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Temperature dependence of single crystal raman spectra of the complex (perylene)3 TCNQ
Chemical Physics 66 (1982) 293-301 North-Holland PubIS& Company
TEMPERATURE DEPEND&E OF SE’JGLE CRYSTAL RAR%ANSPECTRA OF THE CO&&X (PERYLENE)3 %!NQ AD. SANDRA-UK *, KD. TRUONG * and C. CARLONE f Groripede recherchesSWles sem~~onducreurs et ditTecm@es, Facultt?des SciencZs,UCversitPde Sherbrwke, Shdmoke,
P-Q.,
Cbm?a
Ilic
ZRI
Received 1 November 1981
Single crystals of the charge transfer complex @eryIene)s TCNQ (P3Tl) and the deuterated peryiene homoIo~ue Pd3Tl exhibit a fluorescence over a narrow temperature me from ~200 to 250 K. Raman spectra of low and high frequency \%mitions In the crystals were studied as a function of tem_peratz+z.The widths of some intramolecular modes show stroe kotope effects and a temperature dependence which can be attributed to coupes to low frequency modes. Most low frequency modes broaden continuous with temperature, in contrast to the disappearance of the fluorescence above 250 K.
i . Introduction Recently we reported the preparation of a new complex of TCNQ, (peryiene)3 TCNQ denoted as P3Tl [i] _Preliminary Rarnan spectra of powders taken in KBr pellets [2] &owed that for this compound a room temperature Raman spectrum for the peryIene molecules was observable without the interference of the usual intense fluorescence of perylene [3]. A complete Raman spectrum of perylene has been obtained since, from low frequency phonons to high frequency overtones and combinatbns. Only fundamentals are observable in *he case of the PI Tl complexes [h] . in the present work, we examine the temperature dependence ofsome of the modes ofthe P3Tl complexes for which perylene overtones and combinations are readily excited. Our interest in this system stems from the fact that the first ionization potenpal of tetrathiafulvalene, forms an extensive series of conductive, mixed valence compounds with organic acceptors such as TLWQ, is comparabie to that of perflene, i.e. 6.83
w&h
rise to a charge transfer (CT) absorption at 960 nm [1-4].Neutralsystemssuchasperylene-TCNQ(PlT1) are known to undergo neutral to ionic. phase transitions at l&b pressures [7]. Since much of structural information accessible by Raman spectroscopy is contained’in the measurement of the temperature dependence of vibrational frequencies and their Iinewidths [8 ] , we present here a detailed study of temperature dependent Raman spectra of P3T1 for which the crptal structure is now known [9] and is illustrated in fig. l_ Thus from the X-ray work of Hanson, each TCNQ is sandwiched between two perylene molecules, so that the PZT units are stacked aIong the z axis. isolated perylenes, stacked along the x axis, separate nelghbouring cohunns of the P2T units, giving rise to a triclinic structure with2 = 1, i.e. one P3Tl per unit cell. The present work is a first step in a proper understanding of this and other organic mater&& which have low lying ionic states which can influence the physical properties of these materials at high pressures [7] and low temperatures.
versus 6.97 eV [5,6]. Our preliminary work on P3Tl complexes indicates that these are weakly bonded covalent complexes with an excited ionic state giving
2. Experimental
* D&rtement de chimie.
P3Tl and Pd3T1 (deuterated perylene, Pd) cryst& were obtained by a diffusion method fl] . Several of the crystals, about 2 X 3 X 9 mm grew into the shape
f D&ar:e_ment de physique.
0301-0i04/82/0000-0000/~
02.75 0 1982 North-Holland
294
A.D. BcnahixR et al./Ramai! spectra of
(perylenej3
TCWQ
coam&?x
wexe
excited readily with an argon-ion laser witi typical power Ievels below 15 mW at the sample in order
to avoid crystal decomposition, especially ir the resonance region at 4579 nm [2]_ All spectra were taken
Fig. 1. The ilnif ceil of P3T1. -J%he P2T units ciystallize along the z axis, the neutral perylenes akng the x axis.
at 90” and recorded on a strip recorder and simultaneously stored in a multichannel analy?r. A double monochromator, Jobir-Avon Ramanor HG2S, equipped with a scrambler was used_to resolve the lines. A polarizer was also used to select the direction of the electric field incident on the crystaI. Slit width was maintained usually at z?- cm-1 for simultaneous optimum signal intensity and resolution conditions. JIn hewidth measurements, the instrument width was measured and found to be gaussian. True linewidths were obtained by numerical deconvolution, assuming the true lines were lorentzhn.
of the unit cell, i.e., well defined parallelepipeds. The crystals have a deep blue-black colour so that IR transmission experiments were impossible. Rarnan spectra
P3TI 45795.
