- Email: [email protected]
Electronic and photochemical behavior of some conformationally restricted intramolecular charge transfer systems
201
L Photockm. Phoro~fo~.A: Chem., 78 (1994) 201-204
Electronic and photochemical behavior of some conformationally restricted intramolecular charge transfer systems F.D.
Saeva
Cmporate Research Laboratories, Eastman Kodak Company, Rochester, NY 14650 (USA) (Received September 21, 1993; accepted November 2, 1993)
Abstract Two series of conformationally restricted intramolecular charge transfer (CT) systems with very different potential for through-space interaction between an anthracene electron donor (D) and a series of pyridinium acceptor (A) moieties were synthesized. A comparison of the electronic absorption and emission behavior of these two systems indicated that CT absorption and intramolecular exciplex emission were more important in the series in which close proximity and coplanarity between D and A could be achieved. The electron-withdrawing pyridinium moiety was found to be a very efficient quencher of anthracene fluorescence. Cf absorption and emission are dependent on the solvent polarity, the redox behavior of D and A and the ability of D and A to interact electronically either through bond or through space. In general, CT absorption is enhanced and red shifted as the solvent polarity decreases, while the reverse is true for intramolecular exciplex emission, i.e. the wavelength of exciplex emission shifts to the red and decreases in intensity as the solvent polarity increases. All of the intramolecular CT systems investigated were found to be photochemically inert.
1. Introduction In view of the need to detect chemical species in biological assays and industrial processes simply and with a high degree of accuracy and precision [l-3], a luminescence scheme has been proposed to produce exciplex and/or charge transfer (CT) emission as a function of the nature of a pendant electron acceptor. The fundamental basis for the concept is the conformational rigidity of the phenyl ring in 9-phenylanthracene and the ability of ortho substituents on the phenyl ring to interact electronically with the anthracene ring system and modify its fluorescence behavior.
formational change to this situation occurs within the lifetime of the anthracene excited singlet state. This effect has not been observed for systems in which the electron donor (D) and acceptor (A) groups are connected by a flexible hydrocarbon chain [S]. We report the synthesis, spectroscopic and redox behavior of the following two conformationally restricted anthryl systems. In addition, the photochemical behavior of la was investigated. A comparison of the electronic behavior of system 1 and system 2 is provided to determine the effect of through-space orbital overlap between D and A moieties. System 1 can adopt a conformation that places the R group over the anthracene rr system which is not possible for system 2.
2. Experimental We have previously shown that, when R = -N(Et),, intramolecular exciplex luminescence is observed in solution and in polymer I%ns [4]. The observation of exciplex luminescence in polymer films indicated that either a ground state interaction between the nitrogen lone pair and the anthracene ring system exists or a rapid conElsevier Sequoia SSDI 1010-6030(93)03734-X
details
2.1. Materials Compounds la-le and 2a-2e were prepared from 3 [5] and 4 (Aldrich) respectively by reacting the appropriate pyridine derivative either neat or in acetonitrile and refluxing for 1 h. The reaction mixture was then poured Into diethyl ether and the pyridinium halide salt collected by suction
202
F.D. Saeva / Conformation&y rest&ted intramolecular cha%e transfer systems
3. Results and discussion
Series 1 and 2 were synthesized of the corresponding halomethyl the appropriate pyridine as shown nucleophilic counterion was used state chemistry and CI’ effects.
by the reaction derivatives with below. A weakly to avoid ground
CH
\I
qr
TABLE 1. Characterization of FDMS and melting point
compoundsla-le and Za-Zeby 3.2. Electronic absorption spectra
Compound
FDMS
m.p. (“C)
la lb lc Id h? 2a 2b tc 2d 2e
346 389 423 371 42A 270 313 341 295 348
200-202 240-241 174-176 195-197 =5(d) 155-156 190-192 223-224 172(d) 183(d)
d in parenthesesindicates decomposition. filtration. Purification was accomplished by recrystallization from an acetonitrile-diethyl ether solvent mixture. Conversion to the trifluorome-
thanesulfonate counterion was accomplished by dissolving the pyridinium halide in a minimum amount of water and adding to this solution an aqueous solution of sodium trifluoromethanesulfonate. The triflate salt was collected by suction filtration and air dried before being recrystallized from an acetonitrile-diethyl ether solvent mixture. All the compounds were characterized by their field desorption mass spectrum (FDMS), proton nuclear magnetic resonance (IH NMR) spectrum and melting Table 1.
