Volume 72, number 1 CHEMICAL PHYSICS LETl-ERS 15 May 1980

CONSECUTIVE PROCESSES IN THE PHOTOLYSIS OF DIMETHYL+TETRAZINE MOLECULES

IN THE VAPOR PHASE *

Mark PACZKOWSKI, Roberta PIERCE, Amos B. SMITH III l and Robin M. HOCHSTRASSER Department of Chemzt~~, Laboratory for Research on the Structure of Matter and illon2ll Chemical Senses Center. ifnivernty of Pennsylr*ama. Phdadelphra. Pennsyivanm 19101. USA

Recewed 15 Aprd 1980

Experimental studies of the wriatlon of the fluorescence yield and photochemical product yield are presented for &methyl-s-tetrazine (DMT) ac 0.6 TOIT and 300 K. Results arc also reported for the variation of these yields upon adding argon buffer gas. The fluorescence yield decreases with increasing exntition eneq) (0 to 4200 cm-’ vibrational energy excess UI the fist evclted s&et state) but the decrease IS insufficient to account for the corresponding increase in product yield. Added pas (600 Torr argon) quenches the photochemistry but not the fluorescence. The results unply that the photo- rhssoaahon mvolves a bottleneck (B) in the non-radiative smglet decay. The molecules in B can be rehxcd by coll%ion~. relax to photostable DMT molecules wthout collisions, or undergo photodrssociatlon.

1. Introduction

The photochemistry and photophysrcs of s-tetrazme and its derrvatrves present some interesting challenges of a theoretical and experimental nature. The overall photoprocess resulting from singlet excitation generates sunple products, RCN and N2 from di-R-substituted

tetrazine. The relevant low-energy excited states of the tetrazines are moderately well understood in the Franck-Condon region [l-6]. One might therefore expect a reasonably sophistrcated understandmg of the dynamics of this system to develop.

The photochemical_drssociation of tetrazines was first studied in solution [7-91 and low-temperature sohds [5,6,8-101 and matrices [6-9,111. Earher gas- phase studies were carned out without the reahzation that the tetrazines were quite unstable photochemical- ly [l-4,12]. The pathway to photoproducts in the condensed phase is now known to be complex smce the reaction occurs in a two-photon process for certam denvatrves and solvents and monophotonically for

* This research was supported by the NSD and in part by the NSF/MRL program.

* Camdle and Henry Dreyfus Teacher-Scholar, 1978-1983; and NIH Cancer Development Award 1980-1985.

others [7-g]_ It IS apparent that the environment ef- fect on the photochemistry in this instance is cruci&

The first excited mr* state of tetrazme is reached by transitions from the ground state with visrble Iighr from 450-600 pm. The fluorescence hfetime for 57 1.5 nm excitation of the vapor when compared with the radiatke lifetime yields 1 fluorescence quantum

yield of 1.33%_ The fate of the other 98.7% of excited

molecules is photochemistry, recovery of the initial state or formation of triplets_ Because of the multi- photon nature of the photoprocess in the condensed phase, the partrtioning of the excitatron is expected to be dependent on light intensity. We have therefore COR- sidered it necessary to evaluate some molecular param- eters for tetrazines in cucumstances where the photo- chemical reactron occurs by the absorption ofonIy one photon. This IS the case for s-tetrazine in hexane at ambient temperature, and in the present paper we show that this is also the case for low-pressure dimethyl- s-tetrazme (DMT) vapor. An estimate [13] of the photochemical quantum yreld for dimethyl-s-tetrazine of 1 f 0.3 at 5 14.5 nm was the only information exist- mg on the non-radiative pathways for an isolated moIe- cule. In the present work we have measured both the relative fluorescence yield, the absolute photochemicaI

Volume 72, number 1 CHEMICAL PHYSICS LETTERS 15 May 1980

yield at one eucrtatton wavelength and the relatrve yreld of photochemrstry at a variety of excitatron energres in the vrsrble and ultravrolet. We conclude that the earlrer estimate for the photochemrcal yield IS too hrgh, so that assumptrons concerning the photophysics based on a near unit quantum yield must be re-evaluat- ed. Our results also suggest that the photochemrstry does not result drrectly from unimolecular drssocration of the opticahy prepared state

2. Experimental methods

2. I Preparahott of samples

3,6-dunethyl-s-tetrazme @MT) was synthesrzed ac- cordmg to the methods of Skonanetz and Kovdts [ 111 and purrtied by repeated vacuum drstdlatron.

