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Lifetime, triplet-triplet absorption spectrum and relaxation energy of an acyclic conjugated triene triplet: A pulse radiolysis study
Soiumc 97. number -1.5
LlFETtME.
27 May 1983
CHEMICAL PHYSICS LETTERS
TRIPLET-TRIPLET
A~~O~T~ON
SPECTRUM
AND RELAXATiON
ENERGY
OF AN ACYCLIC CONJUGATED TRIENE TRIPLET: A PULSE RADIOLYSIS STUDY A.A. GORhlAN
and I. NAhiBLETT
I’hc rnplet awe of nco-alloocimrne (t = 333 ns:X,,, = 315 nm) Ius been produced in tofuenc by pulse radioIysis_ The c\t.rbtt\hmcnt of a rr.m\icnt st.ttion.rry p&ion for reversible triplet energy transfer between this triene and antbmcene has plxrd the eqmhbrium energy of the triune tripiet 3t 169 f 2 iJ mof-l. 28 + 2 kJ mof-t Iess than that of the km&--
I. Illtroduction
Qu,mtitarme drrra concerning the liferims. geometttt’s and energies of the relaxed triplet states Of simple rrcwronjug.rted and conjuwted Okfins have until recerltly been ut~~v~ii~~~e_ This has been principaily a conscquet~c~
of tlw bet
ttut
S, 4 T, intersystem
crossing rend phosphorescence iwe processes of essenli.diy zc\ro effieiertc~ for such systems. Progress has ILYW~~~ ~WII made by ourselves, using the pulse radroI\ sis tcchniqtw to cfrsractrrise the triplet states of twrbortwne i I j artd .I series of I .3-dienes [3.3]. and II! C.tldwll srtd Singh f41. using eiectrou transfer to twtl$ viologcn to .rIso rx.rminc I $-diene triplets. To &tc. howwer. it has not beeit possible to iiccirrately Jctcrmtw the energy change associtrted with torsional rcla~~~ikui of the trrpfct strile of3 simple 3cyolic ole!intc spurn. 111 this paper we report the dctemirna-
11~ of rhr hktime (333 IIS). triplet-triplet absorp1liWix,,, = 3 I5 ml) aid. in particular, the rei-ssaIWII c‘nergv (7s 5 1 kJ mol - 1) of the triplet state in ~I&IL’IZ of the smtpie trtenc nco-Jloocimene @!A: I). tlflr of thtr gwmetric isomers on the alfoocimene potcrlt;A rr1L’rg.rsurfxe. foflo\vs.
Our tlpproach is outlinerf 3s
Energy absorption from a 10 or 20 ns pulse of highenergy electrons (several (MeV) by an aromatic liquid suciJ as toluenc (T) produces significant yields of shortlived bound excited states of the matrix molecules according to Tz
IT”, 3T* .
(11
After the pulse these species decay via the normal photophysical channels or can, in the presence of suitable additive molecules, pass on their electronic excitation energy. Since the rate of transfer is pureIy concentration dependent for exothermic processes. 2 solute of concentration IOO-foId higher than other additives wil1 be essentirllly excIusively excited. The short lifetiaies of IT*, an excimer state, aJJd 3Tx in liquid toluene, 22 11sand 17 ns respectively [S],and the nomudly short lifetime of the singlet state of the acceptormolecuie, means that the formation of the triplet state Of the Iatter is compkte within 50 ns. Itssubsequent lifetime is governed by the composition of the solution with respect to further low concentration additives. By appropriate choice of concentmtions we have thus been able to populate the triplet manifolds of NA and the aromatic hydrocarbons pyrene (Py), peryicne (?‘e) and anthracene (An)* and to witness triplet energy transfer both to and from NA. This has
allowed deterniination of the triplet energy transfer rate constants and NA energy levels depicted in fig. 1.
%J 1
* Triplet energies quoted for p~renc, anthracenc and perylene are taken from ref. 161, ref. 171 and ref. [8] respectively.
0 009-26 14/S3/0000-0000/S
03-00 0 1983 North-Holland
CHEhfICAL PHYSICS LETTERS
Volume 97, number 4.5
27 Nay 1983
3. Results and discussion &!#+_)-
- -
_c - -
1
l.OXlo7
-
-
----
178-
I
(vide infra). Its formation via the channel 3An=
r 3sxlo* 169-
I 3 I
Pulse radiolysis of a deaerated toluene solution of formaNA (1O-2 mol Q-1) resulted in “immediate” tion of 3 transient species,X,, =315 nm (t-i. 23). confirmed as 3NA* from energy transfer esperiments
3T*+NA-+T+sNA*,
0
was followed by exponential decay with a rate constant of 3 .O X 1O6 s- 1 which was insensitive
to NA
i &
91xlo9
I __--_-_147-
%-
Fig. 1. Triplet energies (kJ mol-’ ) and rate constants (‘2mol-t s-l) for triplet energy tmnsfcr iwolving neo-alloocimcne (NA). pyrene (Pp), antltraccnc (An) and perylene (I+).
