92 1
J. Phys. Chem. 1982, 86, 921-927
the CollPPIX(pyr)l to oxidation and the five-coordinate species rapidly reverts to a Co(II1) species or the ability of ConPPIX to eject the sixth ligand is ifiipaired. However, the oxidation to Co(1II) in either case requires an adequate donor. The toluene used was not redistilled and could have contained such donors. The results in neat pyridine are puzzling, particularly in that the main feature of the spectrum closely resembles
the strongest features of the spectrum of the five-coordinate species.
Acknowledgment. This work was supported by a grant from the North Atlantic Treaty Organization. We are grateful to Lorraine Harris for performing some of the experiments and to Tom Provost for preparing the separated hemoglobin chains.
Spectroscopy of Polyenes. 6. Absorption and Emission Spectral Properties of Linear Polyenals of the Series CH,-(CH=CH),-CHO ParRosh K. Das' Radiatlon Laboratory, Unlverstly of Notre Dame, Notre Dame, Indiana 46556
and Ralph S. Becker Lbpartment of Chemlshy, Unh'erstly of Houston, Houston, Texas 77004 (Received: June 8, 198 I; I n Final Form: October 8, 198 I)
The results of a detailed study are presented concerning the absorption and emission spectral properties of four homologous polyenals of the series, CH3-(CH=CH),-CHO, n = 2-5, under various conditions of solvent and temperature. A total of four different band systems have been recognized in the well-resolvedabsorption spectra. these have been assigned as arising from singlet K* n (very weak), lBU* lA, (very strong), lAg*+ lA, (cis peak, moderately weak), and lA,*lA, (weak) transitions. Except for the lAg*+ state which is always located at a relatively high energy, the relative location of the remaining three states, viz., lB,*, l$*-, and '(n,r*), is seen to be a sensitive function of polyene chain length and appears to be the most important factor in determining the radiative behavior of the polyenals. Thus, the C6 and C8 aldehydes (n = 2 and 3), with a '(n,?r*)state seen as the lowest singlet state in absorption,do not fluoresce under any condition of solvents (polar and nonpolar) and temperature (296-77 K). The Clz aldehyde ( n = 5) has the dipole-forbidden lAg*state as the lowest singlet state and exhibits fluorescence (at 77 K) that is moderately strong is not dependent on excitation wavelength). In the case of the intermediate polyenal, Cloaldehyde (n = 4),the three low-lying states, viz., l(n,?r*),lB,*, and l$*-, appear to be nearly degenerate based on their locations in the absorption spectra. While no fluorescence is observed for Clo aldehyde in 3-methylpentane at 77 K, it fluoresces very weakly in 2-methyltetrahydrofuranat 77 K where C$F is practically independent of excitation wavelengths; moreover, fluorescence is moderately strong in EPA at 77 K where h is very strongly dependent on excitation wavelengths. No phosphorescence is observed for any of the polyenals under study.
-
-
Introduction Because of their importance in absorbing and transducing light energy through various photobiological processes, polyenes have always been subjects of considerable interest for research concerning their spectroscopy and photodynamics. The relatively recent finding~l-~ that in polyenes there exists a dipole-forbidden singlet excited state of primarily l$* character near or below the strongly allowed lB,* state has generated a great deal of renewed vigor in the theoretical and experimental work regarding the excited-state properties of polyenes. In polyene al(1)Hudeon, B. S.;Kohler, B. E. Chem. Phys. Lett. 1972,14,299-304. J. Chem. Phys. 1973,59,4994-5002. (2)Christensen, R.L.;Kohler, B. E. J. Chem. Phys. 1975,63,1837-46, J.Phys. Chem. 1976,80,2197-2200. (3)Schulten, K.;Karplus, M. Chem. Phys. Lett. 1972, 14, 305-9. Schulten, K.;Ohmine, I.; Karplus, M. J. Chem. Phys. 1976,64,4422-41.
Ohmine, I.; Karplus, M.; Schulten, K. Ibid. 1978,68,2298-318. (4)Becker, R.S.;Hug, G.; Das, P. K.; Schaffer, A. M.; Takemura, T.; Yamamoto,N.; Waddell, W. J. Phys. Chem. 1976,80,2265-73,and references therein. (5) Takemura, T.;Das, P. K.; Hug, G.; Becker, R. S. J.Am. Chem. SOC. 1976,98,7099-101,1978,100,2626-30,Takemura, T.;Hug, G.; Das, P. K.; Becker, R. S. Ibrd. 1978,100,2631-4. (6) Song, P. S.; Chae, Q.; Fujita, M.; Baba, H. J. Am. Chem. SOC. 1976, 98.ai9-24.