2.2. Relahi-e jikorescetzce ql~atlhttn yrelds

Gas-phase excrtatron spectra for 3,6-dunethyl-s- tetrazme samples were recorded with a Hitachr Perkin- Elmer hlPF-4 spectrofluortmeter equrpped with a true excnation corrector. Typrcal spectral resolution was of the order of 6 nm. Optical densities for these measure- ments in a 1 cm cell were between 0.01 and 0.03. The fluorescence was studied for excrtatron wavelengths nngmg from 250-590 nm but onIy the propertres of the first excited state are explored m detad m this re- port_

23. Absolute and relahke pltotochemical quanhm yields

A 10 cm gas cell was constructed wrth an addrtronal srdearm and stopcock so that it could be evacuated (to =Z 10m3 Torr), isolated from the vacuum system and fd- led wrth DMT vapor through the stopcock from the srdearm. After eqmhbratron at ambrent temperatures, the stopcock to the srdearm was closed, rsolatmg the contents in the cell. This procedure was repeated for each data pomt. Samples contammg argon-DMT were prepared by first allowmg the DMT to fdl both the cell and manifold and then adding enough argon to obtam the desired ratio.

The relatrve rates of photolysis were obtained by comparison of the ultravrolet absorptron spectra (220- 350 nm regron) of the sample taken on a Car-y 14 spec-

6

trophotometer before and after irradration. The extinc- tton coefficient of 1970 l? mol-1 cm-l at 265 nm was measured by deter-muting the vapor pressure of DMT at 300 K to be 0.6 Torr. Various lines from a Coherent model 52 argon-krypton ton laser were used as excita- tion sources. Absolute excitatron mtensitres were mea- sured by standard potassium ferrioxalate actinometry usmg 1 ,IO-phenanthrohne and measurmg the AOD of the complex at 5 10 run. Relative intensrties of drf- ferent laser lures were measured on a Coherent model 2 10 power meter. The absolute photochemical quantum yield (ap) was measured at 488 MI (A? lure) where the absorptron spectrum IS drffuse. All other photo- chemical quantum yields were made relatrve to thrs point. All rates of photolysrs were corrected for the amount of light absorbed by the sample at the laser frequency (typrcally 4-20s) and the mtensity of the laser. The thoroughly mLxed Ar-DMT samples were treated in the same manner. The results for the relatrve fluorescence yrelds, photochemical quantum yrelds and deduced Internal conversion yields are m table 1.

The IA + lBsU transition of DMT is dominated by a progressi%r of the rmg-stretching mode 6a havmg an excrted-state frequency of 520 cm-l_ The vapor spec- trum origm 1s around 572 run so excitatrons in the region 565 to 573 nm involve the O-O band and low- frequency sequence transitrons. The photochemrcal sources at 520.8 run, 5 14.5 nm, 496.5 run and 488.0 nm corres

B ond to excrtatron of the transrtron regions

6azA, 6a0, 6as and a drffuse regron near 6ad respec- trvely.