2. Experimental Experiments were performed at the Christie Hospital and Holt Radium Institute, Manchester using the Vickers 10 MeV linear accelerator with pulse widths of 1’J or 20 ns. Kinetic absorption measurements were made with the apparatus described by Keene and Hodgson [9]. Voltage waveforms at the anode rjfeither an EM1 9783R photomultiplier or a silicon SH 100 diode coupled to 3 56 I2 load resistance, risetime <5 ns, were transferred to the memory of a modified Commodore 200 l-32 PET via a Tektronix 79 12 AD transient digitizer. Unless otherwise stated irradiations were carried out at ambient temperature -and solutions were deaerated by prolonged argon bubbling. Toluene (AnalaR, BDH) was refluxed over lithium aluminium hydride for 48 h and fractionated_ Neoalloocimene (Fluka, 99%) was fractionated under nitrogen at reduced pressure. Pyrene (ethanol) and perylene (toluene) were recrystallised. Anthracene (scintillation grade BDH) was used as received.
Fig. 2. (I\) TIIW dependence of 3&i* dc~xr nlonitorcd at 315 nnl after absorption of a 20 IIFelectron pulw by liquid toluene containing NA (1-2 x IO-2 mol C-t ). 2.7% ahsorption/div. 100 nsldiv: tb) triplet-triplex abwrption spr+ trum of NA from e.\perintsnts corresponding to (~1. 120 n% delay; (c) time dependence of 3Pcf formation monitored .tt 390 MI after absorption of a 70 ns electron pulse by liquid toluene contGning NA (2.4 x lo-’ mol C-‘) and Pr (2.1 x lo-’ moi Q-t). 1.45 absorption/div. ZOOnddiv: (d) t%storder contains for 3Pe’ formation against Pe concentration for energy transfer in toluenc from NA (2.4 X lo-’ 1i101 E-’ 1; the wperimental point at zero Pe cvwvttration corresponds tokd for ‘NA’ determined from c\puiment (a); (6 rime dependence of 3An* formation monitored at 425 nm after absorption of ;L10 ns electron pulse hy liquid tdutnc contaniq NA (2.0 X 10-t molE‘l)andzmthm~wae(l.l X low3 molC_‘1,
3.0% absorptionldiv. 200 ns/div_
423
Volunir
97.
nwulwr 4.5
C~ICWI~.IIIO~.
Thus
CHCXlICAL
the selfquenching
chamiel
‘NA* + NA --Lloss of 3NA* . IS unin~port,mt 1olurnc.
and the natural
PIIYSICS
for triple: (3
lifetime of 3NA* in
.
(4
IS _J?3us. It should he notcll thLt1 the spcctrulll reCOI~CL~ ti)r ‘NA* (fig. 2b)doe~ not appear compatible with c1.111wd trIplet -triplet absorption spectra (all h lll.l\ = 3hS nni) obI:~iiIcti from flash photolysis \Iudics on .ui .dloocimcnc mixture (77 to and a SICroIt1.d IrIcnc (77 K .md l
1983
27 hlsy
energy transfer
from Py and An to NA
shown in fig. 1 are in good agreement with this value. These rate constants were determined by pulse radiolysis of dearated toluene solutions of Py or An (lo-’ mol E- l) containing varying low concentrations NA followed by data Imnlysis according to
k, ’ ,
k rl ;-I’* t z&Z* --b loss of JNA*
LETTERS
k’ = k& + k,, [NA] .
of
(7)
where k; is the natural decay constant of the psrticular aromatic hydrocarbon triplet in toluene. The rate constant for vcrtical endothermic energy transfer is given by ,
$.I = k,, esp (-LSE@T)
(f9
where AEsr is the ener_gy difference between the O-O Fran&-Condon allowed So -+Tt transitions of donor and acceptor and k, the rate constant for diffusion [ 171. Because triplet energy transfer between NA and An has been demonstrated to be reversible (vide infra) rile rate coIlst;LIlts for transfer have been denoted as
kt.( ‘NA*
A-,=x-,, +k,,[l’cj pvc the
.
