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Laser flash studies of thioindigo and indigo dyes. Evidence for a trans configuration of the triplet state
1
1. introductiot~ In previous papers it b.ts been suggested that the d1rcct rruirs - c& photoisomerization of thioindigo d> es occurs via tripiet intermediates [ I-61 _ T-T absorption spectra hrtve been observed at 77 K by flash photolysis [a] _GrelImann snd I fent~&eI found tltzt the s.une tripfet state can be populated zt room temperature from the maus and the cis form. They proposed ti1.a the cis G= trmn photoisomcrirstion of thioindigo and 6.6’-diethoxythioindigo occurs vi.1 a common triplet state [5.6]. and they conchtded that the obserwd triplet state has a twisted configuwtion (i-e. tile angle of twist is 2 900)- An excited singlet state mechrmism has been proposed by other workers [7,S]_ Very recently, Karstens et al. ,dso suggested a trip!et mechanism for Pam - c-is photoisomerization of thioindigo [V] _ In the present paper experimental results iire presented which suggest that the triptet strtte of thioindigo dyes hrts predominantly the planar Tramsconfiguration (tr’) [IO]. This form is believed to be in thermal equiIibrium with the twisted (i.e. perpendicular) configuration (~3) with the position of the tr; + p3 equilibrium on the rrans side,
October
$979
2_ E\perimentsI The second and third harmonics af a Xd-faser were used for escitation (pulse width 15 ns. pulse energies G 300 mJ at 530 nm and 50 mJ at 353 nm). The laser flash photoiysis system hzs been described elsewhere [ 1 I, I?] _ For irr~dktion the 5-G or 576157s run lines of ;L 1000 W xenon-mercury Iamp were seiected by a monochromztor (ScRoeffeI: bandwidth 10 run)_ The low temperature apparatus and the method were the same as described elsewhere [ 11,12]_ The samples were degassed by purging with argon (except where indicated). At low temperarures the photostationary state was reached from 100% R-0~1s and from a mixture containing = SO% cis. The indigo and thioindigo dyes u we the same as used in previous work [4] _The solvents (Merck) were purified by distillation where necessary. and poiymethyl~methacryI.xte (Phlh1.4) samples were prepared as described in ref_ f 131.
3. Results 3.1. Tn*plet- triplet absorption spectra The transient
absorption
spectra of @aits-.5$-d&t363
I_--
--_-
Cimlpound
._. _ .I_. ---
_
EYI\Lw
xg:xL! I tiIl1) - -_-_--
_ _ -_. ---
-... _-- -_-- ----.
-I
L-q -_ _-___-_______ 6.5 x IO6 sax 10” 2.7x 104
3SSfSSS 39Ol600 37OB90 375lC580
-190
none 39Oi5SO 3Xii580 3651515
+-23 -175
*One S-70/470
i-15 -70 *IO
+%I-
=) ~01) indicates OD ofT-Tabsorption 364
I70
st the frsi\est temperarums
Ice x to4 106 I9 x 104
AOD 4 at xgtxz
.
-__-_-_
0.9: 1.2
25x 2.0 22x
t.ofi.0
3.Sl3.1
515 23 x I-2 x
106 IO4
none retativc to OD at room temperature.
4.6f1.7
l;s3f23
01
EPA (dirtily I ether. isopentane. .I& etltanof: 552) .tt three temperatures is shown in fig_ 3. Only d we& tr.msient Jbsorption ads observed for NJ!‘-dimetbylindigo m EPA at - I75’C. and ilo tr.wsient w.~s f‘otlrid for 55’-di-t-an\ littdigo in EPA (tJb!e 1) in agreement with prekiious work I*] _
In CT and MTHF the optical densities (OD) from T-T absorption for truns isomers of 5.5’-di-t-~rilylttlioindigo (fig. 3) and of tbioindigo and 5.5’-di-neopent> Ithioindigo (table 1) are nor affected by temperature. Since ground state absorption at 530 nm changes less than 10% in the temperature region essmined, OD represents the yield of triplet formation. it folIows tbst in CT and RlTHF this yieId is independent of viscosity and temperature. However. the same reIati\e increase of the two T-T absorption masimz was observed in EPA with decreasing temperature (figs. 2 and 3 and table 1). Since OD immediatefy after tile laser pulse (G20 ns) is not influenced by the triplet lifetime, it is the yield of triplet state which increases with decreasing temperature in EPA.
