Excited state kinetics and stern-Volmer quenching plots in the photolysis of azoisopropane at 366 nm

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  • INTERNATIONAL JOURNAL OF CHEMICAL KINETICS, VOL. VII, 99-107 (1 975)

    Excited State Kinetics and Stern-Volmer Quenching Plots in the Photolysis of

    Azoisopropane at 366 nm

    G. 0. PRITCHARD and F. M. SERVED10 Department of Chemistry, [Jniversity o j California, lranta Barbara, California 93706

    Abstract

    The photochemistry of azoisopropane is reinvestigated at 366 nm over an extended pressure range by using n-butane as an added bath gas, and over a range of temperature from 217" to 180C. The Stern-Volmer type plot of the NS product quantum yield is inter- preted in terms of the decomposition of the vibrationally excited upper singlet and triplet states, with the onset of the dissociation of the vibrationally equilibrated triplet state as the temperature is increased. The energy barrier for the dissociation of the vibrationally equilibrated first triplet state is found to be 8.8 kcal/mole. Triplet sensitization experi- ments with biacetyl correlate with our observations, and it is suggested that the proposed mechanism is generally applicable to the photodissociation of acyclic azoalkanes at 366 nm, based on a comparison of our data with those of Wu and Rice on hexafluoroazomethane.

    Introduction

    The photochemistry of the azoalkanes and their use as free radical sources has interested photochemical kineticists for several decades. A comprehensive review was presented by Collier, Slater, and Calvert in 1968 [ 11, dealing with the nature of the excited states and the primary processes involved in the photolysis of acyclic azoalkanes. However, the role that multiplicity plays in these excited state reactions is not established [l-51. Luminescence studies cannot be used as a diagnostic tool since acyclic azoalkanes do not fluoresce or phosphoresce when irradiated [ 1 , 2, 6, 71. The simplest activated molecule reaction scheme is given by [ I , 3,71

    (a) A + h v - A " (1) A * - + N f + 2R (11) A * + A(M) -+ A + A(M) Although not explicitly stated, it has been generally assumed that the A * mole- cules represent the vibrationally excited upper singlet state (&*) or that decom- position (and perhaps deactivation) occurs from a highly vibrationally excited

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    @ 197.5 by John Wiley & Sons, Inc

  • 100 PRITCHARD AND SERVED10

    ground state molecule (So**) formed via a rapid internal conversion (IC). How- ever, Calvert and Steel and their respective coworkers conclude that the experi- mental evidence is best explained by assuming that the A* molecules in reactions (I) and (11) are in a vibrationally excited triplet state, formed by a rapid inter- system crossing (chat" ISC) from the initial photochemically excited singlet state. The reaction scheme leads to the traditional Stern-Volmer (S-V) type equation

    (A) w 2 - l = 1 + (kII/kI)[AI The published data on acyclic azoalkane photochemistry in the 366-nm region tend to obey the S-V expression very closely [2, 7, 9-14]. All of the data were obtained in the range of 0-200 torr.2 In 1968 Wu and Rice [l6] published an important reinvestigation of the photolysis of hexafluoroazomethane (HFAM). Their data, obtained a t room temperature over an extended pressure range (up to 700 torr), displayed distinct curvature in the S-V plot. I t was concluded that two dissociating states (with different rates) were involved, which were either different electronic states (singlet and triplet) or different configurations of the same electronic state [lob, 16, 171. Probably the most definitive data exist on hexafluoroacetone (HFA) [15, 181, and the curvature in the S-V plots is un- equivocally interpreted in terms of decomposition from both the S3 and TI states, in the excitation range 265 to 334 nm. Based on the discussion of Porter and Connelly [19] on the possible primary kinetic processes of photoexcited ketone and azoalkane molecules, we have recently further evaluated the behavior of S-V plots for photochemical decomposition versus collisional quenching in these sys- tems [ZO]. We have also presented data [21] on the photolysis of HFA in the presence of a triplet quencher, and conclude that both the vibrationally excited and vibrationally equilibrated S1 states decompose. The dissociation of the latter state only becomes important as the temperature is raised.

    Herein we describe a reinvestigation of the gas phase photolysis of azoisopro- pane (AIP) over a sufficient range of pressure to look for curvature in the S-V plots. In the absence of luminescence such plots provide an alternative method of determining the nature of the excited species, as well as the primary processes that they undergo [ZO, 211. The 366-nm photolysis of AIP has been studied previously [3, 111. Durham and Steacie [ l lb] detected some curvature in the S-V plot at about 10 torr and 100OC. The later and much more extensive data

    . .

    It should be noted that Steels group [4, 81 has observed fluoresence with cyclic azoalkanes, with dissociation from both the first singlet (S,) and triplet ( T I ) states.