0022-385418212086-092 1$0 1.2510
-
-
dehydes," there is an additional low-lying singlet excited state, viz., '(n,a*), that plays a very important role in determining the spectral and photodynamical behavior of these polyene systems. The notable dependence of the relative location of the three low-lying singlet excited states, namely, lB,*, 'Ag*-, and '(n,?r*), on polyene chain length and various structural factors and environmental conditions makes the photophysics, photochemistry, and spectroscopy of the polyene aldehydes complex as well as interesting. In the previous papers7+ of this series we have reported on the spectral and photophysical properties of polyene systems ending with various functional groups and related to retinyl polyenes as homologues. One interesting feature of the spectroscopy of retinyl polyenes and their homologues (particularly the shorter ones) is that their spectra, both absorption and emission, are generally broad and unresolvable even in a low-temperature glass matrix. This has been s h ~ w n ~toJ ~ be' related, ~ in one way or other, (7)Das, P.K.;Becker, R. S. J. Phys. Chern. 1978,82,2081-93.1978, 82,2093-105. (8)Das, P.K.;Kogan, G.; Becker, R. S. Photochern. Photobiol. 1979, 30,689-95. (9)Becker, R.S.;Das, P.K. J. Phys. Chern. 1980,84,2300-5.Photochem. Photobiol. 1980,32,739-48.
0 1982 American Chemical Society
922
The Journal of Physical Chemistry, Vol. 86, No. 6, 7982
with the nonbonding steric interaction between the methyl groups of the @-ionylidenering and the hydrogen atoms of the polyene chain (close to the ring). The purpose of the present work is to obtain absorption-emission spectral data concerning four homologues of the polyenal series, CH,-(CH=CH),-CHO (n = 2-5), compare the results with those of similar studies with their retinyl counterparts, and obtain a better insight into the nature, order, and interaction of the low-lying excited states in the polyenals in general. A report', of some of our work on 2,4,6,8,10dodecapentaenal (n = 5) has appeared in the literature. Blout and Fields14studied the absorption spectra of the polyenals at room temperature. Besides this, to our best knowledge, there has not been a previous detailed study of the absorption-emission spectral properties of these compounds nor a consideration of various conditions of solvent and temperature. For the sake of brevity, throughout the rest of the paper, the polyenals will be designated by the number of carbon atoms present in them (e.g., Clo aldehyde for 2,4,6,8-decatetraenal). Experimental Section c 6 aldehyde (2,4-hexadienal, Aldrich) and C4aldehyde (crotonaldehyde, Eastman) were distilled twice under vacuum. For spectroscopic measurements they were distilled again (bulbto-bulb) before use. The longer polyenals were synthesized from hexadienal by procedures' similar to those used for preparing the homologues of retinals. The polyenals (other than c4/c6 aldehyde) were purified by column chromatography on silica gel with mixtures of diethyl ether ( 5 2 0 % ) and petroleum ether, followed by multiple crystallization from n-hexane. For emission spectral measurements, the polyenals (except c4/c6 aldehyde) were subjected to high-pressure liquid chromatography (HPLC) on a r-porasil column with 5-20% diethyl ether in petroleum ether as eluent and spectral measurements were carried out within 1h after removal of solvent from the purified samples. Also, for Ca-C12 aldehydes, thin layer chromatography, using conditions similar to those for column chromatography,was employed for purification. The methods of purification of solvents and reagents and the procedures for obtaining and correcting the absorption-emission-excitation spectra are described in the The absorption spectra were measured previous in a Cary 16 or Cary 219 spectrophotometer; the ones shown in Figures 1-6 were obtained in the latter with 1-nm bandpass. The apparatus used for recording emissionexcitation spectra and determining fluorescence quantum yields consisted of the following: 1000-W dc xenon lamp (operated at 700 W), 0.35-M McPherson (Model EU700) stepping motor driven grating monochromator (exciting), scanning Aminco 1/4 M grating monochromator (analyzing), and 9558 QA EM1 photomultiplier tube, rf shielded and cooled to -20 "C in a housing (Model T E 102RF, Products for Research). The configuration made use of front-face illumination with 20° between the directions of excitation and detection. The output of the photomultiplier tube was amplified and recorded either on an X-Y recorder (Model 2000, Houston Instruments) while scanning the analyzing monochromator manually or me-
-
(10) Warshel, A.; Karplus, M. J. Am. Chem. SOC.1974,96, 5677-89. (11) Christensen, R. L.; Kohler, B. E. Photochem. Photobiol. 1973,18,
293-301. (12) Hemley, R.; Kohler, B. E. Biophys. J. 1977, 20, 377-82. (13) Becker, R. S.; Das,P. K.; Kogan, G. Chem. Phys. Lett. 1979,67, 463-6. (14) Blout, E. R.; Fields, M. J. Am. Chem. SOC.1948, 70, 189-93.