I
1
i
3
5 103/T [K--l B
l‘y_ 3_ Opacai &nut) for ob\erred rrtpkt St&e at the end of the Iaer puke (.tt At? xeras T -* for rrorrs-j.5’-di-t-am Ithioindtgo. = 5 x :Ctm4 11. m- (a) C;T, (b) \lTfIf. and (c) CPA; ;l-\C .I1 530 nm-
log 5- 1 versus T-t is shown for 5.5’-di-t-amyltbioindigo. in GT an Arrbenius pIot was obtained. whereas in ~netI~_vIc~~cIohc~~ne(MCH), RITHF, and EPA two linear dependences of Iog 7-t versus T-l were foundThe temperature (ro) at which these curves intersect, activatton energies. -Jnd A-factors are listed in table 1.
0
-50
* PC1
fs
Triplet decay is of first-order in all cases. The same rate constants of decay (kobs = 7-l) were found r\t the 400 and 600 nm maxima (table I)_ In fig. 4 a plot of
-150
-i80
-ii96
t
3
5
7 idI
3.3. Decav rate constan
-IL0
-100
TlK“I
9
11
1 13
-
Fig. 4. Plot of log ,--I for dec>_v of the observed triplet state versus T-’
for .t-a,zs-5,5’-di-t-amylthioindigo, = 5 x 10s4 11, he. a~). (b) XlTHF (broken line, @I), and (c) EP,\ (full line, a*); open and full symbols refer to X1 and X2. respectively: k,= at 530 nm.
in: Q) CT (dotted
365
The ratgo of Di~ts~rErisorncxs in the photoststionary st;tte. a;t j/rc&. is shown in t$_ 5 for 55’-di-tamylthioindigo
3s a funstion
of tcmperrrturc.
In various
sotvents thii ratio increases with decreasing temperature- Since Ihe C-is-
a7nlS quttnrum yield
(<2,._,) can-
not exceed unity and the ratio of molar extinction coefficients
is only slight&
temperature
dependent
itt GT between 25 and -7CFC represents 3. more than 100 fotd decrease of the r~lirs - cLs quantum yield (Q+,)_ At -7PC, corresponding tc) a viscosity of 5 X 109 po-rx f 14, I5 1. no fraas - cis photoisomrrkttion could be observed, whereas #c-t is still substznthi. In MCff snd MTHF #jf[cl), increases with decre.tsing
temperature
the
incre3se of(ft]/[c]),
even in the region where viscosity Change5
are not important_ This suggests that the increase in
these solvents is due to the decrease in temperature, and in GT due to both the decrease in temperatcre and the increase in viscosity_ For thioindigo ad SS’~-n~~RtyI~~o~Rdi~o shitar results were found (table 3)- Duting irradiation of the thioindigo dyes in benzene in the presence of oxygen, formation of sin&t oxygen was observed using the method fformzttion of ascaridol from a-tcrpinerte as monitor) described elsewhere I’ 63 _ 366
ITS-5. Trzz~tsfcti ratio of the photostationarystate versus T-’ for 5,5’-di-t-;rmylthioin~i~o, -5 X iOes M, in: (a) GT (dotted tine), (b) SSTEIF(broken line). and fc) MCEI (full line): irradiation at 546 nm.
---__-
-_ - -... _ __._ _ _ - _~- --- _ ._____-_ b, Irr.&auon Jf 5761576 nm. 4 Irr&l;tIon At 516 nm.
