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Intramolecular energy transfer between the saturated amine and carbonyl chromophores
Volume 25, number 3
1 April 1974
CHEMICAL PHYSICS LETTERS
INTRAMOLECULAR THE SATURATEDAMINE
ENERGY TRANSFER BETWEEN AND CARBONYL CHROMOPHORES~
Arthur M. HALPERN and Robert 8. WALTER Department of Chemistv, Northeastern University,
Boston. Massachusetts 021 IS, USA Received
21 January 1974
The photophysical properties of the rigid, bicyclic amino-ketone 1-azabicycto(2.2.2)octan-3-one (ABCONE) are compared with similarlystructured aminesand ketones. The fluorescencespectrum and the radiativerate constant, kR, of ABCONE was found to be similar to those of the ketones. It is suggestedthat ABCONE’sabsorption at ca. 23.5 nm correlates with the low-!ying, weak transition observedin the free amines. Energy transfer from the second excited state to the (n* + n) ketone
state was determined
to be about 50%.
Fluorescence has now been observed to be a dominant mode of electronic relaxation for many saturated tertiary amines, both in the vapor phase [l-3] and in
of steric shielding about the nitrogen atom 183. Thus for these amines, only the fluorescent lifetime is concentration dependent. In order to determine the nature of the interaction between the amine chromophore (such as is represented by ABCO) and the carbonyf group, the photophysical properties of I-azabicyclo(22.2)octan3-one (ABCONE, II)
solution as well [4,5] . The cage amine, I-azabicyclo (2.2.2)octane (ABCO, I)
I
and other strain-free cage amines (such as 1-azaadamantane (l-AA)) have been shown to excimerize in nonpolar solvents. ABCO, itself, undergoes excimer
II
formation at room temperature in the vapor phase,
were studied. ABCONE represents a rather special case . of intramolecular interaction between the nitrogen
provided that there is a sufficient partial pressure of some ‘intzrt’gas, eg., n-hexane [S] . Thus, for ABCO, the fluoresce&spectrum
is concentration
atom lone pair and the carbonyl group because the
dependent
” aliphatic framework constrains th? two chroSmophores to a preferred orientation. In the ground state, the over-lap between the N- and 0- nonbonding orbitals cari be readily determined,‘assuming a certain molecular configura@on, but in the excited itgte, the r&&s of such -: a calculationtiould depend upon $he ‘part@lar excited state descriptiorichoseti. .. .. -. .’The absorption spectrum of ABCO@ iri noripolai ..: solvents (e.g., $and cy&he&e) below.cti. 2?5 tim: ._.:-‘: :. ..~ . ...~._. ,.,-, _ :.
and the decay. kinetics of both the monomer and extimer are characterized by the nonexponentiality of an excinier system [S-7]. Many other tertiary amines which do not excimenze undergo selfquenching for whjch the efficiency~s’eems to depend upon the degree ‘t
A portion
of the research tis
carried out at Nichols tabom-
: tory, New York University, University Heights @J-P.). : ._
:
.,.”
I-
.:.
_.:
_,. ,.._
Volume 25. number 3
CHEMICAL
PHYSICS
1 April 1974
LETTERS
Table 1 ---
_-..
Compound
OfX
IO”
.aIwx (rim)
%ls 01 cm)-
c (A=235 nm)
ABC0
209
!6OO
170”)
-
-
273
AM-ONE
204
1460 _
160”
308
25.4
445
294
22
425
;:$I
g::;
427
7.9=)
5.24d)
htorbor~tlone 2ahnantiwtone
__I__
__
‘)shoulh. ykldl
-
bed
-
_ _. _.
-
- _.~
-_-~-~--.-
&ax
290
-_ _.--
hrx
u-luor) ___ -.-_-___
17.5 - _____~_
Frfsec) 55.0
310’)
Wlkieat mpof pressure (.a. 0.13 tom). The O-O band f’ t## svotq transition in ABC0 (S2 * s. or 3p 4- a) Wa ot 2283 nla 19). lbm my be 3ome question as lo wkthor the ‘235. mm” t-tit obsanwd in (nonpolar) sotutions of AK’ONE correlates with the weak, low-lying transftkn goonfn the free antines @peciaUy in the vapor Iute). or wfuMu it is 8 manifesttktkmof the ground sww coupling Mwccn the n-orbital on the Natom as4 the C=O zra&ital [IO). In a related compound,
1o-5
(se?