point. The data are summarized
The absorption spectrum of compounds lb-le exhibits readily discernible CT absorption beyond the anthracene absorption at about 395 nm (see Fig. 1). The wavelength and molecular absorptivity of the CT absorption for compounds lb-le are
dependent on the solvent polarity, the redox behavior of D and A and the steric requirements for the through-space interaction of D with A. In general, A,,_= shifts to longer wavelength as the solvent polarity decreases, indicating that the initially formed photoinduced CT state is less polar than the ground state. For example, Id exhibits CIY absorption at 486 run in chloroform (E= 1000) and at 468 nm in acetonitrile (E = 830). V.3”
1
I
466 nm k = 1000)
lo
in Fig. 1. Absorption spectrum of ld in chloroform
solvent.
203
F.D. Saeva / Conformational& restnbted indrumolecular charge transfer systems
#!ktQmm
value shifts to longer wavelength as The hmanCT the lowest unoccupied molecular orbital (LUMO) of A decreases in energy, as expected. However, this is true only for those acceptors whose physical dimensions will allow efficient orbital overlap with the anthracene r system (see Table 2). The bipyridinium lf, for example, exhibits CT absorption at 470 nm (e=580 nm) in CH,Cl, while le, which exhibits a slightly higher energy, i.e. 20 mV, LUMO level, shows CT absorption at 482 nm (E= 820) in the same solvent system. A rationale for this observation is presented in Section 3.3. With the exception of 2d and 2e, the electronic absorption spectra of the compounds in series 2 are generally similar to one another and to 9methylanthracene in the spectral region between 300 and 400 run. Compounds 2d and 2e exhibit CT absorption which is considerably lower in intensity, i.e. with E< 200, than observed for system 1 in the spectral region around 450 nm. This difference is attributed to the large distance between atoms involved in the CT absorption process.
TABLE 2. Charge transfer absorption and redox behavior of compounds la-le and 2a-2e in C&C&
4 <-->19
2.7 A'
8<-a34
7.7 A'
The molecules in series 2 possess a bent structure with a dihedral angle of approximately 109” with free rotation around the A-C bond. The orientation of the D and A moieties in 2a provides for poor r orbital overlap. The atoms determining D-A overlap are about 3-8 8, apart. 3.3. Redox behavior The oxidation and reduction potentials (Eg and I$,$) for the unsubstituted model for series 1, 9o-tolylanthraccne, are 1.35 V and -2.0 V respectively (z)s. SCE in CD&.). Substitution of an electron-withdrawing group on methyl shifts the oxidation potential to more anodic values, i.e. 1.40 to 1.50 V, depending on the substituent (see Table 2). Reversible one-electron redox potentials were observed for lc, Id and le, while irreversble cathodic peak potentials were observed for la and lb at a scan rate of 100 mV s-l. The irreversibility is presumably due to a competing reductive cleavage process [6]. A plot of the energy of the CT transition 2)s.the reduction potential for the various pyridinium salts is linear with RZ=0.99 when If is excluded from the correlation (see Fig. 2). Inclusion of If produces a less satisfactory correlation. The energy of the CT transition for If is observed at shorter wavelength than would be expected on the basis of the reduction potential for the bipyridinium functionality. A rationale for this observation is proposed based on the effec-
Compound la lb 1C
Id le 9-o-Tolylanthracene 2a 2b 2c 2d 2e
-410 =436 (=500) 445 (490) 482 (820) 470 (580)
450 (200) 450 (40)
(-1.18) (-0.90) - 0.82 - 0.45 - 0.43 ( - 2.07) (-1.12) (-0.87)
1.45 1.50 1.40 1.45 1.46 1.35 (1.25) (1.39)
-0.67 - 0.44 -0.20
(1.20) (1.63) (1.65)
“100 mV s-’ scan rate, vs. saturated calomel electrode (SCE), 0.1 N tetrabutylammonium tetrafluoroborate.