4. Discussion
The similarity of the T-T absorption spectra of the rrairs-thioindigo dyes es.mlined in liquid and rigid media suggests that the triplet state has the same configuration under both conditions. This is true if there is no change in configuration of the thioindlgo dyes after excitation of their tram forms in rigid media and if the absorption spectra of tr3 and p3 are different. Twisting about the C=C or N=N double bonds has rtn important influence on the absorption spectra of the isomers (see e.g. thioindigo dyes [3,17], stilbenes [l&19]. or azobenzenes [17,19]). Due to the strong
configurational differences between tr3 and pj a significant difference in the triplet absorption spectra has to be expected. We have used the decrease of ot _ c with increasing viscosity as a measure for the hindrance of internal rotation. i-e_ twisting about the central double bond. In GT at the highest viscosities available for our measurements practically no tram + cis photcisomerization rouId be achieved_ At -70°C, corresponding to a viscosity of 5 X I OS poise [ 151, +_, is reduced by a factor of more than 100 (e-g_ pt_c = 0.11 for thioindigo in benzene at 25OC [3])- Therefore we propose that the obseped triplet state of the thioindigo dyes
s9 I 65 81 0.5 SO
0 I:! 100 0.51 022 200 0.15
1 IS
100 4.6
s1 1
0.12 100 __-
remains in the tram configuration after excitation in G-T below -70°C. These experimental results constitute clear evidence that the triplet state of the thioindigo dyes should habe d planar rram configuration .dso at room temperature. If the observed triplet state has the planar ~PZWS configuration in liquid and rigid media and if formation of the triplet state does not depend on tempera-
ture. the quantum yield for formation not be inff uenced by the viscosity_ If triplet state has a pj configuration at ture. a decrease of the quantum yield with increasing viscosity should result
of tr3 shouId however. the room temperaof p’ formation due to the hin-
drance of twisting_ The latter case is ruled out since e\perimentalIy the yield of the observed triplet state in CT and hlTHF does not depend on temperature and viscosity even in the range where Q~_= falls off drastically. In solution at room temperature we suggest that the rrz configuration is in thermal equiIibrium with the p3 configuration. The direct tramcis photoisomerization of thioimiigo dyes takes place via a triplet state [2.3], and excitation of the cis form leads in substantial yield to the same triplet intermediate as excitation of the tram form [5,6] _ We propose the
pathway
c1 - p’ + p’ - tr’ for population
of tr3 367
t-f~Irll~~
ip’
31111 c f 3re
riw
I\Li~.tt‘tI .4:1d
‘ &-
rxsit&I
ri%pcai\eI> 1. IX>iXI2airg p3 during Iba ct -- tr3 tr;lm.itittn is unfibeI> _ Tflis Job_ ~I~NIXX not ~\clrtJc Ixtrticip.Gion of the p~:ltr~+ c I __ !,I __$J_ b> p;lssitip p;_ zrtd iurrhrr &a~+- to i-is XXI rnrf~syoxrrd Sillgk1
staw5
Sfzifr‘s.
fp” is rhr iwisisli
grmtrid
sutr’).
The posiricm d the configurrrtiorxtl triplet cqui!ibrium (tr3 = $1 is predomiilant:y on the Ifii~Jsside_ This sonclusion
is supporicd
h> tIw &wction
368
(ssr‘ results) which
of~in~Ie1
has alrczdy been reported by Kirsch and i\-mrtn using mother method 12I_ I-Iuor=zscericequenching b> owgen in air-saturated sohnions 1x1~not been observed 13.5 I_ The energy gap between p3 3nd pn [l’-01 is espeaed to be considerabIF smalier than the 12 k~~i/mol which is needed for fimnation of sir&t oxygen [ f 6 I_ Therefore sir&t ol~gen cxmnot be producsd from trl and pj but on& from tr3_ The shift of the photost~tionary state to the fralI5 side riith increasing oxygen concentration f23.51 is in agreement with this conclusion- IfonIy p3 is quenched by oxygen via a spin-exchange mechanism as p:oposed by Saltie and Thomas for