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Intramolecular excited charge transfer interaction and triplet formation in N2-aryl-N1-anthrylmethylpiperazine
Journal of Photochemistry
and Photobiology,
A: Chemistry, 49 (1989)
143 - 149
143
INTRAMOLECULAR EXCITED CHARGE TRANSFER INTERACTION AND TRIPLET FORMATION IN N2-ARYL-lV ‘-ANTHRYLMETHYLPIPERAZINE J. X. LIU,
Q. YU,
Q. F. ZHOU
and H. J. XU’
Institute of Photographic Chemistry, Academia Sinica, Beijing 100012
(China)
Summary The fluorescence and transient absorption spectra of N*-phenyl-N’anthrylmethylpiperazine and N*-p-methoxyphenyl-N’anthrylmethylpiperazine, a trichromophoric system D*-D’--A, were determined. They form an intramolecular ternary complex. In non-polar solvents, intersystem crossing through charge transfer to produce a localized triplet state is an important pathway of deactivation of the exciplex.
1. Introduction The fluorescence of aromatic hydrocarbons is quenched by tertiary amines. The quenching is accompanied by the appearance of a broad structureless band to the red of the fluorescence of the hydrocarbon monomer in non-polar solvents. This new emission band is associated with the complex formed between the excited hydrocarbon A* and the electron donor amine D, and is called an exciplex. On incorporation of two, three or more chromophores into a single molecule separated by non-conjugated bonds, an intramolecular exciplex forms readily [ 11. Studies of the interactions between an electron donor and an electron acceptor in an intramolecular exciplex may provide insight into the mechanism of photoinduced electron transfer and primary charge separation in complex biological redox systems such as those involved in photosynthesis. Previously, we have reported [ 2] that quenching of 9methylanthracene fluorescence by piperazines and the formation and decay of intermolecular exciplexes can be determined together with the rate constants using the single-photon-counting technique. The results showed that the exciplex formed involved only one amino group, while the second amino group in the piperazine ring can deactivate the exciplex via other radiative and non-radiative processes. In this work, we synthesized N*-phenyl-N1-anthrylmethylpiperazine (I) and N2-p-methoxyphenyl-N1anthrylmethylpiperazine (II), a trichromophoric D*-D1- A system ( D1,
TAuthor lOlO-6030/89/$3.50
to whom
correspondence
should
be addressed.
0 Elsevier Sequoia/Printed
in The Netherlands
146
L
400
500 h
(nrn)
600
400
500 A (rim)
Fig. 1. Fluorescence spectra of I and II in C6HllCH3 (2 X 10e4M). pound I; 3, compound II.
600
1, AnCHs; 2, com-
Fig. 2. Fluorescence spectra of AnCH3 f III and IV in CeHl+ZH3 (2 X 10B4 M). 1, AnCH3; 2, AnCH3 + III; 3, AnCH3 + IV.
cence diminished greatly. It has been demonstrated that AnCHs + III and IV form an intermolecular excited binary complex [Z]. It is believed that I and II form an intramolecular excited ternary complex. It can be seen from Fig. 3 that II exhibits exciplex fluorescence at 490 nm in methylcyclohexane even at 77 K, while I, AnCHs + III and AnCH3 + IV showed no exciplex emission under similar conditions. From the excitation spectra of II, we can see that the anthracene and piperazine moieties in II interact in the ground state as shown in Fig. 4. Therefore it is not surprising that it exhibits intramolecular exciplex fluorescence which reveals the interaction between the anthryl group and the nearest neighbour N1 (D1) at 77 K. Mes et al. [8] pointed out that the piperazine ring in the trichromophoric molecule N1phenyl-N”-pcyanonaphthylpiperazine adopted a chair conformation with N-substituents in the equatorial position. The orientation of the phenyl group is such that there is extensive overlap between its a system and the N2 lone pair. There is no interaction between the two nitrogens in the ground state. Upon excitation at room temperature the relative orientation of the chromophores on a time scale compatible with the photophysical
147
300 h (nm)
Fig. 3. Fluorescence spectra of II in methylcyclohexane 2, compound lI. 3, curve 2 minus curve l-
400 A(nm)
500
(2 X 10e4 M) at 77 K. 1, AnCHs;
Fig. 4. Excitation spectra of AnCH3 and II in methylcydohexane temperature. 1, compound II, emission at 510 nm; 2, compound 3, AnCHs, emission at 410 nm; 4, AnCH3, emission at 510 nm.
