J. Phys. Chem. 1983, 87, 45-49
45
Determination of Molecular Electron Affinities Using the Electron Capture Detector in the Pulse Sampling Mode at Steady State E. C. M. Chen University of Houston at Clear Lake Clty, Houston, Texas 77058
and W. E. Wentworth’ Department of Chemistry, Universlty of Houston, Houston, Texas 77004 (Received March 78, 1982; I n Final Form: August 19, 7982)
Thermal electron attachment to several molecules has been studied by using the electron capture detector operated in the pulse sampling mode at steady state. For molecules where electron detachment is the predominant process, the following ”absolute” electron affinities have been determined NO, 0.1 f 0.1 eV; 02,0.46 f 0.05 eV; CS2, 0.6 f 0.1 eV; benzophenone, 0.64 f 0.05 eV; nitromethane, 0.45 f 0.05 eV; biacetyl, 0.63 f 0.05 eV. For molecules where recombination is predominant,then lower limits to the electron affinity can be obtained from ECD data. The electron affinities of nitrobenzene and naphthoquinone are greater than 0.8 eV while the electron affinities of SFs and COS are greater than 0.7 and 0.4 eV, respectively. From dissociative capture of nitromethane the electron affinity of NOz is found to be 2.11 f 0.2 eV. These electron affmities compare favorably with the available literature values.
Introduction The electron affinity (EA) is a fundamental molecular property with important applications in chemistry, physics, and biology. It is defined as the energy difference between the neutral and the negative ion in their most stable state, i.e. the energy change for the reaction k
AB
+ e- 2ABk-1
(1)
at 0 K. The electrons, e-, have a thermal distribution and AB, for the purposes of this article, is a molecule. Although gaseous molecular negative ions were first observed around 1900 and have been studied extensively since then, the first reliable electron affinities were not obtained until the early 1960’s. Phelps and Pack’ reported a value of 0.46 f 0.02 eV for the electron affinity of O2 which was obtained by measuring the temperature deIn late 1961, pendence of the rate constants, kl and kl. Kraus, Muller-Duysing, and Neuert2 found the electron affinity of SOz to be between 1.0 and 1.12 eV and placed the electron affinity of CSz between that of NHz and SO2. Earlier, Henglein and Muccini3established that the electron affinity of nitrobenzene was less than that of SO2. This latter data were obtained from thermal charge transfer experiments (TCT). In 1962, Wentworth and Becker4related the equilibrium constant for reaction 1 to the response of the electron capture detector and obtained electron affinities for several aromatic hydrocarbon from relative responses at a single temperature. It was also postulated that “absolute” electron affinities could be obtained from the temperature dependence of the electron capture detector response. Wentworth, Chen, and Lovelock5carried out these measurements and, using a kinetic model for the reactions taking place in the ECD, obtained electron affinities for several molecules. The term “absolute” means that the electron affinity is obtained from experimental measure(1) Phelps, A. V.; Pack, J. L. Phys. Reu Lett. 1961, 16, 111. (2) Kraus, K.; Muller-Duysing,W.; Neuert, H. 2. Naturjorsch. A 1961, 16, 1385. (3) Henglein, A.; Muccini, G. A. J . Chem. Phys. 1959, 39,1426. (4) Wentworth, W. E.; Becker, R. S. J . Am. Chem. SOC.1962,84,4263. (5) Wentworth, W. E.; Chen, E. C. M.; Lovelock, J. E. J . Phys. Chem. 1966, 70, 445.