stilbene [3 I 1 then no singlet oxygen should be produced_ It should be addid that z similar contigurational triplet equiii-brium has been found for thz J-nitrostiIbcnes [I I _I 3, 911_ The triplet decay rate constant of the thioindigo dyes depends on temperature and somewhat Iess on viscosity_ The tempcrxture dependence above fO may resuIt from im activation ener,T for the tr3 - p3 step. or an enthalpy difference between trs and p3 and/or 3x1activation en+rCy for the intersystem crossing 9 - p”_ The first case is unlikely at room temperature since the observed triplet state is formed in substantial yield from the cir side as well [5,6]_ In the latter two ~XXS the e_istence of a tr3 * p3 equilibrium is presumed- F&ed on resuits of Creihnann and Hentzschel [S-6] we condude that this equilibrium is established at room temperature much faster than decay_ When suck an equilibrium e.xistsit follows that the A-factor for t&z decay should be predominantIy determined by the intersystem crossing p3 -+ poI i-e_ A < 1012 S- I_ This is indeed the ease (table 2)_ The rate constant of the deactivation step p3 - p” is favoured by the Fmnck-Condon Factors in preference to that of the 3ltemative step tr3 - tr” because the energy difference between p3 and p” is considerably smaller (t@ is the rrans ground state)_ ActuaIIy, the m>gCn
triplet states of 1hc thioindigo dyes deacti\;lrc in soiutions oi la\- viscosity
ria p;
-
pn since of _c :md
_c xrr’ comp.uabIr if the high tluorescence quzmtm” > ieid of illC rruirs f‘orul is taken irito xcount f3 I_ 0
if esr.hibfiment of the lr3 --Lp’ cquiiibrium is prevented_ e-s_ be lowering the twq)erdture or ulcre.~ng 11x rr~~ir\ _intcr~>s?ircmcrossing tr3 -- trn becomes the dominsnt dexti\a:ion step_ Since 0, _u IS dmost /au belo\\ r,, and since fht xtiration energy and the ,l-l>stor are smJIIer t1l.m in the region ttbo\c r, (table 2). NL‘ conclude that below f. dewy of the tripIct occurs \ia fr; - trO without appreciable twisting. For cram isomers of XiS’ifhcetyIindigo in EPA and CT as \rcli xs for 55’4-t-.mi) I- and 5.5’4~neopcnt\Ithioindigo in EPA an increrrse of the triplet yield \+ith dccrcxsing tempcmturc was observed_ This su=ests the existence of an addition31 deactivation process for the excited trQJIZsingIet s:ate under these conditions not leading to triplet states at room temperature (cfrefs_ [7.231)_
Ackno~ iedgemen t The authors wish to thank Professor G.&f_ Wyman for helpful comments and A1r.U. CurrcII for technical assistance. We thank Dr. T. Karstens for a preprint of ref f!l]_ References [ I[ G-M. Wymztn.B-M_ Zarnqr Chem 77 (1973 25I;-t_
and D-G. Whitten, J. Ph) s_
131_&I)- Hirschand G-V. \!‘~nxm.J. Phls. Chem. 81 (1977) 41x 141 D- Schulte-I rohiindr, IL Iltrmann and G.M. Wyman. Z_ Physik_Chem_(Frankfurt am Vain) IO 1 ( 1976) 115. [Si K-IL CrelIm;lnn and p_ Ilentzschel, Chem. Phys. Letters 53 (1978) 545 [6[ P- Ikntzschel, PhD thesis. Unirersit3t GZittin~m (1977). 171 G. Haucke md R_ hetzotd. Nova Acta Leopoldina. SuppL 1 I (1978)_ [81 T. Kxstens, I(. Kobs, R. \lemminS and F_ Schriippel. Cbem- Phys Letters 48 (1977) 540. 19 J T_ Km-stens. R_ Kobs and R- Memming. Ber. Bunsengs. Physik. Chem., submitted for publication. [IOJ D- Schulte-Trohlindeand IL Giirner. Rrsults presented at the Inforrnzl Symposium on Internal Rotation in Excited Orsanic Molecules. Cambridge (1978)_
cI1I-uIcAL
PIl\
1 October
SIC-S LLTTIZKS
1979
[ 1 S i J. Sdtiel. J-T. D’Agostino. L _D_ Vegarity. L_ Uetts. K.R. Seubsr~cr. .\I. \\‘tighton .tnd 0-C. Zati:iou. Org. Photochcm. 3 (1973) 1. [ 191 D. Cicgiou. K_ 1. Vwzhat and 17. i‘ischcr. J. Am. Chem. Sot. 90 I1 9681j907. j2Oj 11. I rlzr. G. ilauchc and R. Pmzofd. 2. Ph?sik. CImn. ( Leipzig) 256 ( 1977) 3 15. [Z! } J. S&w1 and B. Thom.~s. Chcm. Ph! s. Lcrrer\ 37 (1976) 147.