(2 X 10m5 M) at room II, emission at 410 nm;
processes of electron transfer is provided by interconversion around the N1-CH2 bond to attain the closest distances between N2 (D2) and the naphthyl group (A). Compound I and compound II, in which the anthryl group is substituted for the naphthyl group, should acquire similar conformational preferences and dynamics. The observation that the introduction of a methoxy group (compound I + compound II) leads to a bathochromic shift of charge transfer fluorescence by about 20 nm (Table 1) implies that a substantial fraction of positive charge is indeed transferred to N2 (D2) of the anilino chromophore and an intramolecular excited ternary complex (D2-D1-A) is formed so that the fluorescence occurs to the red of that displaced by the related binary system. The transient absorption spectra of I and II in Fig. 5 indicate that the absorption of the flash intermediate measured 50 ns after pulsing is stronger than that of the anthracene monomer triplet at 420 nm. It is concluded that 3An*-CH2- D is generated from the excited charge transfer complex rather than the occurrence of intersystem crossing from the excited singlet ‘An*-CH2-D 191.
148
0.67
3
0.0. 0.4-
0.2-
350
400
~ 450 h Inm) i
~
500
/
550
Fig. 5. T-T absorption spectra of AnCHs, I and II in toluene after the pulse. 1, AnCHs; 2, compound I; 3, compound II.
(5 x low4 M),
taken
50 ns
The exciplex forms and decays in non-polar solvents as follows: An-CH2-D
hu
+
‘An*-CHZ-D fast,
‘(An--CH2-D+)*
+
-%
An-CH,-D
+ hv’
exciplex
I
k rt
k11
slow
\
J
krt
3An*-CH2-D
\
An-CH2-D
+ heat
After excitation of the anthracene moiety in II, the excited anthracene and piperazine in the ground state interact to form a locally excited complex and then to give a relaxed exciplex from which fluorescence may be observed. It can also decay through a radiative and non-radiative pathway to the ground state or it can dissociate to yield the anthracene triplet. Whether the anthracene triplet 3An-CH2-D is generated from the locally excited complex or relaxed exciplex is still a question of controversy. It can be seen from Fig. 6 that there is a gradual slow increase in 3An*-CH2-D then a decay after attaining a maximum value, indicating a “slow” growing in process of 3An*-CH2-D. We therefore conclude that the experimental evidence in deaerated non-polar solvents supports the intersystem crossing
O.D.
Fig. 6. Decay of triplet at room temperature.
absorption
of II in toluene
(5 X 10m4 M),
monitored
at 420 nm,
149
mechanism via the “slow” path within the thermalized exciplex but not through the “fast” mechanism involving a locally excited complex [lo]. The results also explain the low fluorescence quantum yield of the exciplex of compounds I and II in non-polar solvents. Intersystem crossing to produce the localized triplet state is an important pathway of exciplex deactivation besides non-radiative decay to the ground state.
References 1 G. F. Mes, H. J. Van Ramesdonk and J. W. Verhoeven, Reel. J. R. Neth. Chem. Sot., 102 (1983) 55. 2 J. X. Liu, Q. F. Zhou and H. J. Xu, Chem. Plays. Lett., 146 (1988) 382. 3 W. 0. Kermack and T. W. Wight, J. Chem. Sot., (1935) 1425. 4 E. Cigareh, J. Org. Chem., 45 (1980) 1497. 5 G. B. Pollard and L. G. MacDowell, J. Am. Chem. SOC., 56 (1934) 2199. 6 V. Prelog and V. Stepan, Collect. Czech. Chem. Commun., 7 (1935) 93. 7 V. Prelog and V. Stepan, Collect. Czech. Chem. Commun., 6 (1934) 211. 8 G. F. Mes, H. J. van Ramesdonk and J. W. Verhoeven, J. Am. Chem. Sot., 106 (1984) 1335. 9 C. R. Goldschmidt, R. Potshnik and M. Ottolenghi, J. Plays. Chem., 75 (1971) 1025. 10 N. Orbach, J. Novros and M. Ottolenghi, J. Phys. Chem.. 77 (1973) 2831.