ments and fundamental constants, which in this case is only the gas constant. During the middle sixties, Page and co-workers6perfected another technique for measuring the temperature dependence of the equilibrium constant, the magnetron surface ionization method. It is best suited for the determination of values above 1.0 eV while the ECD is limited to values less than 0.8 eV so that the two techniques are complementary. During the seventies, with the advent of lasers, other absolute methods were perfected. These are based upon the reaction AB- + hv AB e(2) The energy of this reaction has been obtained by measurement of the threshold for photodetachment (PD) and by measurement of the energy of the detached electrons, photoelectron spectroscopy (PES). The most precise values of the electron affinities of NO, 02,and NOz have been determined by photoelectron spectros~opy.~-~ A number of relative electron affinities were determined during the seventies by using thermal charge transfer (TCT), endothermic charge transfer (ECT), and alkali metal beam experiments (AMB) in the gas phase.’*21
-
+
(6) Page, F. M.; Goode, G. C. “Negative Ions and the Magnetron”; Wilev-Interscience: New York. 1969. (7j Celotta, R. J.; Bennett, R. A.; Hall, J. L.; Siegel, M. W.; Levine, J. Phys. Reu. A 1972, 6, 631. (8) Siegel, M. W.; Celotta, R. J.; Hall, J. L.; Bennett, R. A. Phys. Reu. A 1972, 6; 607. (9) Herbst, E.; Patterson, T. A.; Lineberger, W. C. J . Chem. Phys. 1974, 61, 1301. (10) Berkowitz, J.; Chupka, W. A.; Gutman, D. J . Chem. Phys. 1971, 55, 2733. (11) Hughes, B. M.; Lifshitz, C.; Tiernan, T. 0. J . Chem. Phys. 1973, 59, 3162. (12) Tiernan, T. 0.; Clow, R. P. Adu. Mass Spectrom. 1974, 6 , 295. (13) Compton, R. N.; Reinhardt, P. W.; Cooper, C. D. J. Chem. Phys. 1975, 63,3821. (14) Disert, H.; Lachmann, K. HMI-B198; Hahn-Meitner Institute for Kernforschung: Berlin, 1975; p 34. (15) Compton, R. N.; Reinhardt, P. W.; Copper, C. D. J. Chem. Phys. 1978,68,4360. (16) Lifshitz, C.;Tiernan, T. 0.; Hughes, B. M. J . Chem. Phys. 1973, 59, 3182. (17) Compton, R. N.; Cooper, C. D. J . Chem. Phys. 1973, 59,4140. (18) Compton, R. N.; Reinhardt, P. W.; Copper, C. D. J . Chem. Phys.
--.
1978. - - - - ,68. 2023. ~ -
~
-
(19) Refaey, K. M. A,; Franklin, J. L. Int. J . Mass Spectrom. Ion Phys. 1978, 26, 125. (20) Leffert, C. B.; Tang, S. Y.; Rothe, E. W.; Cheng, T. C. J. Chem. Phys. 1974, 61, 4929.
0022-3654/63/2087-0045$01.50/00 1983 American Chemical Society
46
The Journal of Physical Chemistry, Vol. 87,
Relative electron affinities can also be obtained from the measurement of half-wave reduction potentials in aprotic solvents. Absolute values can be obtained by empirically relating the solution values to measured gas-phase values.22 Lifetimes of negative ionsz3have yielded gas-phase electron affinities by using theoretical consideration. At present, direct measurements of the electron affinities of about 125 molecules have been reported in the literature, more than half of which have been determined with thermodynamic procedures. However, very few comparisons have been made between these values and those obtained by other methods. For example, only the electron affinities of O2 and NO have been determined by more than one absolute method. In this article, precise values of the electron affinities of NO, 02,CS2, N20, NOz, CH3NOz,biacetyl, benzophenone, and the acetate radical, and lower limits to the electron affinities of SF,, COS, nitrobenzene, and naphthoquinone, as