(23
1 J_ SuIuwr .md I;. Gustav.
3101. Photochem.
S t 197’)
437_
369
1. introductiot~ In previous papers it b.ts been suggested that the d1rcct rruirs - c& photoisomerization of thioindigo d> es occurs via tripiet intermediates [ I-61 _ T-T absorption spectra hrtve been observed at 77 K by flash photolysis [a] _GrelImann snd I fent~&eI found tltzt the s.une tripfet state can be populated zt room temperature from the maus and the cis form. They proposed ti1.a the cis G= trmn photoisomcrirstion of thioindigo and 6.6’-diethoxythioindigo occurs vi.1 a common triplet state [5.6]. and they conchtded that the obserwd triplet state has a twisted configuwtion (i-e. tile angle of twist is 2 900)- An excited singlet state mechrmism has been proposed by other workers [7,S]_ Very recently, Karstens et al. ,dso suggested a trip!et mechanism for Pam - c-is photoisomerization of thioindigo [V] _ In the present paper experimental results iire presented which suggest that the triptet strtte of thioindigo dyes hrts predominantly the planar Tramsconfiguration (tr’) [IO]. This form is believed to be in thermal equiIibrium with the twisted (i.e. perpendicular) configuration (~3) with the position of the tr; + p3 equilibrium on the rrans side,
October
$979
2_ E\perimentsI The second and third harmonics af a Xd-faser were used for escitation (pulse width 15 ns. pulse energies G 300 mJ at 530 nm and 50 mJ at 353 nm). The laser flash photoiysis system hzs been described elsewhere [ 1 I, I?] _ For irr~dktion the 5-G or 576157s run lines of ;L 1000 W xenon-mercury Iamp were seiected by a monochromztor (ScRoeffeI: bandwidth 10 run)_ The low temperature apparatus and the method were the same as described elsewhere [ 11,12]_ The samples were degassed by purging with argon (except where indicated). At low temperarures the photostationary state was reached from 100% R-0~1s and from a mixture containing = SO% cis. The indigo and thioindigo dyes u we the same as used in previous work [4] _The solvents (Merck) were purified by distillation where necessary. and poiymethyl~methacryI.xte (Phlh1.4) samples were prepared as described in ref_ f 131.
3. Results 3.1. Tn*plet- triplet absorption spectra The transient
absorption
spectra of @aits-.5$-d&t363
I_--
--_-
Cimlpound
._. _ .I_. ---
_
EYI\Lw
xg:xL! I tiIl1) - -_-_--
_ _ -_. ---
-... _-- -_-- ----.
-I
L-q -_ _-___-_______ 6.5 x IO6 sax 10” 2.7x 104
3SSfSSS 39Ol600 37OB90 375lC580
-190
none 39Oi5SO 3Xii580 3651515
+-23 -175
*One S-70/470
i-15 -70 *IO
+%I-
=) ~01) indicates OD ofT-Tabsorption 364
I70
st the frsi\est temperarums
Ice x to4 106 I9 x 104
AOD 4 at xgtxz
.
-__-_-_
0.9: 1.2
25x 2.0 22x
t.ofi.0
3.Sl3.1
515 23 x I-2 x
106 IO4
none retativc to OD at room temperature.
4.6f1.7
l;s3f23
01
EPA (dirtily I ether. isopentane. .I& etltanof: 552) .tt three temperatures is shown in fig_ 3. Only d we& tr.msient Jbsorption ads observed for NJ!‘-dimetbylindigo m EPA at - I75’C. and ilo tr.wsient w.~s f‘otlrid for 55’-di-t-an\ littdigo in EPA (tJb!e 1) in agreement with prekiious work I*] _
In CT and MTHF the optical densities (OD) from T-T absorption for truns isomers of 5.5’-di-t-~rilylttlioindigo (fig. 3) and of tbioindigo and 5.5’-di-neopent> Ithioindigo (table 1) are nor affected by temperature. Since ground state absorption at 530 nm changes less than 10% in the temperature region essmined, OD represents the yield of triplet formation. it folIows tbst in CT and RlTHF this yieId is independent of viscosity and temperature. However. the same reIati\e increase of the two T-T absorption masimz was observed in EPA with decreasing temperature (figs. 2 and 3 and table 1). Since OD immediatefy after tile laser pulse (G20 ns) is not influenced by the triplet lifetime, it is the yield of triplet state which increases with decreasing temperature in EPA.