Das and Becker
chanically (for emission spectra and quantum yield) or on a strip-chart recorder (Model EU-205-11, Heath-Schlumberger) synchronized with the exciting monochromator (for excitation spectra). The detecting system was calibrated for relative sensitivity at 300-750 nm by using a standard tungsten lamp (Type 101, Electro Optics Associates) and the relative spectral irradiance of the exciting assembly was measured by using an ethylene glycol solution (3 g/L) with rhodamine B as a quantum counter. The fluorescence spectra, recorded at a band pass of 2-4 nm for excitation and 2 nm for analyzing, are corrected for relative sensitivity of the detector and presented as relative intensity in photons per second per wavenumber vs. wavenumber (Figures 5 and 6). The excitation spectra (Figures 5 and 6), recorded at a band pass of 1 nm for excitation and 15 nm for emission, are corrected for the relative irradiance of the excitation msembly and for inner filter effect by multiplying with the function, A(X)/[l where A is the total absorbance at the wavelength (A) of excitation. The quantum yields are determined at the same spectral resolution as used for recording excitation spectra by using, for reference, a well-degassed solution of 9,lO-diphenylanthracene in 3-methylpentane at 77 K (@p = 1.0). Some of the emission and excitation spectra were also recorded in a Spex Fluorolog emission spectrophotometer; these showed essentially the same features as those observed in the setup described above. Results Before we present and discuss the results, it is necessary to point out two important aspects regarding the properties of the polyenals under examination. First, the polyenals were relatively unstable-far more unstable than their retinyl analogues. The purified compounds deteriorated within 1 or 2 day even on storing at -40 "C under argon or vacuum. The spectral data reported here were obtained, and the authenticity of the various spectral band systems were established, by measurements on samples that were freshly purified by crystallization or chromatography (column, thin-layer, or HPLC, see Experimental Section). Second, the polyenals, particularly the ones with long chain length (Cloand C1J, are only sparingly soluble in hydrogen solvents. Thus, a dilute solution (10-"5 X lo4 M) of Clo or C12aldehyde in 3-methylpentane (3MP) turns turbid on cooling to 77 K and this results in a large reduction of intensity of the absorption spectral bands (along with the appearance of a broad background absorption due to the aggregation of the solute as well as its precipitation from the solution phase at low temperature). Thus, for spectral data in glasses at 77 K, we have mostly used very dilute solutions (5 X 104-5 X M) in EPA (etherisopentane:alcohol, 5:5:2, v/v) and 2-methyltetrahydrofuran (2MTHF). No precipitation occur in EPA and 2MTHF at 77 K in the concentration range used. The absorption and emission spectra of the polyenals under various conditions of solvent and temperature are shown in Figures 1-6. The related data are summarized in Tables I and 11. Absorption Spectra. Figure 1 shows the absorption spectra of crotonaldehyde (C4,n = 1)and 2,4-hexadienal (c6,n = 2). The most prominent feature of the absorption spectra of C4and c6 aldehyde is the clearly defiied location of the singlet x* n transition in the spectral region 300-400 nm. The extinction coefficients (emm) of this transition at the absorption maxima are in the range 70-80 M-' cm-'. On going from C6 aldehyde to Ca aldehyde, the singlet T* n transition is nearly swamped by the strongly ab-
-
-
Spectroscopy of Linear Polyenals
TABLE I:
The Journal of Physical Chemistry, Vol. 86, No. 6, 7982 923
Absorption Spectral Data of Polyenals of the Series, CH,-(CH=CH),-CHO band max, nma (extinction coeff, M - l cm-' x 10-4)b 'Ag*- + 'A, 'B.,,* + 'A, ,Ag*++ 'Ag
n
solvent
temp, K
'(n,n*)
2
PFH EPA
296 296 77
338.5 330-400' 320-400'
253.0 267.0 (3.19) 270.0 (3.48)
3
PFH 3MP EPA
296 296 296 77
340-410' 340-410' 370-410' 370-41 0'
287.0 297.5 310.0 (4.87) 324.0 (5.34)
-210 210 -210 -210
4
PFH 3MP EPA
296 29 6 29 6 77
390-410' 38 5-40 5'
320.0 331.5 347.0 (6.67) 365.0 (7.88)
228.0 233.0 230.0 -230
430-450'
347.5 362.0 375.0 ( 8 . 9 6 ) 382.0 (10.7)
249.5 256.0 2 58 .O 262.0
5
PFDB 3MP EPA
296 296 296 77
a i.0.5 nm. *lo%;extinction coefficients are shown in the parentheses. of strong overlap with the tailing of the intense band system, I & * + ,A,.