2.41 -1.4
I -1.2
I -1.0
I -0.8
I -0.6
I -0.4
I -0.2
Reduction Potential ( V vs SCE) Fig. 2. Charge transfer transition energy vs. reduction potential for compounds la-k in methylene chloride solvent.
F.D. Sueva I Conformutional&
204
restricted intrmwlecular
tiveness of the through-space v overlap between the D and A moieties. Only the pyridinium ring directly attached to the methylene group can interact efficiently with the anthracene ring system as shown below. The electron-accepting ability of the pyridinium moiety, as indicated by cyclic voltammetry, does not reflect the effective electronaccepting behavior of the constrained bipyridinium moiety as felt by the anthracene ring system.
2 m*3
charge transfer system
solvents respectively. The quantum yield (&J of luminescence from the intramolecular exciplex of lb in air-saturated CH2Ci2 is 0.001, while la and lc exhibit less efficient exciplex luminescence. In order to determine the extent of ground state complexation, the luminescence behavior of la was investigated in ethyl cellulose polymer films. Only exciplex emission at 497 nm (&,+ex -0.2) was observed for la in an ethyl cellulose polymer film due to crystallite formation. 3.5. Photochemistry The photochemistry of compounds la-le and 2a-2e was investigated in argon-purged methanol solution at a concentration of lop2 M using a 200 mW Hg-Xe lamp through a Corning O-52 cut-off filter, i.e. hu > 340 nm. Only starting material was recovered in each case after irradiation for 3 h. Deactivation of the charge transfer state by back electron transfer is apparently faster than bond cleavage processes in these systems.
le
3.4. Fluorescence behavior The fluorescence behavior of systems 1 and 2 is dominated by quenching of the monomer fluorescence by the electron-withdrawing pyridinium group. System 2 exhibits the most effective quenching of anthracene fluorescence. The quantum yield of monomer fluorescence ranges from 0.05 to 0.002 for 2b, while 2e is non-fluorescent. No clear trend between & and the electron-withdrawing behavior of the pyridinium moiety is indicated. In addition to the weak monomer fluorescence exhibited by 2a-2d the pyridinium derivative 2b exhibits weak intramolecular exciplex or excited CT luminescence. The luminescence behavior of the compounds in system 1 is also dominated by the quenching of the monomer fluorescence by the pyridinium group. Compounds Id and le, which possess strong electron-withdrawing groups, are non-fluorescent in CH,Cl,, methanol and in an ethyl cellulose polymer film. However, compounds la-lc exhibit intramolecular exciplex (excited CT) emission in methylene chloride at 563, 600 and 647 nm respectively, as well as monomer fluorescence. In addition, the intramolecular exciplex band for compounds la-lc shifts to the red as the polarity of the solvent increases, indicating a greater stabilization of the relaxed CT state by polar solvents than of the ground state. Monomeric emission is observed from lb as well as exciplex emission at 582,600 and 630 nm in CHCI,, CH,Cl, and CH,CN
4. Conclusions A comparison of the electronic absorption and emission behavior of two systems has indicated that CT absorption and intramolecular exciplex emission are more important in the series in which close proximity and coplanarity between D and A could be achieved. The electron-withdrawing pyridinium moiety is very efficient at quenching anthracene fluorescence. CT absorption and emission are dependent on the solvent polarity, the redox behavior of D and A and the ability of D and A to interact electronically either through bond or through space. In general, CT absorption is enhanced and red shifted as the solvent polarity decreases, while the reverse is true for intramolecular exciplex emission, i.e. the wavelength of the exciplex emission shifts to the red and decreases in intensity as the solvent polarity increases. References T. Hirschfeld, J.B. Callis and B.R. Kowalski, Science, 226 (1984) 312. T.M. Freeman and W.R. Se&, Anal. Chem., 50 (1978) 1242. M. Thompson and U.J. Krull, Trends And Chem., 3 (1984) 173. F.D. Saeva, H. Luss and P. Martic, I. Ckm. Sot., Chem. Commun., (1989) 1476. P.A. Martic, R.C. Daly, J.L.R. Williams and S. Farid, J. Polym. Len., I7 (1979) 305. F-D. Saeva, Tetrahedron, 42 (1986) 6123.
