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Fluorescence and absorption spectra of the charge transfer complex in the EDA system exhibiting exciplex fluorescence
Volume 26. number 4
CHEbIICAL PHYSICS LEITERS
lSJune1974
FLUORESCENCE AND ABSORPTION SPECTRA OF THE CHARGE TRANSFER IN THE EDA SYSTEM EXHIBITING EXCIPLEX FLUORESCENCE*
COMPLEX
Received 5 hlarch 1974 Revised manuscript received 1 April 1974
The fluorescence (polarization) and absorption spectra, and lifetimes of the charse transfer (CT) comple?tes between 9,lO-dicyanoanthracene and slkylnaphthaienes in 3-methylpcntane w1tk.b exhibit characteristic esciplex fluerescences above about 160°K were observed below = 125°K. The results demonstrate that CT absorption bands lie in the region of the ’ La band of the acceptor, and that fluorescence lifetimes of the esciples and the CT complcs depend markedly on the electron donors.
1. Introduction Experimental evidence for an identical fluorescent sthte of the exciplex stable only in the excited state and the charge transfer (CT) complex was proposed in the electron donor acceptor system (EDA) of 9,10dicyanoanthracene (DCA) and 2-methylnaphthalene (Z-MN) in previous papers [3,4]. However, no CT
local excitation in the This paper presents polarizations, and also CT complexes between and methylnaphthalenes 77OK. It is demonstrated
DCA. the fluorescence and excitation the absorption spectra, of the DCA and acenaphthene (AcN), in 3-methylpentane (MP) at that the CT absorption
bands
lie in the region 370-480 run. These EDA systems show the exciplex fluorescence above about 160’K in-
absorption band corresponding to the fluorescence of
the same wavelength region as that-of the correspond-
the CT complex which seems to be emitted from the identical fluorescent state with that of the exciplex was observed, though the absorption and fluorescence excitation spectra clearly demonstrated CT-complex formation below = 130°K. Ishida and Tsubomura have suggested a CT absorption band in the 250-300 nm region between anthracene and various aliphatic amines by the adsorption technique [S] _These EDA systems are known to show exciplex fluoreicence in fluid media [6], whereas no fiuorestience due to CT-complex formation was observed. In the.I$A-2-Mti system reported previously ‘[3,+] , the CT absorption band at 77°K was t&tatively predictid to lie almost in the same region as that of ’ 1 the IL, band of the electron acceptor with a small .transition moment; but by ,a strong mixing with.&e
ing CT complex_ Fluorescence lifetimes of both exciplex and CT complex with-various electron donors, as weil as the temperature dependence, have been determined. These results provide further confirmation for an identical fluorescent state of the exciplex stable in the excited state and for the CT complex stable in the ground state.
2. Experimental The purification of materials and solvents has been described previously [113].. Commercial zone refined samples of naphthalerie (Tokyo. Kasei) u;ei& used after reciystallisatibn @vice) from etJiano1. Akenapht+he,
i
Volume 26, number 4
CHEMICAL PHYSICS LETTERS
15 June 1974
Table 1 horescence maxima and lifetimes of the exciples (at room temperature) various nnphth&nes, and ionization potentials of the electron donors
I__-----~-
_-p__P--
-
Donor
and the CT complex (77°K) in MP solutions of DCA and
I a) P
Esciplex
(eV)
xmas
CT complex T
(nsec)
(om)
__
naphthalene 2-methylnaphthalene l,S-dimethylnaphthalene acenaphthcne
8.07 7.90 7.74 7.66 -___
a)
490 510 515 528
36 46 54 95
490 490 494 524 ___-
47 49 56 106
From ref. 19 1.
The methods used for the determination of the fluorescence and absorption spectra, and the fluorescence lifetimes, have been described in the previous papers. For the determination of the fluorescence (excitation) polarization, two polarizers (Pentax filter) were used, and the data were corrected in the way described by Azumi and McGlynn [7].
3. Results and discussion
pletely reversible. Furthermore, no significant change in the fluorescence and absorption spectra was observed between solutions cooled (or warmed) in the dark and during irradiation_ It is important to note that the fluorescence excitation spectrum monitored at 530 nm in an ML’ solution of DCA and AcN at low temperatures (== 120-77°K) is noticeably different from t!lat of the exciplex. Fig.