    ZThis statement excludes the pioneering experiments of Weininger and Rice [loa] on azoethane, who went to almost 2 atmospheres. However, the data are rather scattered and will not be considered further. Careful linear extrapolations to zero pressure often result in inter- cepts that deviate from unity, and this has been the subject of discussion [7, lob, 1 la, 13b, 151. The various mechanistic possibilities need not be reiterated here, and our ensuing discussion will regard them as being not significant.

  • PHOTOLYSIS OF AZOISOPROPANE 101

    of Riem and Kutschke [ l l a ] were interpreted as linear plots with intercepts at zero concentration ((p~$")-- l greater than unity. Both studies [ 1 11 were conducted at less than 100 torr. Steel and coworkers [3] photolyzed AIP in both the gas and solution phases.

    Experimental and Results

    AIP was obtained chromatographically pure from Merck, Sharp, and Dohm of Canada, Limited. Azomethane was synthesized and purified by standard pro- cedures [22, 231, and n-butane was research grade obtained from Phillips Petro- leum Company. Biacetyl was supplied by Chemical Samples Company and purified by bulb-to-bulb distillation. The mercury and grease-free apparatus and experimental procedures are described elsewhere [2 1, 231. The decomposition of AIP was kept to less than 0.1% in all experiments. At pressures above 30 torr of AIP, n-butane was used to increase the pressure in the system. The 366-nm radiation was isolated with a Farrand interference filter, having a peak wavelength at 368 nm and a half-bandwidth of 5.5 nm, with a transmission of 41%. The absorbed intensity never exceeded 15% of the incident light. Blanks were run under all conditions to correct for any dark reactions. The correction was -5-10y0 at 160C. Azomethane was used as an actinometer with ( P N ~ taken as unity [24]. In runs were biacetyl was added as a triplet sensitizer an Oriel inter- ference filter with a peak wavelength at 436 nm, a half-bandwidth of 9 nm, and 52% transmission was used. No detectable direct photolysis of the AIP occurred with t.his filter.

    Detailed tabulations are given elsewhere [23]. I t is assumed that the quenching efliciencies of the AIP and n-CIHlo are the same. Wu and Rice [ 161 found the eficiencies of HFAM and CzFs to be virtually identical in HFAM photolysis. We used n-CrHlo, rather than (CHJ) zCHCH(CH3) 2, due to the former's greater volatility.

    In the photosensitized experiments with biacetyl, no Nz was detected at 27 or 12OoC, but some sensitized Nz product was detected at 160C (see Table I ) .

    The data on AIP photolysis are presented as a S-V plot in Figure 1.

    TABLE I. Biacetyl photosensitized dissociation of AIP at 436 nm and 160C.

    [AIP] x lo3 [Biacetyl] x lo3 [ C ~ H ~ ~ I x lo3 (P(N2)

    1.105 1.105 0.4

    1.105 1.105 6.65 0.4

    The N P yield was corrected for the dark reaction. Concentrations are in mole/l. The expressed quantum yield is based on the production of N z in the photolysis of AIP at 366 nm and 16OoC, see Figure 1. The values are therefore only relative.

  • 102 PRITCHARD AND SERVED10

    Discussion

    Our data on AIP, taken at room temperature over 250 torr, display curvature similar to that found by Wu and Rice [16] for HFAM at room temperature. Our quantum yields at lower pressures generally agree with the previous studies on AIP at less than 100 torr [ 111; we conclude that the data of Riem and Kutschke [ 1 la ] are best interpreted as having some curvature, with ( p ~ * O ) - l = 1 (see [ l la, Fig. 11).

    We propose the following basic mechanism to account for our results, where the superscripts have their usual significance [16, 20, 211.

    (a) A + h v 4 A* (1) A* -+ Nz + 2R (2) A * + M -+ lAo + M (3) A o 4 Nz + 2R (4) 1A* -+ 3A*

    (5) 3A* 4 Nz + 2R (6) 3A* + M -+ 3A0 + M ( 7 ) A0 3 3AO

    (8) 3A0 4 Nz + 2R (9) 3AO 4 A

    Hot ISC of A* molecules is competitive with dissociation, and 3A* mole- cules decompose, but at a slower rate. There is no basis to assume other than the strong collision assumption for deactivation in both the singlet and triplet mani- , folds. The mechanism accounts for the shape of the S-V plots for both AIP and HFAM at room temperature [16]. As the temperature is raised the p~~ values increase for AIP in Figure 1, due to the increasing dissociation of A * and 3A* molecules, and the eventual significant onset of the dissociation of molecules from the 3A0 state. The latter dissociation, being from a vibrationally equili- brated (Boltzmann) state which is not subject to pressure-dependent depopula- tion, is manifested as a pressure-independent leveling off in the S-V plot; compare HFA [15, 18].3 Since HFAM is known to photolyze in solution between 0 and 120C [25], the room temperature S-V plot would presumably level off at high enough pressures, in accord with some decomposition of molecules from the 3A0 state. AIP also photodecomposes slightly in solution at room temperature

    3Reaction (3), the decomposition of the Boltzmann singlet state, is not necessary to the mechanism, and its importance cannot be assessed until triplet quenching experiments are per- formed, cf. HFA [21]. A0 and 3Ao states are equivalent with regard to S-V behavior.