_i_A_i_
235K
Fig. 2. Raman and_Fcence spectra as a function of temperature - emission WISobserved from yz surfaces with .z polarized excitation at 2 cm slit width. (a) P3Tl scale: 1.5 X IO4 cps at 234 K, lOi cps for other temperatures: (II) Pd3Tl scale: 0.5 x lo4 cps at 295 K, IO’ cps for other ?emperatures.
A.D..Bandrauk eral./Raman specna of (pperytene_jsTCNQ complex
_
295
PST1
A complete Run2n spectrum at room temperature was obtained for various polarkation for P3Tl and Pd3T1 single crystals [4] _ SeIective resonance enhance-
ment of f&e perylene vibrations was observed with no fIuorescence evident at the perylene resows wave length at 4572 mu. This is to be contrasted with the usual strong fluorescence of perylene at such excitation wavelength even at 10~ temperature [3]. In fig. 2, we present the resonant F?2m2n spectr2 of our P3Tl and the deuterated perylene Pd3T1 caystak at various temperatures with excitation along the z axis of theyz surface (fig. 1). This axis 2nd surface were determined by the total absence of TCNQ viirations for such scattering conditions. This choice of excitation axis therefore excites mainly the intmmolecular modes of the isolated perylene, thus reducing the number of lines and dimhnishing overlaps especialIy in the high frequency overtone region. This incident pokization also favors coupling of librations of the complexed perylene and TCNQ to the charge transfer bands which are pohu-ized parallel to rhe stacking axis z [4]. We observed a strong fluorescence which vases at low temperatures and at room temperature in both complexes. Temperature dependent measurements of the appearance of this fhrorescence leads us to con&de that in both complexes this fluorescence exists over a range of about SC%! between 200 and 250 K.
This fluorescence interferes with any careful intensity and linewidth measurements for vibrations above 1000 cm-l _For this reason, it was possible to put accent only on intmmolecuhu modes below this frequency and in particular on IO-Wfrequency modes. Only in the case of the deuterated compound did any noticeable intensity changes occur io the fluorescence region. Thus in fig. Zb, the relative intensity of the most intense 1357 cm-l line versus the doublet at 1550 cm-l in Pd3T1 seems to change in the fluorescence region when compared at other temperatures_ The undeuterated complex showed littie re!ati\q ‘mtensity variation, fig. 2a. Previous fluorescent and simdtaneous Raman spectral cku~ges have been obse-ed in the mthrace2e-
Fig. 3. Temperature dependence of linewidths of nerylene vibmtioos - experimental conditions as in fg. 22.
changes, it is likely that the transitions are of continuous type in this complex and our perylene TCNQ CryStalL% In the Mse of P3Tl crystals, only the low frequency mode at 354 cm-l was measurable with any precision_ In fig. 3 we present the temperature width J? of that perylene mode and its overtone at 708 cm-l in addition to the 1367 cm-l fundamental. In fig. 4 we present the widths for the corresponding mode at 340 cm-l and its overtone at 680 cm-’ for the deuterated perylene P, . In addition we were able to measlnre the Ividth of the deuterated 1357 cm-1 vibra-
red lot 9.
8. 7.
I$3Ti x
34omr
2 1957
I 2x540
0 2x1357 a 340.1357
trinitrobenzene complex [lo]. The most likely explanation forsuch temperature dependent changes is thought to be a smwal phase tmnsition causing an alteration of the excited charge tmnsfer geometry [IO]. Due to the gradual nature of these spectral
Fis. 4. Tempenture dependence of liriewidrhs of deutenred perykne vibr2tions - experimental conditions 2s irr fir. 2b.