2 shows the fluorescence and excitation spectra of this solution at room temperature and at 77”K, and also includes their polarizations at 77°K. On t& other hand, the absorption spectra of this solution at 77°K depend on concentration, exhibiting a weak absorption band
The exciplex fluorescences in MP solutions of DCA and alkyl substituted naphthalenes were observed at room temperature, for which the h,,, are summarized in tab!e 1. These exciplex fluorescences were almost quenched at low temperatures, and other fluorescences were observed in the same wavelength region as those of the corresponding exciplexes below z 125°K. Similar effects have been reported in the intramolecular interaction of @,lO-dicyanoanthracene)-(CH2)3-(naphthalene) [I], and also in the DCA--‘>-MN system [3]. Here. the fluorescence intensity ratios of DCA and the exciplex were plotted against f/T, as shown in fig. I_ Over the temperatuie range, these plots are straight lines with a negative slope. the longer wavelength fluorescence being regarded as due to the exciplex. Below = 125°K similar fluorescence intensity ratios exhibit also s straight iin6 with a .positiie slope. In this Fig. 1. Fluorescenke lifetimes (monitored at 5X0-530 nm), lower temperature range the fluorescence (&, +90fluorescknce maxima and fluorekence intensity r?tios of the 530 nm) was @signed to the CT complex. These ternexciplex.(f,, && in 520-540 nm) and the CT complex V,, perature dependenc&of.the fluorescence, and of-the _ .: &,,& in 520-54@ nm).to l?CA cf, A&i in 420-43.0 nm) i? ab3orption,spectra (to be, _mentionr&later) were cornthe MP soluiion of DCA a@ AcN at various. Jemperatures. --
Volume 26, number 4
UIO nm
CHEMICAL PHYSICS LETTERS
500
Fig. 2. Fluorescence (-.-_-, excited at 375 nm) and escitation (---, monitored at 530 nm) of an MP solution of DCA and AcN at room temperature (concentration: DCA, 5 X 10m6 hl; AcN, 4 X lOA2 M). Fluorescence (--, escited at 375 nm) and excitation (---. monitored at 530 nm) at 77°K (concentration: DCA, 3 X 106 hi; AcN, 2.5 X 10-j M). P, and P2 are fluorescence (excited at 470 nm) and excitation (monitored at 530 nm) polarizations of the hlP solution mentioned above ;it -f?K. in a longer wavelength region (430-480nm) than the local excitation (350-430nm) of DCA, as shown in fig. 3. Since the absorption band with a vibrational progression at 350-430 nm may be the shifted local excitation of DCA in the CT complex, P, and P2 shown in fig. 2 demonstrate that the absorption as well as the fluorescence excitation spectra in the 430480 nm region may be ascribed to the CT absorption band between DCA and AcN. In the CT complexes with naphthalene and methylnaphthalenes, it was observed that the fluorescence (A,, 490-5 15 nm) of these CT complexes show negative polarization with an excitation at 376 nm, and that the excitation polar-
c?4-
OCA.
3.5~10%
AcN. ---0 ----
1.6Xlci3M
Fig. 3. Electronic absorption spectra of MP solutions of DCA and AcN at 77°K (3’cm light path).
15 June 1974
izations monitored at 510 nm behave very similarly to those of the DCA-AcN system apart from the longer wavelength region beyond 430 nm. These results suggest that the CT absorption bands between DCA and several naphthalenes seem to lie in the 360-440 nm region and to be overlapped with the absorption band of local excitations in the respective CT complex. The fluorescence lifetimes of the exciplex (room temperature) and the CT complex (77OK) in MP solutions of DCA and AcN, and methylnaphthalenes, are summarized in table 1, together with X,, values and ionization potentials of the electron donors. The longer wavelength shift of.the fluorescence maxima of the CT complexes (table 1) is rather a common feature of the CT complex with decreasing ionization potential of the electron donor [8] _Since the CT absorption band seems to lie at almost the same or at longer wavelengths (in the case of AcN) as does the local excitation of DCA as mentioned above, the red-shift of the fluorescence of the CT complex with decreasing ionization potential of the donor suggests an increase of the CT character of these electronic transitions. These arguments are also consistent with the increases in the fluorescence lifetimes of the CT complexes as summarized in table I_ The most important trend in the data is the larger fluorescence lifetime and the longer wavelength shift of %n, of the exciplex with the smaller ionization potential of the electron donor. In particular, the fluorescence lifetime and &,,= of the exciplex and the CT complex in the DCA-AcN system stand out among those of the exciplexes and CT complexes with other electron donors. However, this is easily understood by taking account of the unexpectedly small ionization potential of AcN compared with those of the other naphthalenes. These exciplex results demonstrate that the fluorescent state of the exciplex has virtually the same electronic structure as that of the corresponding CT complex. The temperature dependence of the fluorescence maxima and lifetimes in an MP solution of II&A and AcN exhibits continuous changes from room temperature to 77”K, as shown in fig. I, which demonstrates the identical fluorescent state of the exciplex and CT_domplex, as mentioned above. The results’reported here are the first observation of a distinct CT absd$tion band co&esponding to the fluorescence of the CT complex in the EDA.sy&m exhibiting exci$ex’ fluqrescentie.: .- ._
Volume
26, number
4
CHEhlICAL
Acknowledgement
PHYSICS
15 June
1974
[?I
The author is greatly indebted to Dr. Keitaro Yoshihara of the Institute of Physical and Chemical Research for the determination of fluorescence lifetimes.