I
1
i
3
5 103/T [K--l B
l‘y_ 3_ Opacai &nut) for ob\erred rrtpkt St&e at the end of the Iaer puke (.tt At? xeras T -* for rrorrs-j.5’-di-t-am Ithioindtgo. = 5 x :Ctm4 11. m- (a) C;T, (b) \lTfIf. and (c) CPA; ;l-\C .I1 530 nm-
log 5- 1 versus T-t is shown for 5.5’-di-t-amyltbioindigo. in GT an Arrbenius pIot was obtained. whereas in ~netI~_vIc~~cIohc~~ne(MCH), RITHF, and EPA two linear dependences of Iog 7-t versus T-l were foundThe temperature (ro) at which these curves intersect, activatton energies. -Jnd A-factors are listed in table 1.
0
-50
* PC1
fs
Triplet decay is of first-order in all cases. The same rate constants of decay (kobs = 7-l) were found r\t the 400 and 600 nm maxima (table I)_ In fig. 4 a plot of
-150
-i80
-ii96
t
3
5
7 idI
3.3. Decav rate constan
-IL0
-100
TlK“I
9
11
1 13
-
Fig. 4. Plot of log ,--I for dec>_v of the observed triplet state versus T-’
for .t-a,zs-5,5’-di-t-amylthioindigo, = 5 x 10s4 11, he. a~). (b) XlTHF (broken line, @I), and (c) EP,\ (full line, a*); open and full symbols refer to X1 and X2. respectively: k,= at 530 nm.
in: Q) CT (dotted
365
The ratgo of Di~ts~rErisorncxs in the photoststionary st;tte. a;t j/rc&. is shown in t$_ 5 for 55’-di-tamylthioindigo
3s a funstion
of tcmperrrturc.
In various
sotvents thii ratio increases with decreasing temperature- Since Ihe C-is-
a7nlS quttnrum yield
(<2,._,) can-
not exceed unity and the ratio of molar extinction coefficients
is only slight&
temperature
dependent
itt GT between 25 and -7CFC represents 3. more than 100 fotd decrease of the r~lirs - cLs quantum yield (Q+,)_ At -7PC, corresponding tc) a viscosity of 5 X 109 po-rx f 14, I5 1. no fraas - cis photoisomrrkttion could be observed, whereas #c-t is still substznthi. In MCff snd MTHF #jf[cl), increases with decre.tsing
temperature
the
incre3se of(ft]/[c]),
even in the region where viscosity Change5
are not important_ This suggests that the increase in
these solvents is due to the decrease in temperature, and in GT due to both the decrease in temperatcre and the increase in viscosity_ For thioindigo ad SS’~-n~~RtyI~~o~Rdi~o shitar results were found (table 3)- Duting irradiation of the thioindigo dyes in benzene in the presence of oxygen, formation of sin&t oxygen was observed using the method fformzttion of ascaridol from a-tcrpinerte as monitor) described elsewhere I’ 63 _ 366
ITS-5. Trzz~tsfcti ratio of the photostationarystate versus T-’ for 5,5’-di-t-;rmylthioin~i~o, -5 X iOes M, in: (a) GT (dotted tine), (b) SSTEIF(broken line). and fc) MCEI (full line): irradiation at 546 nm.
---__-
-_ - -... _ __._ _ _ - _~- --- _ ._____-_ b, Irr.&auon Jf 5761576 nm. 4 Irr&l;tIon At 516 nm.