TABLE 11: Emission Spectral Data for C,, and C,, Aldehyde at 77 K fluor max,a Polyenal solvent C,,
2MTHF
EP A
lo3
obsd lifetime,' ns
cm-'
fluor quantum yieldb
20.4 20.4
-0.0001 ( 3 9 0 ) 0.019 (380)
0-
3MP 17.8 0.008 (400)d 80 2MTHF 17.6 0.046 (408) 3.7 EPA 17.7 0.022 (403) 3.7 168 a + 5 0 0 cm-'. i.20%for QF'S in the range 0.01-0.05; excitation wavelengths shown in the parentheses. i.10%. The actual @ Fis probably higher, because the absorbance (at exciting wavelength) used for calculation of @ Fwas not totally due to the fluorescing monomer.
I
I
I
Band maxima are not well-defined because
m
n
A&,,(cd'I
intrinsic radiative lifetime, ns
C,,
W
-
W
2 a
4-
6.0-
WAVE NUMBER, k K
I
Flgure 2. The absorption spectra of CBaldehyde (purified by multiple crystallization from n-hexane). A. I n perfluoro-n-hexane at room temperature at low (a) and hlgh (b) concentrations. B. I n 3methyipentane at room temperature (a), at 77 K (b), and at 77 K in lo4 M hexafluoro-2-propanol (c). C. I n EPA at the presence of room temperature (a and c) and at 77 K (b).
-
I
-
-
I
3.0-
'5
r 2.0-
=.
.d
4
1.0-
X
\v
50
45
40 35 30 WAVE NUMBER, k K
25
20
Flgure 1. The singlet r' n transition of crotonaldehyde in perfluoro-n-hexane (a) and EPA (b) at room temperature. B. The absorption spectra of Ce aldehyde in perfiuoro-n-hexane at room temperature. C. The absorption spectra of Ce aldehyde in EPA at room temperature (a and c) and at 77 K (b and d). Both crotonaldehyde and Ce aldehyde were purified by vacuum distillation (twice). + -
sorbing 'B,* 'A, transition. The latter moves rapidly to lower energy as the polyene chain length is increased. The absorption spectrum of C8aldehyde at high concentration in perfluoro-n-hexane (PFH) still shows some vibronic structures in the spectral region 350-400 nm (Figure 2A) and these are ascribable to the '(n,r*) state. In 3MP (Figure 2B) and EPA (Figure 2C) the intense band system due to the IBU* 'A, transition is red-shifted relative to that in PFH and overlaps strongly with the weak *r n transition rendering the latter hardly observable beyond the tailing of the strongly absorbing band system ('B,* 'A ). $he singlet x* n transition is not discernible in the absorption spectra of Clo (Figure 3) and C12 (Figure 4) aldehydes, except that the spectrum of Clo aldehyde at relatively high concentration in PFH shows a slight change
-
-
924
The Journal of Physical Chemistry, Voi. 86, No. 6, 1982
Das and Becker
t
8m
6-
I
n
A L n
2
1527 1476 1717
3
2 3
4
W
2-
WAVE NUMBER (kK)
Figure 5. Absorption (A), excitation (B), and fluorescence spectra (D), and the relative quantum yleld (I$:) as a function of excitation wavelength (c), for Clo aldehyde in EPA at 77 K. The Cl0 aldehyde sample was purified by HPLC. WAVENUMBER, kK >
Flgure 3. The absorption spectra of C, aldehyde (purified by multiple crystallization from nhexane). A. I n perfluoro-nhexane at room temperature. B. I n 3-methylpentane at room temperature (a) and in 2-methyltetrahydrofuran at 77 K (b). C. In EPA at room temperature (a) and at 77 K (b).