296
A.D. Bandnnrk eI aL/Raman specfra of @?ylene);
r’
F& 5. Temperature dependence of Lheshpe of the overtone (2 x 1367) cm-’ and the combination (1367 + 1376) cm-* - see fg. 2 for details.
tion, its overtone and combination with the 340 cm-l mode. In the fluorescence region, the fluorescent background was subtracted to obtain the ‘true lines. Tbis was possible for Pi3T1 crystals but not for P3Tl crystals where shoulders were always observed in the line structure of the corresponding overtones and combinations of the 1367 cm-l mode (see fig. 5j. We remark fustly that the wid-hs of the two fundamentals illustrated in tigs. 3 and 4 vary slightly through the fluorescence temperature region, wheras the -widths of the overtones and combinations tend to increase exponentially in and past that region, except for the P3T1 overtone at 708 cm-1 which would seem to peak in the fluorescence region. These two strong fundamentals are attributed to an $ vibration comprised of planar C-C and C-H vi’srations [11], in agreement with our isotope shifts. @f note is that the widths of these fundamentals (354 and 1367 cm-l) of perylene, P, are about mice the widths of the corresponding vibrations (340 and 1357 cm-l) of the deuterated perylene, Pd. The same seems to be true for the overtone of the 354 (340) cm-l vibration. We note that the excitation profde of this 354 CR-I-~
TGVQ complex
vibration has been studied carefully by Hochstrasser and Nyi [3] at 77 K. What we are observing here are isotopic effects on the temperature dependence of the widths of this and other vibrations in the ground state of the perylene molecule when complexed with TCNQ. The single crystals which we obtained enabled us to Iook carefully at very low frequency modes which are usually classified as iibrations, i.e. hindered rotations. Temperature measurements on such lattice modes have helped identify orientational transitions in crystals of anthmccne [12] and naphthalene [13] with tetracyanobenzene (TCNB). These transitions have been discussed in connection with Peierls transitions in organic radical ion salts [14], and are therefore indicative of lattice dynamics. In fig. 6 we show the temperature dependence of the low frequency modes in P3Tl and P,3Tl. For this first compound, we show spectra at two different excitation wavelengths, 457.9 and 514.5 MI. Thus the three intense lines at 38,6.5 and 113 cm-l (attributed to TCNQ) at 514.5 nm excitation become weaker at 4.579 nm excitation where now modes at 45,50 ?, 75 and 84 cm-l (attributed to perylene) become prominent. The attribution of these lines is based on the selective enhancement of perylene vibrations at 457.9 nm 1143. Of note is that at 5 14.5 nm excitation, TCNQ intramolecular modes are very weak whereas the three low frequency modes at 38,65 and 113 cm-1 are prominent. Due to the selective enhancement of perylene vibrations at 457.9 nm observed in the present and previous work [2,4], we attribute the fust three lines at 37,65 and 113 cm-l to TCNQ and the last four to perylene. Gas phase Raman spectra of TCNQ show low lying internal modes at 40,75 and 145 cm-1 [15]. We thus assign the 37 and 6.5 cm-l modes as internal modes of TCNQ. The 113 cm-l line broadens very rapidly and is therefore assigned as a libration. A mode at 110 cm-l has been observed in solid perylene [3] and solid TCNQ [16]. We are witnessing therefore the overlap of modes from different molecules. This could explain the rapid temperature dependence of this mode (fig. 6). The pair of modes of roughly equal intensity at 75 and 84 cm-l as mentioned above is assigned to peryfene and broadens also rapidly with tem7 This mode is masked by a phma line at 457.9 run excitation
only
[4] _
A.D. Bandmuk et al./Rmnm specua of (peiyIem& TCiVQ complex
’i
I
P3TI 5145 A” P3T! 4529A’
k Z
F -Z
L
i-
237%
i43’K
4 k!
50
rm
150
2aJ
250
cn
;
Fe_ 6. TemperaturedependentspectrumofP3Tl low frequency modes from the yz surfaces,= polarized excitation, 2 cm-r speo tral slit, 10 s time constant, 2 cm-t/min scan: (a) 457.9 mn excitation, (b) 514.5 nm excitation.
perature. We tentatively assign these two as perylene librations. The disappearance of the 45 cm-l mode at higher temperature wo?lld indicate it is also a liiration of perylene. Finally, the strong temperature dependent
feature at 189 cm-l (disappearing at 37 K) in both complexes is attributed to a difference band [548 (523)354(340) cm-l] of perylene due to its selectivti enhancement by the 457.9 nm laser line.