)
56 6.5
h, Daonvnlutcd value, horcvcr .seeref. i I 3 1 . ‘) Deconvoluted value. however, see ref. [ 14). on of for xetonc of 0.0009;1 1IS 1. “8asedonOffor tolueneof0.14 (61.
resembles tba( of ABC0 in the same solvent. Above a. 225 nm. ARCONE’s absorption spectrum appears fo be that of a typical relatively rigid, stunted ketone 4c.g.. 2 norbornanone or Zsdamantanone). Excitation inro rirhcr of ABCONE’s transitions (e.g., at 236 nm or 0I 31 I nm) produce5 fluorescence will; A,, a1455 nm. ThS fluorescence band, in conjunclion with the absorpion above 255 nm is typical of a ketone. More0~. like a ketone. ABC0NE has a concentntion-indcpesdrnt fllaarescencespectrum and lifetime. A summary of the spectral and photophysical properties of ABCXXUEand related amines and ketones is contained tntdb I. It an be seen in &is table, that the slight inflexion abwnrd in atnorprfon spectra for ABCONE 8t ca. 235 csmb rbc abrrrvcd in the free amine. ABCO. and may be d* lo the weaker. iow-iying transition which is &irly seen fat ABC0 in the vapot phase f9]. This has Men ass&ted as a 3s * n (or a sub-Rydberg) transi. tiua 191. The vepof phase &sorption spedrum of AKONE at room temperature (IOcm cell) revealed *19spaorsnrly strong transition with a O-O band at 222.8 nm; rhr weaker lowsr-lyisg transition corrcspot&@ W titer235 nm rqion absorption in r4ttlon ws net distinctly observed,probably because of in-
kRX
z:: d, Quantum
I-azabicyclo(3.3.l)nonan-3-one,
Hughes and Hudec 1I I ] ascribe an absorption at ca. 235 nm to such a transition (i.e., the red-shifted member of a split n* + n transition). In such a situation, however, one would expect that the familiar IT*+ n transition would be enhanced, as was observed in this particular aminoketone. It is apparent from the data in table I, that there is no intensification of the n* 4- n transition in ABCONE relative to related ketones?. This may be a result of ABCONE’s symmetry (and rigidity) wherein the N atom n--orbital and the C=O n-orbital could be
orthogonal. Further indication of the apparent independence
of the N- and CO-systems is provided by the fact that the radiative rate constant (kR) for ABCONEis very close in value to those of the related ketones whose k, values are listed in table 1. One would expect that RR would reflect any change in the nature of the excited state (such as the depafling and terminating orbitals). It would be interesting and informative to determine whether in those amino-ketones which do illustrate enhanced II+ + n transitions, the respective k~ values appropriately reflect such orbital interaction. By comparing the fluorescence intensity of AJICONE when excited at 3 I 1 nm (the S* + n transition) and at 236 nm (presuma bly the S2 + So transition), relative to the amount of radiation absorbed at these wavelengths.; it is possiile to determine the effkiency of energy bans- ” fur between the two states populated at these respec-
tive wavele?igths.That is,
t Marerigorously the osciUatarstrength instead of emax should be ased. The arguments would be unchanged. howover.