bf. Itoh, T. Mimura and T. Okamoto, Bull. Chem. Sot. Japan 47 (1974) 1078. [3 J hl. ltoh and T. hlimura, Chem. phys. Letters 24 (1974) 551. [4]
[5] [6] [7]
References
[Sl
[ I] hf. Itoh. T. hlimun, H. Usui and T. Okamoro, Chem. Sot. 95 (1973) 4388.
LETTERS
J. Am. [9]
hl. Itoh, T. hlimurn arid T. Okamoto, J. Am. Chem. Sot., to be submitted for publication. H. Ishida and H. Tsubomura. Chem. Phys. Letters 9 (1971) 296. A. Nakajima, Bult. Cbem. Sot. Japan 42 (1969) 3409. T. Azumi and S.P. hIcGtynn, J. Chem. Phys. 37 (1962) 2413. T. Kobapashi, I(. Yoshihora and S. I%gnkur+ Bull. Chem. Sot. Japan44 (1971) 2603. J.B. Aldekomo and J.B. Birks, Proc. Roy. Sot. A284 (1965) 551.
CHEbIICAL PHYSICS LEITERS
lSJune1974
FLUORESCENCE AND ABSORPTION SPECTRA OF THE CHARGE TRANSFER IN THE EDA SYSTEM EXHIBITING EXCIPLEX FLUORESCENCE*
COMPLEX
Received 5 hlarch 1974 Revised manuscript received 1 April 1974
The fluorescence (polarization) and absorption spectra, and lifetimes of the charse transfer (CT) comple?tes between 9,lO-dicyanoanthracene and slkylnaphthaienes in 3-methylpcntane w1tk.b exhibit characteristic esciplex fluerescences above about 160°K were observed below = 125°K. The results demonstrate that CT absorption bands lie in the region of the ’ La band of the acceptor, and that fluorescence lifetimes of the esciples and the CT complcs depend markedly on the electron donors.
1. Introduction Experimental evidence for an identical fluorescent sthte of the exciplex stable only in the excited state and the charge transfer (CT) complex was proposed in the electron donor acceptor system (EDA) of 9,10dicyanoanthracene (DCA) and 2-methylnaphthalene (Z-MN) in previous papers [3,4]. However, no CT
local excitation in the This paper presents polarizations, and also CT complexes between and methylnaphthalenes 77OK. It is demonstrated
DCA. the fluorescence and excitation the absorption spectra, of the DCA and acenaphthene (AcN), in 3-methylpentane (MP) at that the CT absorption
bands
lie in the region 370-480 run. These EDA systems show the exciplex fluorescence above about 160’K in-
absorption band corresponding to the fluorescence of
the same wavelength region as that-of the correspond-
the CT complex which seems to be emitted from the identical fluorescent state with that of the exciplex was observed, though the absorption and fluorescence excitation spectra clearly demonstrated CT-complex formation below = 130°K. Ishida and Tsubomura have suggested a CT absorption band in the 250-300 nm region between anthracene and various aliphatic amines by the adsorption technique [S] _These EDA systems are known to show exciplex fluoreicence in fluid media [6], whereas no fiuorestience due to CT-complex formation was observed. In the.I$A-2-Mti system reported previously ‘[3,+] , the CT absorption band at 77°K was t&tatively predictid to lie almost in the same region as that of ’ 1 the IL, band of the electron acceptor with a small .transition moment; but by ,a strong mixing with.&e
ing CT complex_ Fluorescence lifetimes of both exciplex and CT complex with-various electron donors, as weil as the temperature dependence, have been determined. These results provide further confirmation for an identical fluorescent state of the exciplex stable in the excited state and for the CT complex stable in the ground state.
