12
D. A. Lerner, P. M. Horowitz, and E. M. Evleth
Comparative Photophysics of lndolizine and Related Heterocyclics D. A. Lerner,' P. M. Horowitr,* and E. M. Evleth" Centre de Mhcanique Ondulatoire AppliqGe, 23 rue du Maroc, 750 19, France, and the University of Californla, Santa Cruz, California 95064 (Received June 11, 1976)
The absorption-emission characteristics of six indolizines are reported. Analysis of the fluorescence lifetime-quantum yield data indicates that as a class of materials these substances exhibit slower nonradiative decay than do their indole counterparts and a number of aromatic hydrocarbons. A series of molecular orbital calculations indicate that the spacing and number of triplets states lying below the S1states in the indoles are much different than in the indolizines. Calculations indicate that the Tzstates in the indolizines could lie above SI so that intersystem crossing from S1 and T1 occurs without the benefit of communicating triplets.
Introduction Indolizine (1) is an isomer of indole (2) conceptually obtained by the transposition of adjacent carbon and nitrogen atoms. There are also several other isomers of indole, 3-5, obtained by the placing of the nitrogen atom in various positions of the basic 5-6 ring s t r ~ c t u r e . ~ 9
I R
I
Y
1 indolizine
2 indole
3 1-pyrindine
4
5
2-pyrindine
isoindole
A large number of aza and polyaza derivatives of structures 1-5 are possible. From a photophysical standpoint what makes these systems interesting for study is the wide variation in spectral properties that occurs as a function of the position of these additional nitrogen atom^.^-^ However, even in the basic series of structures 1-5 the variations of spectral features are dramatic. Indole exhibits two close lying singlet states, S1 and S2, transitions to which from So lie in the 260-290-nm region.6 In contrast to indole the 0-0 bands of 1 for the Sz So and S1 So transitions lie at 295 and 380 nm, respectively. Although aza substitution into indole leaves the positions of the two lowest lying T--a* transitions largely in the same energy region7the spectra of azaindolizines are strongly dependent on the position of nitrogen sub~titution.~ Finally, the Sz-Sl energy gap in derivatives of 3 and 4 (ca. 1.5 eW4 appears even larger than that for 1. Visually the absorption spectra of 1,3, and 4 resemble azulene4i8even though the magnitude of the Sz-S1 energy gap is not as large. This resemblance provokes the idea that perhaps these heterocyclics share some of the other photophysical properties of azulene, namely, either SZ So emissiongor dual fluores~ence.~~ Unfortunately, of this group only the indolizines have extensive preparative l i t e r a t ~ r e . Here ~ we report studies on 1, together with 1-azaindolizine (6), 2-azaindolizine (7), 18-diazaindolizine (8)) and the 2-phenyl derivatives of 1 and 6, these being 9 and 10, respectively.
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The Journal of Physical Chemistv, Vol. 8 1, No. 1, 1977
10
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Preliminary reports have been presented on the photophysical properties of 1 and 6-8.8J0 While no Sz So emission was detected from any of these materialss it was determined that the intersystem crossing rate constant and quantum yield had anomalous values.IOThus, we will also present theoretical calculations and attempt to rationalize the photophysical properties of the materials dealt with here.
Experimental Section A. Materials. All materials were synthesized by well-established literature methods or obtained from commerical sources. Indolizine (l), known in the older literature as pyrrocoline,6J1was synthesized by the given procedure.lla The material appears unstable on long storage in the dark but is relatively easily purified by multiple microsublimation to yield a sharp melting point material. It is a colorless volatile material with an odor resembling naphthalene. A gas phase spectrum was easily obtained in a 10-cm gas cell at room temperature.8 Compound 6, 1-azaindolizine (imidaza[1,2a]pyridine), is known under the alternate names of [1,3a]diazaindene5" and pyrimidazole. It was commercially obtained from Aldrich Chemical Co. under the latter name. However, it was easily synthesized in one step from 2-amino~yridine.~" Both synthesized and commerical materials were spectroscopically identical after purification by GLC. The material is a colorless liquid at room temperature. Both 7 and 8, imidaza[l,5a]pyridine and imidaza[l,2a]pyrimidine, respectively, are reported in the literature as [2,3a]diazaindene and [ 1,3~,7]triazaindene. Their syntheses were easily affected by the given method^.^**^ As with indolizine purification of these materials was best achieved by multiple microsublimation immediately before use. Both 9 and 10 were purchased from Aldrich Chemical Co. and purified by recrystallization from ethanol. These latter two materials proved somewhat less soluble in hydrocarbon solvents. All materials were checked for purity using mass spectral analysis. All physical properties agreed closely with the literature values, especially their rather complex ultraviolet
13
Photophysics of Indolizine and Related Heterocyclics
