J. Phys. Chem. 1982, 86, 514-518
514
assumed to be significant only for molecules in the bc plane. Thus, the crystal data do, indeed, suggest an effective two-dimensional topology for exciton transport in DCBP, a topology which is probably responsible for the weak exciton emission and extremely short exciton lifetime. These properties will make difficult, but not impossible, future experiments which probe the dynamics of energy migration, band equilibration, and the nature of the density of states function in this interesting system.
Acknowledgment. We thank J. A. Mucha, of Bell
Laboratories, for initially stimulating our interest in the possibility of detecting triplet excitons in DCBP, A. J. van Strien and J. F. C. van Kooten, of the Huygens Laboratorium, R. U. Leiden, for assistance with the time-resolved experiments, B. M. Craven, of the Department of Crystallography, University of Pittsburgh, for expert advice and helpful discussion, and a referee for bringing ref 33 to our attention. This work was supported in its initial stages by the Department of Energy, under Contract No. DEAC02-76ER03435,and by the A. W. Mellon Foundation, through graduate fellowship support to S.B.S.
Dicyanoacetylene Cation. Laser-Induced Fluorescence and Photoelectron-Photon Coincidence Studies John P. Maler," Llubomlr Mlsev, and Frltr Thommen PhysNailsch-ChemlschesInstltut der Unlversttijt Basei, Cn-4056 Basel, Switzerland (Received: August 4, 198 1; I n Final Form: October 15, 198 1)
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Dicyanoacetylenecation has been studied in ita excited electronic states by means of laspinduced fluorescence and photoelectron-photon coincidence spectroscopies. The excitation spectrum of the A2n, X2n, transition yields the vibrational frequencies of the three fundamentals in the excited state, and these complement the ground cationic state values obtained from the emission spectrum. The coincidence measurements provide the fluorescence quantum yields and lifetimes of the excited cation in levels defined byJhe energy of the photoelectrons. True coincidences were also detected on production of the cation in the D2n, state. These data as well as the excitation and emission spectra are considered in conjunction with each other, and the decay behavior of such an excited open-shell cation under collision-freeconditions is discussed.
Introduction Dicyanoacetylene cation, N=C-C=C-C=N+, is the smallest aza-containing polyatomic organic cation for which an emission spectrum has been found.' This was excited by a low-energy (=30eV) electron impact, and the band system lying inJhe 580-720-nm wavelength region was assigned to the A2Zl 7X211, _transition,2which is, however, now reassigned as A211, X2n,(see Discussion). In the related smaller cations, HCN+, NCCN', and HCCCN+, radiative relaxation of their excited electronic states has not been d e t e ~ t e d . ~On the other hand, the emission spectra of the cations of cyanodiacetylene, HCCCCCN+ A211 X211, and of its methyl and ethyl d,eri~ativef3~ as well as of dicyanodiacetylene, NCCCCCCN+ A211, X211g,shave been obtained. In all of these cases the analysis of the emission spectra yielded vibrational frequencies of most of the totally SymmeJric fundamentah for the cations in their ground states, 2X,and provided a means to measye their lifetimes in the lowest vibrational level in their 2A excited states.
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(1) For reviews of this field, see: J. P. Maier, Chimia, 34,219 (1980);
J. P. Maier in 'Kinetics of Ion-Molecule Reactions", P. Ausloos, Ed., Plenum Press, New York, 1979. (2) J. P.Maier, 0. Marthaler, and F. Thommen, Chem. Phys. Lett., 60, 193 (1979). (3)M. Allan, E. Kloster-Jensen, and J. P. Maier, Chem. Phys., 17,11 (1976). (4) G. Bieri, E. Kloster-Jensen, S. Kvisle, J. P. Maier, and 0. Marthaler, J . Chem. SOC.,Faraday Trans. 2,76,676 (1980). (5)E. Kloster-Jensen, J. P. Maier, 0. Marthaler, and M. Mohraz, J. Chem. Phys., 71, 3125 (1979). 0022-3654/82/2086-05 14$01.25/0
The detection o,f the radiative decay of such cations allows also their 2A excited electronic states to be investigated. The spectroscopic details can be obtained by the technique of laser-induced fluorescence! and the decay behavior under collision-free conditions can be_followed quantitatively for the populated levels of the 2A state by determining their fluorescence quantum yields and lifetimes by means of the photoelectron-photon coincidence approach.' The results of these studies on dicyanoacetylene cation are reported in this article. The laser-induced excitation spectra were recorded for the gaseous dicyanoacetylene cations cooled to liquid-nitrogen temperature as well as at room temperature. These data yield the vibrational frequencjes of the three Z: fundamentals of the cation in the A2ngstate and thus complement the three X211, state frequencies obtained from the emission spectrum. The photoelectron-photon coincidence measurements revealed unexpected features apart from yielding- the fluorescence quantum yields and lifetimes for the A211g state. True coincidences were also detected for the D211, state, and the wavelength of the emitted photons could be ascertained. Thus, this cation provides an unusual example in that a higher excited electronic state also leads to a radiative decay to the ground state. (6) T.A. Miller and V. E. Bondybey,J.Chim. Phys. Phys.-Chim.Biol., 77, 696 (1980), review this area. (7) M. Bloch and D. W. Turner, Chem. Phys. Lett., 30,344 (1975); E. W.Schlag, R. Frey, B. Gotchev, W. B. Peatman, and H. Pollak, ibid., 51, 406 (1977).