3. Results and discussion

The fluorescence hfetune of dunethyl tetrazine was previously measured to be 6 ns [ 13 ] for excrtatron at 57 1.5 MI in the region of the O-O band of the lBsU + lA, electromc transitron. The hfetune at 568.2 nm, corresponding to one of the Kr+ lines used in the photolysis, was found to be 7 ns [IS]. The natural ra- diative hfetrme obtained from the integrated absorption in solution 1s 450 ns [ 161 so that the fluorescence quantum yreld for 568.2 nm excrtatron is estimated to be 0.016. The second column in table 1 shows the rela- trve quantum yields of fluorescence assuming a value of 0.016 at 568.2 run. The numbers in thrs column should not doffer from the absolute yields of fluorescence by

Volume 72, number 1 CHEMICAL PHYSICS LETTERS 15 May 1980

Table 1 Quantum yrelds of fluorescence (@PI, photochemistry (Qp) and return to the ground state (@RI for DMT in the vapor phase at 06Torr

hevc Urn0 Rel. @F X lo3 a) *P @R Rel. @P@P X IO4

457.9 l-5 0 22 0.78

476.5 25 0.33 0.67 8.3

482.5 1.3 0 35 0.65 4.9

488.0 2.0 0.27 e) 0.73 5.4 490 0 1.7 -

496.3 2.3 0.37 0.63 85 500 0 2.0 - -

510.0 2.5 - -

514.5 2.8 0.24 0.76 6.7 520.0 46 -

520.8 s-4 b) 0.15 0.85 8.1 530 0 4.8 - 540 0 7.0 -

550.0 12 - -

560 0 23 -

568.2 t6b) 0041 0.94 6.4 16 d) 0004 0.98 -

570 0 16 - -

5715 13 b-c) - -

572.0 14 - -

a) The values tn thus column are deduced from relatrve-yreld data urth 6 run bandwrdth elcitatton, and an assumed value 0FO.016 for the absolute yield at 568.2 nm.

b) Ref. [ 151 (from Uq= = TF/T~). c) Ref [ 13 ] (From OF = q=/rO). d) In the presence of 600 Torr of argon. e, The photochemtcal yield at 488 nm aas srgmficantly reduced as additional amounts of argon were added (up to a Factor of 10

at 100 Torr of argon).

more than 20% although it should be remembered that they represent averages over vibrational and rotational states mcorporated in the 6 nm bandwrdth of the ex- citation. By means of a tunable dye laser, we have found that the fluorescence hfetune does not change srgnificantly as the excrtation frequency is vaned a few run around the position of the Ar-Kr+ laser lines [ 15 ] _

Although there are osctllatrons m the manner the fluorescence yteld vanes with energy excess, especially rn the lower-energy region where the spectrum is not so congested, the general trend for the first 3500 cm-t of vrbrattonal energy excess IS that the yield drops as the excitation energy IS increased_ However the de- crease IS not nearly sufficrent to account for the mag-

nitude of the increase tn the photochemical yield. For

example, between 568.2 nm and 482.5 nm excitatron, aF changes by 0.015, representmg a factor of 12 de-

crease_ On the other hand the photochemical yield changed by 0.3 1, representing a factor of 8.5 increase_

These results indicate that the photochemicd reaction does not proceed in direct competitzorz with the fruo- rescence process.

As indicated m table 1, the effects of argon on the photochemical yield following excitation at 568.2 MI are significant. +p drops by a factor of about 10 for 600 Torr added gas whrle the Iifetune remains un- changed at its collision-free value of 7 ns. A detailed study of this pressure dependence IS in preparation

WI. The quantity +p, in table 1 is (1 - +F - ap) and

represents the overall yield of stable ground-state mofe- cules. In the condensed phase the triplet yield was found [IO] to be less than 104. For s-tetrazine vapor, the triplet yield was close to zero both in the presence and absence of collisions [ 121. Therefore we assume that the total triplet yield is zero at our precision in which case the measured aR refers to the overall internal conversion into the ground state. Thus the results in

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Volume 72. number 1 CHEMICAL PHYSICS LE-I-I-ERS 15 hlay 1980

table 1 indicate that non-radiative non-photochemlcal rela\atlon IS the dommant deexcltatron pathway m DhlT for e\cltatlons rangmg up to ca. 4300 cm-* v1- brational energy excess m the first esclted singlet state.

