- Email: [email protected]
Induced birefringence as a method for studying the electronic transitions and polarizability of molecules in excited electronic states
,VoIume 6. number 5
CHEMICAL PHYSICS LETTERS
INDUCED THE
BIREFRINGENCE
ELECTRONIC MOLECULES
AS
TRANSITIONS IN
EXCITED
A METHOD AND
1 September 2970
FOR
STUDYING
POLARIZABILITY
ELECTRONIC
OF
STATES
K. B. EISENTHAL and J. E. RIECKHOFF * IBM Research
Laboratory.
San 30~6, California
95114. USA
Received 15 May 1970
The measurement of excited state polarizabilities and the determination of both the symmetries and energies of excited electronic states using an induced birefringence method is described. This technique is applied to study the triplet states of naphthalene - $3.
The excitation by polarized light of randomly distributed molecules embedded in a rigid environment induces a birefringence in the medium. This birefringence which results from the anisotropy in the distribution of excited and ground state molecules can be used to obtain direct information on excited state polarizabilities [I]. Furthermore, the frequency dependence of the excited state polarizability reveals the presence and symmetries of more highly excited electronic states. This method is used here to study triplet states of tiphthalzne-d8 in the energy region 15625 cm-l (6400 A) to 23810 cm-l (4200 A) above the lowest tripret state. A forbidden transition from Tl (3B$u) having long axis (y) polarization is observed at 18180 cm-l and the allowed transition 3Biu --i 3Big having y axis polarization is also observed at higher energy. For naphthalene-d8 (5X 1O-3 M in EPA at 77OK) the quantity obtained is A = 2Acvz - Acry - APT where ACY~(i =x, y, z) is the difference in the polarizability along the molecular i axis between the lowest triplet state and the ground state of the molecule. The naphthalene-d8 molecules in the 3B1u state are produced by intersystem crossing following kxcitation of the lAlg-+ lBlu transition (z axis) with a suitably filtered Hg-Xe light source. The induced birbfriqgence is obtained by a measurement of the change in polarization of a p$obe beam on passing @rough the sample. This measurement coupled with a-determination of the triplet state populatioxi’yields the optical anisotropy A. : * Presen; address: Sfmon Fraser University, Bun-&y .. 2, B.C.,
Canada-
As a resonance is approached from the lowenergy side that component of the polarizability tensor which corresponds to the transition direction increasas very rapidly. Thus the marked decrease in A with increasiry energy in the neighborhood of 4200 A (see fig. I) can be ciue to increases in the ground state a$ or triplet comT , a~‘IS’ of the polarizabilivzut not to the pyents cyu aZ component. The dispersion of CL~ in this energy region is too small to explain the charge of A with energy and is therefore neglected [Z]. The dispersion of az &II also be neglected since the transitions contributing to az occur at much greater energies than the energy re ion studied % component here. Hence the increase is in the o! and therefore indicates that a co&r .bution due to a triplet state which has a transition moment with respect to Tl C3Bru) along the molecular y axis is re onsible for the observed changes in A as 4200s is approached. The y axis assignment is in accord wit$ the well-known 3+-.3 - at 4150 A (3,4]. BJ~ Blu Superimposed upon the strong change in A near 4200 A there appear-s exteosive_structure in the region between 4500 A and 5900 A. This indicates the presence of weak transitions betieerr. Tl and other electronic stat& in this region. The transitions are observed at 18180 cm-l, 18640 cm-l, 20180 cm-1 and 21280 cm’1 above T1. Using the same ar ments advanced for the dispersion in-the 4200 F region leads to the conclusion that changes ii~ a$ zre responsible for the +ru&ure at the longer wave-lengths. Eence, the transitions in this region are polarfzed along the molecular y axi* The existence of exci&@ trip&t states in this
Volume 6: number 5
CHEMICAL PBYSICS LEl”,rERS ... ‘_.
