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The temperature dependence of proton relaxation times in triplet exciton ion radical salts
Volume 25, number 2
+EMICAL
iwsrcs wrrms
:
-15 March !974
.
‘_
THE TEMPERATUREDEPENDENCE OF PROTON RELAxATI~NTI~~ES INTRIPLETEXCITONION RADICALSALTS GM. SEMENIUK and D.B. CHESNUT
Received 17 January 1974
Proton spin lattice relaxation times have been measured for a variety of single crystal orientations of the j@sAsCH3)?TCNQ)5 ion radical salt over the temperature range of approximately 90 to 370°K. It is shown that the relaxation rate is dire&y proportion& to the triplet exciton density where the exciton production energy is significantfy dependent on temperature. The data suggests that exciton-exciton exchange is an important aspect in
the relaxation mechanism.
1. Introduction The ion radical salts of TCNQ (tetracyanoquinodemethane) constitute an extremely interesting class of materials due to their unusual electrical, magnetic, and structural properties*. They exhibit some of the highest electrical conductivities known for organic solids [2] and have been suggested 131 as candidates for Little’s proposal [4] for a high-temperature superconductor. Many of these salts exhibit low lying, thermally accessible magnetic states [5,6 ] which can be approximately described in terms of tightly bound (Frenkel) triplet excitons [I, 71. These unusual properties are reflected in the cryst$ structures which show a one-dimensional plate-like stacking of the planar TCNQ molecules with short interplanar iritermolecular distances and, in some cases, evidence of clustering of sup&molecular species_@] **_ In the subclass of TCNi! salts which shows the spin correlated, mobile excitations there has been considerable interest in the exciton dynamics. The triplet excitons can achieve ver$ high densities (ca. 2X 1020 CmM3 for &AsCH3(TCNQ), at 300%) and lead to most interesting and still only incompletely-understood ., * A comprehensive referencelist is g&enin ief. [ 11, ** Ref. 191 contains a review of structures published through January’I, 1972.
phenomena. The purpose of this no&eis to report the first part of a single crystal study of proton Iongitudinal relaxation times (T,) which corrects an earlier ~te~retation of similar but limited work [ 101 and which adds to the already large list of data illustrating the unusual nature of the TCNQ salts.
2. Results and discussion In 1969 Nyberg et al. [ 101 reported the proton Tl. values of several powder and a few single-crystal orientations of TCNQ salts which display the presence.
of triplet excitons. ft was shown that the triplet excitons clearly dominated the proton Tt values but., as discussed below, the analysis of the data has proven
to be, we believe, in error. Since that time the detailed .crystal structures of both the (#sPCH$(TCNQ)F and (+3A~CH3)+(TCNQ)z salts have been worked out [8]and with this information in hand it was decided to-carefuUy remeasure TI both as a function of tern-. perature and orientation for the arsonium salt, probab$y-the best characterize? compound of the excit$ ,. subclass of TCNQ salts. As before t$e, re!axatjon time? were measured by _ .ther na pulse technique [ 1 l] at 35 MHz. ‘J’he-&ngle ex$dnential behavior of the signql reckrery shows that a single relaxation’ t*e is b&g obseived; We: :-’ .I: . .,.. .. .. : . .’ ‘. 25 1 .-, .XI
.’
.-
:
.
._
:
‘,
:
.’
::
,+.
:,
.
Volume25, number 2
.:; ‘15 March 1974, _: .:
CHEMICALPHYSICSLETTERS
I
II
2
t
I’II)
4
1
f?
II1
6 a ‘&C-y.:tOOOfT . i'K"' f
Pig. 1. Logarithmicplots of Tt (left) and (Tt p)-’ (&girt) as a function of inverse temperature for a varZety of rotation angles. Tr is expressed in miIIireconds in each plot:Successively higher curves are offset numerically by factors of three for clarity of presentation with the lowernkt curve representing no offset. Data.fromseveraldifferentexperimentsat each orientation are contained
in the figures.
. -~ report here. the .temperaiure behavior for seven rotation &gIes about one axis of rotation_ The rotation axis is parallel to the (I, I;?) direction in tlie ,crystal -: ai determined by the &le’sexternal morphology; a fulled dekiption of the &ystal orientations &ii1 ap,
,.