The results mdlcate, regardless of detailed assump- tions. that the three processes of fluorescence, photo- chemistry and non-photochemical relaxation do not result from parallel ummolecular decays of the optlcal- ly pumped levels On the other hand. the results are consistent with the occurrence of a non-radiative bot- tleneck (B) state (see fig. 1) from which the partltlonmg

into the non-radlatlve pathways represented by Qp and *RR occurs The quantum yield of formation of the

B-state IS not sensitive to excltatlon energy m the proposed model. It IS the partltmrziilg mto @RR and a, that depends on elcltatlon energy with ap = 1 - +R for all levels. The fluorescence hfetlm-? m this picture gives a hmltmg rate of formatlon for the bot-

FIN. I Ewlratron and decay pathuals of DhlT

tleneck state. There is no evidence pertaming to the equdlbrlum structure of the B-state; e g., whether it is an isomeric form, or a vlbrattonally excited molecule near to the ground- and excited-state equilibrium con- figuration. The partitlonmg of the overall internal con- version @R into @RS and @RR (see fig. 1) remains to

be evaluated. The fluorescence data conform theoretIcally to

nolated-molecule condltlons neglecting processes havmg collision cross sections larger than 10 times gas hnetlc. In practice we obtamed dtfferent fluorescence spectra and yields for each of the excltatlon wave- lengths. The phorochemlcal yield vanes with excitation energy mdlcatmg that the dissoclatlve state also Is not colhsIonally relaxed, and this IS confirmed by the ef- fect of added gas. The hfetlme of the B-state is there- fore consldered to be shorter than the colhslonal fre- quency at 0 6 Torr while long enough for the slate to be almost completely relaxed by 600 Torr argon. At present the effect of added gas IS not known to regener- ate directly ground-state DMT molecules. It 1s possible that B has a relaxed equlllbrlum structure that 1s rela- tlvely stable to dlssoclatlon.

The last column of table 1 mdrcates that ar 0.6 Torr the product of the photochemlcal and fluorescence 1 telds IS roughly constant_ For the model suggested here this result lmphes that the excess energy depen- dences mvolved m the formatlon of B and the photo- dlssoclatlon of B are slmllar. The relative fluorescence yields. the measured hfetime at 568.2 nm, and the as-

90 1’ ’ 0

r% u” 00 mm -0 5

m 0 I

Log,, k‘ 0 n

0

85 0

‘; Loglo%

-I 0

@ 0

80 o -I 5 I I I I

0 1000 2000 3000 4000

nE/hc (cm-‘)

kg. 2 Ewess energy dependencc of the photochemcal yield

scale) (@p, ., n&t-hand scale) and rate of &ht emisnon (kf. 0, left-hand

8

Volume 72, number 1 - CHEhflCAL PHYSICS LE-l-TERS 15 May 1980

sumed constant radratlve lifetime can be used to cal- culate the rate of non-radrative decay of the singlet state as a function of energy excess. Fig. 2 shows that

Q’p has a very simrlar dependence on energy excess. Tlus result is consrstent wrth +, bemg close to unity. and the rate constants for photodrssociatron and for- mation of B having srmrlar dependence on excess ener-

gy- The results presented here suggest an explanatron for

the dependence of the photoreaction rnechamsm on the medrum and on the substituents (drmethyl, diphenyl, etc.)_ Smce the fluorescence lIfetImes are comparable

m gases [ 171, solution [ 181 and sohds [ 191 the B- state of our model is very likely formed in each case. In the condensed phase the reactive B-state will relax

very raprdly to a photochemically stable configuration (either DMT ground state or a stable intermedrate). The rearrangement of B towards photoproducts may also depend critrcally on stenc restnctrons introduced by tire medium Thus the quantum yield of photolysis can be understood to be much lower in condensed phases. The btphotonic nature of the photoprocess 111 solution or in the solid phase (for DMT), and m the sohd phase for tetrazme, mrght then arise by the absorption of a second photon by either relaxed B or by the singlet excited state of DMT dependmg on which of these species has the largest product of absorptton cross sec- tion and hfettme at the pumpmg frequency.