determined with the ECD, will be presented and compared with literature values. These are the only compounds with electron affinity values obtained from both the ECD and other techniques. The results for 02,24 N20,25SF6,26and the acetate radicalz7have been presented earlier but new literature values have been subsequently reported. Most of the ECD data in this paper was obtained in our laboratory as a part of graduate research project^^^-^^ but one set of Oz data,32the NO2 data,33and the benzophenone data34 were taken from the literature. Experimental Section The basic experimental procedures have been described in the l i t ~ r a t u r e .The ~ , ~NO, ~ 02,and benzophenone data were obtained with a Ni-63 concentric electrode detector while the rest of the data were obtained with a tritium parallel plate detector. The carrier gas was Ar-10% CH4 at a flow rate of 150 mL min-I. The data were taken at steady-state conditions. Two procedures were used to obtain the data as a function of temperature. In the discrete temperature mode, the temperature is stabilized and the response measured as a function of concentration. The limiting slope gives the capture coefficient. In the temperature programmed mode, the detector is raised to the maximum value and samples injected as the temperature drops. The fractional capture generally is kept low. The temperature was measured with a thermometer inserted in the heating block directly adjacent to the ECD. Kinetic Model for the ECD The reaction sequence in the kinetic model for the ECD (21) Rains, L. J.; Moore, H. W.; McIver, R. T. J . Chem. Phys. 1978, 60,3709. (22) Chen, E. C. M.; Wentworth, W. E. J . Chromatogr. 1981,217,151. (23) Christophorou, L. G. Adu. Electron. Electron Phys. 1978,46, 55. (24) Chen, E. C. M.; Wentworth, W. E.; Ayala, J. A. J . Chromatogr. 1980, 188, 89. (25) Wentworth, W. E.; Freeman, R. R.; Chen, E. C. M. J . Chem. Phys. 1971,55, 2075. (26) Chen, E. C. M.; Wentworth, W. E.; George, R. D. J. Chem. Phys. 1967.49. 1973. (27) Wentworth, W. E.; Steelhammer, J. C.; Chen, E. C. M. J . Phys. Chem. 1968, 72, 2671. (28) Freeman, R. R., Doctoral Dissertation, University of Houston, Houston TX, 1971. (29) Steelhammer, J. C., Master’s Thesis, University of Houston, Houston TX, 1969. (30) Chen, E. C. M., Doctoral Dessertation, University of Houston, Houston TX, 1966. (31) Chen, E. C. M., unpublished data. (32) Van de Wiel, H. J.; Tommassen, H. J . Chromatogr. 1972, 71, 1. (33) Morrison, M. E.; Corcoran, W. H. Anal. Chem. 1967, 39, 255. (34) Vessman, J.; Hartvig, P. Acta Pharm. Suec. 1972, 9, 463. ~
.-
Chen and Wentworth
No. 1, 1983
is summarized in reactions 3-10, where AB represents any
p
-+
+ (Ar-10% CHI) e- + @
’$8
e-
e-
kD’
+ AB e- + AB
ABAB-
+ p*
(3)
neutrals
(4)
+ B-
(5)
‘12
A
k,
AB-
(6)
+ eA + B-
(7)
AB
-
+@ kN21 B- + @
AB-
@
h i ’
neutrals
neutrals
(8) (9) (10)
polyatomic molecule capable of capturing or attaching an electron and where p* designates the p particle with reduced energy as a result of the ion pair formation. Generally p* contains sufficient energy to cause subsequent ion pairs since each ion pair formation consumes about 40 eV and each particle has an energy in the keV or MeV range. Direct dissociative capture occurs via reaction 5 and is typically the mechanism for attachment to aliphatic halides. For the compounds in this study we assume that electron attachment occurs via reaction 6 and the negative ion AB- can then undergo reaction via reactions 7 , 8, or 9.
