Chapter 28
An Overview of Photochemistry in Ionic Liquids
Downloaded by STANFORD UNIV GREEN LIBR on July 19, 2012 | http://pubs.acs.org Publication Date: August 26, 2003 | doi: 10.1021/bk-2003-0856.ch028
Richard M. Pagni Department of Chemistry, University of Tennessee, Knoxville, TN 37996-1600
Photochemistry in ionic liquids, taken mainly from the authors own work, will be described. It will be seen that ionic liquids provide interesting, unusual and often unique environments in which to carry out photochemistry.
Introduction Organic, inorganic, organometallic, catalytic, and polymer chemistry is currently being actively studied in ionic liquids. " Surprisingly, very little of this activity deals with photochemistry. What has been published, however, is very interesting, even unique in some cases. This suggests a rich future for this subject. This article will give an overview of what is presently known about the photophysical and photochemical properties of ionic liquids and solutes therein, with emphasis on work from the author's laboratory. Only photochemistry in imidazolium- and pyridinium-based ionic liquids, the mostly widely used, will be considered. 1
344
6
© 2003 American Chemical Society In Ionic Liquids as Green Solvents; Rogers, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2003.
345
Results and Discussion Salt Solutions A good place to begin is by examining what effect dissolved salts have on the properties of solutions because ionic liquids may be considered "neat" salt solutions. Dissolving a salt in a liquid has the effect of increasing the internal pressure , a thermodynamic property, and polarity , an empirically derived property, of the resulting solution. Internal pressure, which is known for a few molten salts but not a single imidazolium- or pyridinium-based ionic liquid, does not mimic applied external pressure and has little effect on the rates of reactions. The large internal pressures of lithium perchlorate in diethyl ether, an ionic liquid when the lithium perchlorate concentration is above 4.2S M , has little effect on the lowest B6B* absorption and fluorescence bands of the nonpolar anthracene and the second B6B* absorption and fluorescence bands of the polar azulene. Increases in solution polarity due to added salts does affect the rates of all sorts of reactions including the S 2 reaction, however. Photoreactions proceeding through radical ion pairs formed via photoinduced electron transfer are often influenced by added salts. This often occurs by exchange of the anion in the ion pair with the much less basic anion, often C10 ~, of the added salt. Many ionic liquids in current use contain weakly basic anions such as P F \ BF ", and (CF S0 ) N" (Tf N). 7
8
Downloaded by STANFORD UNIV GREEN LIBR on July 19, 2012 | http://pubs.acs.org Publication Date: August 26, 2003 | doi: 10.1021/bk-2003-0856.ch028
9
10
8
7
10
11
N
11
4
6
4
3
2
2
2
Relevant Properties of Ionic Liquids Polarity is a useful, well-known, if poorly defined, property of liquids whose magnitude depends on non-specific electrostatic interactions between charged and dipolar sites on a solute and dipoles on a solvent. It can be defined and thus quantitated in numerous ways using the spectroscopic properties of selected probe solutes. Many of these polarity indices such as Ε » V, 3, B*, and P , which involve electronic transitions (absorption, emission), have been applied to ionic liquids and molten salts. " Several pertinent results arise from these studies. (1) [Emim][Tf N] has a very low dielectric constant, a property which cannot be measured directly for ionic liquids. One may presume that other ionic liquids have low dielectric constants as well. (2) By and large, the imidazolium- and pyridinium-based ionic liquids have polarities similar to those of CH CN, DMSO, and the lower alcohols. " If an aliphatic side chain is replaced with one containing an hydroxyl group, the polarity of the ionic liquid goes up significantly. (3) Adding small amounts of water to an ionic liquid affects the polarity of the ionic liquid. (4) The organic cation seems to be the major contributor to the polarity of the ionic liquid. (5) Polarity indices do not reflect polarity alone. Properties such as viscosity and polarizability are also influential. τ
y
13
25
2
17
1318
21
3
25
19,22,24
In Ionic Liquids as Green Solvents; Rogers, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2003.