4. Discussion
The similarity of the T-T absorption spectra of the rrairs-thioindigo dyes es.mlined in liquid and rigid media suggests that the triplet state has the same configuration under both conditions. This is true if there is no change in configuration of the thioindlgo dyes after excitation of their tram forms in rigid media and if the absorption spectra of tr3 and p3 are different. Twisting about the C=C or N=N double bonds has rtn important influence on the absorption spectra of the isomers (see e.g. thioindigo dyes [3,17], stilbenes [l&19]. or azobenzenes [17,19]). Due to the strong
configurational differences between tr3 and pj a significant difference in the triplet absorption spectra has to be expected. We have used the decrease of ot _ c with increasing viscosity as a measure for the hindrance of internal rotation. i-e_ twisting about the central double bond. In GT at the highest viscosities available for our measurements practically no tram + cis photcisomerization rouId be achieved_ At -70°C, corresponding to a viscosity of 5 X I OS poise [ 151, +_, is reduced by a factor of more than 100 (e-g_ pt_c = 0.11 for thioindigo in benzene at 25OC [3])- Therefore we propose that the obseped triplet state of the thioindigo dyes
s9 I 65 81 0.5 SO
0 I:! 100 0.51 022 200 0.15
1 IS
100 4.6
s1 1
0.12 100 __-
remains in the tram configuration after excitation in G-T below -70°C. These experimental results constitute clear evidence that the triplet state of the thioindigo dyes should habe d planar rram configuration .dso at room temperature. If the observed triplet state has the planar ~PZWS configuration in liquid and rigid media and if formation of the triplet state does not depend on tempera-
ture. the quantum yield for formation not be inff uenced by the viscosity_ If triplet state has a pj configuration at ture. a decrease of the quantum yield with increasing viscosity should result
of tr3 shouId however. the room temperaof p’ formation due to the hin-
drance of twisting_ The latter case is ruled out since e\perimentalIy the yield of the observed triplet state in CT and hlTHF does not depend on temperature and viscosity even in the range where Q~_= falls off drastically. In solution at room temperature we suggest that the rrz configuration is in thermal equiIibrium with the p3 configuration. The direct tramcis photoisomerization of thioimiigo dyes takes place via a triplet state [2.3], and excitation of the cis form leads in substantial yield to the same triplet intermediate as excitation of the tram form [5,6] _ We propose the
pathway
c1 - p’ + p’ - tr’ for population
of tr3 367
t-f~Irll~~
ip’
31111 c f 3re
riw
I\Li~.tt‘tI .4:1d
‘ &-
rxsit&I
ri%pcai\eI> 1. IX>iXI2airg p3 during Iba ct -- tr3 tr;lm.itittn is unfibeI> _ Tflis Job_ ~I~NIXX not ~\clrtJc Ixtrticip.Gion of the p~:ltr~+ c I __ !,I __$J_ b> p;lssitip p;_ zrtd iurrhrr &a~+- to i-is XXI rnrf~syoxrrd Sillgk1
staw5
Sfzifr‘s.
fp” is rhr iwisisli
grmtrid
sutr’).
The posiricm d the configurrrtiorxtl triplet cqui!ibrium (tr3 = $1 is predomiilant:y on the Ifii~Jsside_ This sonclusion
is supporicd
h> tIw &wction
368
(ssr‘ results) which
of~in~Ie1
has alrczdy been reported by Kirsch and i\-mrtn using mother method 12I_ I-Iuor=zscericequenching b> owgen in air-saturated sohnions 1x1~not been observed 13.5 I_ The energy gap between p3 3nd pn [l’-01 is espeaed to be considerabIF smalier than the 12 k~~i/mol which is needed for fimnation of sir&t oxygen [ f 6 I_ Therefore sir&t ol~gen cxmnot be producsd from trl and pj but on& from tr3_ The shift of the photost~tionary state to the fralI5 side riith increasing oxygen concentration f23.51 is in agreement with this conclusion- IfonIy p3 is quenched by oxygen via a spin-exchange mechanism as p:oposed by Saltie and Thomas for stilbene [3 I 1 then no singlet