r11plc1 wc~gy
((3) traiisfel
r.ite const.eiI
sliown
.uld dn ulterccpt.k,l = 3.1 X 1Oh s- I. vmu.111?idenIic.tl Iu IhaI dcrcrmined from the decay of >NA* slmrp~~on (cf tig . I). ?‘hr data co11lh1 lt:Jl wc‘ arc de.thng with the trienc triplet st.Ite. hi .~ddirlon Ilie v.11~ ot’9.1 X IO’) P mol-l s-I for energy I~dmfi’r IO 1% is clcdrl~ rcp:esrnIaIive of a process whch 1s subst,uiti.dly e~othermic (cf. refs. 12.1 1.I?]) .Ind Icylures no ZIC~IV;LI~OI~ of jNA* IO SOIIW less ~l&lbk gmllc~n’. 11 lml p~cvlousl> been concluded that mhfh7nic r~tpkl cnrr~~ lrmsftr IO, for instance. acyclic 1,3d~enrs 15non-vrrtic.il in character [13.1-l]. i.e. requires lesh xtiv.Ition th.uI antIcip.Ited on the b&s of So ‘T, absorptron spectra. Although \%c have shown that this is not the ase for such dwnes 121, it was of import.mcc to cxuninc the possibility of non-vertical transfcr in tlWc3~ ofN.4 .ts J prellllliilary to interpretation of our triplet energy Irvlsfer data. The lowest energy b.md (607 nm) :n the St, + Tt absorption spectrum of N.4. me.Isured JS previously drscnbed [ 151. is close to published value for Irienes [ 15,161 and corresponds IO a tr.msition of 197 kJ mol-I. The rate constants 111lig. 1
+ An -
NA
+
;.4n*
,
and li;I x-2 3An* + NA An + “NA*(vertical)
.
(10)
For the ldtter rate constant the esperimental and calculated values are 1.0 X lo7 and 8.6 X lo6 Qmole1 s-t, ill good agreement with the operation of a vertical process. This conclusion has been confirmed by vxiable temperature experiments. In fig. 3 are reproduced plots of the first-order constant for 3An* decay,k’, versus [NA] according to eq. (7).k,, = kkt. for the temperature range 25-75°C. The resulting Arrhenius plot (fig. 3) gives a U+ value of 17.6 kJ m01-~. in excellent agreement with the spectroscopic 4ET value of 18.4 kJ mol-' .We conclude from these data that triplet energ)r transfer processes involving NA are essentially vertical in nature. Pulse radiolysis of a deaerated toluene solution of NA (2.0 X 10-l mol Q-t) containing An (10-j mol Q-l) resulted in initial grow-in of 3An*, identified from its characteristic spectrum. followed by its exponential decay (fig. 2e) with a rate constant of 3.1 X lo6 s-l, virtually identical to k, for 3NA*. This clearly indicates that energy transfer is reversible and
Volume 97. nun&.x 4.5
CHEhlICAL
PHYSICS
LETTERS
kfet /kbet = exp{-[(AE-t.)f
- (AET)b]/Rl)
27 hlap
1983
,
(12)
should hold where kf,/kE! is not equivalent to an equilibrium constant and the AET tenw ark the sctivation requirements of the energy transfer steps (9) and (10). Our value for (AETjb is 18.4 or 17.6 kJ m01-~ depending on whether the energy of the FranckCondon state of NA is taken from its So + Tt spectrum (determined as a 50 : 50 mixture with methylene iodide [ 151) or from the activation energy detcrmined for process (10) in toluene. These values. together with the experimental k,f& yield (AET’lf values of 9.7 + 1 A and SA + 1 .-I kJ mol-t _This places the relaxed 3NA* at 169 + 2 kJ mol-*, 2s +-Z kJ mol-t lower than the Fmnck-Condon state.