and Photobiology,
A: Chemistry, 49 (1989)
143 - 149
143
INTRAMOLECULAR EXCITED CHARGE TRANSFER INTERACTION AND TRIPLET FORMATION IN N2-ARYL-lV ‘-ANTHRYLMETHYLPIPERAZINE J. X. LIU,
Q. YU,
Q. F. ZHOU
and H. J. XU’
Institute of Photographic Chemistry, Academia Sinica, Beijing 100012
(China)
Summary The fluorescence and transient absorption spectra of N*-phenyl-N’anthrylmethylpiperazine and N*-p-methoxyphenyl-N’anthrylmethylpiperazine, a trichromophoric system D*-D’--A, were determined. They form an intramolecular ternary complex. In non-polar solvents, intersystem crossing through charge transfer to produce a localized triplet state is an important pathway of deactivation of the exciplex.
1. Introduction The fluorescence of aromatic hydrocarbons is quenched by tertiary amines. The quenching is accompanied by the appearance of a broad structureless band to the red of the fluorescence of the hydrocarbon monomer in non-polar solvents. This new emission band is associated with the complex formed between the excited hydrocarbon A* and the electron donor amine D, and is called an exciplex. On incorporation of two, three or more chromophores into a single molecule separated by non-conjugated bonds, an intramolecular exciplex forms readily [ 11. Studies of the interactions between an electron donor and an electron acceptor in an intramolecular exciplex may provide insight into the mechanism of photoinduced electron transfer and primary charge separation in complex biological redox systems such as those involved in photosynthesis. Previously, we have reported [ 2] that quenching of 9methylanthracene fluorescence by piperazines and the formation and decay of intermolecular exciplexes can be determined together with the rate constants using the single-photon-counting technique. The results showed that the exciplex formed involved only one amino group, while the second amino group in the piperazine ring can deactivate the exciplex via other radiative and non-radiative processes. In this work, we synthesized N*-phenyl-N1-anthrylmethylpiperazine (I) and N2-p-methoxyphenyl-N1anthrylmethylpiperazine (II), a trichromophoric D*-D1- A system ( D1,
TAuthor lOlO-6030/89/$3.50
to whom
correspondence
should
be addressed.
0 Elsevier Sequoia/Printed
in The Netherlands
146
L
400
500 h
(nrn)
600
400
500 A (rim)
Fig. 1. Fluorescence spectra of I and II in C6HllCH3 (2 X 10e4M). pound I; 3, compound II.
600
1, AnCHs; 2, com-
Fig. 2. Fluorescence spectra of AnCH3 f III and IV in CeHl+ZH3 (2 X 10B4 M). 1, AnCH3; 2, AnCH3 + III; 3, AnCH3 + IV.
cence diminished greatly. It has been demonstrated that AnCHs + III and IV form an intermolecular excited binary complex [Z]. It is believed that I and II form an intramolecular excited ternary complex. It can be seen from Fig. 3 that II exhibits exciplex fluorescence at 490 nm in methylcyclohexane even at 77 K, while I, AnCHs + III and AnCH3 + IV showed no exciplex emission under similar conditions. From the excitation spectra of II, we can see that the anthracene and piperazine moieties in II interact in the ground state as shown in Fig. 4. Therefore it is not surprising that it exhibits intramolecular exciplex fluorescence which reveals the interaction between the anthryl group and the nearest neighbour N1 (D1) at 77 K. Mes et al. [8] pointed out that the piperazine ring in the trichromophoric molecule N1phenyl-N”-pcyanonaphthylpiperazine adopted a chair conformation with N-substituents in the equatorial position. The orientation of the phenyl group is such that there is extensive overlap between its a system and the N2 lone pair. There is no interaction between the two nitrogens in the ground state. Upon excitation at room temperature the relative orientation of the chromophores on a time scale compatible with the photophysical
147
300 h (nm)
Fig. 3. Fluorescence spectra of II in methylcyclohexane 2, compound lI. 3, curve 2 minus curve l-
400 A(nm)
500
(2 X 10e4 M) at 77 K. 1, AnCHs;
Fig. 4. Excitation spectra of AnCH3 and II in methylcydohexane temperature. 1, compound II, emission at 510 nm; 2, compound 3, AnCHs, emission at 410 nm; 4, AnCH3, emission at 510 nm.