0 1982 American Chemical Society
The Journal of Physical Chemistry, Vol. 86, No. 4, 1982 515
Spectroscopic Studies of Dicyanoacetylene Cation
Experimental Section Laser-Induced Fluorescence. The laser-induced excitation spectra were obtained according to the following overall scheme:
Hef/He
NX-CZC-CZN
xlx:
NEC-C:C-CIN@
R2nu
The energy of the helium metastables (23S,19.8 eV; 2lS, 20.6 eV) is sufficient to produce dicyanoacetylene cation in its ground and various excited states by Penning ionization, but these are subsequently deactivated by collisions with the helium gas (at =l torr) which is either at around room or liquid-nitrogen temperature! The apparative setup and the on-line data acquisition with a transient digitizer (Tektronix 7912 AD) and microcomputer (LSI 11/03) have already been described.8 The photon excitation is by means of a nitrogen pumped dye laser, and two dye solutions were used to cover the 510-600-nm egion. The respective portions of the spectra were normaized by recording common bands in the overlapping region. The wavelength scale was calibrated by using the band heads of the C2 Swan transitions apparent in the spectra. The output bandwidth of the dye laser was 0.02 nm. Photoelectron-Photon Coincidences. The principle of these measurements is based on defining the internal energy of the cation by the energy of the ejected e€ectron following photoionization with He&) or Ne(1) radiation. Any emitted photons are sampled in delayed coincidence with the electrons, e.g.
1
1 NSC-CSC-CIN
0
x- 2 nu
True coincidences are evidenced by a decay curve superimposed on a uniform background due to random coincidences (cf. Figure 3). The experimental details have also been discussed earlier; in the present measurements the energy band-pass for the photoelectrons was 80 or 40 meV, using a He(1a) or Ne(1) source, respectively. The cascade-free lifetimes, r(u3, are extracted from the decay part of the coincidence curve by a weighted leastsquares fit. The fluorescence quantum yield, c$~(u')is obtained via the relationship N T / N e= h ( X ) $&?. The rate of true coincidences, NT, is inferred from the area under the decay curve, and the rate of detection of true electrons, Ne, is directly measured (and corrected if necessary for stray electrons). The overall collection efficiency of photons, f h v ( X ) , has been absolutely determined as a function of X as de~cribed.~The errors of the reported data were evaluated from the coincidence statistics and from the reproducibility of the values. In all of the measurements, the pressure of the dicyanoacetylene gas-in the ionization region was C$F(A 0'1, because the majority of true electrons counted in the coincidence measurements correspond to the formation of the cations in the ?Dstate. This would then also be inconsistent with the k,(u? rate increase. The determined C$&? values show however that the radiative decay is a minor relaxation channel as the nonradiative decay begins to dominate. The k,(v? rate constants given in Table I11 were evaluated by assyming the same k,(v? value as for the lowest level of the A2ngstate. l'he nonraiiative rate determined is presumed to be the 2X* internal conversion. In fact, if the k&? 2A* values listed in Table 111are logarithmically pl_otted against the excess internal energy with respect to the Az% Oo level, a monotonous increase is apparent. Because of the error limits of the knr(v?values, it is not clear whether the dependence is strictly linear. A linear or exponential increase in the k,(v? rate with excess energy is usually associated
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J. Phys. Chem. 1982, 86,518-524
with a decay of a species in the statistical limit.16 The density of the 2x state lev& is estimated to be on the order of l@-lO'O/cm-l at the 2A state energies, 13.9-15.3 eV, so the decay is irreversible. It is a p m e d that a relaxation mechanism similar to that of the D2n, state is followed on formation of dicyanoacetylene cation in the B and C states. Again, in the emission spectrum,2there is no evidence of bands from the initially populated levels (eg., 3) to the X2n, Oo level which are expected because of the Franck-Condo? shjpe of the photpelectron bands (Figure 2). Thus,the %, 2C 2A* 2X* decay pathway is suggested, causing a shift in the emission to wavelengths X > 550 nm. It is also interesting to no-te that the B2Zi state can be populated directly from the X211ustate in the laser-induced fluorescence experiment. There are unidentified broad
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(16) K. F. Freed, Top. Appl. Phys., 15,23 (1976); P. Avouris, W. M. Gelbart, and M. A. El-Sayed, Chem. Reu., 77, 793 (1977).