Volume 25, number 3
CHEMICAL PHYSICS LETTERS
where T, E. and I are, respectively, the fractional transmission of the sample, the relative intensity of the exciting radiation, and the intensity of fluorescence produced by exciting at a particular wavelength. Subscripts 1 and 2 refer, respectively, to excitation wavelengths of 3 11 nm and 236 mn. The relative excitation intensities were determined from the front-surface ihumination of a rhodamine-B solution [12] _ From such an analysis, EET was determined to be approximately 50%. It is premature to speculate about the nature of the energy transfer mechanism in ABCONE. The close proximity of the donor and acceptor chromophores (ca. 4 A) suggests that exchange interactions would be important. Moreover, because of the rigidity of the two chromophores, the orientation factor would be specified and fmed by the respective transition dipoles. An analysis is complicated by the fact that the polarization of the transitions in the saturated amines has not yet been determined. The inability to observe distinct emission from the amine chromophore in ABCONE solutions makes it impossible to determine the rate of energy transfer from the N to the CO group. However, if energy transfer to the carbonyl group were the on& additional decay mode introduced to the ‘ABCO-type’ system in ABCONE, the fluorescence quantum yield of the amine portion of the molecule would still be appreciable, i.e., ca. 15% (see table 1). One conclusion from the failure to detect amine-type fluorescence is that some other nonradiative channel (possible a photochemical process), in addition to energy transfer, is enhanced in ABCONE relative to ABCO.
The authors are grateful to the donors of the Petroleum Research Fund of the American Chemical Society for partial support of this work. A grant from the New
York University Institutional Grants Committee is also acknowledged. Appendix: Experimental All materials were procured from Aldrich Chemical Co. ABCO, obtained as the hydrochloride salt, was liberated by combining very concentrated aqueous soiu..
1 April 1974
tions of ABC0 HCI and NaOH. The free base was then sublimed from a solid mixture of anhydrous BaO. The same procedure was used to obtain ABCONE from its hydrochloride. ABCONE, which tends to become slightly yellow upon standing, was resubhrned immediately prior to use. 2-norbomanone and 2-adamantanone’were sublimed prior to use. Quantum yields were obtained with a conventional fluorimeter described elsewhere [2] _ The solutions were prepared such that their optical densities at the exciting wavelength (3 11 nm) were equivalent to within 3%. The standard chosen was acetone in cyclohexane, for which a quantum yield of 0.00094 was used [15]. For the ERT measurement, the ABCONE concentration was kept low enough such that inner filter effects were avoided at both exciting wavelengths. Lifetimes were measured using the time-correlated single photon method. The lifetimes reported for ABCONE and the ketones are deconvoluted values.
References [I] A.M. Halpem, Cbem. Pbys. Letters 6 (1970) 296. [2] A.M. Halpem and R.M. Danaiger, Cbem. Pbys. Letters 16 (1972) 72. [3] C.G. Freeman, M.J. McEwan, R.F.C. Clairidge and L.F. Phiips, Chem. Phys. Letters 8 (1971) 77. [4] Y. Muto, Y. Nakato and H. Tsubomura, Cbem. Pbys. Letters 9 (1971) 597. [S] A_M. Halpem and E. Meratos. J. Am. Chem. Sot. 84 (1972) 8273. [6] AM. Helpem end RJ. Sternfels, to be submitted for ‘. Ipublication. [7] J.B. Birks, Pbotophysics of aromatic molecules (Wiley, New York, 1970) ch. 7. [S] A.M. Halpernand T.. Gartrnan. submitted for publications [9] A.M. Halpem, J-L_ Roebber and K. Weiss, J. Cbem. Pbys. 49 (1968) 1348. [IO] J. Hudec. Cbem. Commun. (i970) 829. and references therein; private communication. [ll] h1.T. Hughes and J. Hudec, C&em: Commum. (1970),831. .. [12] CA. Parker,-Photduminescence of solutions (Elsetier. . . Amsterdam, 1968) pp. 204-205. [13] F.S, Wettack. G.D. Renkes. M.G. Rockley. NLTurro .. and J.C. Dalton, J_ Am. Cbem. Soc.92 (1970) 1793; .. .. ]14] J.C. Dalton, D.M. Pond and N.J. Tuna, J. Am. Chem.: : Sot. 92 (1970) 2173:. [IS] AMi Halpem and WiR. Ware, I. Cbem. whys. 54 (L&L) ..:. 1271.; .. : ‘.
-. -:
I
:
.‘~.,.
,:
~:
: T ,. _:. . ..-.-
. . :. -,_._-..:._ ,
1 : -. :
.-..
_,
,-. .:. ___._r.. .: .,
_.-~_._.,__. ,..