4. i.Xsction 4.1. High frequency modes The most striking features of this frequency regime as illukated in figs. 3-5 are: (i) An important isotope effect on the linewidths of
the fundamentals of perylene in P3Tl as compared to deuterated perylene in Pd3T1.
(ii) An exponential increase .of the linewidths of combinations and overtones in the fluorescence region. Since the fundamentals at 354(340) cm-l and
1367(1357) cm-l (deuterated frequencies in parentheses) contain some C-H vibration, the amplitude of that vibration will decrease upon deuteration. Tem-
perature effects arise from anharmonic terms corresponding to thermal expansion as well as phonon-phonon scattering [17-181. One can therefore expect a reduction in coupling with the environment~in the case of the deuterated molectde. The linewidths for the 354 cm-! fundamental and its overtone decrease by about a factor of two in going from the perylene to deuterated perylene complex. This wo-uldsuggest that it is maidy the C-H component of the vibration wbicb is
298
A.D. Bmahu.k et aL/Raman spectra of (psylene)s TC2VQcomplex
responsiile for the width. This same decrease in linewidtfr is observed for the fundamental at 1367 cm-l _ Unfortunately, due to overlapping bands, fig. 5, accurate lintxvidt&s of the overtone at 2730 cm-l (2 X 1367) and the combination of 2743 cm-l (1367 + 1376) were not measurable.
Table 1 Activation
enei%es Ei obtained
from tin- 8 (accuracy *20 cm-l)
Ei (cm-' j w (cm-lj
IilAr
(2 x 340) 340 c 1357
212
hAw
NormaBy, C-H stretch&g
modes are nearly as sensitive to lattice perturbations as are lattice modes whereas carbon skeletal modes are Ie& SeIlS3ii~ [19]. We thus see fiom the &ovc temperature belxkour of the linewidth?s that a small C-H bond component can lead to important isotope effects as it is probing tie emironment external to the molecule.
More sign&ant is the iinewidtb bebaviour of the overtones and combinations in bo& deuterated and undeuterated compound. Althou& the measure widths are not exceptionaliy accurate due to the fkorescence backgouad which bad to be subtracted in ‘Lhe200250 K region, nevertheless a noticeable increase in bnewidth was observed, figs. 3-5. As the beha%jour appeared exTonentiai Gke, the temperature dependent increments of widths, At?, and of frequency shifts, Aw, were obtained from the experimental data by subtracting the essentially temperature independent values at 37 k. This corresponds to the assumption that Ar and Aw can be represented as the exponential forms 217,201
where 4, b are temperature independent constants and Ei is m acthation energy. In fig. 8 we present plots (least-squares fits) of In Aw and in AI’ versus i/T for the complex Pd3T1. The activation energies obtained by this method are tabuiated in table 1 (no due is given from mzvurements of Aw for the overtone at 2 X 340 cm-l s&e shSs were smali and therefore inaccurate). From tabie 3 we conciude that the actirrltion energies obtained from two different physical measurements (AI’ and Aw) coneiate rather well, thus leading credence to the applicability of eq.
(2 x 1357)
coupled to a 136 cm-l mode {close to the TCNQ mode at 144 cm-l ar?d the perylene mode at 135 cm-l, fig. 6) and the 1357 cm-1 mode k coupled to a 207 cm-l mode (fig. 7) we can expJ,zin the activetion energy Ei as the combination of these two low frequency modes (339 = 136 f 297, table 1). This would imply that the A overtones and combinations mainly are coupled to B overtones and combinations. This is reasonable since in some cases even Raman fundamentals have been observed to be coupled to low frequency combination bands [20]. Furthermore, the above hot band coupEng model tiplies an anharmonic coupiing of the form &p2 f20]. Thus broadening wiu be proportional to (eA? > &!A being
1
25
50
Ido
ax,
150
LL 76ii
(1). Although various models have been proposed to explain such experimental temperature dependence for fundamentais [17,20], the present data are more consistent with the model of interactions of high frequency modes A and hot bands of low frequency modes B. Thus assuming that the 340 cm-l mode is
329 426
339 418
25
53
co
150
ma
car’
Fig. 7. Teqemtwe dependent spectrum Of Pd3Tl Iow frequency modes - experimenta: dttails as in fg_ 6 (50 cm-’ - p&ma line).