spectra in hydrocarbon solvents. Either at room temperature or in glassy solvents (EPA, MCH-IP) at 77 K we observed no anomalous emission which could be attributed to either impurity emission or the searched for S2 SO fluorescence. B. Equipment. UV-visible spectra were recorded on either a Cary 14 or Beckman Acta V spectrophotometers. Fluorescence-phosphorescence was measured on either Perkin-Elmer-Hitachi MPF-2A or 3A units. Polarization measurements were obtained using Polacoat UV 105 coatings on Suparsil plates and were corrected for using standard procedures." Variable temperature studies were carried out using a Varian variable temperature acessory E-4540 mounted in the fluorimeter. Fluorescence lifetimes were measured from nanosecond flashes using a Tektronix 547 scope with a 1S1 sampling unit. C. Techniques. All liietime measurementswere checked using as standards quinine sulfate, chrysene, biphenyl, and anthracene. The literature values13were assumed correct. The quantum yields of fluorescence were obtained using as standards quinine sulfate14 (af= 0.55), PP0l2 (1.00), and tryptophane15 (0.14). Measurements were taken on degassed solutions having optical densities lower than 0.05. An independent check of the quantum yield of anthracene gave a value of 0.31 in n-hexane, identical with the average of the literature v a l u e ~ . ~ ~ J ~ Phosphorescence was searched for in these materials both instrumentally and visually under both mild and intense radiation. Unsensitized phosphorescence was only found for 6 and 10 (EPA, 77 K) with yields estimated to be below 0.01. In addition, sensitized phosphorescence was only obtained for 6 and 10 (benzophenone). The quantum yields for photodisappearance for 1 and 61° were found to be less than 0.005. D. Theoretical Calculations. Computational estimates of the spectral features of the materials dealt with here were carried out using both the Pariser-Parr-Pople (PPP-CI)17 and CNDO/S18 methods. The latter method generally gave poorer agreement with experiment with regard to the positions of the lowest lying transitions. The CNDO/S calculations did predict that in every structure the lowest lying transition was of the mr* type and that n r * states are not the lowest lying even in the case of 8. A complete analysis of the variation of the n r * states with structure will be published at a later date for the azaindolizines, azaindoles, and related structures. We will only report the results of the PPP-CI computations. These were carried out using 20 single and 17 double excitations. The 0integrals were calibrated by fitting the calculations to produce the positions of the 0-0 transitions for indole (2), 1-azaindolizine (61, and 1-pyrindine (3,R = m e t h ~ l ) ~ " to within 0.2 eV. This 0parameterization scheme yielded good results for a number of azaindoles and aminonaphthalenes but as will as seen does not reproduce well the spectra of the polyazaindolizines to within 0.2 eV. The resulting 0integrals for various bond types were as follows: C-C, -2.07 eV; C=C, -2.20; C-N, -1.84; C=N, -2.55; C=C (aromatic), -2.15; and C=N (aromatic) -2.18 eV. The Coulomb and electron repulsion parameters were as previously used.3c For bond distances we used the standard bond distances of Pople and Beveridge.lg The triplet-triplet spectra were estimated by both CI treatment of the SCF ground state orbitals and by direct minimization of the lowest energy triplet followed by CI using both single and double excitations. E. Estimates of Photophysical Constants. The oscillator strengths of the transitions were estimated from the integrated intensities using the standard equation.20a
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FREQUENCY, CM-1
x 10-3
Figure 1. Absorption-emission spectrum of indolizine (1) in nhexane. 250
./I
WAVELENGTH, 350
300
n~
I/
,
NM
I
400
,
450
,
550
I
METHANOL
it
I ++E ~
z
Ab
,
t l
40
35
FL
Le
30 25 F R E Q U E N C Y , C M - I x 10-3
20
Flgure 2. Absorption-emission spectrum of indolizine (1)in methanol.
250
I
A,
I
40
WAVELENGTH, NM 300
35
30
25
20
F R E Q U E N C Y , C M - ~ X10-3
Fgure 3. Absorption-emission spectra of l-azaindolizine (6) in nhexane and methanol.
Likewise, the radiative (natural) lifetimes, were estimated by direct integration.20b The measured radiative lifetime, T,, was taken in standard fashion from
(1)
7, = T f / @ f
where 7fand afare the measured fluorescence lifetimes and quantum yields. The radiative rate constant, k,, is the inverse of 7,. The nonradiative rate constant, k,,, is estimated from k , and 7f using
1- @ f = k n r / ( k +
knr)
(2)
Results and Discussion A. Absorption and Emission Properties. Table I and Figures 1-5 show the experimental radiative and nonradiative rate constants and spectral properties of 1 and 6-10 The Journal of Physical Chemktry, Vol. 81,No. 1, 1977
14
D. A. Lerner, P. M. Horowitz, and E. M. Evleth
TABLE I : Computed and Measured Photophysical Properties of Various Indolizinesa h,,
f
Test
@f
7f
rr
383
0.027
42
0.84 0.72
42 41 19 21 40 42 25 23 22 28 6
50
Compd Solvent
H M H M H M H M H M H
1 6
7 8 9
10
336
0.037
24
0.80 1.00
380
0.026
43
0.98 0.98
368
0.037
28
1.00 0.77
386
0.039
30
346
0.10
0.60 0.67 0.73
9
57 23 21 40 42 25 30 36 42 8
k,, s-' 2.0 1.8 4.3 4.8 2.5 2.5 4.0 3.3
x
107 107 107
x x x 107 x 107 x 107 x 107 x 107 1.9 x 107 2.4 x 107 1.3 X
lo8
k n n s-' 4 x 7 x
lo6
lo6 lo6
1x 107