t ln z
W
t
z
W 0
z
W
0
cn a
8-
W
0
3 J
LL
W
3
W
2 a -I
t
cz
W
a
a I
I
1
I
W
1
t-
4
W
6-
WAVE NUMBER
4-
Flgure 6. Absorption (A), excitation (B), and fluorescence spectra (D), and the relative quantum yield (I$F') as a function of excitation wavelength (C), for CI2 aldehyde in 2-methyltetrahydrofuran at 77 K. The C, aldehyde sample was purified by HPLC.
2-
-
-
10.0
&
6.0-
-'E
8.0-
5 4.0G
2.050
45
40 35 30 WAVE NUMBER,kK
25
Figure 4. The absorption spectra of CI2aldehyde (purlfled by multiple crystallizationfrom nhexane). A. I n perfluorodirnethylcyclobutane at room temperature. B. I n 3-methylpentane at room temperature (a) and in 2-methyltetrahydrofuran at 77 K (b). C. In EPA at room temperature (a) and at 77 K (b).
in the slope at 390 nm (Figure 3A). The most interesting feature in the absorption spectra of Clo and C12aldehyde is that, on cooling dilute solutions of these compounds in EPA and 2MTHF to 77 K, a weak band system developes on the low-energy side of the main band system. This is shown in Figure 3, B and C, for Clo aldehyde (385-405-nm region) and Figure 4, B and C, for C12 aldehyde (430450-nm region). Experiments done at various concentra-
tions from 1 X to 5 X M in both EPA and BMTHF at 77 K show that the relative intensity of the lower-energy band system with respect to the intensity of the peaks of the main band system remains practically unchanged. Thus, in 2MTHF at 77 K t349/t400= 21 f 3 for Clo aldehyde and t386/tq44= 30 f 5 for Clz aldehyde over the concentration range given above. These results clearly establish the intrinsic nature of the lower-energy band system; that is, it is not due to any extrinsic phenomena such as reversible formation of dimer (aggregate) or hydrogen-bonded complex at low temperature. For Clo and C12 aldehydes and to some extent for C8 aldehyde, a relatively weak band system is observed at energies 11000-14000 cm-' greater than the main band system. We assign this higher-energy band system in each of Clo and C12aldehyde as an lA,*lA, transition (cis peak). This as well as the assignment of ?r* n and lBU* lA, transitions is based on comparison with related polyene ~ y s t e m s ~ ~where J ~ " similar band systems have
-
-
+-
(15) Zechmeister, L. "Cis-TransIsomeric Carotenoids, Vitamin A and Aryl Polyenes", Academic Press, New York, 1962.
The Journal of Physical Chemistry, Vol. 86, No.