bands in the excitation spectrum (Figure l),e.g., around 17 000,17 600, and 19 000 cm-', which may b e j u e to the excitation of the various vibronic levels in the B2Z: state. Their broad nature would subscribe to the notion that they are rapidly-depleted -by nonradiative decay. In other words, the A, B, and C states are considerably mixed by vibronic coupling. In contrast, the D2n, state is not accessible from the cationic ground state in a dipole transition by the laser excitation. Thus, these studies on dicyanoacetylene cation illustrate the complementary nature of the two techniques which, on the one hand, rely on the photoionization process with the molecular species or, on the other hand, excite the already generated cations in an absorption process. Acknowledgment. This work is part of project no. 2.2622-0.80 of the Schweizerischer Nationalfonds zur Forderung der wissenschaftlichen Forschung. Ciba-Geigy SA, Sandoz SA, and F. Hoffman-La Roche & Cie SA, Basel, are also thanked for financial support.
Electron Spin Resonance Spectra of Vanadyl Acetylacetonate In +Alkanes. Rotatlonal Relaxation Marco Patron,+ Danlel Klvelson;
A Study of
and Robert N. Schwartd
Depa~mentof Chemlstry, Unlverslty of Callfornie at Los Angeles, Los Angeles, California 90024 (Received: August 7, 1981: In Final Form: October 16, 1981)
The ESR spectrum of vanadyl acetylacetonate has been studied as a function of temperature in a number of n-alkanes. The analysis has been carried out in order to obtain information concerning the rotational correlation spectrum J(o).The rotational correlation time T = J(0)has been studied in terms of a quasi-hydrodynamic model, and in addition non-Lorentzian behavior of J(w) has been considered.
Introduction The ESR spectra of dilute solutions of the free radical vanadyl acetylacetonate (VOAA) in liquids has been extensively studied.'* This free radical has an electron spin (S = 1/2) which interacts with the vanadyl nucleus which has spin I = 7/2; therefore, the ESR spectrum consists of eight lines, each corresponding to a nuclear quantum number M: M = I,I - 1, ...,-I. The line width, AHM,of each of these Lorentzian hyperfine lines can be described by an expansion in M: l9' A H M = a + O M + yw + 61M3 + ... (1) The coefficients a,p, y, and 6 are functions of molecular magnetic parameters, the applied dc magnetic field, and various correlation times; in particular, /3 and y are dependent upon molecular parameters which can be independently determined from solid-state measurements, and upon a rotational correlation time, 7. We shall assume that the molecular parameters are independent of solvent and of temperature. The rotational correlation time 7 can be well approximated by the quasi-hydrodynamic relation: Universidad Autonoma Metropolitana-Iztapalapa Phys. Dept., Iztapalapa, Mexico 13, D. F. *Hughes Research Laboratories, 3011 Malibu Canyon Rd., Malibu, CA 90265. 0022-3654/82/2086-05 18$0 1.25/ 0
7
= (Vpf&.ick/kB)(v/T)C
(2)
where V, is the volume of VOAA, 9 the coefficient of shear viscosity, kB the Boltzmann constant, T the absolute temperature, and Cfdd a dimensionless friction constant. The factor fatickis defined as the dimensionless friction coefficient in the hydrodynamic limit (where the solvent is taken to be a continuous, homogeneous fluid) provided stick boundary conditions apply; in this hydrodynamic case, it follows that C$fk = 1. The hydrodynamic factor fstick was determined for ellipsoids by Perrid and for arbitrary shapes by Youngren and A c r i v ~ s . ~ For slip boundary conditions in the hydrodynamic limit, Cb'z = 0 for spheres, C $! = 1 for a very long, thin prolate ellipsoid rotating about a short axis, and, for various intermediate (1) Wilson, R.; Kivelson, D. J. Chem. Phys. 1966, 44, 154. (2) Kivelson, D.;Lee, S. J. Chem. Phys. 1964, 41, 1896. (3) Hwang, J.; Plachy, W. Z.; Kivelson, D. J. Chem. Phys. 1973, 58, 1753. (4) Kowert, B.; Kivelson, D. J. Chem. Phys. 1976, 64, 5206. (5) Hoel, D.;Kivelson, D. J. Chem. Phys. 1975, 62, 4535. (6) Cambell, R. F.; Freed, J. H. J. Phys. Chem. 1980, 84,2668. (7) Kivelson, D. J. Chem. Phys. 1960, 33, 1094. Kivelson, D. In 'Electron Spin Relaxation in Liquids"; Muus, L.T., Atkins, P. W., Eds.; Plenum Press: New York, 1972. (8) Perrin, F.J. Phys. Radium 1934, 5, 497. (9) Youngren, K.; Acrivos, A. J. Chem. Phys. 1975, 63, 3846.
0 1982 American Chemical Society