I
I
I
4ZQo .4400
4600
4800
:
1 Septembfx 1970
I
5000
YM)
5400
4
5330
5800
6000
6200
6400
WAVELENGTH. x tit,
,Fig. Ii Optical anisotropy (-a) x 1024cm3 as a functionof the wavelengthof the probe light. + -and 3BIu are energy region of symmetries ’Alg predicted by theoretical calculations. The positions of these levels above the lowest triplet according to de Gmot and’Hoytink [5] and Pariser + ( 16370 cm-l, 18175 cm-l) and 3Biu C6] are 3Alg
i7170 cm-l,
16440 cm--l), respectively *; Since
Tl is of B$,, symmetry, transitions to these states are forbidden and are made allowed by vibronic mixing with electronic states of suitable symmetry**. Since the nearest state of appropriate symmetry is the 3Big (24100 cm-l) then the resulting transition from the Biu would be y axis polarized. This prediction is in agreement with our data. Direct .experimental~evidence of naphthalene transitions inthis region prior to the present work were the observations of very, weak triplettriplettransitions in a polymethyimetbacryiate Bru&d B~~symbclsusedhere correspond to the Bzu and Blsnotation used in refs. [5] and [S]. .-** .The 3l&+ +3Ai R.-forbiddeness arises from the ._ pius * plu&pro lbltlon. Even if this transitionwere .weakly allowed; its polarization.wouldbe along the -z a.s which Is contrary to our experimental obser: ’ __ :‘ -. ,. - va@ns~ -* The
,442 ._
: ;
:,_
._ .
‘. ___ . .
_‘. _-
_-
..
host and in an alcohol glass [7,8]. However, the only experimental work which indicates the symmetry of the triplet state or states in this energy region is the two-photon absorption work on a-ClOH7C1[8]. The singlet state at 42000 cm-l observed in the two-photon spectrum of * cr-ClOH7Cl was assigned as a lKt, in the Platt nomenclature whi.ch would correspond to a state of Alg symmetry in naphthaiene. Since the respective singlet and triplet states of naphthalene and cY-chloronaphthalene are very close in energy, we can conclude that there should be a 3A1 in naphthaleie at about 20700 cm-l or lower dP. pending on the singlet-triplet split, Combining the theoretical predictions with the two-photon measurements we conclude that the state we are observing in the present study is most likely a ._ %g . The authors thank J. H. Smith Jr. for his assist%nce.’Th’e authors wish to thank the referee , for bringing to their attention the paper by K&all et ai. [9]:hi which the induced birefringence _ method [l] is-also used to study exited state ..anistropy-~...-‘~ I ~ ‘: . .: : ,--. ,. .. / -. ..i_. ,_ .:. _.‘ ~., ,. ‘. . --__.__ ~,_ :‘_:_ .,_ ‘_I’ ..< .: -. : _.: -. ‘. _. _, _ .-/_ -‘.
.,
:
-
_._,-,,
..
./
_-
_,
._
Volume 6. number 5
CHEMICAL PHYSICS LETTERS
REFERENCES flf K.B.Eisenthd and K.E.Riec’khoff, Phys. Rev. Letters 20 (1968) 309. [2] A.Bree and T.Thirunamachsndrsn, Mol. Phys. 5 (1962) 397. f3] M.A.El-Sayed and T.Pavlopaulos, J. Chem. Phys. 39 (1963) 834. [4] D.P.Craig and I.G.Ross, J.Chem. Sot. (1954) 1589.
1. September 1970
[5] R;L.de Groat and G.tI.Hoyfink. J. Chem. Phys. 46 (1967) 4523. [S] R. Pariser. J. Chem. Phys. 24 (1956) 250. [7] W.H.Melhuisb, J. Chem. Phys, 50 (1969) 2779. [81 R.Astier. A.Bokobza and Y.R.Meyer, private communication. $91 Kuball, Ewing and Ustens, Ber. Bunsenges. Physik. Chem. 74 (1970) 316.