(2 X 10m5 M) at room II, emission at 410 nm;
processes of electron transfer is provided by interconversion around the N1-CH2 bond to attain the closest distances between N2 (D2) and the naphthyl group (A). Compound I and compound II, in which the anthryl group is substituted for the naphthyl group, should acquire similar conformational preferences and dynamics. The observation that the introduction of a methoxy group (compound I + compound II) leads to a bathochromic shift of charge transfer fluorescence by about 20 nm (Table 1) implies that a substantial fraction of positive charge is indeed transferred to N2 (D2) of the anilino chromophore and an intramolecular excited ternary complex (D2-D1-A) is formed so that the fluorescence occurs to the red of that displaced by the related binary system. The transient absorption spectra of I and II in Fig. 5 indicate that the absorption of the flash intermediate measured 50 ns after pulsing is stronger than that of the anthracene monomer triplet at 420 nm. It is concluded that 3An*-CH2- D is generated from the excited charge transfer complex rather than the occurrence of intersystem crossing from the excited singlet ‘An*-CH2-D 191.
148
0.67
3
0.0. 0.4-
0.2-
350
400
~ 450 h Inm) i
~
500
/
550
Fig. 5. T-T absorption spectra of AnCHs, I and II in toluene after the pulse. 1, AnCHs; 2, compound I; 3, compound II.
(5 x low4 M),
taken
50 ns
The exciplex forms and decays in non-polar solvents as follows: An-CH2-D
hu
+
‘An*-CHZ-D fast,
‘(An--CH2-D+)*
+
-%
An-CH,-D
+ hv’
exciplex
I
k rt
k11
slow
\
J
krt
3An*-CH2-D
\
An-CH2-D
+ heat
After excitation of the anthracene moiety in II, the excited anthracene and piperazine in the ground state interact to form a locally excited complex and then to give a relaxed exciplex from which fluorescence may be observed. It can also decay through a radiative and non-radiative pathway to the ground state or it can dissociate to yield the anthracene triplet. Whether the anthracene triplet 3An-CH2-D is generated from the locally excited complex or relaxed exciplex is still a question of controversy. It can be seen from Fig. 6 that there is a gradual slow increase in 3An*-CH2-D then a decay after attaining a maximum value, indicating a “slow” growing in process of 3An*-CH2-D. We therefore conclude that the experimental evidence in deaerated non-polar solvents supports the intersystem crossing
O.D.
Fig. 6. Decay of triplet at room temperature.
absorption
of II in toluene
(5 X 10m4 M),
monitored
at 420 nm,
149
mechanism via the “slow” path within the thermalized exciplex but not through the “fast” mechanism involving a locally excited complex [lo]. The results also explain the low fluorescence quantum yield of the exciplex of compounds I and II in non-polar solvents. Intersystem crossing to produce the localized triplet state is an important pathway of exciplex deactivation besides non-radiative decay to the ground state.
References 1 G. F. Mes, H. J. Van Ramesdonk and J. W. Verhoeven, Reel. J. R. Neth. Chem. Sot., 102 (1983) 55. 2 J. X. Liu, Q. F. Zhou and H. J. Xu, Chem. Plays. Lett., 146 (1988) 382. 3 W. 0. Kermack and T. W. Wight, J. Chem. Sot., (1935) 1425. 4 E. Cigareh, J. Org. Chem., 45 (1980) 1497. 5 G. B. Pollard and L. G. MacDowell, J. Am. Chem. SOC., 56 (1934) 2199. 6 V. Prelog and V. Stepan, Collect. Czech. Chem. Commun., 7 (1935) 93. 7 V. Prelog and V. Stepan, Collect. Czech. Chem. Commun., 6 (1934) 211. 8 G. F. Mes, H. J. van Ramesdonk and J. W. Verhoeven, J. Am. Chem. Sot., 106 (1984) 1335. 9 C. R. Goldschmidt, R. Potshnik and M. Ottolenghi, J. Plays. Chem., 75 (1971) 1025. 10 N. Orbach, J. Novros and M. Ottolenghi, J. Phys. Chem.. 77 (1973) 2831.