4. Conclusions
1‘1~~ 3. First-order
constants for 3An* dewy rtgainst NA concentruion 3t values of 103/Twllicll varied from 2.90 to 3.35 K-l at interv.tls of 0.05 K-t : [An] = (O-96- 1.01) X lo-’
ma1 ‘2-l. Inset: Arrheniu
& t3NA*] [An] = kit[3An*]
[NA] ,
(11)
should hold (cf. eqs. (9) and (10); 3An* natural decay insignificant). By determination of the concentrations of 3NA* [( 1.1 * 0.4) X 10m5 mol Q-l ] and 3An* [( 1.9 +- 0.5) X 10V6 mol Q-l] at the stationary position we have evaluated X-it/kzt = 35 2 15 which establishes& as (3.5 f 1.5) X 10” Q 11101-l s-l_ This is one to two orders of magnitude down on values typical of exothennic processes [3,11,12] and indicates that energy transfer from 3NA* to An [eq_ (9)] is endothermic (cf. fig. 1). Determination of 3NA* and 3 Art* concentrations was based on extinction coefficients for 3NA*, 3Pe* and 3An* determined for toluene by use of the methodology and extinction coefficients for benzene published by Bensasson and Land [ 18]_ It is io be emphasised that the transient validity of eq. (11) is strictly a kinetic phenomenon and the equation
For an unsaturated hydrocarbon. I loss of triplet energy of 28 kJ mol-t via a process which does not at least include torsion with respect to the n system would be both esperimentally and theoretically unprecedented [2,19-23]_ Our data are thus incompatible with a proposal [II] that plrurar 3NA* represents the lowest niinitnum on the alloocimene triplet potential energy surface. In addition, the vertical nature of the energy-transfer processes involved. together with the fact that 3N_4* requires no activstion to a less stable geometry before enera transfer takes place, argues against a fully twisted perpettdictrlar geometry for 3NA* which cannot be anything like 169 kJ mole1 above the corresponding ground-state maximum, irrespective of whether twisting is about a terminal or central double bond [ 19--22]. We therefore conclude that the relaxed 3NA* possesses a parfiaZ[s misted geometry which is intermediate with respect to torsion between the planar and perpendicu-
lar species (cf. fig. 4) and that geometrical isomerism within the triplet manifold proceeds via a low barrier corresponding to the perpendicular species. It is to be emphasised that fig. 3 is purely schematic and no attempt has been made to differentiate between geometrically isomeric species on the So and Tl surfaces. It is also recognised that the species referred to
here as “relaxed 3NA*” may predominantly occupy a well corresponding to the partially twisted foml of some other geometric isomer of alloocimene. This 425
CHEMICAL
Volume 97. number 4.5
PHYSICS LETTERS
27 hIay 1953
References [ 1 ] A.J.G. Barwise, A.A. Gorman and JLAJ. Phyr Letters 36 (1976) 313.
3
i IF_4. SC~WI~IIC rcprcccnt.ttmn of the Sn and l-1 pofcntial :nrrp> rdrhriuns resulfmg iron1 torsionil ch.mgr dt one of 111efwmal double bounds of N.A (1).
dews no1 affect pxtwul.tr]y
ibe arguments
since St, + T,
put forward referred
spectra
here. to in this
rwrA .xld clsewhrre [ 15.16 ] indicate that all essenr1_1]1? planar geonwtric isomers exhibit triplet energies N !thin 4 k J mol- I of e.~ch other.
Rodgers. Chem.
[Z] A.A. Got-man, 1-R. Gould and I. Hamblett, J. Am_ Chem. sot. 103 (1981) 4553. [3] A.A. Gormsn, 1-R. Gould and I. Hamblett. J. Photochem. 19 (1982) 89. 141 R.A. Caldwell and Xl. Sin& J_ Xrn. Chem. SW_ 104 (1982)6121. [S] J.H. Balendale and E-J. Rasburn, J. Chem. Sot. Faraday I69 (1973) 771. [ 61 Z. CIar and Il. Sander, Chem. Ber. S9 (1956) 749. 17 ] MR. Padhpe, S.P. 5icClynn and M. Kasha, J. Chem. Phys. 24 (1956) 588. [8] S.L. Murov. Handbook of photochemistry (Dekker. Sev. York. 1973). (91 J.P. Keene, 1. Sci. Instrum. 41 (1965) 493; B_\v. Iiodgson and J-P. Keene. Rev_ Sci. Instrum. 43 (1972) 493_ [ 101 R.S.H. Liu. Y _ Butt and W. Herkstrotcr. J. Chem. Sot. Chem. Commun. (19731799. ill] P.J. \vagner and 1. Kocherar. J. Am. Chem. Sot. 90 (1966) 1232. [ 121 J. S.dtiel, P.T. Shannon, O.C. Zafuiou and AX. Uriarre, J. Am. Chem. Sot. 107 (1980) 6799. [ 131 K.S.H. Liu. N.J. Turro and G.S. H.mlmond. J. Am. Chem. Sot. 87 (1965) 3406. [l-l] A.J. Fry. R.S.H. Liu and G.S. H.nnmond. J. Am. Chem. Sot. 88 (1966) 4781. [ 151 H.J.C. Jacobs and E. Havin~a. Advan. Photo&em. 11 (1979) 305. [ 161 Y. Butr, AK Sinsh, B.H. Baretz and R.S.H. Liu, J. Phys. Chem. 85 (1981) 2091. [ 171 K. Sandros. Acta Chcm. Stand. 1S (1964) 2355. [ 161 R. Bensasson and E.J. Land, Trans. Faraday Sot. 67 (1971) 1904. 1191 R. Hoffmann, Tetmhedron 22 (1966) 521[ZO] NC. Baird and R.N. N’est. J. Am. Chem. Sot. 93 (1971) -1327. [ 2 1 ] V. Bona%-Kouteckc and Shingo-lshimaru, J. Am. Chem. Sot. 99 (1977) 8134. [ZZ] I. Ohmine and Xi. Morokuma, J. Chem. Phys. 73 (1960) 1907. [?3 ] 1. Bona?i&Kouteckj.. 51. Persico. D. Dahnert and A. Srkin, J. Am. Chem. Sot. 104 (1982) 6900.