,‘.
-. -.
tion and that the TI ~minirn~ &curs when w;i = .I,, &I interpretation Giseswbich does not aflow a direct dependence”of Ti’ ‘tin p, the excitc$ density. ?I& is. ‘-not-readily understood-and Was t&en at-the time to.::
Volume 25, number 2
CHEMICAL
PtiYSICS
.I5 March’1974.
LETTERS
Table 1
Leastsquares values for (Tr p)-’ =A exp(pE) as a function of orientation. The data was tit over the temperature range ofap, proximately
90 to 370°K.
The standard deviations are denoted by o
Orientation
E (eV) X lo3
ffE (eVj X lo3
~__ A (set-‘)
I-O” I-30” I-60” I-100” I-1300 i-l& I-1700
69.5 66.3 68.5 69.3 68.7 71.0 71.6
0.46 0.46 0.52 0.43 0.47 0.50 0.51
6.11 7.43 5.08 6.48 4.75 3.55 4.57
averages
69.3
0.48
S-42
T;’ =pq.dj,
(1)
S(w) being the spectral density function involved. In order to match the observed qualitative behavior of T, it tias found to be necessary to invoke a temperature dependence of J = J(T) such that (2a) + 0.005 14 ( lo3 T-l)
(in eV)
(Tb)
f&r the.temperature range of +&oximatel+ 100 to 400°K. Throti& correspondence with M-T_ Jones [ 121 it was learned that this was fi%ecis&ythe temperature dependencetf+md necessary in un&blishedwork+ required to fit the observed ESR intensity-behavior over t&i same temperature range. Our owni&depeAdent co&l&ion of. this temp&ture &p&den& of 3
-.
::;.
,__;_
-.
0.24 0.28 --0.21
_‘0.22 0.18 0.14 0.19
-0.21
..
_..____--_
logarithmic correlation time plots which showed considerable curvature, continuing to suggest a flaw in the model and that T, should depend explicitly on p. Accordingly a model presuming an explicit p term was proposed where
J(T) = 0.0286
‘.
1.82
1.7
aave
bA (xc-‘)
Table 1 shows that A definitely depends on orienta-‘tion while E probably does not. The standard deviation of the average A-value is some-nine times-the standard deviation of that of any on& orientation -. while the similar ratio for the E data is only about ..’ three. The anisotropy in T, as reflected by the least squares fit .of A to the temperature data agrees with the anisotropy measured directly, as indeed it shpuld, and shows that the anisotropy is virtually indepen: dent of temperature. It is not obvious that the activation energy should be anisotropic; the least squares anisotropy does not show any correlation withthe _ amsotropy of A and its relatively. s&l1 value is probably within the range ofexperimental uncertainty. The most obvious mechanism fck enhancing the proton.relaxation’ratek the direct interaction of the -large intern+ magnetic field provided by the triplet. exciton’s magnetic moment modulated by the motion -. of.the,exciton &rough .tie cjstal. The.motion of the exciton is refledted in the absence of nuclear hyper-, c. fine structure in thk ESR spectfa [iland estimate? of
Volume 2.5. number 2
CHEMICAL PHYSICS LETTERS
vated. The fact that the observed E value in this study is of comparable magnitude to the predicted exchange barrier is strongly suggestive of involvement of exciton-
exciton
exchange
coupling
in the nuclear relaxation. Jn addition, accurate prior data is available on the temperature dependence of the (Cs’),(TCNQ)y2 salt inwhichJ(T)=0.185 -0.146(103 T-l)-’ ineV (note the linear dependence here of J on 7’) and for the (Et3NHf) (TCNQ)z salt where J is approximately constant and equal to 0.04 1 eV [6]. Using our previous proton T, powder data for these two salts the density-normalized relaxation rates indicate E values of 0.060 and 0.04-0.05 eV, respectively, for the cesiurn and triethyiammonium compounds. The necessary data for J for the 1: 1 morpholinium TCNQ complex [ 1] is probably less accurate but also leads to an E value of approximately 0.05 eV. The disparity of the cations involved in these salts but the common, essentially