L Photockm. Phoro~fo~.A: Chem., 78 (1994) 201-204
Electronic and photochemical behavior of some conformationally restricted intramolecular charge transfer systems F.D.
Saeva
Cmporate Research Laboratories, Eastman Kodak Company, Rochester, NY 14650 (USA) (Received September 21, 1993; accepted November 2, 1993)
Abstract Two series of conformationally restricted intramolecular charge transfer (CT) systems with very different potential for through-space interaction between an anthracene electron donor (D) and a series of pyridinium acceptor (A) moieties were synthesized. A comparison of the electronic absorption and emission behavior of these two systems indicated that CT absorption and intramolecular exciplex emission were more important in the series in which close proximity and coplanarity between D and A could be achieved. The electron-withdrawing pyridinium moiety was found to be a very efficient quencher of anthracene fluorescence. Cf absorption and emission are dependent on the solvent polarity, the redox behavior of D and A and the ability of D and A to interact electronically either through bond or through space. In general, CT absorption is enhanced and red shifted as the solvent polarity decreases, while the reverse is true for intramolecular exciplex emission, i.e. the wavelength of exciplex emission shifts to the red and decreases in intensity as the solvent polarity increases. All of the intramolecular CT systems investigated were found to be photochemically inert.
1. Introduction In view of the need to detect chemical species in biological assays and industrial processes simply and with a high degree of accuracy and precision [l-3], a luminescence scheme has been proposed to produce exciplex and/or charge transfer (CT) emission as a function of the nature of a pendant electron acceptor. The fundamental basis for the concept is the conformational rigidity of the phenyl ring in 9-phenylanthracene and the ability of ortho substituents on the phenyl ring to interact electronically with the anthracene ring system and modify its fluorescence behavior.
formational change to this situation occurs within the lifetime of the anthracene excited singlet state. This effect has not been observed for systems in which the electron donor (D) and acceptor (A) groups are connected by a flexible hydrocarbon chain [S]. We report the synthesis, spectroscopic and redox behavior of the following two conformationally restricted anthryl systems. In addition, the photochemical behavior of la was investigated. A comparison of the electronic behavior of system 1 and system 2 is provided to determine the effect of through-space orbital overlap between D and A moieties. System 1 can adopt a conformation that places the R group over the anthracene rr system which is not possible for system 2.
2. Experimental We have previously shown that, when R = -N(Et),, intramolecular exciplex luminescence is observed in solution and in polymer I%ns [4]. The observation of exciplex luminescence in polymer films indicated that either a ground state interaction between the nitrogen lone pair and the anthracene ring system exists or a rapid conElsevier Sequoia SSDI 1010-6030(93)03734-X
details
2.1. Materials Compounds la-le and 2a-2e were prepared from 3 [5] and 4 (Aldrich) respectively by reacting the appropriate pyridine derivative either neat or in acetonitrile and refluxing for 1 h. The reaction mixture was then poured Into diethyl ether and the pyridinium halide salt collected by suction
202
F.D. Saeva / Conformation&y rest&ted intramolecular cha%e transfer systems
3. Results and discussion
Series 1 and 2 were synthesized of the corresponding halomethyl the appropriate pyridine as shown nucleophilic counterion was used state chemistry and CI’ effects.
by the reaction derivatives with below. A weakly to avoid ground
CH
\I
qr
TABLE 1. Characterization of FDMS and melting point
compoundsla-le and Za-Zeby 3.2. Electronic absorption spectra
Compound
FDMS
m.p. (“C)
la lb lc Id h? 2a 2b tc 2d 2e
346 389 423 371 42A 270 313 341 295 348
200-202 240-241 174-176 195-197 =5(d) 155-156 190-192 223-224 172(d) 183(d)
d in parenthesesindicates decomposition. filtration. Purification was accomplished by recrystallization from an acetonitrile-diethyl ether solvent mixture. Conversion to the trifluorome-
thanesulfonate counterion was accomplished by dissolving the pyridinium halide in a minimum amount of water and adding to this solution an aqueous solution of sodium trifluoromethanesulfonate. The triflate salt was collected by suction filtration and air dried before being recrystallized from an acetonitrile-diethyl ether solvent mixture. All the compounds were characterized by their field desorption mass spectrum (FDMS), proton nuclear magnetic resonance (IH NMR) spectrum and melting Table 1.
