Volume 38. ni&ber 3 _ &EMI&+P&CS LETTERS ._

15 hfasch‘1976.

EMISSION EXCITATION %PECTRA AND PHOrOCHlZMISTRY OF TKE 3820 A BAND SYSTEM OF PROPYNAL

Received 15 October 1475

The ffuorcscence and phosphorescence excitation spectra ofpropynaf i~t 0.60 torr with ;md without an added 100 torr He WXP measured up to an excess vibrational energy of 6000 cm-‘ above the zero-point level of Sl(*A”). Over the same ener,y range, the relative quantum yield of the photopIoduct CO ws determined as a function of the excess encr_g. From these data it is suggested that the singlet Sr is the photochcmi&y active state, and that the coifision-induced intersystem crossing process governs the photochemistry.

2. Experimental I. introduction

Propyna! (HCXK%Kl) is a particuiarty suitable system for investi;;ation of the dynamic processes of single vibronic levels of electronically excited states in poiyatomic molecules. Yardley and co-workers [l-3] have recently published a series of papers about this molecule dealing with time resotved measurements of luminescence decay rates and intensities of selected vibronic states of the first excited singlet S, (‘A”). in determining the important deactivation rates, they have been able to give a very comprehensive picture of energy dissipation in a potyatomic molecule in the gas phase.

Given this extensive knowledge of the photophys- ical properties of propynal we chose to investigate its photochemical behavior. By means of excitation spectroscopy and mass spcctrometry the fluorescence and phosphorescence excitation spectra tip to 6000

cm -’ above the vibrationfess level of S, have been measured as well as the excess energy dependence of the relative quantum yields of the photochemically produced CO. In the present communication we report on the resuits obtained at a propynal pressure of 0.60 torr with and without an added 100 torr He.

Propynal was synthesized according to a published procedure [43. The sampfe was thoroughly dried, using map&urn sulphate, and ~il~uurn distilled sever- aI times to eliminate any remaining impurities. After purification, the sample was stored under vacuum at 77 K. The mass spectrum of the substance revealed no peak, which could not be accounted for as a frag- ment of pure pfopynd itself.

The s~e~lrophoto~uor~meter employed for emis- sion and excitation spectra consists of a 450-W xenon arc excitation source, a Spex Model 1670 fl. 220 mm, f/4 mo~oe~romator, and a Spex Model 1702 fl- 750

mm, f/i’ analysing spectrometer equipped with a cooled RCA C3 1034 photomultiplier tube. The sig- nals are fed into a pulse preamplifier and a ratemeter which are interfaced to an Interdata Model 70 mini- computer. The processed data i.e. tie corrected emis- sion and excitation spectra were piotted on a Calcomp Model 565 recorder. The sample was excited in a T- shaped blackened Pyrex cell (diam. 5 cm) with Suprasil quartz windows. Phosphorescence excitation spectra at 0.6 torr of propynal (with and without 100 torr of helium) were monitoied at the O-O band of the phosphorescence. Owing to the iow intensity of the 0-O band the fluorescence excitation spectra were monitored at the 4: band (Cc3 stretch) of the fluores-

537

VoIumc 38, iiumbcr 3’ ). . . ( .I : . . CHEhfICAL PHiiiCS LETTERS' '. ' 15 March 1976 .- , . . . . _:.. - _.:. -. I

cencel To Cmprov&~the sip&to-noise ratio of-the ex~

.citation spek a’mtiitiscanning technique was em- _ windows and connected to a quadrupole tiass spec-

ployid; :’ . trometer B&ers QMG 3 I1 whose. injection system was heated to 150°C. The pressure in the reaction

. . Absorption specka in-the gas phase at 10 torr of

. ‘~piOpyr@ were measurqd ori a Caiy f 7 spectrometer.. vessel was controlled by an MKS Baratron Type 210

The photolysis of @rcjpynal: was performed in a heated (MKS !&uments, Mass., USA) pressure gauge_ ‘fhe

_ _ pyrex~cell of 3 cm diameter hnd 2 path kngth of 50 light source for photolysis was a high pressure mer- CU_~ lamp (Osram, HBO 200 W) used in conjunction

cm; The’cell was equipped with two Suprasil quartz with a hi& intensity Spex Model 1670 il. 220 mm,

~ 29

a

o-o

0 26OtKl 29OUO 31m 32am am0 3mxl 35

Fig. i..f‘orrecte:! phosphorescence excitation spectra (solid line) of propjm;tl monitoied at 24127 cm-’ and recorded over the fist absorption band at rook tempcmtore and (a) 0.60 torr,.@) 0.60 torr + 100 torr He. Sirperimposed on this spectrum is the

-a&orption spectrum (dotted Me) measured at 30 torr. ., -_

;Volume 38, number 3 CHEhIlCAL PHYSICS LETTERS 15 hlirch 1976

f/4 monochromator. The light jntensity was moni&d with a calibrated lE28 photomultiplier tube.