2. Experimental The purification of materials and solvents has been described previously [113].. Commercial zone refined samples of naphthalerie (Tokyo. Kasei) u;ei& used after reciystallisatibn @vice) from etJiano1. Akenapht+he,
i
Volume 26, number 4
CHEMICAL PHYSICS LETTERS
15 June 1974
Table 1 horescence maxima and lifetimes of the exciples (at room temperature) various nnphth&nes, and ionization potentials of the electron donors
I__-----~-
_-p__P--
-
Donor
and the CT complex (77°K) in MP solutions of DCA and
I a) P
Esciplex
(eV)
xmas
CT complex T
(nsec)
(om)
__
naphthalene 2-methylnaphthalene l,S-dimethylnaphthalene acenaphthcne
8.07 7.90 7.74 7.66 -___
a)
490 510 515 528
36 46 54 95
490 490 494 524 ___-
47 49 56 106
From ref. 19 1.
The methods used for the determination of the fluorescence and absorption spectra, and the fluorescence lifetimes, have been described in the previous papers. For the determination of the fluorescence (excitation) polarization, two polarizers (Pentax filter) were used, and the data were corrected in the way described by Azumi and McGlynn [7].
3. Results and discussion
pletely reversible. Furthermore, no significant change in the fluorescence and absorption spectra was observed between solutions cooled (or warmed) in the dark and during irradiation_ It is important to note that the fluorescence excitation spectrum monitored at 530 nm in an ML’ solution of DCA and AcN at low temperatures (== 120-77°K) is noticeably different from t!lat of the exciplex. Fig.
2 shows the fluorescence and excitation spectra of this solution at room temperature and at 77”K, and also includes their polarizations at 77°K. On t& other hand, the absorption spectra of this solution at 77°K depend on concentration, exhibiting a weak absorption band
The exciplex fluorescences in MP solutions of DCA and alkyl substituted naphthalenes were observed at room temperature, for which the h,,, are summarized in tab!e 1. These exciplex fluorescences were almost quenched at low temperatures, and other fluorescences were observed in the same wavelength region as those of the corresponding exciplexes below z 125°K. Similar effects have been reported in the intramolecular interaction of @,lO-dicyanoanthracene)-(CH2)3-(naphthalene) [I], and also in the DCA--‘>-MN system [3]. Here. the fluorescence intensity ratios of DCA and the exciplex were plotted against f/T, as shown in fig. I_ Over the temperatuie range, these plots are straight lines with a negative slope. the longer wavelength fluorescence being regarded as due to the exciplex. Below = 125°K similar fluorescence intensity ratios exhibit also s straight iin6 with a .positiie slope. In this Fig. 1. Fluorescenke lifetimes (monitored at 5X0-530 nm), lower temperature range the fluorescence (&, +90fluorescknce maxima and fluorekence intensity r?tios of the 530 nm) was @signed to the CT complex. These ternexciplex.(f,, && in 520-540 nm) and the CT complex V,, perature dependenc&of.the fluorescence, and of-the _ .: &,,& in 520-54@ nm).to l?CA cf, A&i in 420-43.0 nm) i? ab3orption,spectra (to be, _mentionr&later) were cornthe MP soluiion of DCA a@ AcN at various. Jemperatures. --
Volume 26, number 4
UIO nm
CHEMICAL PHYSICS LETTERS
500
Fig. 2. Fluorescence (-.-_-, excited at 375 nm) and escitation (---, monitored at 530 nm) of an MP solution of DCA and AcN at room temperature (concentration: DCA, 5 X 10m6 hl; AcN, 4 X lOA2 M). Fluorescence (--, escited at 375 nm) and excitation (---. monitored at 530 nm) at 77°K (concentration: DCA, 3 X 106 hi; AcN, 2.5 X 10-j M). P, and P2 are fluorescence (excited at 470 nm) and excitation (monitored at 530 nm) polarizations of the hlP solution mentioned above ;it -f?K. in a longer wavelength region (430-480nm) than the local excitation (350-430nm) of DCA, as shown in fig. 3. Since the absorption band with a vibrational progression at 350-430 nm may be the shifted local excitation of DCA in the CT complex, P, and P2 shown in fig. 2 demonstrate that the absorption as well as the fluorescence excitation spectra in the 430480 nm region may be ascribed to the CT absorption band between DCA and AcN. In the CT complexes with naphthalene and methylnaphthalenes, it was observed that the fluorescence (A,, 490-5 15 nm) of these CT complexes show negative polarization with an excitation at 376 nm, and that the excitation polar-
c?4-
OCA.
