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The photo-isomerization of stilbene
Volu&
38, number 3
CHEbIIi3AL
The Sclttrs@r Laboratory, University
PHYSICSLEtiERS
of Mmcizester. Manchester.
15 March 1976
UK
Received 7 November 1975
The SOpotential of stilbcne has minima in the trarls (t) and cis (c) confiiuntions and a maximum in the pfrp @) configuration. The St potential has minima at t, p and c due to crossing of the *B, snd t As potentials. and it is proposed that the Tt potential has similar minima due to crossing of the 3B, and “Ag potentials. Five alternative isomerization mecba~sms are considered, tke yields of which depend on eompetirion between vertical radiative and radiationless transitions to lo~r states, and horizontal radiationless transitions betwe% the t, p and c confiiurations of St and Tr . ’
The efiergies of the electronic states of stilbene‘(diphenylethylene) depend on the angle 8 of rotation of the phenyl groups about the ethylenic bond. The following symbols are used to describe the states and their energies. t, p and c refer to the tram (0 = O”), perp (0 = 90”) and cis (0 = I 8U6) con~gur~tions. Subscripts G, T, K, S and H refer to the So, TI, TZ, Sy and S2 states, respectively. Superscripts A and B refer to As and B, symmetries, respectively. It is proposed that the S,, T,, T,, S, and S, potentials of stilbene depend on 0 in the manner shown in ftg. 1. The energies (in 103 cm-‘) of each state are indicated, where known. The values of $5, $, fg, c$, ct and cg are from the review by Saltiel et al. El], who list four determinations of @t - c$, the activation energy of thermal cis -+ tram So isomerization: 36.7 and 46 F 2 kcal/moie in solution, and 42.8 * 2 and 42.6 * 1 kcal/mole in the vapour phase. Because of the experimental uncertainty, the mean value of 42 kcaIf mole is taken in preference to that of 46 kc&mote chosen by SaItiei et al. {l J , so that the value of g(j = 15700
cm-’
replaces that of 17 100 CIXI-~ used pre-
viously [I .2] _The crossing of the ’ 3, afid ’ Ag potentials, proposed on theoretical grounds by Orlandi and Siebrand 131, has been confirmed experimentally.by Birch and Birks [2], who determined p$ = t! f 610 cm-l = 31010 cm-l. it is proposed that .there is a similar I;rossing of the 3Bti and 3Ag potentials, so that the T, potential has minima at $.B , pi and CT;with intermediate maxinia. . ._ . :
.: ..
~
Fig. 1. Schematic diagram of potential cuties of etect&nic sta:es of stilbcne as a function of internal rotational angle 0. Energies in I@ csrFt _
’ ‘:’ Vofurne38; num@r 3
15 hfarch I976
CHEMICALPHYSICS LETTERS
: The triplet mech&sm.of
stilbenc isomerizztion [4,5 J , - discussed below, requires a T, potential minimum at 8 = 90°. The absence of efficient c; -+ f:! isomer&ration, folloving triplet se~it~ation of cis-stilbene [ 1 ] , despite the 3 500 cm-I energy excess, shows that there is at Ieast one maximum in the ‘I-1 potential, Saltiel [6J proposed empirically. that both the St and T, potentials have minima ai t, p and c with intermediate maxima, The S, potential has been shown to have this form [2f, due to the crossing of the 1B, and ‘A8 potentials f3] . By analogy it is proposed that crossing of the 313, and 3+a potent&& occurs and produces a similar form of TI potential. Fig. 2 shows the radiative (solid line) and radiationless (broken line) transitions and their rate parameters. ‘B,, + As radiative transitions are symmetry-allowed, so that fluorescence can occur from f: [rate k&t)] and cg [rate k&c)] and phosphorescence can occur from Ae + 5 radiative fg [rate $&II and ct [rate k&c)], transitions are symmetry-forbidden, so that ps is nonffuorescenf and J$! is non-phosphorescent. There are two types of radiationless transition: (a) vertical transitions to lower electronic states; and