In order to measure the concentration of products

in the mass spectrometer, the prqduct peak in the spectrum must be corrected for contributions of the -mass spectral frasentation of propynal (nz/e = 54). The concetitration of the photoproduct CO (m/e = 28) was thus determined by

hZrr_ = hf8 - (hplii”)h;” , (1) where h is the relative peak height at a particular mass/charge ratio. The index f refers to values taken after the irradiation, whereas i indicates values mcas- ured before irradiation.

3. Results

The excitation spectrum of propynal at 0.60 torr monitored at the O-O band (24 I27 cm-‘) of the

phosphorescence is shown in fig. la*. Superimposed’ on this spectrum is the absorption spectrum at 10 torr. The spectral resolution of the excitation and absorp

tion spectrum is <IO a and ~2 A, respectively. The

phosphorescence excitation spectrum follows the absorption spectrum closely up to 29 115 cm-’ _ At

this point, which corresponds to an excess energy

Em%_ of 2952 cm-‘, the intensity of the excitation spectrum starts to collapse and virtually disappears at 30500 cm-l _ From these data, the phosphorescence quantum yield d, as a function of Eyib_ may be de- duced. The values 9, (F) displayed in fig. 4b were determined relative to 9, excited at 382 1 A (O-O

band of the absorption) using the relation

x 1 -exp[-2.303 E(3821)cxj

1 -exp[-2.303 e(X) cx] ’ (2)

where IP@) is the intensity of the phosphorescence

at the monitoring wavelength if excited at h and [,a) is the intensify of the incident light at X. c

denotes the concentration in M/P, x the cell length in cm and E the decadic molar extinction coefficient.

* The specirum remains ynchanged if the pressure of propy- nal is reduced to 0.20 torr.

-The value of +,(3821) = 0.12 was taken from the data

of Thayer tidyardley [2] extrapolated to a pressure of 0.60 torr.

As is shown in fig. lb She adhition bf 100 torr of

helium- to 0.60 torr propynal has a marked effect.

Under these conditions the excitation spectrum foi- lows the absorptioti spectrum up to about 30300- 30500 cm-’ and vanishes at around 38000 cm‘-‘. A comparison of the two excitation spectra (cf. fig. 2) indicates that below an excess energy df 3000 cm-‘, in the region where both spectra are-similar to the absorption spectrum, the addition of helium gas in-

creases the intensity by a factor of 1.8 + 0. l_ In the upper half of fig. 2 the ratio of the intensities of the IWO excitation spectra up to an excess energy of

7000 cm-’ is displayed, which shows the energy

region where the influence of the foreign gas is most effective.

The fluorescence excitation spectrum of 0.60 torr propynaI monitored at 24460 cm-’ is presented in

fig. 3. With respect to the absorption spectrum, the intensity of the excitation spectrum is dramatically

reduced in going to higher enerw. Evidently with

increasing vibrational excess energy a nonradiative

channel successfully competes with the fluorescence. Under the present conciitions resonance fluorescence was not found to be significant thus indicating that

vibrational relaxation within S, is faster than fluores-

cence. The fluorescence qvantum yield +&> versus

E v%_ shown in fig. 4a, was determined in a similar fashion as $,Q [cf. eq. (2)j with the Q~(383,i A) value derived from previous work [2]. The addition of 100 torr of helium diminishes the intensity of the O-O band of the fluorescence excitation spectrum 12 times with respect to 0.60 torr propynal.

The photolysis experiments of propynal at a pres- sure of 5 torr were performed at 100°C with an exci- tation band pass of =lO A. Since CO was the photo- product most easily detectable in the mass spectrum, its formation was taken as a measure of photodecom-

position. (Besides CO, acetylene was found, but only

in small amounts, and a polymer was observed on the celi walls.) Acetone at 16 torr, irradiated at 3130 a, was employed as an actinometer. Its quantum yield

oft20 fqrmation is given as $$3 130 A, 100°C) = 0.72 IS]. The corresponding value for propynal $&(3 !30) was thus found with (1) and the rel?tion

539

Voluine 38, number ,3_ - ‘.- : CN‘EMICAL PHYSIQ LEFERS_ *

15 hlarch 1976.

. . .

22 noco 2aoi30 29000 32ooo 33000 34wo 3:

Fig. 2. Comparison of the phosphoressence escitation spectra at 0.60 torr (lower curve) and 0.60 torr f 100 torr He (upper cuive). The curve on ihe top represents the relative phosphorescsnce enhancement due to 100 torr He versus excess energy.

. .

.I’ .

I 26000 Row ~2wua 29m 3oooa 3lcm 32000 33a30 3LGuO 3SaoO . Cllf’

:.Eig.-3..ci’drrected fluorescknck excitatkm SpeCtrum of propynal rno!i$red at 24460 cm”’ and recorded’oypr the fire -acnrntinn

band ?t room tehperatlire ancf0;60 torr. - 1 .- .,j&) ‘_ ‘:...’ .I .:._- ,_ .