A.D. Ban&auk et al_,fRamzm spectra of @rylene)3
290
TCNQ complex
fiequencymodes.
Theseactivation energiesarethere-
fore .larSer ‘&XI for fimdamentals and are therefore more readily measurable as they lead to stronger temperature dependences of A.o and AI’. 4.2. Low jiequency
n?odes
Interpretation of low frequency modes is usually based on the assumption of separation of external (translations, hbrations) from the internal molecular modes. Kecent measures of Grtineisen parameters in molecular [21] and quaai-one&nenaionai crystals [IS] indicate this distinction is blurred in complex
-1.5 t
Fig_ 8. Temperature dependence of lo&t.hmic increments in(a) frequency-Awj(b)tieWidth -Al-.
the mode amplitude) which is proportional to 11~ where JI is the reduced mass of the mode. Since the two modes studied in table I con’Z+.inconsiderable C-H bond component as witnessed by the isotopic shift of the fundamentals, we can thus explain the j&or of two enhancement of width of the fundamentals and overtones in table I for the perylene as compared to the deutemted perylene (fig. 4) as arising from the C-H bond component. Activation energies were not obtainable from the shift Ao of the fundamentals & these shifts were q---1cm-1 in general and difficult to measure v&h accuracy. What is interesting, is that for overtones and combinations of these Jndmentak, the activation energies correspond toexcitation of overtones and combiiations of low
loo
b)
2po
3GQK
3% r 120
t
pb
1
100
-1
’
3&K
Fig: 9. Temperature dependence of low frequencies at 514.5 nm excitation df (a) P3T1, (bj i’d3n
300
AD_ Eandmk et aL/Ramanspectraof [per/he)3
crystals with different bonding strengths due to the
strong dependence of the Griineisen parameters on anhronicity effects [21]. The temperature dependence of mode frequencies arises also from thermal expansion and anharmonicity effects [17,18]. Thus in fig. 9 we illustrate the temperature dependence of the fundamental at 354(340) cm-1 and the low freqttency modes observed below 120 cm-l in both complexes. The fundamental is only slightly down shifted (up to 1 crnl) by the thermal expansion and phononphonon scattering. As discussed above, it is the iinewidth which is a more sensitive measure of coupling of fundamentals to the lattice. All low frequency modes show a normal linear decrease in frequency with increasing temperature [17,18], including the intenzaI low frequency modes assigned at 65 and 37 cm-1 to TCXQ_ (This assignment was made on the resonance behaviour of their intensities, previous jettion.) S&e the thermal expansion coefficient and compressibility are not known for P3T1, no sepaation of the thermal expansion and anharmonicity contributions is feasible [ 17, IS] _ The present temperature measurements do not permit a clear distinction between internal and external modes; the similarity in temperature dependence of their frequer.zies woirid imply that these are mixed. On the other hand, the similar resonance behaviour of the intensities of the modes at 35,65 and 113 cm-l implies they are activated by the same excited election&z state, so that they manifest vestiges of TCNQ intiaamotecdar motion (see fig. 6). ln addition, due to the z axis polsrizstion of the excitation, the coupling would be to the charge transfer states appearing at 410 and 960 nm in ihe optical absorption spectra of PlTl and P3Tl I4 j _The rapid broadening of the 75-84 CXII-~doubler and the 113 cm_1 line sugests dramatic changes in coupling 2nd therefore in structure in tie region above 15O.K where fluorescence begins. Observed silangn in fluorescence with temperature are usually indicative of a structural phase transition [10,22] _ Sirxc the law temperature Raman spectrum is sharp and consistent with an ordered stmcture, the rapid broadening of the above low frequency mode in the korescence region (above 150 K) suggests a gradual order-disorder transition as in naphthalene-TCNB [2Zj. The curious disappearance of the fluorescence above 250 K and the complete collapse of most low frequency modes and some Kigh frequency modes
TCNQ mptex
(fig. 5) is however diffic*ult tc reconcLle w&b the above interpretation_
5. CoilcllLsion
A study of the temperature dependence of widths and frequency shifts of the 354(340) cm-l and 1367(1357) cm-l fundarrentals, overtones and their combinations in crystals of P3Tl show a strong isotope’ effect when compared to Pd3Tl. Thus the linewidth of perylene vibrations was found to be usually twice that of undeuterated perylenes. A detailed temperature study of the increase of the widths and shifts of the overtones and combinations of these two viirations gave estimates of activation energies for these temperature effects. These activation energies were found to correlate well with observed low frequency modes which are coupling strongly to the above high frequency modes. Of note is that these activation energies involve overtones and combinations of the low frequency modes, rendering temperature increases in widths and shifts of high frequency overtones and combinations more significant than for fundamentals. An onset of fhrorescence was found at ~200 K which disappeared above ~250 K. An attempt was made to correlate changes in the low frequency (librations) spectrum to the onset of the fluorescence. Most low frequency modes broaden continuously, becoming extremely dampened at room temperature. Hence no light is shed on the onset and disappearance of this fluorescence, in terms of dynamics of the lattice as a function of temperature. A measure of the decay time as done in pure perylene [3] would be interesting to elucidate the drastic effects on the relaxation times in the excited states of this complex.