(b) horizontal transitions to states of the same multiplicity and similar energy, but differing in configuration.and symmetry. The vertical mdj~tionless transition rates depend on
the energy gap and relative multiplicity of the in&l and final states [7]. From compariscn v&h otiter aromatic hydrocarbons it is expected that kT&) 3- kcs(f) and that J+(c) Z+$.8(c). If there are higher triplet states lying above but adjacent to fg and c& kT-(t) and kn(c) may include temperature-dependent components, but this has not yet been resolved i2]. For the transitions frompg, kGs(p) (= 5.8 x 108 s-I fZ]> > &.S(p) * kTs(t) = kTs(cj. For the transitions from T, k,,b) > kGT(t) % &.(c)_ Thus &....@), and to a lesser extent k&t), kTs(p) and kTs(c), are the principal vertical radiationless transitions from SI, and k&(p), and to a lesser extent kGT(t) and k,,(c),
are the yrincipai ones from T,. The horizontal radiationless transitions in St and ‘I’, correspond to thermally-activated reversible internal conversion due to intramolecular rotation between dif-
ferent configurations. The corresponding t$ +- & + ce processes in So are irreversible at normal temperatures. The ti * p& process in methylcyclohexanefisohexane (MCIif1r-i) solution has an enthalpy AH= 1.75 kc-al mole‘-’ = 610 cm-t and an entropy&‘= IO.6 cal deg -1 mofC1 [3_]. At 25’C the forward rate kss@) exceeds the backward rate kss(tp) by a factor of a 10, so that the fractions of S1 excited molecules in the two ~on~gurations are [f,“] = 0.09 and [p$] = 0.9 1. pending similar data about the c: +& process, it is proposed that under similar conditions (low-v~cosity solutions at room temperature) kssfpc) P k&q), so that Ip$] P [c.$fJ . Thus the St horizontal transitions from tt and c8 are each considered to favour the forrnation of pt. It is similarly pro osed that the T, horizontal transitions from r: and cTf: .favour the formation of P&
Following t$ + tg excitation of tmns-stilbene, there are five potential channels leading to cis-isomerization: (i) the singlet mechanism, f;=p;+p
6 --+;
fW
(ii) the triplet mechanism, f$f+p+
F& 2. Principalradiative(solid line) aqd radiationless‘@roken line) transitians between S 1, T1 md SCJstates of stilbene.
438-.._ _. . . -
. .. ..
1 .,
(iii) the
-+p$-+&;
singlet-triplet
ts” =+&-tP$-V~-+.
: I .
.
. CM
mechanism, A;.
(3c)
Volume 38, number 3
CHEMICAL
(iv) the S, horizontal mechanism, B ts” =p7 =c +p 2;
(v) the T, horizontal
t;++At$L.A
PHYSICS LETTERS
15 March 1976
I
10
w
S?
mechanism,
.
k TS (SC)
T T G’ Following c$ + ci excitation of cis-stllbene, there are five similar potential channels leading to trans-isomerization. These channels (1 t)-(5t) are obtained by substituting c for t and t for c in (1 c)-(5~). The singlet mechanism (1 c), proposed by Orlandi and Siebrand [3] following earlier suggestions by Saltie1 [1,6] , has been confirmed experimentally by Birch and Birks [2] who have shown it to be the dominant isomerization channe1 in MCH/IH solutions at -40’C and above. Evidence for the triplet mechanisms (2c, 2t), proposed by Forster [4], has been obtained by Malkin and Fischer [S] and others [l] , using such methods as indirect triplet sensitization, heavy-atom substitution, or oxygen perturbation. The singlet/triplet mechanism (3~) has a lower yield than (1 c) since kTs(P) < &-.8(p), but it will have a higher yield than (2~) when [p&] > [t!] . The S, horizontal mechanisms (4c, 4t) are potential isomerization channels, unless both the tg + p: andpt + cg processes are effectively unidirectional as proposed above. Similarly the T, horizontal mechanisms (SC, St) are potential isomerization channels, unless both the t: *p$ and p$ f c :p recesses are effectively unidirectional. In