Differential rate expressions can be written for the production of free electrons, e-, and the molecular negative ion, AB-. The solution to these differential equations is simplified by using steady-state conditions and this is realized at long pulse intervals of about 1000-2000 ps. The solution has been obtained by assuming constant positive ion concentration (5) and also a variable positive ion c ~ n c e n t r a t i o n .These ~ ~ solutions both lead to the same result if the concentration of the capturing species, AB, is sufficiently low to have less than 15-20% capture of electrons. The response at steady state under this restriction can be expressed as
where [e-] and [b] are the electron concentrations in the presence and absence of AB, and [@I is the positive ion concentration. K‘ is defined as the electron capture coefficient. From this expression, the value of K’ depends upon the relative values of kl, k z , and kN1’[@]. Three kinetic mechanisms or temperature regions have been defined for the combinations of these rate constants that have been observed experimentally. These are designed cy, p, and y and can be easily identified from a plot of In K’PiZvs. 1/ T.35 If there is a positive slope with an intercept in the range of 10-16, then there is an a region where k-, 1 kN1’[@]and kz = 0. When the statistical mechanical expression is used for K,, In K’TJ/2 = In A/2 + In (kNl’/kD’) + EA/kT (12) where A is a combination of fundamental constants. From (35) Wentworth, W. E.; Chen, E. C. M. J. Chromatogr. 1979, 186,99.
The Journal of Physical Chemistry, Vol. 87, No. 1, 7983 47
Determination of Molecular Electron Affinities
TABLE I: ECD and Literature Values of Electron Affinities EA, eV molecule o r radical
ECD
lit. values A. Diatomics
NO
0.1
f
0.1 (28)
0
0.5
i
0.1 (32)
2
0.46
i
0.05 (28)
NO2
2.11
I
0.2” (28, 29)
CH,COO
3.36
I 0.05“
0.028 (rate constants, 41) 0.024 i. 0.01, -0.008 (PES, 8) 0.43 i 0.01 (rate constants, 42) 0.46 i 0.02 (rate constants, 1) 0.440 i 0.008 (PES, 7 )
B. Radicals 2.36 i 0.10 (PD, 9 ) 3.1 0.05 (PD, 4 3 ) 3.30 (EI, 38) 3.39 (TCT, 39)
*
(27)
C. Polyatomics 0.6
f
0.1 (31)
*
>0.4 0.1 (31) 0.27 0.2” (25) 0.45 I 0.05 (28) >0.8 0.2 (30) >0.7 f 0.2 (26)
*
a
C,H,O, benzophenone naphthoquinone Dissociative capture.
0.63 f 0.05 (29) 0.64 f 0.1 (23, 34) >0.8 f 0.2
a graph of In K P I 2 vs. 1 / T the EA is found from the slope. In principle kNl’/kD’ can be calculated from the intercept although frequently there is considerable error due to the extrapolation to 1 / T = 0. This ratio could vary due to variations in kNl’ but also kD’ can vary due to the experimental conditions. In particular kD’ will include other electron loss processes such as attachment to impurities in the carrier gas or gas chromatographic system. For this reason kD’ should be determined immediately at the time of the electron attachment study. If the slope is small or zero and the intercept is in the 28-35 region, then there is a fl region where generally kN1’[@] >> kl > k,. The fl region will always occur at a lower temperature than the a region since ItN1’[@] is essentially temperature independent. The temperature dependence for the capture coefficient in a In K’TJ12vs. 1/ T graph is given by In K’TJ12 = In (kl/2kD’[@])
+ y2 In T
(13)
If there are data only in the fl region, a lower limit to the electron affinity can be estimated from the slope of the line from the highest temperature data to an average value for the intercept. At high temperatures the molecular negative ion, AB-, may undergo dissociation via reaction 8 and the rate constants are in the order It-, > I t 2 > I t N l ’ [ e ] In this case we have the y region and this will show a negative slope in a graph of In K’P/* vs. 1 / T and the intercept will range from 28 to 35. In certain cases, which have been designated by mechanism IV,36the activation (36)Wentworth, W. E.; Steelhammer, J. C. ‘Radiation Chemistry”, American Chemical Society: Washington, DC, 1968; Chapter 4. Adv. Chem. Ser. No. 82.