point. The data are summarized
The absorption spectrum of compounds lb-le exhibits readily discernible CT absorption beyond the anthracene absorption at about 395 nm (see Fig. 1). The wavelength and molecular absorptivity of the CT absorption for compounds lb-le are
dependent on the solvent polarity, the redox behavior of D and A and the steric requirements for the through-space interaction of D with A. In general, A,,_= shifts to longer wavelength as the solvent polarity decreases, indicating that the initially formed photoinduced CT state is less polar than the ground state. For example, Id exhibits CIY absorption at 486 run in chloroform (E= 1000) and at 468 nm in acetonitrile (E = 830). V.3”
1
I
466 nm k = 1000)
lo
in Fig. 1. Absorption spectrum of ld in chloroform
solvent.
203
F.D. Saeva / Conformational& restnbted indrumolecular charge transfer systems
#!ktQmm
value shifts to longer wavelength as The hmanCT the lowest unoccupied molecular orbital (LUMO) of A decreases in energy, as expected. However, this is true only for those acceptors whose physical dimensions will allow efficient orbital overlap with the anthracene r system (see Table 2). The bipyridinium lf, for example, exhibits CT absorption at 470 nm (e=580 nm) in CH,Cl, while le, which exhibits a slightly higher energy, i.e. 20 mV, LUMO level, shows CT absorption at 482 nm (E= 820) in the same solvent system. A rationale for this observation is presented in Section 3.3. With the exception of 2d and 2e, the electronic absorption spectra of the compounds in series 2 are generally similar to one another and to 9methylanthracene in the spectral region between 300 and 400 run. Compounds 2d and 2e exhibit CT absorption which is considerably lower in intensity, i.e. with E< 200, than observed for system 1 in the spectral region around 450 nm. This difference is attributed to the large distance between atoms involved in the CT absorption process.
TABLE 2. Charge transfer absorption and redox behavior of compounds la-le and 2a-2e in C&C&
4 <-->19
2.7 A'
8<-a34
7.7 A'
The molecules in series 2 possess a bent structure with a dihedral angle of approximately 109” with free rotation around the A-C bond. The orientation of the D and A moieties in 2a provides for poor r orbital overlap. The atoms determining D-A overlap are about 3-8 8, apart. 3.3. Redox behavior The oxidation and reduction potentials (Eg and I$,$) for the unsubstituted model for series 1, 9o-tolylanthraccne, are 1.35 V and -2.0 V respectively (z)s. SCE in CD&.). Substitution of an electron-withdrawing group on methyl shifts the oxidation potential to more anodic values, i.e. 1.40 to 1.50 V, depending on the substituent (see Table 2). Reversible one-electron redox potentials were observed for lc, Id and le, while irreversble cathodic peak potentials were observed for la and lb at a scan rate of 100 mV s-l. The irreversibility is presumably due to a competing reductive cleavage process [6]. A plot of the energy of the CT transition 2)s.the reduction potential for the various pyridinium salts is linear with RZ=0.99 when If is excluded from the correlation (see Fig. 2). Inclusion of If produces a less satisfactory correlation. The energy of the CT transition for If is observed at shorter wavelength than would be expected on the basis of the reduction potential for the bipyridinium functionality. A rationale for this observation is proposed based on the effec-
Compound la lb 1C
Id le 9-o-Tolylanthracene 2a 2b 2c 2d 2e
-410 =436 (=500) 445 (490) 482 (820) 470 (580)
450 (200) 450 (40)
(-1.18) (-0.90) - 0.82 - 0.45 - 0.43 ( - 2.07) (-1.12) (-0.87)
1.45 1.50 1.40 1.45 1.46 1.35 (1.25) (1.39)
-0.67 - 0.44 -0.20
(1.20) (1.63) (1.65)
“100 mV s-’ scan rate, vs. saturated calomel electrode (SCE), 0.1 N tetrabutylammonium tetrafluoroborate.