trons-stilbene the relative importance of the singlet (lc) and triplet (2 c) mechanisms depends on the relative magnitudes of kss(pt) and k&t). For MCH/IH soiutions at -40°C and above kss@t) > kTs(t) and the singlet mechanism (1 c) is dominant. With more viscous solvents and/or lower temperatures kss(pt) is reduced, and kTs(f) may exceed I&&X), so that the triplet mechanism (2~) becomes significant. The corresponding reduction in k&pt). which also occurs, is less important because the T, lifetime is longer than the S1 lifetime. k&t) can alternatively be increased, without reducing k88(Pt) or I&&r), by heavy-atom substitution (e.g. bromination), by heavy-atom solvents or quenchers, or by oxygen perturbation. The excited state behaviour of an organic molecule is often described by the following form of reaction scheme :
\ kFS
9
k,
kGT
i
l /
so
SO+
/;/ hv,
kG/ 50
,
km
k RG R
\
so-f-
hVP
(6)
For trQ)zs-stilbene, So = t& SG = p& and R (the photoproduct) = cc. At high temperatures, where the horizontal radiationless transition rates exceed the vertical radiative and radiationless transition rates, the parameters of (6) are related to those of fig. 2 as follows [2,7] (7)
63) (9) 00) (11) (12) (13)
Since [PSA] > [ti] and probably [PTA]P [t: J , rhe main radiationless transitions k G&)9 +8(P)* ‘G,@), ,+C(rp) and kc&p) al1 occur from the p configuration, while the radiative transitions k&t) and km(t) occur from the c configuration. The singlet mechanism (1 c) is dominant, the singlet-triplet mechanism (3~) also occurs, but the triplet mechanism (2~) is unimportant under these conditions. At low temperatures where the horizontal St transition rates are small compared to the vertical St transition rates, kFS = k&t), kTs = km(f) and kGs = k&t) 7 0. However, if the horizontal TI transition rates exceed the vertical T, transition rates (since k,, kGT +Z 439
.;
-._ ::_
.~.
.-
; fi$~~)theL (lO)‘$nd (1 i) rem&i vaiid. Undei these _<.circumstan&s the-s&&et (1~) and &ngfet-triptet (3~) mechMsms are ~~bited~~~t .~js-isome~zat~o~ can oci$r. via the triplet mectinism (2~); . .A~-in~e~e~iate ~ernp~rature~ where the horizantaI -. and-vertical transirion rates,are cornparable in magnitude, the behaviour is m&g complex and cannot be described by a simple re&tion scheme like (6), and a more detailed scheme like fig. 3, must be introduced. A pmhminary reaction kinetic analysis of the S, behaviour of trans-stilbene solutions-at intermediate temperatures @ been given previously fZ] . Further work is required to-extend this type of analysis to other processes in fig. 2. .The unusual photophysical and photochemical bkhaviour.of stilbene originates from the three minima in the S, and T, potentials. Higher diphenyl and retinol -polyenes probably have a similar series of minima in -their S, and_T, potentials associated with rotation about &e polyene bonds, and responsible for their photo-iso-
,
..
‘.
:
mdrization_ In’treattig the.excited state propertiesof tht%+eand. other no%-rigid.organic mole&s which have more than one S1 .or T, potential rn~~urn, the Iirrii-
tations of the conventional single-state reaction kinetic scheme (6) shoutd be real&d, so that ii can be replaced by a more general scheme like fig. 2. .
References 1I 1 J. Saitief, LiYAgostino, E.D. Megarity, L. Metts, KR. Neu-
bcro,er,hl. Wrighton and O.C. Zafariou, Org. Photo&em. 3 (1971) I. [21 D.J.S. Birshand J.B. Birks,Chem.Phys. titters 38 (1976) 432. [ 3 3 G. Orlandiand W. Sicbrand, Chem. Phys. Letters 30 ( 1975) 352. 141 Th. Fbster, Z. Elektrochem. 56 (19.52) 716. [S 3 S. Ma&in and E. Fischer, J. Phys Chem. 68 (1964) 1153. [6] J. Saltiel. J. Am. Chem. Sot. 89 (1967) 1036; 90 (1968) 4394. 171 J.B. Birlrs, Photophysicsof aromatic molecuies&‘iicyInterscience,New York, 1970).