0.50 ?: 0.2 (ECT, 11) 1.0 i 0.2 (AMB, 1 3 ) 0.62 I 0.2 (AMB, 1 4 ) 0.75 Q EA Q 1.0 (ECT, 2 ) 0.46 ? 0.2 (AMB, 1 3 ) 0.22 I 0.1 (Cow, 44) 0.6 i 0.2 (ECT, 1 2 ) 0.44 i 0.2 (AMB, 1 5 ) 0.7 < EA < 1.10 (TCT, ECT, 3, 1 6 ) 0.54 I 0.2 (AMB, 1 7 ) 0.46 * 0.2 (AMB, 1 8 ) 0.6 t 0.1 (TCT, 1 6 ) 0.75 i 0 . 1 (TCT, 20) 1.05 (ECT. 1 9 ) 1.49 {magnetron,. 61, >0.41 (lifetime, 23) 1.10 2 EA; 1.59 < EA (TCT, 21) 1.86 i 0.3 (TCT, 21)
I
;bpr-
I
20
x Y
215
1
2 r-l
3 K-’]
Figure 1. In KT3” vs. l / T f o r O2 and NO.
energy is given by E* = DA-B - EAB, so that the electron affinity of the dissociating radical B can be determined if the bond dissociation energy DA-B is known. This is not an absolute method but has been applied to the acetate radical from acetic anhydride and NOzfrom nitromethane. In other cases of dissociative capture, semiempirical potential energy curves for the neutral and the negative ion of the Morse form can be used to obtain approximate molecular electron affinities.
Results Nitric Oxide and Oxygen. The ECD data for O2and NO are shown in Figure 1where the CY and 6 regions are apparent. The electron affinities obtained from the CY region are shown in Table IA. The good agreement of the O2results is especially important because there are two ECD values obtained from different laboratories and there are two other absolute estimates. The differences in the p region for the two sets of oxygen data are due to different values of ItD’[@]. The line drawn through the data of Tommassen and Van de Wie132has a slope corresponding
48
The Journal of Physlcal Chemlstry, Vol. 87, No. 1, 1983
I
30 -
\
F n " u --
Chen and Wentworth
1
\
\ 1 .
25-
1 .
--
-Y
f
__
.-_
-20-
15
I
5
1
,-I
21,"-3K-lI 3
14
15b"
I
Flgure 2. In KT3" vs. 1 / T for triatomic molecules.
to 0.5 eV. If this slope is modified to 0.46 eV, then the intercepts will be the same and equal to 11.5 indicating that the ratio kNl'/kD' = 1.0. Triatomic Molecules. The ECD data for some triatomic molecules are shown in Figure 2. The molecules CS2,COS, C02, and N20 all have 16 valence electrons and go from a linear neutral to a bent negative ion. However, the temperature dependences of these molecules are quite different. Both a and /3 regions are present for CS,; COS has only a /3 region; N 2 0 has only a y region but the response for COPis difficult to explain. The NO2 data are in the /3 region. From other studies it is known that the kl values for COS and NO2 are low. On the basis of the literature values of the electron affiiities, the a region for COS should appear at about 600 K whereas NO2 should not show an a region until 2000 K. The CS2data show unique fine structure in the region of 473 K. I t may be due to the excitation of bending vibrational modes. The ECD value for the electron affinity is in general agreement with the literature values which range from 0.5 to 1.0 eV. The activation energy for the N 2 0 data has been interpreted in terms of a barrier to the formation of the negative ion due to bending. It has been used to obtain a molecular electron affinity by using semiempirical potential energy curves for the negative ion and the neutral as a function of angle. The results as shown in Table IC agree well with the literature estimate, which could be coincidental but could also support the use of these empirical curve~.~b The mechanism for the response of C02 is unknown but it is probably not due to the formation of CO;. These data are included to show that C02has a slight electron capture detector response about lo00 times less than 02,and that the temperature dependence is small. Most likely this weak response is due to electron scattering. For moderately to strong electron capturing molecules the contribution of electron scattering is negligible. Polyatomic Molecules. The ECD data for some polyatomic molecules are shown in Figure 3. Biacetyl, nitromethane, and benzophenone have cy regions so that absolute electron affinities can be obtained. Nitrobenzene, SF,, and naphthoquinone only have /3 regions so that only lower limits to the electron affinity can be obtained. The data for nitromethane also extend into the dissociative y region at high temperatures. If this dissociation occurs along a single potential energy curve, then the electron affinity of NO2 can be obtained. The data for acetic anhydride from a previous study2' is shown for comparison, from which the EA of the acetate radical was found to be 3.30 f 0.2 eV. The good agreement for these ECD values and the literature values is shown in Table I. The only exception is the value for benzophenone. The lower limits for the
/'
'
L
I d r-1 l l p I 2i - l l
3
Flgure 3. In KT3" vs. 1/T for polyatomlc molecules.
