluorescence quenching data (Table 4) [l]. It is evident that the thermodynamic stability determines whether the Hammett correlation will be observed. This observation is
TABLE 4 of temperature on the Stem-Volmer quenching constants of the para-substituted TS by amines in n-hexane (excitation: 313 mu)
Effect
Amine
Temptratwe
p-x-l-s
(“Cl 3
DIPEA
5 15 25 35 45
7.83 7.65 7.46 6.20 6.11
5
18.79 16.64 12.07 11.71 10.53
DIPMA
15
25 35 45
L;
.-: E
5
6
4.53 4.38 4.15 4.25 3.37
4.43 3.69 3.16 2.89 2.73
3.61 3.24 3.79 2.59 2.46
3.78 4.40 4.31 3.52 2.70
0.89 0.69 0.41 0.29 0.31
23
E
.-E
4
0 tsopropy l Chloro A Methyl
I
2t
W Y
Li
0.20
0.50
o-40
0.30
2 -_e-l 2e+l
Fig. 4. Frequency of p-X-TS-DIPEA
0.60
n2-1 2n2+1
exciplex fluorescence maxima W. solvent polarizability.
consistent with the I-methylnaphthalene-triethylamine and naphthalene-triethylamine exciplex systems [13]. The substituent on the TS also influences the dipole moment of the exciplex. The dipole moment of the excipiex can be estimated from the variation in the exciplex * . emlssIon m-mum v,, with solvent polarity [I91 %nax=~o-
2P2 E-i
ha3
( -
2~+1
-
-
n2--l
4n2+2 1
(3)
where v. is the hypothetical gas phase emission frequency, p is the dipole moment, a is the solvent cavity radius (approximately 4.5 A), h is Planck’s constant and n is the refractive index. From the linear plot of the v_ data (Fig. 4), dipole moments
for TS-DIPEA systems are obtained (Table 5). The dipole moment increases with increasing reduction potential of the substituted TS with the exception of p-cyano-TS (6.56 debye). This abnormal value may be due to the delocalization of the transferred charge into the cyano triple bond. The photochemistry of para-substituted TS (X = CN, 0CH3, CH3, CH(CH&, Cl) with diisopropylmethylamine is shown in Scheme I. p-X-TS
+ DIPMA
---&
p-X-ck-stilbene
(7)
+ C6H5CH2CH@-X-C6H4)CH2N(CH(CH3)2)2
(8)
+ @-X-C,H,)CH,CH(C,H,)CH,N(CH(CH,),),
(9)
+ (p-X-CSH~)CH&H(CaH~)CH(C6H&H&-X-C&,)
(10)
+ (p-X-C,H,)CH,CH(CaEF,)CH(p-X-CJL,)CH,C,H,
(11)
+ CBHSCH2CH@-X-C6H4)CH@-X-C6H4)CH2C6H5 -t C,jH,CH,CH,@-X-C&L) Scheme
(12)
(13)
1.
The products include cis-stilbenes, regioisomers of the amine adduct (8 and 9), regioisomers of the tetraphenylbutanes (10, I1 and 12) and the reduction product 1,2diphenylethane (13). The ratio of 8 to 9 is about unity for most of the substituted TS. For p-cyano-TS, the two regioisomers are separated by chromatography. For most of the substituents (X = CH30, CH3, CH(CH& and Cl), regioisomeric 8 and 9 cannot be separated by chromatographic methods or any other physical method. The NMR spectra of the p-methyl-TS-amine and p-methoxy-TS-amine adducts indicate that the two regioisomers are present in a ratio of 1:1 (Fig. 5). For the p-methoxy-TS-amine adduct and the p-methyl-TS-amine adduct the two singlets (at 3.7 ppm and 2.2 ppm respectively) are of equal intensity; this indicates that the regioisomers are formed in equal amounts. This implies that the protonation from the aminium radical ion to the TS anion radical is not selective and therefore the structure of the TS anion radical is delocalized (III) rather than localized (I and II) (X=CN, OCH3, CH3, CH(CH&, Cl).
TABLE
5
Dipole moments substituted TS
of para-substituted
Substituent
Slope”
X=CN X=Cl X=H X=CH3 X = CI-I(CH& X = OCH,
2647 6430 4681 4455 4181 76.5
“Least-squares
TS-DIPEA
(cm - ‘)
exciplexes and reduction potentials
of para-
CL(debye)
E(A/A-)
6.56 10.23 8.73 8.51 8.25 3.53
0.95 0.87 0.67 0.62 0.60 0.51
slope of plot of f(e, n) vs. exciplex maximum emission (in cm-‘).
(eV>
307 (A)
9 ppm
6
7
6
5
4
3
2
1
0
‘C%
Fig. 5. Proton NMR spectra for the amine adducts of p-methyl-TS with diisopropylmethylamine.
(A) and p-methoxy-TS
(B)
308 TABLE
6
Quantum yields of amine adducts at various amine concentrations
P’IPM-4 (Ml
0.13 0.26 0.39 0.52 0.65 0.78 1.30
in acetonitrile
p-X-TS 4
5
6
0.019 0.053 0.087 0.113 0.132 0.143 0.226
0.010 0.031 0.075 0.086 0.099 0.135 0.162
0.0014 0.0070 0.0200 0.0266 0.0403 0.0587 0.0615
1
0.240
2
3
0.154
0.023 0.041 0.083 0.102 0.113 0.116 0.125
This
is contrary to the selective deprotonation of the aminium radicals [5]. Quantum yields @ of total amine adduct formation at several amine concentrations in acetonitrile are given in Table 6. The double reciprocal plots of @-’ vs. [amine]-’ are linear with correlation coefficients greater than 0.96 (r=0.99 for p-methyl-TS, r = 0.96 for p-methoxy-TS, I= 0.98 for p-isopropyl-TS, r = 0.99 for p-chloro-TS). This is in agreement with the previously proposed mechanism of reversible exciplex formation [I]. There is no obvious substituent effect on the quantum yields of the amine adduct.
4. Conclusions The exciplex fluorescence maxima are well correlated with the Rehm and Weller theoretical calculations for substituted TS with DIPEA, DIPMA and DMA. For the TEA-TS system, the correlation is less linear because of the smaller exciplex dissociation enthalpies and weakly bound exciplexes. Thus the thermodynamic stability of the exciplexes determines the degree of linear correlation. A linear correlation is also observed between exciplex energy and Hammett ap constants for DIPEA-TS, DIPMA-TS and DMA-TS systems. For the TEA-TS system, the correlation is less pronounced.
Acknowledgments The authors are grateful for the financial of ROC {Taiwan). T.-I. Ho wishes to express of Professor T. M. Su and Mr. Sen Hsu.
support of the National Science Council his gratitude for the technical assistance
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