38, number 3
CHEbIIi3AL
The Sclttrs@r Laboratory, University
PHYSICSLEtiERS
of Mmcizester. Manchester.
15 March 1976
UK
Received 7 November 1975
The SOpotential of stilbcne has minima in the trarls (t) and cis (c) confiiuntions and a maximum in the pfrp @) configuration. The St potential has minima at t, p and c due to crossing of the *B, snd t As potentials. and it is proposed that the Tt potential has similar minima due to crossing of the 3B, and “Ag potentials. Five alternative isomerization mecba~sms are considered, tke yields of which depend on eompetirion between vertical radiative and radiationless transitions to lo~r states, and horizontal radiationless transitions betwe% the t, p and c confiiurations of St and Tr . ’
The efiergies of the electronic states of stilbene‘(diphenylethylene) depend on the angle 8 of rotation of the phenyl groups about the ethylenic bond. The following symbols are used to describe the states and their energies. t, p and c refer to the tram (0 = O”), perp (0 = 90”) and cis (0 = I 8U6) con~gur~tions. Subscripts G, T, K, S and H refer to the So, TI, TZ, Sy and S2 states, respectively. Superscripts A and B refer to As and B, symmetries, respectively. It is proposed that the S,, T,, T,, S, and S, potentials of stilbene depend on 0 in the manner shown in ftg. 1. The energies (in 103 cm-‘) of each state are indicated, where known. The values of $5, $, fg, c$, ct and cg are from the review by Saltiel et al. El], who list four determinations of @t - c$, the activation energy of thermal cis -+ tram So isomerization: 36.7 and 46 F 2 kcal/moie in solution, and 42.8 * 2 and 42.6 * 1 kcal/mole in the vapour phase. Because of the experimental uncertainty, the mean value of 42 kcaIf mole is taken in preference to that of 46 kc&mote chosen by SaItiei et al. {l J , so that the value of g(j = 15700
cm-’
replaces that of 17 100 CIXI-~ used pre-
viously [I .2] _The crossing of the ’ 3, afid ’ Ag potentials, proposed on theoretical grounds by Orlandi and Siebrand 131, has been confirmed experimentally.by Birch and Birks [2], who determined p$ = t! f 610 cm-l = 31010 cm-l. it is proposed that .there is a similar I;rossing of the 3Bti and 3Ag potentials, so that the T, potential has minima at $.B , pi and CT;with intermediate maxinia. . ._ . :
.: ..
~
Fig. 1. Schematic diagram of potential cuties of etect&nic sta:es of stilbcne as a function of internal rotational angle 0. Energies in I@ csrFt _
’ ‘:’ Vofurne38; num@r 3
15 hfarch I976
CHEMICALPHYSICS LETTERS
: The triplet mech&sm.of
stilbenc isomerizztion [4,5 J , - discussed below, requires a T, potential minimum at 8 = 90°. The absence of efficient c; -+ f:! isomer&ration, folloving triplet se~it~ation of cis-stilbene [ 1 ] , despite the 3 500 cm-I energy excess, shows that there is at Ieast one maximum in the ‘I-1 potential, Saltiel [6J proposed empirically. that both the St and T, potentials have minima ai t, p and c with intermediate maxima, The S, potential has been shown to have this form [2f, due to the crossing of the 1B, and ‘A8 potentials f3] . By analogy it is proposed that crossing of the 313, and 3+a potent&& occurs and produces a similar form of TI potential. Fig. 2 shows the radiative (solid line) and radiationless (broken line) transitions and their rate parameters. ‘B,, + As radiative transitions are symmetry-allowed, so that fluorescence can occur from f: [rate k&t)] and cg [rate k&c)] and phosphorescence can occur from Ae + 5 radiative fg [rate $&II and ct [rate k&c)], transitions are symmetry-forbidden, so that ps is nonffuorescenf and J$! is non-phosphorescent. There are two types of radiationless transition: (a) vertical transitions to lower electronic states; and