electron affinities determined from the /3 region are valid only if there is no dissociative capture. For nitrobenzene and naphthoquinone, the dissociative processes are strongly endothermic. In the case of SF,, Siegel and Fite3' have studied the ECD response of SF, in an atmospheric pressure ionization source for a mass spectrometer and report that, at 493 K, the predominant ion is SF, so that the lower limit of 0.7 eV obtained from the /3 region should be valid. The literature values vary from 0.43 to 1.05 eV and there is even a magnetron value of 1.49 eV. The different values could be due to multiple negative ion states. For nitromethane, the a region intercept is clearly lower than the average and has been so drawn. The electron affiity is in very good agreement with the literature value, supporting the lower intercept. The ratio kNi/kD' is about 0.2. From the y region, the electron affinity of NO2 has been determined to be 2.11 f 0.2 eV, about 0.25 eV lower than the best photodetachment value. The electron affinity of the acetate radical was obtained from the acetic anhydride data in an analogous manner. The agreement with a literature value of 3.30 eV reported by Tsuda and Hamill%was noted at the time of original publication. It also agrees with a more recent value of 3.39 eV reported by Yamdgani and Kebarle.39 From the a region data, the electron affinity of benzophenone is 0.64 f 0.1 eV. This agrees well with a value obtained from data presented by L o v e l ~ c kand ~ ~with an estimate from half-wave reduction potentials.22Thermal charge transfer experiments2' place the electron affinity less than that of nitrobenzene but greater than that of the methoxy radical (1.59 eV). Since the electron affinity of nitrobenzene is 1.1 eV, these two results are not consistent. If the nitrobenzene limit is used, then the ECD result agrees with the TCT value. The electron affinities of several substituted benzophenones have been determined from ECD data and support the lower value. The electron affinity for biacetyl obtained from the a region is consistent with a lower limit obtained from (37) Siegel, M. W.; Fite, W. L. J. Phys.Chem. 1976,80, 2871. (38) Tsuda, S.; Hamill, W. H., Adu. Mass Spectrom. 1974, 6, 295. (39) Yamdgani, R.; Kebarle, P. J. Am. Chem. SOC.1973, 96, 5940. (40) Lovelock, J. E.Nature (London) 1961, 189, 729. (41) McFarland, M.; Dunkin, D. B.; Fehsenfeld, F. C.; Schmeltekopf, A. L.; Ferguson, E.E.J. Chem. Phys. 1976,56, 2358. (42) Pack, J. L.; Phelpe, A. V. J. Chem. Phys. 1966, 45,4316. (43) Warnek. P. Chem. Lett. 1969.3. 532. (44) Hooper,'D. G.; Wahl, A. C.; Wu, R. C:L.; Tiernan, T. 0.J. Chem. Phys. 1976,65,5404.
J. Phys. Chem. 1983, 87, 49-54
Figure 4. Electron affinities determined with the ECD vs. electron affinities from the literature: 1, NO: 2, N,O; 3, COS; 4, 02;5, CH,N02: 6, CdHB02; 7, CSp; 8, NO?:9, C2H302.
negative ion lifetimes.23 Since it is an absolute determination, the ECD value should be considered the “best” one.