one-dimensiona stacking of the planar TCNQ molecules lends further confirmation to the involvement of the triplet excitons migrating along the TCNQ stacks and suggest that the process dominating the relaxation is common to them all. Should this be so, it is noteworthy that for the best characterized materials (the arsonium and cesium salts) one contains a complex, proton-containing cation ($,AsCHj in the arsonium salt) while the other contains a simple cation with no nuclear moment (the cesium salt). The detailed physical interpretation of the empirical relation satisfied by the density-normalized relaxation rate [es_ (3)J must await the determination of the proper correlation function for this system. Em and co-workers [ 161 have pointed out that the essentially two-dimensional nature of exciton propagation in anthracene predicts a spectral density that must diverge as 0 + 0 in contrast to the usual lorentzian form S(w) 0~ (1 -f-w2t2)-l which is constant in the iimit of small W. Soos’ calculations [IS] would predict divergence as o- “* for a one-dimensional system as w + 0. The experimental frequency dependence is, unfortunately, unknown. Although Kawamori [ 171 has reported data.at 8 and 16 MHz. it was obtained with the less precise saturation technique. Her relarive data is cgnsistent with an w2 dependence at low temperature, characteristic of the lorentzian form, but is not consistent with our 38 MHz data. While the lack of an extremum in our density normalized data does not allow one to experimet&lly rule out the possibili-: ty of the lorentzian form, we feel that the theoretical 254 __ : :i
15 hlarch 1974
arguments against it are valid and that further pulse work at different frequencies is needed to properly resolve the matter. Furthermore, the treatments of correlation functions in one or two dimensions mentioned previously implicitly assume a ZUWdensity exciton limit. While this may be appropriate for our compounds at very low temperatures it is certainly not appropriate at the higher part.of the temperature range studied and we feel strongly that the fact that these systems are densely excited must be taken into
account in an adequate phenomena.
description
of the relaxation
Acknowledgement This work was supported in part by the National Science Foundation Grant GP-22546. We are indebted to the Research Triangle Institute for use of the spinecho pulse equipment, to Dr. B. Crist for aid in its operation and for helpful discussions, and to Professor M.T. Jones of the University of Missouri at ST. Louis for sending us his unpublished results.
References [ 1 J J.C. BaiIey and D-B. Chesnut. J. Chem. Phys. 5 1 (1969) 5118. [2] W.J. Siemons, P.E. Bierstedt and R.G. Kepler, J. 0em. Phys. 39 (1963) [3
3523.
J L-B. Coleman, M.J. Cohen, D.J. Sandman, F.G.
14) [5] [6] [7]
IS]
Yamagishi. A.F. Garito and A.J. Heeger, Solid State Commun. 12 (1973) 1125. W.A. Little, Phys. Rev. 134 (1964) 1416. D-B. Chesnut and W-D. Phillips, J. Chem. Phys 35 (196 1) 1002. R-G. Kepler, J. Chem. Phys. 39 (1963) 3528. D.B. Cbesnut and P. Arthur Jr., J. Chem. Phys. 36 (1962) 2969. A.T. McPhail, G.M. Semeniuk and D.B. Chesnut, J.
Chem. Sot. A (1971) 2174. and references therein. (9 J R.P. Shibaeva and L-0. Atovmyan, J. Struct. Chem. 13 (1972) 514.
[lo] G. Nyberg, D-B. Cbesnut and B. Crist, J. Chem. Phys. 50 (1969) 341. [ll] H.Y. Carr and E.M. Purcell, whys. Rev. 94 (1954) 630. 1121 M.T. Jones, private communication (1973). [13] M.T. Jones, J. Chem. Phys. 40 (1964) 1837. [I4 J 2-G. Soos, I. ChemI Phys. 44 (1966) 1729. [IS] Z.G. Soos and H-M. McConnell, J. Chem. Phye 43 (1965) 3780.
[16] V. Em. A; Suna. Y. Tomkiewicz. &. Ayakian and R.P. Groff, Phys. Rev; BS (1972) 3222. 1171 .A. Kawamori, J. Chem. Phys. 47 (1967) 3091..