There were three reports of transient absorptions that are perhaps relevant to the B-state and the photo- chemical reactron mechamsm. The first concerned the exerted singlet-state absorptton [18] which was shown to occur m the same general spectral region as the ground-state absorptron for s-tetrawne. The other two concerned longer-hved transrents that were presumed to srgnal the presence of a metastable Intermediate in the photochemrcal reactron [7,8]. In recent work we have shown that the prevrously reported [7] long-lived absorption transients are probably not generated in suf- ficient amounts to be involved in the princrpal photo- chemical pathway [20] t_ These studies are not con- clusrve in themselves and we have not distmgulshed

t Absorption measurements have mdlcated that at 632.8 nm the product of the quantum yield of formation and molar eutmction coeffiaent of any transient generated by a 530 nm YAG pulse irradiation of DMT IS less than 60 cm-t_ We obtained slmdar results with 353 nm eucltation. The time scale investigated ranged from 10 ns to 1 ms.

between the two-photon absorption

hv, lA, - 1B3u(nrr*)

huz - tX + products ,

and the process

hvr lAg--+ 1B3u(ns*) + 8% products,

where B has as yet an unidentified spectrum_ Simrlar experiments to those reported here are

being carned out for s-tetrazine and phenyl-s-stetrazine (PI). In the latter case the phenyl group acts as a heat bath into wluch the vibrational energy excess can be transferred and so PT provrdes a model for understand- mg the condensed-phase properties through studies of the isolated molecule.

References

[ I] G.H Spencer, PC. Cross and K EL Wiberg, 1. Chem. Phys. 35 (1961) 1925.

[2] J. Koemgsburg and K. Vogt. Pbys. Z. 14 (1913) 1269. 131 S F. hfason. J. Cbem. Sot. (1959) 1240. [4] A-J. hlerer and K-K. lrmes. Proc. Roy. Sot. A 302 (1968)

271.

[S] R hi. Hochstrasser and D.S. King, J. Am. Cbem. Sot. 97 (1975) 4760.

161 R.hf. Hochstrasser and D-S. Kmg, Lasers in physical chemistry and btophysics (Elsevier, Amsterdam, 1975).

[ 71 B. Delbnger, M.A. Paczkowski, A-B. Smith llL and R.M. Hochstrasser, J. Am. Cbem. Sot. 100 (1978) 3242.

[8] D.hf. Burfand and F. Carmona. hfol Cryst. Liquid Cryst. 50 (1979) 279.

[ 9 ] D hl. Burland. J. Pacansky and F. Cannom. Cbem. Pbys. Letters 56 (1978) 221.

[lo] R.hl. Hochstrasser, D.S. King and A.B. Smith ill, I. Am. Chem. Sot. 99 (1977) 3923.

[ 111 J. Pacansky, J. Phys. Cbem. 81 (1977) 2240. [ 12 ] J.R. McDonald and L.E. Brus. J. Cbem. Pbyr 59 (1973 j

4966. [ 13 1 S H. Meyhng, R.P. van der Werf and D.A. Wiersma, Chem.

Pbys. Letters. 28 (1974) 364. [ 141 W. Skorianetz and ESz. Kov5ts. Helv. Chun. Acta 54

(1971) 1929. [ 151 J.R. Andrews and hf. Paczkowski. unpublished_ [ 161 R.M. Hochstrasser and D S. King. J. Am. Chem. Sot.

98 (1976) 5443. [ 17 ] H. de Vries. D. Bebelaar and J. Langelaar. Opt. Commun.

18 (1976) 24. [ 181 R.M. Hochstrasser. D.S. Kmg and AC. Nelson. Cbem.

Pbys. Letters 42 (1976) 8. [ 191 H. de Vnes and D.A. Wrersma. Cbem. Phys Letters SE

(1977) 565. [20] hf. Paczkowski. A-B. Smith Ill and R.M. Hocbstrasser.

to be published.

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