Conclusions So that these results could be summarized, the electron affinity values obtained from the ECD are plotted against
49
the literature values in Figure 4. If there is a clearcut “best” value, this has been used. The line is drawn with unit slope and zero intercept and thus has no adjustable parameters. This figure illustrates the following conclusions: 1. If the ECD data has an a region, then “absolute” electron affinities can be obtained from the slope of a plot of In K P I 2 vs. 1/T. Examples are Oz, NO, CSz,biacetyl, benzophenone, and nitromethane.2 2. If the ECD data has a /3 region or a limited a region, and if dissociative capture does not occur, then a lower limit for the electron affinity can be obtained by using the average value for the a region intercept and the highest temperature data point to establish a slope. Examples are COS, SF6, NOz, nitrobenzene, and naphthoquinone. 3. If the ECD data has a y region and if E* = DA-BEAB, then the electron affinity of the radical can be obtained from the bond dissociation energy, DA+ Examples are the electron affinities of NOz from nitromethane data and the acetate radical from acetic anhydride data. Acknowledgment. The authors recognize the financial support of a Robert A. Welch Foundation Grant E095 and a grant of the University of Houston, Clear Lake City Summer Research.
Specific Hydrogen-Bonding Effects in the Photophysics of 6-Ape14’-carotenal. Static and Dynamic Aspects of Fluorescence and Triplet Yield Quenching’ P. K. Das’ and G. L. Hug Radiation Laboratory, University of Notre Dame, Notre Dame, Indiana 46556 (Received: Merch 26, 1982; I n Final Form: September 9, 1982)
Both fluorescence and triplet yields of P-apo-l4’-carotenal(Czzaldehyde) in cyclohexane at room temperature are quenched effectively by hydrogen bonding with alcohols. With 1,1,1,3,3,3-hexafluoro-2-propanol (HFIP) and 2,2,2-trifluoroethanol(TFE), the quenching interaction is separable into a static component arising from ground-statecomplexation and a dynamic component arising from hydrogen-bond formation in the singlet excited state; the contribution from the latter is found to be as important as that from the former. The Stern-Volmer constants (KsVF)for fluorescence quenching (dynamic) are found to be smaller than those (KsVT)for triplet yield quenching (dynamic). This observation concerningKsVF< KsvTis discussed in the light of schemes involving two kinetically distinguishable excited-singlet-state species-one responsible for fluorescence and the other acting as the predominant precursor of triplet.
Introduction @-Apo-l4’-carotenal (I), oftentimes called Cz2aldehyde,
I. C, aldehyde (all-trans)
11. all-trans-retinal
is a long-chain polyenal with the carbonyl oxygen atom as a specific site where hydrogen bonding can occur in both (1)The work described herein is supported by the Office of Basic Energy Sciences of the Deparment of Energy. This is Document No. NDRL-2338 from the Notre Dame Radiation Laboratory.
ground and excited states, affecting the photophysical properties of the polyene chromophore in a profound manner. This polyenal is related to all-trans-retinal (II), a molecule of photobiological interest, as its immediate higher homologue. The spectral and photophysical properties of Czzaldehyde under various conditions of solvent and temperature have been characterized by several previous low-resolution s t u d i e ~ . ~ - ~ One attractive feature of the photophysics of CzZaldehyde is that, in nonpolar hydrocarbon solvents at room temperature, this polyenal exhibits relatively pronounced 0.01 in 3-methyl~entane)~~ in contrast fluorescence (& to all-trans-retinal (II),whose room-temperature fluores347-nm laser stimulated cence is very weak5y6(4F=
-
(2) (a) Des, P.K.; Becker, R. S. J.Phys. Chem. 1978,82,2093-105. (b) Ibid. 1978,82, 2081-93. (3) Das, P. K.; Becker, R. S. J. Am. Chem. Sac. 1979, 101, 6348-53. (4) Becker, R. S.; Bensasson, R. V.; Lafferty, J.; Truscott, T. G.; Land, E. J. J. Chem. Sac., Faraday Trans. 2 1978, 74, 2246-55.
0022-3654/03/2007-0049$01.50/00 1983 American Chemical Society