: .., : &:~1,;~__ . .. -. _..,.: .:_:-; i,. ..,,. :-,:,-%,I _
1 April 1974
CHEMICAL PHYSICS LETTERS
INTRAMOLECULAR THE SATURATEDAMINE
ENERGY TRANSFER BETWEEN AND CARBONYL CHROMOPHORES~
Arthur M. HALPERN and Robert 8. WALTER Department of Chemistv, Northeastern University,
Boston. Massachusetts 021 IS, USA Received
21 January 1974
The photophysical properties of the rigid, bicyclic amino-ketone 1-azabicycto(2.2.2)octan-3-one (ABCONE) are compared with similarlystructured aminesand ketones. The fluorescencespectrum and the radiativerate constant, kR, of ABCONE was found to be similar to those of the ketones. It is suggestedthat ABCONE’sabsorption at ca. 23.5 nm correlates with the low-!ying, weak transition observedin the free amines. Energy transfer from the second excited state to the (n* + n) ketone
state was determined
to be about 50%.
Fluorescence has now been observed to be a dominant mode of electronic relaxation for many saturated tertiary amines, both in the vapor phase [l-3] and in
of steric shielding about the nitrogen atom 183. Thus for these amines, only the fluorescent lifetime is concentration dependent. In order to determine the nature of the interaction between the amine chromophore (such as is represented by ABCO) and the carbonyf group, the photophysical properties of I-azabicyclo(22.2)octan3-one (ABCONE, II)
solution as well [4,5] . The cage amine, I-azabicyclo (2.2.2)octane (ABCO, I)
I
and other strain-free cage amines (such as 1-azaadamantane (l-AA)) have been shown to excimerize in nonpolar solvents. ABCO, itself, undergoes excimer
II
formation at room temperature in the vapor phase,
were studied. ABCONE represents a rather special case . of intramolecular interaction between the nitrogen
provided that there is a sufficient partial pressure of some ‘intzrt’gas, eg., n-hexane [S] . Thus, for ABCO, the fluoresce&spectrum
is concentration
atom lone pair and the carbonyl group because the
dependent
” aliphatic framework constrains th? two chroSmophores to a preferred orientation. In the ground state, the over-lap between the N- and 0- nonbonding orbitals cari be readily determined,‘assuming a certain molecular configura@on, but in the excited itgte, the r&&s of such -: a calculationtiould depend upon $he ‘part@lar excited state descriptiorichoseti. .. .. -. .’The absorption spectrum of ABCO@ iri noripolai ..: solvents (e.g., $and cy&he&e) below.cti. 2?5 tim: ._.:-‘: :. ..~ . ...~._. ,.,-, _ :.
and the decay. kinetics of both the monomer and extimer are characterized by the nonexponentiality of an excinier system [S-7]. Many other tertiary amines which do not excimenze undergo selfquenching for whjch the efficiency~s’eems to depend upon the degree ‘t
A portion
of the research tis
carried out at Nichols tabom-
: tory, New York University, University Heights @J-P.). : ._
:
.,.”
I-
.:.
_.:
_,. ,.._
Volume 25. number 3
CHEMICAL
PHYSICS
1 April 1974
LETTERS
Table 1 ---
_-..
Compound
OfX
IO”
.aIwx (rim)
%ls 01 cm)-
c (A=235 nm)
ABC0
209
!6OO
170”)
-
-
273
AM-ONE
204
1460 _
160”
308
25.4
445
294
22
425
;:$I
g::;
427
7.9=)
5.24d)
htorbor~tlone 2ahnantiwtone
__I__
__
‘)shoulh. ykldl
-
bed
-
_ _. _.
-
- _.~
-_-~-~--.-
&ax
290
-_ _.--
hrx
u-luor) ___ -.-_-___
17.5 - _____~_
Frfsec) 55.0
310’)
Wlkieat mpof pressure (.a. 0.13 tom). The O-O band f’ t## svotq transition in ABC0 (S2 * s. or 3p 4- a) Wa ot 2283 nla 19). lbm my be 3ome question as lo wkthor the ‘235. mm” t-tit obsanwd in (nonpolar) sotutions of AK’ONE correlates with the weak, low-lying transftkn goonfn the free antines @peciaUy in the vapor Iute). or wfuMu it is 8 manifesttktkmof the ground sww coupling Mwccn the n-orbital on the Natom as4 the C=O zra&ital [IO). In a related compound,
1o-5
(se?