346 Viscosity, which is the resistance to flow in a liquid, is known as a function of temperature for all common liquids. The rates of diffusion-controlled bimolecular reactions such as the reaction of N 0 with arenes, quenching of excited states, and photoinduced electron transfer are very much solvent dependent because viscosity and rate of diffusion are interrelated mathematically. Often the internal motions of molecules, excited states, and transients are also viscosity dependent. " The viscosities of many imidazolium- and pyridinium-based ionic liquids are known and are much higher than for most common solvents. " The higher viscosities are undoubtedly due to intermolecular associations between cations and anions by hydrogen bonding, electrostatic attraction and other effects. Ionic liquids thus are excellent media to look for effects where diffusion and molecular motion are important. The effects may be unusual because ionic liquids of necessity are made up of a 1:1 molar ratio of cations and anions. For example, the cations and anions respond differently to rapid changes in structure and polarity of a solute brought about by electronic excitation, Fluorescence quenching of the lowest singlet excited states of six alternant polycyclic aromatic hydrocarbons (PAHs) by nitromethane occurred at the rate of diffusion in [bmim][PF ], presumably by an electron transfer mechanism. Six non-alternant PAHs, on the other hand, were not so quenched. This lack of quenching is interesting because quenching of the same PAHs did occur under other conditions. MacLean and Gordon have also observed diffusion-controlled quenching in several ionic liquids by transient spectroscopy. Several pulse radiolysis studies have been carried out in ionic liquids. A wealth of kinetic data on the reactions of transient species has thus been generated. The reaction of C F X with pyrene in (CH )(n-Bu) N (Tf N), for example, occurs at the rate of diffusion. Organic molecules rarely phosphoresce in fluid solution at room temperature because their long-lived triplet states are easily quenched (selfquenching, trace 0 ). 1 -Bromonaphthalene, surprisingly, phosphorseces in degassed [emim][Tf N] at room temperature. This may be due to the fact that the rates of the biomolecular quenching reactions are retarded in the viscous medium. Added 0 quenches the phosphorescene of 1 -bromonaphthalene, as expected, but not its fluorescence because of the short lifetime of the singlet state. Virtually no photophysical data are available for any ionic liquid or molten salt. It is generally believed that imidazolium- and pyridinium-based ionic liquids absorb strongly below 300 nm. [ Emim][Cl/AlCl ] starts absorbing around 300 nm but [bmim][Tf N] doesn't absorb until around 240 nm. Based on the absorption spectrum of the pyridinium ion, pyridinium-based ionic liquids will absorb starting around 300 nm. This is the reason that only substrates having absorption bands above 300 nm have been studied. Owing to the expected large singlet and triplet excitation energies of the ionic liquids, 26
+
2
27
28
30
Downloaded by STANFORD UNIV GREEN LIBR on July 19, 2012 | http://pubs.acs.org Publication Date: August 26, 2003 | doi: 10.1021/bk-2003-0856.ch028
31
34
23,34
35
6
36,37
+
3
3
3
2
2
17
2
2
42
3
43
44
2
45
4 7
In Ionic Liquids as Green Solvents; Rogers, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2003.
347 quenching of substrates' excited states by ionic liquids is not likely. In fact there is no published evidence that any photoreaction is inhibited by the ionic liquids in any way. Sensitization by the ionic liquids is possible but unlikely because the high concentration of the imidazolium and pyridinium salts in the ionic liquids ensures that self-quenching will dominate. The ionic liquids may themselves be photoactive. There is one report that photolysis of [bmim]fPF ] results in the formation of unidentified products. Pyridinium and imidazolium salts in solution are known to be photoactive. 48
6
49,50
Downloaded by STANFORD UNIV GREEN LIBR on July 19, 2012 | http://pubs.acs.org Publication Date: August 26, 2003 | doi: 10.1021/bk-2003-0856.ch028
Photoreactions in Ionic Liquids One can loosely divide photoreactions in ionic liquids into two categories: those in which the ionic liquid is not directly involved in the chemistry and those in which they are. The discussion will begin with chemistry in the first category. Gordon and McLean have shown that election transfer from the excited state of the bipyridyl complex, Ru(bypy)* , to methylviologen (MV ) occurs at the rate of diffusion in butylmethylimidazolium hexafluorophosphate [bmim][PF ] (Scheme 1, line l ) . The energy transfer from the triplet excited state of benzophenone (BP *) to naphthalene (N), which does not involve electron transfer, occurs at the rate of diffusion in several imidazoliumcontaining ionic liquids (Scheme 2, line 2). 2
+2
36
6
3
37
+2
Ru(bpy)$ * + M V 3
BP * + Ν
+2
+3
^ » »
Ru(bpy)$
+
MV
+ #
3
BP + N *
Scheme 1: Electron and energy transfer in ionic liquids The photochemistry of anthracene (AN) in deoxygenated, basic [emim][Cl/AlCl ] afforded the 4 + 4 dimer, a reaction identical to that seen in more conventional solvents (Scheme 2). In oxygenated, basic [emimJCl/AlCy, on the other hand, A N yielded anthraquinone, 9chloroanthracene, and 9,10-dichloroanthracene. Although the details of this chemistry have not been worked out, it is clear that the chlorinated products arise, in part, by the reaction of the radical cation of A N (AN* ) with CI". The A N * likely is formed by the electron transfer from the excited state of anthracene to 0 . The ionic liquid thus provides a polar medium to facilitate the electron transfer. The photochemistry of A N in acidic [emimHCl/AlCy is quite different than seen in the basic melt, yielding an array of monomeric and dimeric 51
3
52
53
X
X
2
In Ionic Liquids as Green Solvents; Rogers, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2003.