AcknGwIedgement
We thank the National Research Council of Canada for equipment‘grants and the Defense Research Board of Canada for grants supporting this research.
Refererxes [l J K_D_Tmorz and ADD.Bandrauk, G-mn Phys Lettexo 44 (1976) 232.
A.D. Bdndrouk et al/Roman spectra of (pegfenej3 TCNQ complex [2] A-D. Bandrauk, K.D. Truong, V.R. Salves and H.I. Bemsteh, I. Raman Spestry. 8 (1979) 5. [3] RX. Hochstrasser and CA. Nyi, J. Chem. Phys 72 (1980) 2591. [4] AD. Bandrauk, K-D. Truong and C. Carlone, submitted to Car-i.J. Chem (1981); K-D_Truong, Ph_DD. Thesis, to be submitted, Universit& de Sherbrooke (1981). [S] .JB. Torxaoce, AccountsChem. Res 12 (1973) 7s. 161 J.B. Torrance, AM. Acad. Sciences N.Y. 313 (1978) 210. 171 Jd. Torrance, JE.Va.zquez, JJ. Mayerle and V.Y. Lee, Phys. Rev. Letters46 (1981) 253. 181 S.P. Cramer, B. Hudson and D&l. Burland, J. Chem. Phys. 64 (1976) 1140. [9 ] A.W. Hanson, Acfa Cryst. B34 (1978) 2339. [IO] .R.L. Beckma;l, J-M. Hayes and G.J. Small, Chem. Phys 21 (1977) 135. [ll] M.A. Kovper, A.A. Terekhov and L.M. Babkov, Opt. Spectry. 33 (1972) 38.
301
[ 121 H. hfiihwald, E. Erdle and A. Thaer, Chem. Phys. 27 (1978) 79. [13] HJ. Bernstein, NS. DaIal, W.F. Murphy, AH. Reddoch, S. Sunder and D.F. Warns, Chem. Phys. Letters 57 (1978) 159. [14] H. Morawitz, Phys. Rev. Letters 34 (1975) 1096. [15] C. Carlone et al., 3. Chem. Phys. 75 (1981) 3220. [16] A. Girlando and C. PeciIe, Spectrochim. Acta 29A (1973) 1859. [17] J.C. Bellows and P-N. Prasad, J. Chem. Phys. 70 (1979) 1864. [lS] F.J. Owens and 2. Iqbal, J. Chem Phys. 74 (1981) 4242. [19] M. Ni&ol, hf. Vernon and J-T. Woo, I. Chem Phys. 63 (1975) 1992.
[20] S. Marks, P.A. Cornelius and C.B. Harris, J. Chem. Phys. ?3 (1980) 3069. [21] R. Zallen, Phys Rev. B9 (1974) 4485. [223 R.M. Macfarlane and S. Ushioda, J. Chem. Phys. 67 (1977) 3214.