oxygen should be produced_ It should be addid that z similar contigurational triplet equiii-brium has been found for thz J-nitrostiIbcnes [I I _I 3, 911_ The triplet decay rate constant of the thioindigo dyes depends on temperature and somewhat Iess on viscosity_ The tempcrxture dependence above fO may resuIt from im activation ener,T for the tr3 - p3 step. or an enthalpy difference between trs and p3 and/or 3x1activation en+rCy for the intersystem crossing 9 - p”_ The first case is unlikely at room temperature since the observed triplet state is formed in substantial yield from the cir side as well [5,6]_ In the latter two ~XXS the e_istence of a tr3 * p3 equilibrium is presumed- F&ed on resuits of Creihnann and Hentzschel [S-6] we condude that this equilibrium is established at room temperature much faster than decay_ When suck an equilibrium e.xistsit follows that the A-factor for t&z decay should be predominantIy determined by the intersystem crossing p3 -+ poI i-e_ A < 1012 S- I_ This is indeed the ease (table 2)_ The rate constant of the deactivation step p3 - p” is favoured by the Fmnck-Condon Factors in preference to that of the 3ltemative step tr3 - tr” because the energy difference between p3 and p” is considerably smaller (t@ is the rrans ground state)_ ActuaIIy, the m>gCn
triplet states of 1hc thioindigo dyes deacti\;lrc in soiutions oi la\- viscosity
ria p;
-
pn since of _c :md
_c xrr’ comp.uabIr if the high tluorescence quzmtm” > ieid of illC rruirs f‘orul is taken irito xcount f3 I_ 0
if esr.hibfiment of the lr3 --Lp’ cquiiibrium is prevented_ e-s_ be lowering the twq)erdture or ulcre.~ng 11x rr~~ir\ _intcr~>s?ircmcrossing tr3 -- trn becomes the dominsnt dexti\a:ion step_ Since 0, _u IS dmost /au belo\\ r,, and since fht xtiration energy and the ,l-l>stor are smJIIer t1l.m in the region ttbo\c r, (table 2). NL‘ conclude that below f. dewy of the tripIct occurs \ia fr; - trO without appreciable twisting. For cram isomers of XiS’ifhcetyIindigo in EPA and CT as \rcli xs for 55’4-t-.mi) I- and 5.5’4~neopcnt\Ithioindigo in EPA an increrrse of the triplet yield \+ith dccrcxsing tempcmturc was observed_ This su=ests the existence of an addition31 deactivation process for the excited trQJIZsingIet s:ate under these conditions not leading to triplet states at room temperature (cfrefs_ [7.231)_
Ackno~ iedgemen t The authors wish to thank Professor G.&f_ Wyman for helpful comments and A1r.U. CurrcII for technical assistance. We thank Dr. T. Karstens for a preprint of ref f!l]_ References [ I[ G-M. Wymztn.B-M_ Zarnqr Chem 77 (1973 25I;-t_
and D-G. Whitten, J. Ph) s_
131_&I)- Hirschand G-V. \!‘~nxm.J. Phls. Chem. 81 (1977) 41x 141 D- Schulte-I rohiindr, IL Iltrmann and G.M. Wyman. Z_ Physik_Chem_(Frankfurt am Vain) IO 1 ( 1976) 115. [Si K-IL CrelIm;lnn and p_ Ilentzschel, Chem. Phys. Letters 53 (1978) 545 [6[ P- Ikntzschel, PhD thesis. Unirersit3t GZittin~m (1977). 171 G. Haucke md R_ hetzotd. Nova Acta Leopoldina. SuppL 1 I (1978)_ [81 T. Kxstens, I(. Kobs, R. \lemminS and F_ Schriippel. Cbem- Phys Letters 48 (1977) 540. 19 J T_ Km-stens. R_ Kobs and R- Memming. Ber. Bunsengs. Physik. Chem., submitted for publication. [IOJ D- Schulte-Trohlindeand IL Giirner. Rrsults presented at the Inforrnzl Symposium on Internal Rotation in Excited Orsanic Molecules. Cambridge (1978)_
cI1I-uIcAL
PIl\
1 October
SIC-S LLTTIZKS
1979
[ 1 S i J. Sdtiel. J-T. D’Agostino. L _D_ Vegarity. L_ Uetts. K.R. Seubsr~cr. .\I. \\‘tighton .tnd 0-C. Zati:iou. Org. Photochcm. 3 (1973) 1. [ 191 D. Cicgiou. K_ 1. Vwzhat and 17. i‘ischcr. J. Am. Chem. Sot. 90 I1 9681j907. j2Oj 11. I rlzr. G. ilauchc and R. Pmzofd. 2. Ph?sik. CImn. ( Leipzig) 256 ( 1977) 3 15. [Z! } J. S&w1 and B. Thom.~s. Chcm. Ph! s. Lcrrer\ 37 (1976) 147.
(23
1 J_ SuIuwr .md I;. Gustav.
3101. Photochem.
S t 197’)
437_
369