2.41 -1.4
I -1.2
I -1.0
I -0.8
I -0.6
I -0.4
I -0.2
Reduction Potential ( V vs SCE) Fig. 2. Charge transfer transition energy vs. reduction potential for compounds la-k in methylene chloride solvent.
F.D. Sueva I Conformutional&
204
restricted intrmwlecular
tiveness of the through-space v overlap between the D and A moieties. Only the pyridinium ring directly attached to the methylene group can interact efficiently with the anthracene ring system as shown below. The electron-accepting ability of the pyridinium moiety, as indicated by cyclic voltammetry, does not reflect the effective electronaccepting behavior of the constrained bipyridinium moiety as felt by the anthracene ring system.
2 m*3
charge transfer system
solvents respectively. The quantum yield (&J of luminescence from the intramolecular exciplex of lb in air-saturated CH2Ci2 is 0.001, while la and lc exhibit less efficient exciplex luminescence. In order to determine the extent of ground state complexation, the luminescence behavior of la was investigated in ethyl cellulose polymer films. Only exciplex emission at 497 nm (&,+ex -0.2) was observed for la in an ethyl cellulose polymer film due to crystallite formation. 3.5. Photochemistry The photochemistry of compounds la-le and 2a-2e was investigated in argon-purged methanol solution at a concentration of lop2 M using a 200 mW Hg-Xe lamp through a Corning O-52 cut-off filter, i.e. hu > 340 nm. Only starting material was recovered in each case after irradiation for 3 h. Deactivation of the charge transfer state by back electron transfer is apparently faster than bond cleavage processes in these systems.
le
3.4. Fluorescence behavior The fluorescence behavior of systems 1 and 2 is dominated by quenching of the monomer fluorescence by the electron-withdrawing pyridinium group. System 2 exhibits the most effective quenching of anthracene fluorescence. The quantum yield of monomer fluorescence ranges from 0.05 to 0.002 for 2b, while 2e is non-fluorescent. No clear trend between & and the electron-withdrawing behavior of the pyridinium moiety is indicated. In addition to the weak monomer fluorescence exhibited by 2a-2d the pyridinium derivative 2b exhibits weak intramolecular exciplex or excited CT luminescence. The luminescence behavior of the compounds in system 1 is also dominated by the quenching of the monomer fluorescence by the pyridinium group. Compounds Id and le, which possess strong electron-withdrawing groups, are non-fluorescent in CH,Cl,, methanol and in an ethyl cellulose polymer film. However, compounds la-lc exhibit intramolecular exciplex (excited CT) emission in methylene chloride at 563, 600 and 647 nm respectively, as well as monomer fluorescence. In addition, the intramolecular exciplex band for compounds la-lc shifts to the red as the polarity of the solvent increases, indicating a greater stabilization of the relaxed CT state by polar solvents than of the ground state. Monomeric emission is observed from lb as well as exciplex emission at 582,600 and 630 nm in CHCI,, CH,Cl, and CH,CN
4. Conclusions A comparison of the electronic absorption and emission behavior of two systems has indicated that CT absorption and intramolecular exciplex emission are more important in the series in which close proximity and coplanarity between D and A could be achieved. The electron-withdrawing pyridinium moiety is very efficient at quenching anthracene fluorescence. CT absorption and emission are dependent on the solvent polarity, the redox behavior of D and A and the ability of D and A to interact electronically either through bond or through space. In general, CT absorption is enhanced and red shifted as the solvent polarity decreases, while the reverse is true for intramolecular exciplex emission, i.e. the wavelength of the exciplex emission shifts to the red and decreases in intensity as the solvent polarity increases. References T. Hirschfeld, J.B. Callis and B.R. Kowalski, Science, 226 (1984) 312. T.M. Freeman and W.R. Se&, Anal. Chem., 50 (1978) 1242. M. Thompson and U.J. Krull, Trends And Chem., 3 (1984) 173. F.D. Saeva, H. Luss and P. Martic, I. Ckm. Sot., Chem. Commun., (1989) 1476. P.A. Martic, R.C. Daly, J.L.R. Williams and S. Farid, J. Polym. Len., I7 (1979) 305. F-D. Saeva, Tetrahedron, 42 (1986) 6123.