LlFETtME.
27 May 1983
CHEMICAL PHYSICS LETTERS
TRIPLET-TRIPLET
A~~O~T~ON
SPECTRUM
AND RELAXATiON
ENERGY
OF AN ACYCLIC CONJUGATED TRIENE TRIPLET: A PULSE RADIOLYSIS STUDY A.A. GORhlAN
and I. NAhiBLETT
I’hc rnplet awe of nco-alloocimrne (t = 333 ns:X,,, = 315 nm) Ius been produced in tofuenc by pulse radioIysis_ The c\t.rbtt\hmcnt of a rr.m\icnt st.ttion.rry p&ion for reversible triplet energy transfer between this triene and antbmcene has plxrd the eqmhbrium energy of the triune tripiet 3t 169 f 2 iJ mof-l. 28 + 2 kJ mof-t Iess than that of the km&--
I. Illtroduction
Qu,mtitarme drrra concerning the liferims. geometttt’s and energies of the relaxed triplet states Of simple rrcwronjug.rted and conjuwted Okfins have until recerltly been ut~~v~ii~~~e_ This has been principaily a conscquet~c~
of tlw bet
ttut
S, 4 T, intersystem
crossing rend phosphorescence iwe processes of essenli.diy zc\ro effieiertc~ for such systems. Progress has ILYW~~~ ~WII made by ourselves, using the pulse radroI\ sis tcchniqtw to cfrsractrrise the triplet states of twrbortwne i I j artd .I series of I .3-dienes [3.3]. and II! C.tldwll srtd Singh f41. using eiectrou transfer to twtl$ viologcn to .rIso rx.rminc I $-diene triplets. To &tc. howwer. it has not beeit possible to iiccirrately Jctcrmtw the energy change associtrted with torsional rcla~~~ikui of the trrpfct strile of3 simple 3cyolic ole!intc spurn. 111 this paper we report the dctemirna-
11~ of rhr hktime (333 IIS). triplet-triplet absorp1liWix,,, = 3 I5 ml) aid. in particular, the rei-ssaIWII c‘nergv (7s 5 1 kJ mol - 1) of the triplet state in ~I&IL’IZ of the smtpie trtenc nco-Jloocimene @!A: I). tlflr of thtr gwmetric isomers on the alfoocimene potcrlt;A rr1L’rg.rsurfxe. foflo\vs.
Our tlpproach is outlinerf 3s
Energy absorption from a 10 or 20 ns pulse of highenergy electrons (several (MeV) by an aromatic liquid suciJ as toluenc (T) produces significant yields of shortlived bound excited states of the matrix molecules according to Tz
IT”, 3T* .
(11
After the pulse these species decay via the normal photophysical channels or can, in the presence of suitable additive molecules, pass on their electronic excitation energy. Since the rate of transfer is pureIy concentration dependent for exothermic processes. 2 solute of concentration IOO-foId higher than other additives wil1 be essentirllly excIusively excited. The short lifetiaies of IT*, an excimer state, aJJd 3Tx in liquid toluene, 22 11sand 17 ns respectively [S],and the nomudly short lifetime of the singlet state of the acceptormolecuie, means that the formation of the triplet state Of the Iatter is compkte within 50 ns. Itssubsequent lifetime is governed by the composition of the solution with respect to further low concentration additives. By appropriate choice of concentmtions we have thus been able to populate the triplet manifolds of NA and the aromatic hydrocarbons pyrene (Py), peryicne (?‘e) and anthracene (An)* and to witness triplet energy transfer both to and from NA. This has
allowed deterniination of the triplet energy transfer rate constants and NA energy levels depicted in fig. 1.
%J 1
* Triplet energies quoted for p~renc, anthracenc and perylene are taken from ref. 161, ref. 171 and ref. [8] respectively.
0 009-26 14/S3/0000-0000/S
03-00 0 1983 North-Holland
CHEhfICAL PHYSICS LETTERS
Volume 97, number 4.5
27 Nay 1983
3. Results and discussion &!#+_)-
- -
_c - -
1
l.OXlo7
-
-
----
178-
I
(vide infra). Its formation via the channel 3An=
r 3sxlo* 169-
I 3 I
Pulse radiolysis of a deaerated toluene solution of formaNA (1O-2 mol Q-1) resulted in “immediate” tion of 3 transient species,X,, =315 nm (t-i. 23). confirmed as 3NA* from energy transfer esperiments
3T*+NA-+T+sNA*,
0
was followed by exponential decay with a rate constant of 3 .O X 1O6 s- 1 which was insensitive
to NA
i &
91xlo9
I __--_-_147-
%-
Fig. 1. Triplet energies (kJ mol-’ ) and rate constants (‘2mol-t s-l) for triplet energy tmnsfcr iwolving neo-alloocimcne (NA). pyrene (Pp), antltraccnc (An) and perylene (I+).