Spectroscopy of Linear Polyenals
been previously assigned as well as on the results of theoretical calculations3~17-21 regarding the relative location and intensity of the cis peak. Emission Spectra and Lifetimes. No emission that can be attributed as the fluorescence or phosphorescence of c 6 and C8 aldehyde is observed in 3MP, EPA, and 2MTHF at room temperature and 77 K (bF, 4p 5 5 X IO4). The lack of emission is also true for Clo and Cl2 aldehyde at room temperature in any of the three previously mentioned solvents, and for Clo aldehyde in 3MP at 77 K. The longer polyenals (Clo and Clz) can exhibit weak or moderately strong fluorescence at 77 K, the intensity of which is dependent upon the nature of the solvent. Clo aldehyde fluoresces very weakly in 2MTHF at 77 K (h lo4); the fluorescence under these conditions show practically no excitation wavelength dependence. In EPA a t 77 K, the fluorescence of Clo aldehyde becomes more intense, and, interestingly, $F is very strongly dependent on excitation wavelength. The absorption-emission-excitation spectra of Clo aldehyde (purified by HPLC) in EPA a t 77 K are shown in Figure 5. The excitation spectrum is seen to deviate strongly from the absorption spectrum and the relative quantum yield (4F') of fluorescence tends to increase sharply as the excitation wavelength is increased toward the long-wavelength tail of the main absorption band. The observed shape and location of the fluorescence spectra of Clo aldehyde are practically identical in EPA and 2MTHF (both at 77 K). At 77 K, the fluorescence of Clz aldehyde is more intense than that of Clo aldehyde and, unlike the latter compound, 4F shows practically no dependence upon excitation wavelengths. The excitation-emission spectra in PMTHF are shown in Figure 6. The weak, lower-energy band system in the 430-450-nm region becomes more prominent in the excitation spectrum than in the absorption spectrum. The excitation of the compound in the spectral region 430-450 run gives emission spectra that are identical in shape and location with those obtained by excitation into the main band system. This again provides strong evidence for the intrinsic origin of the lower-energy absorption band system. Similar observation has also been made in the case of Clo aldehyde in 2MTHF at 77 K although the result in this case is not as conclusive as in the case of Clz aldehyde because of the facts that the fluorescence is very weak and that the lower-energy band (385-405 nm) is never really separated out from the tailing of the main band system. The fluorescence lifetime data (Table 11),determined only for C12aldehyde, show that the observed radiative lifetime (r0)are much longer than that expected from the observed oscillator strength of the main absorption band system ('B,* '4).A sample calculation for Cl2aldehyde in EPA at 77 K gives T~ 0.7 ns based on the latter transition; that is, about 240 times shorter than the observed radiative lifetime, 168 ns.13 A polarization study was carried out with C12aldehyde under photoselective conditions in 2MTHF at 77 K. The
-
+
-
(16) Auerbach, R. A.; Granville, M. F.; Kohler, B. E. Biophys. J . 1979, 25,443-54. (17) H o e , B.; Dinur, V.; Birge, R. R; Ebrey, T. G. J. Am. Chern. Soe. 1980,102,488-94. (18)(a) Inuzuka, K.; Becker, R. S. Bull. Chem. SOC.Jpn. 1974, 47, 88-91. (b) Becker, R. 5.; Inuzuka, K.; Balke, D. E. J. Am. Chem. SOC. 1971,93,38-42. (c) S c M e r , A. M.; Waddell, W. H.; Becker, R. S. J.Am. Chern. SOC.1974, 96,2063-8. (19) Wiesenfeld, J. R.; Abrahamson, E. W. Photochem. Photobiol. 1968,8,487-93. (20) Weimann, L. J.; Maggiora, G. M.; Blatz, P. E. Znt. J. Quantum Chem. Quantum Biol. Symp. 1975, No. 2,9-24. (21) Birge, R. R.; Schulten, K.; Karplus, M. Chem. Phys. Lett. 1976, 31, 451-4.
6, 1982 925
degree of polarization was found to be in the range 0.45-0.5 throughout the emission spectrum on excitation into the main and the lower-energy band system as well as throughout the excitation spectrum on monitoring at the maximum of fluorescence. These results suggest that the fluorescing state has its polarization parallel to the long axis of the polyene.