.’
+EMICAL
iwsrcs wrrms
:
-15 March !974
.
‘_
THE TEMPERATUREDEPENDENCE OF PROTON RELAxATI~NTI~~ES INTRIPLETEXCITONION RADICALSALTS GM. SEMENIUK and D.B. CHESNUT
Received 17 January 1974
Proton spin lattice relaxation times have been measured for a variety of single crystal orientations of the j@sAsCH3)?TCNQ)5 ion radical salt over the temperature range of approximately 90 to 370°K. It is shown that the relaxation rate is dire&y proportion& to the triplet exciton density where the exciton production energy is significantfy dependent on temperature. The data suggests that exciton-exciton exchange is an important aspect in
the relaxation mechanism.
1. Introduction The ion radical salts of TCNQ (tetracyanoquinodemethane) constitute an extremely interesting class of materials due to their unusual electrical, magnetic, and structural properties*. They exhibit some of the highest electrical conductivities known for organic solids [2] and have been suggested 131 as candidates for Little’s proposal [4] for a high-temperature superconductor. Many of these salts exhibit low lying, thermally accessible magnetic states [5,6 ] which can be approximately described in terms of tightly bound (Frenkel) triplet excitons [I, 71. These unusual properties are reflected in the cryst$ structures which show a one-dimensional plate-like stacking of the planar TCNQ molecules with short interplanar iritermolecular distances and, in some cases, evidence of clustering of sup&molecular species_@] **_ In the subclass of TCNi! salts which shows the spin correlated, mobile excitations there has been considerable interest in the exciton dynamics. The triplet excitons can achieve ver$ high densities (ca. 2X 1020 CmM3 for &AsCH3(TCNQ), at 300%) and lead to most interesting and still only incompletely-understood ., * A comprehensive referencelist is g&enin ief. [ 11, ** Ref. 191 contains a review of structures published through January’I, 1972.
phenomena. The purpose of this no&eis to report the first part of a single crystal study of proton Iongitudinal relaxation times (T,) which corrects an earlier ~te~retation of similar but limited work [ 101 and which adds to the already large list of data illustrating the unusual nature of the TCNQ salts.
2. Results and discussion In 1969 Nyberg et al. [ 101 reported the proton Tl. values of several powder and a few single-crystal orientations of TCNQ salts which display the presence.
of triplet excitons. ft was shown that the triplet excitons clearly dominated the proton Tt values but., as discussed below, the analysis of the data has proven
to be, we believe, in error. Since that time the detailed .crystal structures of both the (#sPCH$(TCNQ)F and (+3A~CH3)+(TCNQ)z salts have been worked out [8]and with this information in hand it was decided to-carefuUy remeasure TI both as a function of tern-. perature and orientation for the arsonium salt, probab$y-the best characterize? compound of the excit$ ,. subclass of TCNQ salts. As before t$e, re!axatjon time? were measured by _ .ther na pulse technique [ 1 l] at 35 MHz. ‘J’he-&ngle ex$dnential behavior of the signql reckrery shows that a single relaxation’ t*e is b&g obseived; We: :-’ .I: . .,.. .. .. : . .’ ‘. 25 1 .-, .XI
.’
.-
:
.
._
:
‘,
:
.’
::
,+.
:,
.
Volume25, number 2
.:; ‘15 March 1974, _: .:
CHEMICALPHYSICSLETTERS
I
II
2
t
I’II)
4
1
f?
II1
6 a ‘&C-y.:tOOOfT . i'K"' f
Pig. 1. Logarithmicplots of Tt (left) and (Tt p)-’ (&girt) as a function of inverse temperature for a varZety of rotation angles. Tr is expressed in miIIireconds in each plot:Successively higher curves are offset numerically by factors of three for clarity of presentation with the lowernkt curve representing no offset. Data.fromseveraldifferentexperimentsat each orientation are contained
in the figures.
. -~ report here. the .temperaiure behavior for seven rotation &gIes about one axis of rotation_ The rotation axis is parallel to the (I, I;?) direction in tlie ,crystal -: ai determined by the &le’sexternal morphology; a fulled dekiption of the &ystal orientations &ii1 ap,
,.