2. Experimental Experiments were performed at the Christie Hospital and Holt Radium Institute, Manchester using the Vickers 10 MeV linear accelerator with pulse widths of 1’J or 20 ns. Kinetic absorption measurements were made with the apparatus described by Keene and Hodgson [9]. Voltage waveforms at the anode rjfeither an EM1 9783R photomultiplier or a silicon SH 100 diode coupled to 3 56 I2 load resistance, risetime <5 ns, were transferred to the memory of a modified Commodore 200 l-32 PET via a Tektronix 79 12 AD transient digitizer. Unless otherwise stated irradiations were carried out at ambient temperature -and solutions were deaerated by prolonged argon bubbling. Toluene (AnalaR, BDH) was refluxed over lithium aluminium hydride for 48 h and fractionated_ Neoalloocimene (Fluka, 99%) was fractionated under nitrogen at reduced pressure. Pyrene (ethanol) and perylene (toluene) were recrystallised. Anthracene (scintillation grade BDH) was used as received.
Fig. 2. (I\) TIIW dependence of 3&i* dc~xr nlonitorcd at 315 nnl after absorption of a 20 IIFelectron pulw by liquid toluene containing NA (1-2 x IO-2 mol C-t ). 2.7% ahsorption/div. 100 nsldiv: tb) triplet-triplex abwrption spr+ trum of NA from e.\perintsnts corresponding to (~1. 120 n% delay; (c) time dependence of 3Pcf formation monitored .tt 390 MI after absorption of a 70 ns electron pulse by liquid toluene contGning NA (2.4 x lo-’ mol C-‘) and Pr (2.1 x lo-’ moi Q-t). 1.45 absorption/div. ZOOnddiv: (d) t%storder contains for 3Pe’ formation against Pe concentration for energy transfer in toluenc from NA (2.4 X lo-’ 1i101 E-’ 1; the wperimental point at zero Pe cvwvttration corresponds tokd for ‘NA’ determined from c\puiment (a); (6 rime dependence of 3An* formation monitored at 425 nm after absorption of ;L10 ns electron pulse hy liquid tdutnc contaniq NA (2.0 X 10-t molE‘l)andzmthm~wae(l.l X low3 molC_‘1,
3.0% absorptionldiv. 200 ns/div_
423
Volunir
97.
nwulwr 4.5
C~ICWI~.IIIO~.
Thus
CHCXlICAL
the selfquenching
chamiel
‘NA* + NA --Lloss of 3NA* . IS unin~port,mt 1olurnc.
and the natural
PIIYSICS
for triple: (3
lifetime of 3NA* in
.
(4
IS _J?3us. It should he notcll thLt1 the spcctrulll reCOI~CL~ ti)r ‘NA* (fig. 2b)doe~ not appear compatible with c1.111wd trIplet -triplet absorption spectra (all h lll.l\ = 3hS nni) obI:~iiIcti from flash photolysis \Iudics on .ui .dloocimcnc mixture (77 to and a SICroIt1.d IrIcnc (77 K .md l
1983
27 hlsy
energy transfer
from Py and An to NA
shown in fig. 1 are in good agreement with this value. These rate constants were determined by pulse radiolysis of dearated toluene solutions of Py or An (lo-’ mol E- l) containing varying low concentrations NA followed by data Imnlysis according to
k, ’ ,
k rl ;-I’* t z&Z* --b loss of JNA*
LETTERS
k’ = k& + k,, [NA] .
of
(7)
where k; is the natural decay constant of the psrticular aromatic hydrocarbon triplet in toluene. The rate constant for vcrtical endothermic energy transfer is given by ,
$.I = k,, esp (-LSE@T)
(f9
where AEsr is the ener_gy difference between the O-O Fran&-Condon allowed So -+Tt transitions of donor and acceptor and k, the rate constant for diffusion [ 171. Because triplet energy transfer between NA and An has been demonstrated to be reversible (vide infra) rile rate coIlst;LIlts for transfer have been denoted as
kt.( ‘NA*
A-,=x-,, +k,,[l’cj pvc the
.