CHEMICAL PHYSICS LETTERS
INDUCED THE
BIREFRINGENCE
ELECTRONIC MOLECULES
AS
TRANSITIONS IN
EXCITED
A METHOD AND
1 September 2970
FOR
STUDYING
POLARIZABILITY
ELECTRONIC
OF
STATES
K. B. EISENTHAL and J. E. RIECKHOFF * IBM Research
Laboratory.
San 30~6, California
95114. USA
Received 15 May 1970
The measurement of excited state polarizabilities and the determination of both the symmetries and energies of excited electronic states using an induced birefringence method is described. This technique is applied to study the triplet states of naphthalene - $3.
The excitation by polarized light of randomly distributed molecules embedded in a rigid environment induces a birefringence in the medium. This birefringence which results from the anisotropy in the distribution of excited and ground state molecules can be used to obtain direct information on excited state polarizabilities [I]. Furthermore, the frequency dependence of the excited state polarizability reveals the presence and symmetries of more highly excited electronic states. This method is used here to study triplet states of tiphthalzne-d8 in the energy region 15625 cm-l (6400 A) to 23810 cm-l (4200 A) above the lowest tripret state. A forbidden transition from Tl (3B$u) having long axis (y) polarization is observed at 18180 cm-l and the allowed transition 3Biu --i 3Big having y axis polarization is also observed at higher energy. For naphthalene-d8 (5X 1O-3 M in EPA at 77OK) the quantity obtained is A = 2Acvz - Acry - APT where ACY~(i =x, y, z) is the difference in the polarizability along the molecular i axis between the lowest triplet state and the ground state of the molecule. The naphthalene-d8 molecules in the 3B1u state are produced by intersystem crossing following kxcitation of the lAlg-+ lBlu transition (z axis) with a suitably filtered Hg-Xe light source. The induced birbfriqgence is obtained by a measurement of the change in polarization of a p$obe beam on passing @rough the sample. This measurement coupled with a-determination of the triplet state populatioxi’yields the optical anisotropy A. : * Presen; address: Sfmon Fraser University, Bun-&y .. 2, B.C.,
Canada-
As a resonance is approached from the lowenergy side that component of the polarizability tensor which corresponds to the transition direction increasas very rapidly. Thus the marked decrease in A with increasiry energy in the neighborhood of 4200 A (see fig. I) can be ciue to increases in the ground state a$ or triplet comT , a~‘IS’ of the polarizabilivzut not to the pyents cyu aZ component. The dispersion of CL~ in this energy region is too small to explain the charge of A with energy and is therefore neglected [Z]. The dispersion of az &II also be neglected since the transitions contributing to az occur at much greater energies than the energy re ion studied % component here. Hence the increase is in the o! and therefore indicates that a co&r .bution due to a triplet state which has a transition moment with respect to Tl C3Bru) along the molecular y axis is re onsible for the observed changes in A as 4200s is approached. The y axis assignment is in accord wit$ the well-known 3+-.3 - at 4150 A (3,4]. BJ~ Blu Superimposed upon the strong change in A near 4200 A there appear-s exteosive_structure in the region between 4500 A and 5900 A. This indicates the presence of weak transitions betieerr. Tl and other electronic stat& in this region. The transitions are observed at 18180 cm-l, 18640 cm-l, 20180 cm-1 and 21280 cm’1 above T1. Using the same ar ments advanced for the dispersion in-the 4200 F region leads to the conclusion that changes ii~ a$ zre responsible for the +ru&ure at the longer wave-lengths. Eence, the transitions in this region are polarfzed along the molecular y axi* The existence of exci&@ trip&t states in this
Volume 6: number 5
CHEMICAL PBYSICS LEl”,rERS ... ‘_.