-Voi&m& 38, &&er 3 : CHEMICAL PHYSICS LETTER.9 .-. -_ I$ March 1976 .

,_- .’ ::.

., Before going irito further details, it isjnformative to consider the fluorescence excitation sp(?ctrum at 9.60 torr. ,This spectrum demon&rates that Qf is strongly dependent on the vibrational level excited - even far below I& = 2900 c-m-l i a property al- read~~described bykardleyet al. [2,3]. Accordingly, a strong increase in the rate constants for internal conversion~k&(S~~S8) and intersystem crossing. kkC(S,-T,) with in&easing Ea. is responsible for this behavior. Without quantitative data at hand it is not.imrnediately obvious why a decrease in @f is not accompanied by a corresponding increase of q$,. Bxtrap&&g the datqof ref_ [3] to 0.60 torr shows that for the vibrationless state of the tirst excited singlet, f$O = 0.018 and internal conversion SLeSu and intersystem crossjng S1-T1 with @F: % q$!C z 0.5 dominate the deactivation of SJ . Since it was shown above- that 9, is roughly constant up to Ed_ = 2900 &I-‘, this observation together with the above data implies that k,,(E,,& and k,,C(E,,.) increase by about the same amount between the zero level of S, and the predissociation limit at 2952 cm-J.

pressed if collisions with He take place. Evidently; under these conditions; one or more pressure depen- dent deactivation paths successfully compete with the photochemical channel for Eti, between 2900

and 6000 cm-‘. In the S, state, intersystem crossing

and internal conversion can account for such an effect, while in TJ only vibrational relaxation within this state might be considered a candidate. (A pos-

sible radiationless iutersystem crossing route Tr(v*) *So is excluded since this process would lead to a reduction in fip, contrary to what is observed.) Thus it would Seem that a decision whether S1 or T1 is photochemical active is not possible. However, if

we consider the additional fact that the predissocia- tion limit of SJ(n,n*) reported by Brand et al. [7] agrees so well with our measurements of the CO production, we are inclined to believe that most of the photochemical activity for Evis. between 3000 and 6000 cm-’ is from S, and that the collision-in- duced k~JSp+TI) process governs this photo- chemistry.

In propynal, both internal conversion and intcr- sys tern crossing are greatly influenced by collisions. At the relatively low pressure of 0.60 torr the colli- sion induced rate constants kz and k& are already one order of magnitude larger than the corresponding collision free parameters [2,3]. The 12 fold decrease in fioo t upon addition of 100 torr of He signifies a corresponding 12 foId increase in the sum k$ + k&

from the zero-point vibrational level over that at 0.60 torr*. Moreover, the collision induced enhancement of $ demonstrates that k& is more pressure de- . . pendent than kg_ While at 0.60 torr $.C/kE = I [3], the presence of 100 torr He raises this ratio to z9, a value directly derived from the increase in 9,.

The phosphorescence excitation spectra (figs. la and I b), the relative production of CO (fig. 3c), and the arguments advanced above all point towards the conclusion that photodecomposition following exci- tation into the vib:ationaI levels 2l and- above is sup-

Acknowledgement

Support of this work by the Deutsche Forschungs- gemeinschaft (Hu 210/4) is greatly appreciated. We

sincerely thank Dr. J.V. Morris for his suggestions in writing the manuscript. Thanks are also due to Ms.

M. Mahaney, Drs. H.J. Haink and 0. Serafimov for

helpful discussions and to Mrs. 1. Henle for synthesis- ing propynal.

References

[ 11 C.A. Thayer and J.T. Yardley, J. Chem. Phys. 57 (1972) 3992.

[3] CA. Thayer and J-T. Yardley, J. Chem. Phys. 61 (1974) 2487.

* Vibkional relaxation within S1 is not appreciable with respect to IC and ISC even with 100 torr of He. The evi- denck for this is found in the similar vibr;ltional pattern

of the fluorescence exci‘tation spectra with and without the foreign gas. (Ef vibrational relqation were dominant, the excitation spectm~ should fqllow the absorption spec- tmm at !eaSt Up_tO Evib. =.2900 Cm-l-)

[3] C.A. Thayer, A.V. Pociusand J-T. Yar$ey, J. Chem. Phys. 62 (1975) 3712.

(41 J.C. Sauer, Org. Syn. Collection 4 (1953) 823.

[S] H.M. Frey and I.C. Vinall, I. Chem. Sot. A (1970) 3010.

[6] B.T. Connelly and GX Porter. Can. J. Chem. 36 (1958) 1640; W.A. Noyes Jr., G.D. Porter and J.E. Jolley, Chcm. Rev. 56 (1956) 49.

[7] J.C.D. Band, J.JJ. Callomon and J.K.G. Watson, Can. I. Phys. 39 (1961) 1.508.

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