  • PHOTOLYSIS OF AZOISOPROPANE 103

    8.0 4 I I

    6.0

    4.0 - 160C 0

    2.0

    I .o

    -

    I I I I I I 1 I I I I I 2.0 4.0 6.0 8.0 10.0 12.0

    [MI x lo3 Figure 1 . Stern-Volmer plot for AIP at various temperatures. Points above [MI = 1.6 X lop3 mole/l. have n-butane added to achieve the indicated con- centrations. The vertical arrows show where average data are taken to con- struct Figure 2.

    with ( c p ~ ~ ~ ) - l = 40 [3]. The rates of the respective infinite pressure decomposi- tions will be largely dependent on the triplet energy barriers ET. Reaction (9) is included for completeness to allow for isomerization, and other nondissociative return processes to the ground state (e.g., IC via ISC). The fact that biacetyl triplet sensitizes the decomposition of AIP at 160C but not at lower temperature is in accord with our observations and mechanism and the triplet energies involved. Calvert and coworkers [ 11 estimate the triplet energy level for 1 , 1-azoisobutane a t 53 i 3 kcallmole, and presumably AIP is much the same. The energy of triplet biacetyl is about 55 kcal/mole [ 11. Similarly Calvert and coworkers [ 11 found that there was no detectable Nz formed when 1 , 1-azoisobutane was sensi- tized with biacetyl between 25 and 112C at 436 nm, even though the phospho- rescence quenching showed that energy transfer was complete. We can make the reasonable assumption that the energy barriers to triplet dissociation are very similar for the two azocompounds.

    We may now consider the alternative representations of the A * molecule in reaction (I). If it is a very hot ground state molecule formed by a rapid IC, the S-V plot would be linear, assuming that quenching (of either S1* or SO** molecules) occurs on a single collision. If the quenching of So** molecules was

    The photoisomerization of AIP is a triplet state reaction and Steel and coworkers [3]

    _______

    suggest a 37r, a* rather than a 3n, a* state.

  • 104 PRITCHARD AND SERVED10

    multistage, the curvature would be concave rather than convex [ 191. Theoreti- cal considerations and interpretations of the data on acyclic azoalkane photo- decomposition also effectively rule out the participation of the So** state [ I , 4, 5, lob].

    If the molecules in reaction (I) are "hot'' triplet state molecules, formed via a very rapid ISC, we have as the total mechanism reactions (4), (5), (6), (8 ) , and (9). The decomposition of the Boltzmann state would lead to a leveling off in the S-V curve, which only occurs at the highest temperatures. However this is not the case at low temperatures in the pressure range examined. The bi- acetyl sensitizing experiments further show that 3A0 molecules have insufficient thermal energy to decompose up to 120C which should result in linear S-V plots if only 3A* molecules were decomposing. This again is clearly not the case. Multistage deactivation of 3A * molecules would lead to a concave S-V plot, which is not observed.

    As we have seen in the complete mechanism, reaction (I) incorporates the simultaneous decomposition of both the vibrationally excited ' A * and 3A * states, reactions (1) and ( 5 ) , with the dissociation of the 3A0 state at higher temperatures. The mechanism also necessitates the postulate that the photochemically excited HFAM molecules are still not vibrationally equilibrated at almost one atmosphere pressure [16] in the presence of an efficient vibrational relaxer. Acetone and HFA, which are more energetic triplet sensitizers than biacetyl, sensitize the de- composition of azomethane and HFAM, respectively, at room temperature [6, 211, which occurs due to dissociation from the 3A* state. For example we observed [21] a 7570 reduction in the Nz product yield in going from 60 to 200 torr in the sensitized decomposition of HFAM by HFA at 27C. I t should be noted that the molecules formed initially in the direct photolysis contain approximately 6 kcal/mole additional excitation energy over those in the HFA sensitized experi- ment [26], so that their lifetime may well be short enough for decomposition to occur at 700 torr. I t is known for instance that no collisional relaxation of azo- methane occurs below one atmosphere IS].

    A steady-state treatment on reactions ( l ) , (2), (4), (5), and (6) leads [16] to an equation of the form

    (B) where a, b, and c are composite rate constants.

    CPN,-' = 1 + [(WI + b[MIZ)/(1 + c[Ml)l At sufficiently high pressures eq. (B) takes the form

    C P N ~ - ' = 1 + Q / C + (b/c)[Ml (C) Substituting the rate constants [16] we have

    (D) CPX,-' = 1 + (kg + k5) / k1 + (kz/kJ[Ml which is the simple S-V expression with an intercept > unity. The high-pressure decomposition is therefore due primarily to the dissociation of ' A * molecules,

  • PHOTOLYSIS OF AZOISOPROP...

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