)
56 6.5
h, Daonvnlutcd value, horcvcr .seeref. i I 3 1 . ‘) Deconvoluted value. however, see ref. [ 14). on of for xetonc of 0.0009;1 1IS 1. “8asedonOffor tolueneof0.14 (61.
resembles tba( of ABC0 in the same solvent. Above a. 225 nm. ARCONE’s absorption spectrum appears fo be that of a typical relatively rigid, stunted ketone 4c.g.. 2 norbornanone or Zsdamantanone). Excitation inro rirhcr of ABCONE’s transitions (e.g., at 236 nm or 0I 31 I nm) produce5 fluorescence will; A,, a1455 nm. ThS fluorescence band, in conjunclion with the absorpion above 255 nm is typical of a ketone. More0~. like a ketone. ABC0NE has a concentntion-indcpesdrnt fllaarescencespectrum and lifetime. A summary of the spectral and photophysical properties of ABCXXUEand related amines and ketones is contained tntdb I. It an be seen in &is table, that the slight inflexion abwnrd in atnorprfon spectra for ABCONE 8t ca. 235 csmb rbc abrrrvcd in the free amine. ABCO. and may be d* lo the weaker. iow-iying transition which is &irly seen fat ABC0 in the vapot phase f9]. This has Men ass&ted as a 3s * n (or a sub-Rydberg) transi. tiua 191. The vepof phase &sorption spedrum of AKONE at room temperature (IOcm cell) revealed *19spaorsnrly strong transition with a O-O band at 222.8 nm; rhr weaker lowsr-lyisg transition corrcspot&@ W titer235 nm rqion absorption in r4ttlon ws net distinctly observed,probably because of in-
kRX
z:: d, Quantum
I-azabicyclo(3.3.l)nonan-3-one,
Hughes and Hudec 1I I ] ascribe an absorption at ca. 235 nm to such a transition (i.e., the red-shifted member of a split n* + n transition). In such a situation, however, one would expect that the familiar IT*+ n transition would be enhanced, as was observed in this particular aminoketone. It is apparent from the data in table I, that there is no intensification of the n* 4- n transition in ABCONE relative to related ketones?. This may be a result of ABCONE’s symmetry (and rigidity) wherein the N atom n--orbital and the C=O n-orbital could be
orthogonal. Further indication of the apparent independence
of the N- and CO-systems is provided by the fact that the radiative rate constant (kR) for ABCONEis very close in value to those of the related ketones whose k, values are listed in table 1. One would expect that RR would reflect any change in the nature of the excited state (such as the depafling and terminating orbitals). It would be interesting and informative to determine whether in those amino-ketones which do illustrate enhanced II+ + n transitions, the respective k~ values appropriately reflect such orbital interaction. By comparing the fluorescence intensity of AJICONE when excited at 3 I 1 nm (the S* + n transition) and at 236 nm (presuma bly the S2 + So transition), relative to the amount of radiation absorbed at these wavelengths.; it is possiile to determine the effkiency of energy bans- ” fur between the two states populated at these respec-
tive wavele?igths.That is,
t Marerigorously the osciUatarstrength instead of emax should be ased. The arguments would be unchanged. howover.