3.5~10%
AcN. ---0 ----
1.6Xlci3M
Fig. 3. Electronic absorption spectra of MP solutions of DCA and AcN at 77°K (3’cm light path).
15 June 1974
izations monitored at 510 nm behave very similarly to those of the DCA-AcN system apart from the longer wavelength region beyond 430 nm. These results suggest that the CT absorption bands between DCA and several naphthalenes seem to lie in the 360-440 nm region and to be overlapped with the absorption band of local excitations in the respective CT complex. The fluorescence lifetimes of the exciplex (room temperature) and the CT complex (77OK) in MP solutions of DCA and AcN, and methylnaphthalenes, are summarized in table 1, together with X,, values and ionization potentials of the electron donors. The longer wavelength shift of.the fluorescence maxima of the CT complexes (table 1) is rather a common feature of the CT complex with decreasing ionization potential of the electron donor [8] _Since the CT absorption band seems to lie at almost the same or at longer wavelengths (in the case of AcN) as does the local excitation of DCA as mentioned above, the red-shift of the fluorescence of the CT complex with decreasing ionization potential of the donor suggests an increase of the CT character of these electronic transitions. These arguments are also consistent with the increases in the fluorescence lifetimes of the CT complexes as summarized in table I_ The most important trend in the data is the larger fluorescence lifetime and the longer wavelength shift of %n, of the exciplex with the smaller ionization potential of the electron donor. In particular, the fluorescence lifetime and &,,= of the exciplex and the CT complex in the DCA-AcN system stand out among those of the exciplexes and CT complexes with other electron donors. However, this is easily understood by taking account of the unexpectedly small ionization potential of AcN compared with those of the other naphthalenes. These exciplex results demonstrate that the fluorescent state of the exciplex has virtually the same electronic structure as that of the corresponding CT complex. The temperature dependence of the fluorescence maxima and lifetimes in an MP solution of II&A and AcN exhibits continuous changes from room temperature to 77”K, as shown in fig. I, which demonstrates the identical fluorescent state of the exciplex and CT_domplex, as mentioned above. The results’reported here are the first observation of a distinct CT absd$tion band co&esponding to the fluorescence of the CT complex in the EDA.sy&m exhibiting exci$ex’ fluqrescentie.: .- ._
Volume
26, number
4
CHEhlICAL
Acknowledgement
PHYSICS
15 June
1974
[?I
The author is greatly indebted to Dr. Keitaro Yoshihara of the Institute of Physical and Chemical Research for the determination of fluorescence lifetimes.
bf. Itoh, T. Mimura and T. Okamoto, Bull. Chem. Sot. Japan 47 (1974) 1078. [3 J hl. ltoh and T. hlimura, Chem. phys. Letters 24 (1974) 551. [4]
[5] [6] [7]
References
[Sl
[ I] hf. Itoh. T. hlimun, H. Usui and T. Okamoro, Chem. Sot. 95 (1973) 4388.
LETTERS
J. Am. [9]
hl. Itoh, T. hlimurn arid T. Okamoto, J. Am. Chem. Sot., to be submitted for publication. H. Ishida and H. Tsubomura. Chem. Phys. Letters 9 (1971) 296. A. Nakajima, Bult. Cbem. Sot. Japan 42 (1969) 3409. T. Azumi and S.P. hIcGtynn, J. Chem. Phys. 37 (1962) 2413. T. Kobapashi, I(. Yoshihora and S. I%gnkur+ Bull. Chem. Sot. Japan44 (1971) 2603. J.B. Aldekomo and J.B. Birks, Proc. Roy. Sot. A284 (1965) 551.