I
I
I
4ZQo .4400
4600
4800
:
1 Septembfx 1970
I
5000
YM)
5400
4
5330
5800
6000
6200
6400
WAVELENGTH. x tit,
,Fig. Ii Optical anisotropy (-a) x 1024cm3 as a functionof the wavelengthof the probe light. + -and 3BIu are energy region of symmetries ’Alg predicted by theoretical calculations. The positions of these levels above the lowest triplet according to de Gmot and’Hoytink [5] and Pariser + ( 16370 cm-l, 18175 cm-l) and 3Biu C6] are 3Alg
i7170 cm-l,
16440 cm--l), respectively *; Since
Tl is of B$,, symmetry, transitions to these states are forbidden and are made allowed by vibronic mixing with electronic states of suitable symmetry**. Since the nearest state of appropriate symmetry is the 3Big (24100 cm-l) then the resulting transition from the Biu would be y axis polarized. This prediction is in agreement with our data. Direct .experimental~evidence of naphthalene transitions inthis region prior to the present work were the observations of very, weak triplettriplettransitions in a polymethyimetbacryiate Bru&d B~~symbclsusedhere correspond to the Bzu and Blsnotation used in refs. [5] and [S]. .-** .The 3l&+ +3Ai R.-forbiddeness arises from the ._ pius * plu&pro lbltlon. Even if this transitionwere .weakly allowed; its polarization.wouldbe along the -z a.s which Is contrary to our experimental obser: ’ __ :‘ -. ,. - va@ns~ -* The
,442 ._
: ;
:,_
._ .
‘. ___ . .
_‘. _-
_-
..
host and in an alcohol glass [7,8]. However, the only experimental work which indicates the symmetry of the triplet state or states in this energy region is the two-photon absorption work on a-ClOH7C1[8]. The singlet state at 42000 cm-l observed in the two-photon spectrum of * cr-ClOH7Cl was assigned as a lKt, in the Platt nomenclature whi.ch would correspond to a state of Alg symmetry in naphthaiene. Since the respective singlet and triplet states of naphthalene and cY-chloronaphthalene are very close in energy, we can conclude that there should be a 3A1 in naphthaleie at about 20700 cm-l or lower dP. pending on the singlet-triplet split, Combining the theoretical predictions with the two-photon measurements we conclude that the state we are observing in the present study is most likely a ._ %g . The authors thank J. H. Smith Jr. for his assist%nce.’Th’e authors wish to thank the referee , for bringing to their attention the paper by K&all et ai. [9]:hi which the induced birefringence _ method [l] is-also used to study exited state ..anistropy-~...-‘~ I ~ ‘: . .: : ,--. ,. .. / -. ..i_. ,_ .:. _.‘ ~., ,. ‘. . --__.__ ~,_ :‘_:_ .,_ ‘_I’ ..< .: -. : _.: -. ‘. _. _, _ .-/_ -‘.
.,
:
-
_._,-,,
..
./
_-
_,
._
Volume 6. number 5
CHEMICAL PHYSICS LETTERS
REFERENCES flf K.B.Eisenthd and K.E.Riec’khoff, Phys. Rev. Letters 20 (1968) 309. [2] A.Bree and T.Thirunamachsndrsn, Mol. Phys. 5 (1962) 397. f3] M.A.El-Sayed and T.Pavlopaulos, J. Chem. Phys. 39 (1963) 834. [4] D.P.Craig and I.G.Ross, J.Chem. Sot. (1954) 1589.
1. September 1970
[5] R;L.de Groat and G.tI.Hoyfink. J. Chem. Phys. 46 (1967) 4523. [S] R. Pariser. J. Chem. Phys. 24 (1956) 250. [7] W.H.Melhuisb, J. Chem. Phys, 50 (1969) 2779. [81 R.Astier. A.Bokobza and Y.R.Meyer, private communication. $91 Kuball, Ewing and Ustens, Ber. Bunsenges. Physik. Chem. 74 (1970) 316.