(b) horizontal transitions to states of the same multiplicity and similar energy, but differing in configuration.and symmetry. The vertical mdj~tionless transition rates depend on
the energy gap and relative multiplicity of the in&l and final states [7]. From compariscn v&h otiter aromatic hydrocarbons it is expected that kT&) 3- kcs(f) and that J+(c) Z+$.8(c). If there are higher triplet states lying above but adjacent to fg and c& kT-(t) and kn(c) may include temperature-dependent components, but this has not yet been resolved i2]. For the transitions frompg, kGs(p) (= 5.8 x 108 s-I fZ]> > &.S(p) * kTs(t) = kTs(cj. For the transitions from T, k,,b) > kGT(t) % &.(c)_ Thus &....@), and to a lesser extent k&t), kTs(p) and kTs(c), are the principal vertical radiationless transitions from SI, and k&(p), and to a lesser extent kGT(t) and k,,(c),
are the yrincipai ones from T,. The horizontal radiationless transitions in St and ‘I’, correspond to thermally-activated reversible internal conversion due to intramolecular rotation between dif-
ferent configurations. The corresponding t$ +- & + ce processes in So are irreversible at normal temperatures. The ti * p& process in methylcyclohexanefisohexane (MCIif1r-i) solution has an enthalpy AH= 1.75 kc-al mole‘-’ = 610 cm-t and an entropy&‘= IO.6 cal deg -1 mofC1 [3_]. At 25’C the forward rate kss@) exceeds the backward rate kss(tp) by a factor of a 10, so that the fractions of S1 excited molecules in the two ~on~gurations are [f,“] = 0.09 and [p$] = 0.9 1. pending similar data about the c: +& process, it is proposed that under similar conditions (low-v~cosity solutions at room temperature) kssfpc) P k&q), so that Ip$] P [c.$fJ . Thus the St horizontal transitions from tt and c8 are each considered to favour the forrnation of pt. It is similarly pro osed that the T, horizontal transitions from r: and cTf: .favour the formation of P&
Following t$ + tg excitation of tmns-stilbene, there are five potential channels leading to cis-isomerization: (i) the singlet mechanism, f;=p;+p
6 --+;
fW
(ii) the triplet mechanism, f$f+p+
F& 2. Principalradiative(solid line) aqd radiationless‘@roken line) transitians between S 1, T1 md SCJstates of stilbene.
438-.._ _. . . -
. .. ..
1 .,
(iii) the
-+p$-+&;
singlet-triplet
ts” =+&-tP$-V~-+.
: I .
.
. CM
mechanism, A;.
(3c)
Volume 38, number 3
CHEMICAL
(iv) the S, horizontal mechanism, B ts” =p7 =c +p 2;
(v) the T, horizontal
t;++At$L.A
PHYSICS LETTERS
15 March 1976
I
10
w
S?
mechanism,
.
k TS (SC)
T T G’ Following c$ + ci excitation of cis-stllbene, there are five similar potential channels leading to trans-isomerization. These channels (1 t)-(5t) are obtained by substituting c for t and t for c in (1 c)-(5~). The singlet mechanism (1 c), proposed by Orlandi and Siebrand [3] following earlier suggestions by Saltie1 [1,6] , has been confirmed experimentally by Birch and Birks [2] who have shown it to be the dominant isomerization channe1 in MCH/IH solutions at -40’C and above. Evidence for the triplet mechanisms (2c, 2t), proposed by Forster [4], has been obtained by Malkin and Fischer [S] and others [l] , using such methods as indirect triplet sensitization, heavy-atom substitution, or oxygen perturbation. The singlet/triplet mechanism (3~) has a lower yield than (1 c) since kTs(P) < &-.8(p), but it will have a higher yield than (2~) when [p&] > [t!] . The S, horizontal mechanisms (4c, 4t) are potential isomerization channels, unless both the tg + p: andpt + cg processes are effectively unidirectional as proposed above. Similarly the T, horizontal mechanisms (SC, St) are potential isomerization channels, unless both the t: *p$ and p$ f c :p recesses are effectively unidirectional. In trons-stilbene the relative importance