r11plc1 wc~gy
((3) traiisfel
r.ite const.eiI
sliown
.uld dn ulterccpt.k,l = 3.1 X 1Oh s- I. vmu.111?idenIic.tl Iu IhaI dcrcrmined from the decay of >NA* slmrp~~on (cf tig . I). ?‘hr data co11lh1 lt:Jl wc‘ arc de.thng with the trienc triplet st.Ite. hi .~ddirlon Ilie v.11~ ot’9.1 X IO’) P mol-l s-I for energy I~dmfi’r IO 1% is clcdrl~ rcp:esrnIaIive of a process whch 1s subst,uiti.dly e~othermic (cf. refs. 12.1 1.I?]) .Ind Icylures no ZIC~IV;LI~OI~ of jNA* IO SOIIW less ~l&lbk gmllc~n’. 11 lml p~cvlousl> been concluded that mhfh7nic r~tpkl cnrr~~ lrmsftr IO, for instance. acyclic 1,3d~enrs 15non-vrrtic.il in character [13.1-l]. i.e. requires lesh xtiv.Ition th.uI antIcip.Ited on the b&s of So ‘T, absorptron spectra. Although \%c have shown that this is not the ase for such dwnes 121, it was of import.mcc to cxuninc the possibility of non-vertical transfcr in tlWc3~ ofN.4 .ts J prellllliilary to interpretation of our triplet energy Irvlsfer data. The lowest energy b.md (607 nm) :n the St, + Tt absorption spectrum of N.4. me.Isured JS previously drscnbed [ 151. is close to published value for Irienes [ 15,161 and corresponds IO a tr.msition of 197 kJ mol-I. The rate constants 111lig. 1
+ An -
NA
+
;.4n*
,
and li;I x-2 3An* + NA An + “NA*(vertical)
.
(10)
For the ldtter rate constant the esperimental and calculated values are 1.0 X lo7 and 8.6 X lo6 Qmole1 s-t, ill good agreement with the operation of a vertical process. This conclusion has been confirmed by vxiable temperature experiments. In fig. 3 are reproduced plots of the first-order constant for 3An* decay,k’, versus [NA] according to eq. (7).k,, = kkt. for the temperature range 25-75°C. The resulting Arrhenius plot (fig. 3) gives a U+ value of 17.6 kJ m01-~. in excellent agreement with the spectroscopic 4ET value of 18.4 kJ mol-' .We conclude from these data that triplet energ)r transfer processes involving NA are essentially vertical in nature. Pulse radiolysis of a deaerated toluene solution of NA (2.0 X 10-l mol Q-t) containing An (10-j mol Q-l) resulted in initial grow-in of 3An*, identified from its characteristic spectrum. followed by its exponential decay (fig. 2e) with a rate constant of 3.1 X lo6 s-l, virtually identical to k, for 3NA*. This clearly indicates that energy transfer is reversible and
Volume 97. nun&.x 4.5
CHEhlICAL
PHYSICS
LETTERS
kfet /kbet = exp{-[(AE-t.)f
- (AET)b]/Rl)
27 hlap
1983
,
(12)
should hold where kf,/kE! is not equivalent to an equilibrium constant and the AET tenw ark the sctivation requirements of the energy transfer steps (9) and (10). Our value for (AETjb is 18.4 or 17.6 kJ m01-~ depending on whether the energy of the FranckCondon state of NA is taken from its So + Tt spectrum (determined as a 50 : 50 mixture with methylene iodide [ 151) or from the activation energy detcrmined for process (10) in toluene. These values. together with the experimental k,f& yield (AET’lf values of 9.7 + 1 A and SA + 1 .-I kJ mol-t _This places the relaxed 3NA* at 169 + 2 kJ mol-*, 2s +-Z kJ mol-t lower than the Fmnck-Condon state.
4. Conclusions
1‘1~~ 3. First-order
constants for 3An* dewy rtgainst NA concentruion 3t values of 103/Twllicll varied from 2.90 to 3.35 K-l at interv.tls of 0.05 K-t : [An] = (O-96- 1.01) X lo-’
ma1 ‘2-l. Inset: Arrheniu
& t3NA*] [An] = kit[3An*]
[NA] ,
(11)
should hold (cf. eqs. (9) and (10); 3An* natural decay insignificant). By determination of the concentrations of 3NA* [( 1.1 * 0.4) X 10m5 mol Q-l ] and 3An* [( 1.9 +- 0.5) X 10V6 mol Q-l] at the stationary position we have evaluated X-it/kzt = 35 2 15 which establishes& as (3.5 f 1.5) X 10” Q 11101-l s-l_ This is one to two orders of magnitude down on values typical of exothennic processes [3,11,12] and indicates that energy transfer from 3NA* to An [eq_ (9)] is endothermic (cf. fig. 1). Determination of 3NA* and 3 Art* concentrations was based on extinction coefficients for 3NA*, 3Pe* and 3An* determined for toluene by use of the methodology and extinction coefficients for benzene published by Bensasson and Land [ 18]_ It is io be emphasised that the transient validity of eq. (11) is strictly a kinetic phenomenon and the equation
For an unsaturated hydrocarbon. I loss of triplet energy of 28 kJ mol-t via a process which does not at least include torsion with respect to the n system would be both esperimentally and theoretically unprecedented [2,19-23]_ Our data are thus incompatible with a proposal [II] that plrurar 3NA* represents the lowest niinitnum on the alloocimene triplet potential energy surface. In addition, the vertical nature of the energy-transfer processes involved. together with the fact that 3N_4* requires no activstion to a less stable geometry before enera transfer takes place, argues against a fully twisted perpettdictrlar geometry for 3NA* which cannot be anything like 169 kJ mole1 above the corresponding ground-state maximum, irrespective of whether twisting is about a terminal or central double bond [ 19--22]. We therefore conclude that the relaxed 3NA* possesses a parfiaZ[s misted geometry which is intermediate with respect to torsion between the planar and perpendicu-
lar species (cf. fig. 4) and that geometrical isomerism within the triplet manifold proceeds via a low barrier corresponding to the perpendicular species. It is to be emphasised that fig. 3 is purely schematic and no attempt has been made to differentiate between geometrically isomeric species on the So and Tl surfaces. It is also recognised that the species referred to
here as “relaxed 3NA*” may predominantly occupy a well corresponding to the partially twisted foml of some other geometric isomer of alloocimene. This 425
CHEMICAL
Volume 97. number 4.5
PHYSICS LETTERS
27 hIay 1953
References [ 1 ] A.J.G. Barwise, A.A. Gorman and JLAJ. Phyr Letters 36 (1976) 313.