,‘.
-. -.
tion and that the TI ~minirn~ &curs when w;i = .I,, &I interpretation Giseswbich does not aflow a direct dependence”of Ti’ ‘tin p, the excitc$ density. ?I& is. ‘-not-readily understood-and Was t&en at-the time to.::
Volume 25, number 2
CHEMICAL
PtiYSICS
.I5 March’1974.
LETTERS
Table 1
Leastsquares values for (Tr p)-’ =A exp(pE) as a function of orientation. The data was tit over the temperature range ofap, proximately
90 to 370°K.
The standard deviations are denoted by o
Orientation
E (eV) X lo3
ffE (eVj X lo3
~__ A (set-‘)
I-O” I-30” I-60” I-100” I-1300 i-l& I-1700
69.5 66.3 68.5 69.3 68.7 71.0 71.6
0.46 0.46 0.52 0.43 0.47 0.50 0.51
6.11 7.43 5.08 6.48 4.75 3.55 4.57
averages
69.3
0.48
S-42
T;’ =pq.dj,
(1)
S(w) being the spectral density function involved. In order to match the observed qualitative behavior of T, it tias found to be necessary to invoke a temperature dependence of J = J(T) such that (2a) + 0.005 14 ( lo3 T-l)
(in eV)
(Tb)
f&r the.temperature range of +&oximatel+ 100 to 400°K. Throti& correspondence with M-T_ Jones [ 121 it was learned that this was fi%ecis&ythe temperature dependencetf+md necessary in un&blishedwork+ required to fit the observed ESR intensity-behavior over t&i same temperature range. Our owni&depeAdent co&l&ion of. this temp&ture &p&den& of 3
-.
::;.
,__;_
-.
0.24 0.28 --0.21
_‘0.22 0.18 0.14 0.19
-0.21
..
_..____--_
logarithmic correlation time plots which showed considerable curvature, continuing to suggest a flaw in the model and that T, should depend explicitly on p. Accordingly a model presuming an explicit p term was proposed where
J(T) = 0.0286
‘.
1.82
1.7
aave
bA (xc-‘)
Table 1 shows that A definitely depends on orienta-‘tion while E probably does not. The standard deviation of the average A-value is some-nine times-the standard deviation of that of any on& orientation -. while the similar ratio for the E data is only about ..’ three. The anisotropy in T, as reflected by the least squares fit .of A to the temperature data agrees with the anisotropy measured directly, as indeed it shpuld, and shows that the anisotropy is virtually indepen: dent of temperature. It is not obvious that the activation energy should be anisotropic; the least squares anisotropy does not show any correlation withthe _ amsotropy of A and its relatively. s&l1 value is probably within the range ofexperimental uncertainty. The most obvious mechanism fck enhancing the proton.relaxation’ratek the direct interaction of the -large intern+ magnetic field provided by the triplet. exciton’s magnetic moment modulated by the motion -. of.the,exciton &rough .tie cjstal. The.motion of the exciton is refledted in the absence of nuclear hyper-, c. fine structure in thk ESR spectfa [iland estimate? of
Volume 2.5. number 2
CHEMICAL PHYSICS LETTERS
vated. The fact that the observed E value in this study is of comparable magnitude to the predicted exchange barrier is strongly suggestive of involvement of exciton-
exciton
exchange
coupling
in the nuclear relaxation. Jn addition, accurate prior data is available on the temperature dependence of the (Cs’),(TCNQ)y2 salt inwhichJ(T)=0.185 -0.146(103 T-l)-’ ineV (note the linear dependence here of J on 7’) and for the (Et3NHf) (TCNQ)z salt where J is approximately constant and equal to 0.04 1 eV [6]. Using our previous proton T, powder data for these two salts the density-normalized relaxation rates indicate E values of 0.060 and 0.04-0.05 eV, respectively, for the cesiurn and triethyiammonium compounds. The necessary data for J for the 1: 1 morpholinium TCNQ complex [ 1] is probably less accurate but also leads to an E value of approximately 0.05 eV. The disparity of the cations involved in these salts but the common, essentially