of the singlet (lc) and triplet (2 c) mechanisms depends on the relative magnitudes of kss(pt) and k&t). For MCH/IH soiutions at -40°C and above kss@t) > kTs(t) and the singlet mechanism (1 c) is dominant. With more viscous solvents and/or lower temperatures kss(pt) is reduced, and kTs(f) may exceed I&&X), so that the triplet mechanism (2~) becomes significant. The corresponding reduction in k&pt). which also occurs, is less important because the T, lifetime is longer than the S1 lifetime. k&t) can alternatively be increased, without reducing k88(Pt) or I&&r), by heavy-atom substitution (e.g. bromination), by heavy-atom solvents or quenchers, or by oxygen perturbation. The excited state behaviour of an organic molecule is often described by the following form of reaction scheme :
\ kFS
9
k,
kGT
i
l /
so
SO+
/;/ hv,
kG/ 50
,
km
k RG R
\
so-f-
hVP
(6)
For trQ)zs-stilbene, So = t& SG = p& and R (the photoproduct) = cc. At high temperatures, where the horizontal radiationless transition rates exceed the vertical radiative and radiationless transition rates, the parameters of (6) are related to those of fig. 2 as follows [2,7] (7)
63) (9) 00) (11) (12) (13)
Since [PSA] > [ti] and probably [PTA]P [t: J , rhe main radiationless transitions k G&)9 +8(P)* ‘G,@), ,+C(rp) and kc&p) al1 occur from the p configuration, while the radiative transitions k&t) and km(t) occur from the c configuration. The singlet mechanism (1 c) is dominant, the singlet-triplet mechanism (3~) also occurs, but the triplet mechanism (2~) is unimportant under these conditions. At low temperatures where the horizontal St transition rates are small compared to the vertical St transition rates, kFS = k&t), kTs = km(f) and kGs = k&t) 7 0. However, if the horizontal TI transition rates exceed the vertical T, transition rates (since k,, kGT +Z 439
.;
-._ ::_
.~.
.-
; fi$~~)theL (lO)‘$nd (1 i) rem&i vaiid. Undei these _<.circumstan&s the-s&&et (1~) and &ngfet-triptet (3~) mechMsms are ~~bited~~~t .~js-isome~zat~o~ can oci$r. via the triplet mectinism (2~); . .A~-in~e~e~iate ~ernp~rature~ where the horizantaI -. and-vertical transirion rates,are cornparable in magnitude, the behaviour is m&g complex and cannot be described by a simple re&tion scheme like (6), and a more detailed scheme like fig. 3, must be introduced. A pmhminary reaction kinetic analysis of the S, behaviour of trans-stilbene solutions-at intermediate temperatures @ been given previously fZ] . Further work is required to-extend this type of analysis to other processes in fig. 2. .The unusual photophysical and photochemical bkhaviour.of stilbene originates from the three minima in the S, and T, potentials. Higher diphenyl and retinol -polyenes probably have a similar series of minima in -their S, and_T, potentials associated with rotation about &e polyene bonds, and responsible for their photo-iso-
,
..
‘.
:
mdrization_ In’treattig the.excited state propertiesof tht%+eand. other no%-rigid.organic mole&s which have more than one S1 .or T, potential rn~~urn, the Iirrii-
tations of the conventional single-state reaction kinetic scheme (6) shoutd be real&d, so that ii can be replaced by a more general scheme like fig. 2. .
References 1I 1 J. Saitief, LiYAgostino, E.D. Megarity, L. Metts, KR. Neu-
bcro,er,hl. Wrighton and O.C. Zafariou, Org. Photo&em. 3 (1971) I. [21 D.J.S. Birshand J.B. Birks,Chem.Phys. titters 38 (1976) 432. [ 3 3 G. Orlandiand W. Sicbrand, Chem. Phys. Letters 30 ( 1975) 352. 141 Th. Fbster, Z. Elektrochem. 56 (19.52) 716. [S 3 S. Ma&in and E. Fischer, J. Phys Chem. 68 (1964) 1153. [6] J. Saltiel. J. Am. Chem. Sot. 89 (1967) 1036; 90 (1968) 4394. 171 J.B. Birlrs, Photophysicsof aromatic molecuies&‘iicyInterscience,New York, 1970).