Downloaded by STANFORD UNIV GREEN LIBR on July 19, 2012 | http://pubs.acs.org Publication Date: August 26, 2003 | doi: 10.1021/bk-2003-0856.ch028
348
Scheme 2: Photochemistry ofantracene in basic [emim][Cl/AlC.3]
ANH* Figure 1: Photoproducts from anthracene in acidic [emim][CUAICI3]
In Ionic Liquids as Green Solvents; Rogers, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2003.
349 reduced, neutral and oxidized products (Figure 1). This is a consequence of the fact that residual HC1 is a powerful Br0nsted acid ' and protonates A N to yield a small amount of A N H , a good electron acceptor from excited states. Furthermore, with the powerful Lewis acid A1 C1 " present in the ionic liquid, none of the myriad positively charged and neutral molecules generated in the photochemistry will react with the solvent; they will only react with each other. Thus the reaction is inititated by electron transfer from the singlet excited state of A N to A N H followed by a series of bimolecular electron transfer, hydrogen transfer and coupling reactions (Scheme 3). It was possible to mimic this photochemistry using a mixture of the strong acid, trifluoromethanesulfonic acid, and the much weaker acid, trifluoroacetic acid. Clearly the type of chemistry described in this paragraph will not occur in the contemporary onecomponent ionic liquids, but it might if a Lewis acid were added - [emim][BF ] + BF , for example 43 54
+
2
7
Downloaded by STANFORD UNIV GREEN LIBR on July 19, 2012 | http://pubs.acs.org Publication Date: August 26, 2003 | doi: 10.1021/bk-2003-0856.ch028
+
55
4
3
AN
+
ANVt c r
HC1
ANH*
+
AN
ΑΝΗ·
+
AN**
h V
»
ΑΝΗ·
— • — •
+
AN
+ #
Products
Scheme 3: Initial reactions of anthracene in acidic [emim][Cl/AlCl
Jones and coworkers have described a synthetically useful photoreaction in imidazolium-containing ionic liquids, the reduction of benzophenones to benzhydrols by primary amines (Scheme 4). The reaction is initiated by hydrogen abstraction by the ketone to form a radical pair. Electron transfer then yields an ion pair, a reaction facilitated by the polarity of the ionic liquid. Proton transfer within the ion pair completes the reaction. Surprisingly, benzpinacols are formed instead in ionic liquids not containing the imidazolium ring, ^ec-butylammonium trifluoroacetate and /^-propylammonium nitrate. Perhaps the imidazolium cation plays a more active role in the chemistry than the mechanistic scheme implies, as is seen in several cases described below. McLean, Gordon and coworkers have investigated the kinetics of the hydrogen atom abstraction by benzophenone triplet excited state (BP*) from several imidazolium-containing ionic liquids (Scheme 5). Although no products were isolated, it is reasonable to assume that the hydrogen abstraction occurs on the alkyl substituents of the imidazolium cations as shown below. Interestingly, the energy of activation for the abstraction is considerably higher than seen for abstraction from 1-butanol, cyclohexane and toluene. Why there is a difference is currently unclear. UC1 ", which is stable in acidic [emim][Cl/AlCl ] is reduced when photolyzed into its ligand-to-metal change transfer band, yielding UC1 , CI* and C1X. The chlorine atom apparently reacts with emim to form unidentified 56
57
6
3
4
58
+
In Ionic Liquids as Green Solvents; Rogers, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2003.
350
Downloaded by STANFORD UNIV GREEN LIBR on July 19, 2012 | http://pubs.acs.org Publication Date: August 26, 2003 | doi: 10.1021/bk-2003-0856.ch028
• ^ « >
Q
ΝΗ2
PtiT>h
OH
ΝΗ2
!