Volume 25, number 3
CHEMICAL PHYSICS LETTERS
where T, E. and I are, respectively, the fractional transmission of the sample, the relative intensity of the exciting radiation, and the intensity of fluorescence produced by exciting at a particular wavelength. Subscripts 1 and 2 refer, respectively, to excitation wavelengths of 3 11 nm and 236 mn. The relative excitation intensities were determined from the front-surface ihumination of a rhodamine-B solution [12] _ From such an analysis, EET was determined to be approximately 50%. It is premature to speculate about the nature of the energy transfer mechanism in ABCONE. The close proximity of the donor and acceptor chromophores (ca. 4 A) suggests that exchange interactions would be important. Moreover, because of the rigidity of the two chromophores, the orientation factor would be specified and fmed by the respective transition dipoles. An analysis is complicated by the fact that the polarization of the transitions in the saturated amines has not yet been determined. The inability to observe distinct emission from the amine chromophore in ABCONE solutions makes it impossible to determine the rate of energy transfer from the N to the CO group. However, if energy transfer to the carbonyl group were the on& additional decay mode introduced to the ‘ABCO-type’ system in ABCONE, the fluorescence quantum yield of the amine portion of the molecule would still be appreciable, i.e., ca. 15% (see table 1). One conclusion from the failure to detect amine-type fluorescence is that some other nonradiative channel (possible a photochemical process), in addition to energy transfer, is enhanced in ABCONE relative to ABCO.
The authors are grateful to the donors of the Petroleum Research Fund of the American Chemical Society for partial support of this work. A grant from the New
York University Institutional Grants Committee is also acknowledged. Appendix: Experimental All materials were procured from Aldrich Chemical Co. ABCO, obtained as the hydrochloride salt, was liberated by combining very concentrated aqueous soiu..
1 April 1974
tions of ABC0 HCI and NaOH. The free base was then sublimed from a solid mixture of anhydrous BaO. The same procedure was used to obtain ABCONE from its hydrochloride. ABCONE, which tends to become slightly yellow upon standing, was resubhrned immediately prior to use. 2-norbomanone and 2-adamantanone’were sublimed prior to use. Quantum yields were obtained with a conventional fluorimeter described elsewhere [2] _ The solutions were prepared such that their optical densities at the exciting wavelength (3 11 nm) were equivalent to within 3%. The standard chosen was acetone in cyclohexane, for which a quantum yield of 0.00094 was used [15]. For the ERT measurement, the ABCONE concentration was kept low enough such that inner filter effects were avoided at both exciting wavelengths. Lifetimes were measured using the time-correlated single photon method. The lifetimes reported for ABCONE and the ketones are deconvoluted values.
References [I] A.M. Halpem, Cbem. Pbys. Letters 6 (1970) 296. [2] A.M. Halpem and R.M. Danaiger, Cbem. Pbys. Letters 16 (1972) 72. [3] C.G. Freeman, M.J. McEwan, R.F.C. Clairidge and L.F. Phiips, Chem. Phys. Letters 8 (1971) 77. [4] Y. Muto, Y. Nakato and H. Tsubomura, Cbem. Pbys. Letters 9 (1971) 597. [S] A_M. Halpem and E. Meratos. J. Am. Chem. Sot. 84 (1972) 8273. [6] AM. Helpem end RJ. Sternfels, to be submitted for ‘. Ipublication. [7] J.B. Birks, Pbotophysics of aromatic molecules (Wiley, New York, 1970) ch. 7. [S] A.M. Halpernand T.. Gartrnan. submitted for publications [9] A.M. Halpem, J-L_ Roebber and K. Weiss, J. Cbem. Pbys. 49 (1968) 1348. [IO] J. Hudec. Cbem. Commun. (i970) 829. and references therein; private communication. [ll] h1.T. Hughes and J. Hudec, C&em: Commum. (1970),831. .. [12] CA. Parker,-Photduminescence of solutions (Elsetier. . . Amsterdam, 1968) pp. 204-205. [13] F.S, Wettack. G.D. Renkes. M.G. Rockley. NLTurro .. and J.C. Dalton, J_ Am. Cbem. Soc.92 (1970) 1793; .. .. ]14] J.C. Dalton, D.M. Pond and N.J. Tuna, J. Am. Chem.: : Sot. 92 (1970) 2173:. [IS] AMi Halpem and WiR. Ware, I. Cbem. whys. 54 (L&L) ..:. 1271.; .. : ‘.
-. -:
I
:
.‘~.,.
,:
~:
: T ,. _:. . ..-.-
. . :. -,_._-..:._ ,
1 : -. :
.-..
_,
,-. .:. ___._r.. .: .,
_.-~_._.,__. ,..
: .., : &:~1,;~__ . .. -. _..,.: .:_:-; i,. ..,,. :-,:,-%,I _