2971
Structural Effects on Photophysical Properties in Saturated Amines. IV. Intramolecular Excimer Arthur M. Halpern*2aand P. P. Chan Contribution from the Photochemistry and Spectroscopy Laboratory, Department of Chemistry, Northeastern University, Boston, Massachusetts 021 15. Received September 30, 1974
Abstract: The fluorescence properties of two saturated diamines, N,N,N’,N‘-tetramethylethylenediamine (TMED) and N,N,N’,,V’-tetramethyl-l,3-propanediamine (TMPD), are compared with another diamine, N,N’-dimethylpiperazine ( N N D M P ) , and a monoamine, trimethylamine ( T M A ) . It is concluded that for the diamines studied, the lowest excited state is characterized by an interaction between the two nitrogen centers. The absorption spectra and the ionization potential values of the diamines are very similar to the respective properties of T M A . The fluorescence maxima of the diamines, however, are red shifted relative to monoamines, and, more significantly, the radiative rate constants for the diamines, k R . are all considerably smaller than that for T M A . These data are interpreted in terms of these diamines as having lowest excited states which are excimeric nature; i.e., these states are not directly reachable via a radiative transition from the ground state(s). For T M E D , the pressure dependence of @f and the decay curves have been analyzed from ca. 0.05 Torr to ca. 12 Torr. From these data, eight rate constants have been elucidated which characterize the dynamic properties of the excited state. I t was found that the radiative rate constant of the relaxed (excimeric) state was pressure dependent, having a pressure coefficient of ca. 2.4 X lo9 (M-I sec-I). The vapor phase fluorescence spectrum of T M P D is very sensitive to excitation wavelength; two distinct emission bands can be observed. One band at 290 nm is assigned as monomer fluorescence, and the other at 365 nm is interpreted as the excimer emission spectrum. I n a condensed medium, the ratio of the excimer-monomer intensities exhibits the usual temperature dependence. T h e binding energy of the T M P D intramolecular excimer is ca. 2.7 kcal/ mol.
er quantitative comparison can be made. e (227 nm) for In a previous paper, the photophysical properties of the T M E D and T M A is respectively 2150 and 910 M - ] cm-I, symmetrically bifunctional amine, N,N-dimethylpiperazagain in a ratio of about 2:l. These spectroscopic data are ine ( N N D M P ) , were examined and compared with those of summarized in Table I. several monoamine^.^ O n the basis of the spectral properties Photoelectron spectroscopic data of T M E D 9 provide anof this diamine, as well as the value of its radiative rate conother basis of comparison with other amines. T h e vertical stant, it was concluded that the nonbonding orbitals cenionization potential (IP) of 8.34 eV is close to the IP’s of tered on the nitrogen atoms became coupled in the excited both N N D M P 3 and TMA:Io 8.41 and 8.5 eV, respectively. (singlet) state. This paper is one of a series dealing with the These data as well as the optical spectroscopic data imply l-azabicyclo[2.2.2]octane symmetrical cage amines that in the diamines T M E D and N N D M P , the nonbonding (ABCO), 1,4-diazabicyclo[2.2.2]octane (DABCO), and orbitals a r e unsplit by any ground state coupling.lf It seems structurally analogous compounds. This work reports the therefore a good approximation to view these two diamines results of a further investigation of the photophysical properties of two related diamines: N,N,N’,N’-tetramethylethy-as noninteracting bonded dimers of T M A . The IP values for these amines are also contained in Table I. lenediamine ( T M E D ) and N,N,N’,N’- tetramethyl- 1,3-proThe emission properties of T M A , T M E D , T M P D , and panediamine (TMPD). Both N N D M P and T M E D bear a N N D M P , on the other hand, a r e rather dissimilar. The fluorescence spectra of T M E D and N N D M P maximize a t 304 for T M A is a t 287 nm. and 313 nm, respectively, while A,, A,, for T M P D is 365 nm. The fluorescence properties of NNDMP TMED TMPD T M P D will be discussed below. More striking differences formal resemblance to the rigid bicyclic cage amine 1,4-diare encountered in the comparison of the radiative rate constants ( k R ) of these amines. The k R values a r e contained in azabicyclo[2.2.2]octane (DABCO), in which one (two) of the ethylene bridges is (are), respectively, opened. In Table I along with the zero-pressure lifetime and (absolute) DABCO, the fixed orientation of the two nitrogen atoms requantum yield values. Whereas k R for T M A (2.1 X l o 7 sults in ground state i n t e r a ~ t i o n ~with - ~ the consequent absec-l) is typical for a monoamine, the k R values for the disorptive7 and emissive8 spectroscopic characteristics. amines are significantly smaller (although k R for T M E D is about ten times greater than that for N N D M P ) . T h e wide Spectroscopic Properties difference in the k R values of these diamines will be disThe vapor phase absorption spectra of T M E D and cussed below. Figures 1 and 2 respectively show the absorpT M P D , like N N D M P , resemble that of a monofunctional tion and emission spectra of T M E D and T M P D (vapor) a t amine, e.g., trimethylamine (TMA). For example, the cmax ambient temperature. values are very similar. Furthermore, E,,,,, values of T M E D The diamines also differ from T M A with respect to their (7540 M - ’ cm-I) and T M P D (6470 M - ’ cm-l) are fluorescence decay characteristics. T M A fluorescence folroughly twice that of T M A (3460 M-I cm-I). It should be lows simple (first-order) decay kinetics a t all pressures realized that in the nonrigid tertiary amines, the absorption above 1 Torr (for A,, > ca. 230 nm). O n the other hand, band (which is structureless) a t ca. 195-200 nm correthe fluorescence decay properties of N N D M P are rather SOtransition. T M A is the only amine in sponds to the Sz complicated (see ref 3). T M E D is also characterized by this class for which the SI SO transition is discernible, nonexponential decay kinetics a t all pressures examined having a shoulder a t ca. 227 nm. A t this wavelength, anoth(0.3-15.5 Torr) and a t all exciting wavelengths used (up to
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Halpern. Chan
Intramolecular Excimer Formation in Saturated Amines
2972 Table I. Summary of Spectroscopic and Fluorescence Data of Amines Amine TMA TMED
Amax, nm
emax,M-' sec-'
IP, eV
3460 7540 6800 6470
8.5 8.34 8.41
198 195 205 202
NNDMP
hma,(fluor) nm
T~ -+
287 304 313 365
oa (nsec)
op -,oa
45 71 770 42
kR,a SeC-' 2.1 x 10' 3.2 X lo6
1.0 0.23b 0.23 0.11"
3.2 x 105
TMPD 2.6 X lo6 aValues refer to the vibrationally relaxed lowest excited state. bcalculated from k R 1 /(kR, + k N R l 1, see Figure 6. "Obtained from a $f calculated by extrapolation using kQ= 8.5 X lo9 M-' sec-' . measured at 5.0 Torr; QP -+
IO00
*
,
....
IO0
. , ... , ....,.... . -... ._. . .
IF 30
40
35
45
50
3 , K CM-'
',.
Figure 1. Vapor phase absorption and (corrected) emission spectra of 3.0 Torr of TMED in the presence of 100 Torr of n-hexane; A,, = 265 nm.
400
300
I
I
.
IO
20:
NM
7
X '
20
IO0
60 80 CH #
40
120
Figure 3. Fluorescence decay curve of TMED vapor (5.0 Torr). The exciting wavelength is 265 nm (1.6 nm bandpass), and the channel width is 1.29 nsec. Open circles correspond to the flash lamp profile. PF(ES"SUHE. T O R R
4 6
2 I
I
3.3Q
I