one-dimensiona stacking of the planar TCNQ molecules lends further confirmation to the involvement of the triplet excitons migrating along the TCNQ stacks and suggest that the process dominating the relaxation is common to them all. Should this be so, it is noteworthy that for the best characterized materials (the arsonium and cesium salts) one contains a complex, proton-containing cation ($,AsCHj in the arsonium salt) while the other contains a simple cation with no nuclear moment (the cesium salt). The detailed physical interpretation of the empirical relation satisfied by the density-normalized relaxation rate [es_ (3)J must await the determination of the proper correlation function for this system. Em and co-workers [ 161 have pointed out that the essentially two-dimensional nature of exciton propagation in anthracene predicts a spectral density that must diverge as 0 + 0 in contrast to the usual lorentzian form S(w) 0~ (1 -f-w2t2)-l which is constant in the iimit of small W. Soos’ calculations [IS] would predict divergence as o- “* for a one-dimensional system as w + 0. The experimental frequency dependence is, unfortunately, unknown. Although Kawamori [ 171 has reported data.at 8 and 16 MHz. it was obtained with the less precise saturation technique. Her relarive data is cgnsistent with an w2 dependence at low temperature, characteristic of the lorentzian form, but is not consistent with our 38 MHz data. While the lack of an extremum in our density normalized data does not allow one to experimet&lly rule out the possibili-: ty of the lorentzian form, we feel that the theoretical 254 __ : :i
15 hlarch 1974
arguments against it are valid and that further pulse work at different frequencies is needed to properly resolve the matter. Furthermore, the treatments of correlation functions in one or two dimensions mentioned previously implicitly assume a ZUWdensity exciton limit. While this may be appropriate for our compounds at very low temperatures it is certainly not appropriate at the higher part.of the temperature range studied and we feel strongly that the fact that these systems are densely excited must be taken into
account in an adequate phenomena.
description
of the relaxation
Acknowledgement This work was supported in part by the National Science Foundation Grant GP-22546. We are indebted to the Research Triangle Institute for use of the spinecho pulse equipment, to Dr. B. Crist for aid in its operation and for helpful discussions, and to Professor M.T. Jones of the University of Missouri at ST. Louis for sending us his unpublished results.
References [ 1 J J.C. BaiIey and D-B. Chesnut. J. Chem. Phys. 5 1 (1969) 5118. [2] W.J. Siemons, P.E. Bierstedt and R.G. Kepler, J. 0em. Phys. 39 (1963) [3
3523.
J L-B. Coleman, M.J. Cohen, D.J. Sandman, F.G.
14) [5] [6] [7]
IS]
Yamagishi. A.F. Garito and A.J. Heeger, Solid State Commun. 12 (1973) 1125. W.A. Little, Phys. Rev. 134 (1964) 1416. D-B. Chesnut and W-D. Phillips, J. Chem. Phys 35 (196 1) 1002. R-G. Kepler, J. Chem. Phys. 39 (1963) 3528. D.B. Cbesnut and P. Arthur Jr., J. Chem. Phys. 36 (1962) 2969. A.T. McPhail, G.M. Semeniuk and D.B. Chesnut, J.
Chem. Sot. A (1971) 2174. and references therein. (9 J R.P. Shibaeva and L-0. Atovmyan, J. Struct. Chem. 13 (1972) 514.
[lo] G. Nyberg, D-B. Cbesnut and B. Crist, J. Chem. Phys. 50 (1969) 341. [ll] H.Y. Carr and E.M. Purcell, whys. Rev. 94 (1954) 630. 1121 M.T. Jones, private communication (1973). [13] M.T. Jones, J. Chem. Phys. 40 (1964) 1837. [I4 J 2-G. Soos, I. ChemI Phys. 44 (1966) 1729. [IS] Z.G. Soos and H-M. McConnell, J. Chem. Phye 43 (1965) 3780.
[16] V. Em. A; Suna. Y. Tomkiewicz. &. Ayakian and R.P. Groff, Phys. Rev; BS (1972) 3222. 1171 .A. Kawamori, J. Chem. Phys. 47 (1967) 3091..
.’