3
i IF_4. SC~WI~IIC rcprcccnt.ttmn of the Sn and l-1 pofcntial :nrrp> rdrhriuns resulfmg iron1 torsionil ch.mgr dt one of 111efwmal double bounds of N.A (1).
dews no1 affect pxtwul.tr]y
ibe arguments
since St, + T,
put forward referred
spectra
here. to in this
rwrA .xld clsewhrre [ 15.16 ] indicate that all essenr1_1]1? planar geonwtric isomers exhibit triplet energies N !thin 4 k J mol- I of e.~ch other.
Rodgers. Chem.
[Z] A.A. Got-man, 1-R. Gould and I. Hamblett, J. Am_ Chem. sot. 103 (1981) 4553. [3] A.A. Gormsn, 1-R. Gould and I. Hamblett. J. Photochem. 19 (1982) 89. 141 R.A. Caldwell and Xl. Sin& J_ Xrn. Chem. SW_ 104 (1982)6121. [S] J.H. Balendale and E-J. Rasburn, J. Chem. Sot. Faraday I69 (1973) 771. [ 61 Z. CIar and Il. Sander, Chem. Ber. S9 (1956) 749. 17 ] MR. Padhpe, S.P. 5icClynn and M. Kasha, J. Chem. Phys. 24 (1956) 588. [8] S.L. Murov. Handbook of photochemistry (Dekker. Sev. York. 1973). (91 J.P. Keene, 1. Sci. Instrum. 41 (1965) 493; B_\v. Iiodgson and J-P. Keene. Rev_ Sci. Instrum. 43 (1972) 493_ [ 101 R.S.H. Liu. Y _ Butt and W. Herkstrotcr. J. Chem. Sot. Chem. Commun. (19731799. ill] P.J. \vagner and 1. Kocherar. J. Am. Chem. Sot. 90 (1966) 1232. [ 121 J. S.dtiel, P.T. Shannon, O.C. Zafuiou and AX. Uriarre, J. Am. Chem. Sot. 107 (1980) 6799. [ 131 K.S.H. Liu. N.J. Turro and G.S. H.mlmond. J. Am. Chem. Sot. 87 (1965) 3406. [l-l] A.J. Fry. R.S.H. Liu and G.S. H.nnmond. J. Am. Chem. Sot. 88 (1966) 4781. [ 151 H.J.C. Jacobs and E. Havin~a. Advan. Photo&em. 11 (1979) 305. [ 161 Y. Butr, AK Sinsh, B.H. Baretz and R.S.H. Liu, J. Phys. Chem. 85 (1981) 2091. [ 171 K. Sandros. Acta Chcm. Stand. 1S (1964) 2355. [ 161 R. Bensasson and E.J. Land, Trans. Faraday Sot. 67 (1971) 1904. 1191 R. Hoffmann, Tetmhedron 22 (1966) 521[ZO] NC. Baird and R.N. N’est. J. Am. Chem. Sot. 93 (1971) -1327. [ 2 1 ] V. Bona%-Kouteckc and Shingo-lshimaru, J. Am. Chem. Sot. 99 (1977) 8134. [ZZ] I. Ohmine and Xi. Morokuma, J. Chem. Phys. 73 (1960) 1907. [?3 ] 1. Bona?i&Kouteckj.. 51. Persico. D. Dahnert and A. Srkin, J. Am. Chem. Sot. 104 (1982) 6900.