Discussion Assignment of Absorption Band Systems. Dependence on Chain Length and Comparison with Retinal Homologues. Except for the weak, low-energy band system observed near the long-wavelength tail of the main absorption band system of Clo and Clz aldehyde in EPA and 2MTHF at 77 K, the assignment of the other observed band systems has been quite straightforward. The onset n transition (very weak) is located at of the singlet ?r* 400-410 nm in the shorter polyenals and moves very slightly to lower energy (by -500 cm-') on increasing the chain length on going from C4aldehyde to C8 aldehyde (in PFH at room temperature). On the other hand, the lB1* 'A, transition, responsible for the main, intense absorption band system, moves rapidly to lower energy with increase in chain length (the net decrease in the energy of its location being about 11000 cm-l on going from c6 to Clz aldehyde). The cis peak lAg*+ lA, transition maintains an energy separation of 11000-14000 cm-' from the location of the lBU* IA, transition in the three polyenals, C8-C12,and shows a tendency to move to lower energy at a slightly faster rate than the latter as the polyene chain length is increased. To be more specific, the energy separation between the maxima of the two transitions is 12 610 cm-' for Clo aldehyde in PFH, while that for Clz aldehyde is 11303 cm-l, in the same solvent. Two alternative assignments are possible for the weak band systems observed at 385-405 and 430-450 nm for Clo and Cl2 aldehyde, respectively (in polar solvents at 77 K). These are singlet ?r* n and '$*- '4 transitions. We rule out the assignment as singlet r* n transition on the following grounds. Firstly, the observed intensity of the band system is too large compared with the ?r* n transitions of the shorter polyenals (C4,C& Even after correcting generously for the overlapping tail of the main absorption band system, the extinction coefficients in EPA at 77 K are -3000 and 1500 M-' cm-' for Clo aldehyde at 390 nm and Clz aldehyde at 440 nm, respectively. These numbers are 20-40 times higher than the extinction coefficients (70-80 M-' cm-') of the maxima of the band systems that have been assigned to the singlet ?r* n transition in &cyclocitral' and C4 and c6 aldehydes, vide supra. Secondly, the location of the band system under consideration is shifted to lower energy by about 2500 cm-' on going from Clo to Clz aldehyde. Such a large red-shift with increase in polyene chain length by one double band is not compatible with the trend shown by the r* n transition in shorter polyenals. Thirdly, some members of the polyene Schiff base series,22CH3-(CH=CH)nCH=NBu, also exhibit similar weak band systems at locations comparable to those for the corresponding members of the polyenal series. The singlet r* n transition is, however, blue-shifted by 2000-3000 cm-' on going from a polyenal to i'ts Schiff base derivative.22 The assignment for the weak, lower-energyband system in terms of lAg*- lAg transition appears to be more plausible. There is now ample evidence for the dipoleforbidden IAg*- state being responsible for either the
-
+
-
-
+
-
-
+-
N
-
-
-
-
~~~
~
(22) Das, P. K.; Becker, R. S., manuscript in preparation.
020
The Journal of Phy8ical Chemistry, Vol. 86, No. 6, 1982
Das and Becker
loweat-lying singlet transition or one of the low-lyingsinglet spectra of the retinal h o r n o l ~ g u e s , ~ Jparticularly ~J~ the transitions in many polyenes. While most of thie evidence lower members. The assignment of band system I1 is still a subject of controversy. The two alternative explanations came from h i g h - r e s o l u t i ~ n and ~ ~ ~two-photon *~~ excitaJ ~ ~ ~(1)assignment tionM*2s spectroscopy, and photophysical ~ t u d i e s , ' ~ *it~ ~ * ~ ~that are currently a ~ a i l a b l e ' ~include has been possible to observe the l$*- state directly in the as a cis bandi7930that owes its intensity, to a large extent, case of 2,lO-dimethyl~ndecapentane~~ to the distorted s-cis configuration of the retinal homoby relatively lowlogues with respect to the c647 bond and (2) assignment7+ resolution, one-photon absorption spectroscopy. The '4)by 6-s-trans conformers in terms of absorption ('B,* reason why we have been able to observe it in the present present in solution as a minor species. The present data study in the case of Clo and Cl2 aldehyde is probably a combination of the facts that (1) the spectra of these with linear polyenals clearly indicate that the band system polyenals are relatively well-resolved at 77 K and (2) the I1 owes ita origin to the presence of the p-ionylidene moiety in retinal homologues although the results are not necesl$*'$ transitions possess sufficiently large intensity because of symmetry breakdown in the polyene aldehydes sarily definitive in resolving the issue of its assignment. Emission Spectral Behavior. Nature of the Lowest (relative to polyenes) as well as intensity borrowing from Singlet State and Excitation Wavelength Dependence of the closely lying, strongly allowed lB,* lA, transition. Fluoroescence Quantum Yield. The lack of fluorescence A comparison of the absorption spectral properties of in the case of c6 and C8 aldehydes confirms our previous the linear polyenals under study with those of the retinal homologues7 is in order. Any difference in the spectral finding that the polyene derivatives,4~~J~ where the '(n,?r*) state is the lowest singlet state via absorption, do not behaviors of these two series of polyenals should presumfluoresce. This is explained by the fact the radiative rate ably arise from the fact the retinal homologues exist priconstant, k,, associated with the '(n,?r*) state is small in marily as distorted s-cis conformers10-12*28 with respect to magnitude (