TEMPERATURE DEPEND&E OF SE’JGLE CRYSTAL RAR%ANSPECTRA OF THE CO&&X (PERYLENE)3 %!NQ AD. SANDRA-UK *, KD. TRUONG * and C. CARLONE f Groripede recherchesSWles sem~~onducreurs et ditTecm@es, Facultt?des SciencZs,UCversitPde Sherbrwke, Shdmoke,
P-Q.,
Cbm?a
Ilic
ZRI
Received 1 November 1981
Single crystals of the charge transfer complex @eryIene)s TCNQ (P3Tl) and the deuterated peryiene homoIo~ue Pd3Tl exhibit a fluorescence over a narrow temperature me from ~200 to 250 K. Raman spectra of low and high frequency \%mitions In the crystals were studied as a function of tem_peratz+z.The widths of some intramolecular modes show stroe kotope effects and a temperature dependence which can be attributed to coupes to low frequency modes. Most low frequency modes broaden continuous with temperature, in contrast to the disappearance of the fluorescence above 250 K.
i . Introduction Recently we reported the preparation of a new complex of TCNQ, (peryiene)3 TCNQ denoted as P3Tl [i] _Preliminary Rarnan spectra of powders taken in KBr pellets [2] &owed that for this compound a room temperature Raman spectrum for the peryIene molecules was observable without the interference of the usual intense fluorescence of perylene [3]. A complete Raman spectrum of perylene has been obtained since, from low frequency phonons to high frequency overtones and combinatbns. Only fundamentals are observable in *he case of the PI Tl complexes [h] . in the present work, we examine the temperature dependence ofsome of the modes ofthe P3Tl complexes for which perylene overtones and combinations are readily excited. Our interest in this system stems from the fact that the first ionization potenpal of tetrathiafulvalene, forms an extensive series of conductive, mixed valence compounds with organic acceptors such as TLWQ, is comparabie to that of perflene, i.e. 6.83
w&h
rise to a charge transfer (CT) absorption at 960 nm [1-4].Neutralsystemssuchasperylene-TCNQ(PlT1) are known to undergo neutral to ionic. phase transitions at l&b pressures [7]. Since much of structural information accessible by Raman spectroscopy is contained’in the measurement of the temperature dependence of vibrational frequencies and their Iinewidths [8 ] , we present here a detailed study of temperature dependent Raman spectra of P3T1 for which the crptal structure is now known [9] and is illustrated in fig. l_ Thus from the X-ray work of Hanson, each TCNQ is sandwiched between two perylene molecules, so that the PZT units are stacked aIong the z axis. isolated perylenes, stacked along the x axis, separate nelghbouring cohunns of the P2T units, giving rise to a triclinic structure with2 = 1, i.e. one P3Tl per unit cell. The present work is a first step in a proper understanding of this and other organic mater&& which have low lying ionic states which can influence the physical properties of these materials at high pressures [7] and low temperatures.
versus 6.97 eV [5,6]. Our preliminary work on P3Tl complexes indicates that these are weakly bonded covalent complexes with an excited ionic state giving
2. Experimental
* D&rtement de chimie.
P3Tl and Pd3T1 (deuterated perylene, Pd) cryst& were obtained by a diffusion method fl] . Several of the crystals, about 2 X 3 X 9 mm grew into the shape
f D&ar:e_ment de physique.
0301-0i04/82/0000-0000/~
02.75 0 1982 North-Holland
294
A.D. BcnahixR et al./Ramai! spectra of
(perylenej3
TCWQ
coam&?x
wexe
excited readily with an argon-ion laser witi typical power Ievels below 15 mW at the sample in order
to avoid crystal decomposition, especially ir the resonance region at 4579 nm [2]_ All spectra were taken
Fig. 1. The ilnif ceil of P3T1. -J%he P2T units ciystallize along the z axis, the neutral perylenes akng the x axis.
at 90” and recorded on a strip recorder and simultaneously stored in a multichannel analy?r. A double monochromator, Jobir-Avon Ramanor HG2S, equipped with a scrambler was used_to resolve the lines. A polarizer was also used to select the direction of the electric field incident on the crystaI. Slit width was maintained usually at z?- cm-1 for simultaneous optimum signal intensity and resolution conditions. JIn hewidth measurements, the instrument width was measured and found to be gaussian. True linewidths were obtained by numerical deconvolution, assuming the true lines were lorentzhn.
of the unit cell, i.e., well defined parallelepipeds. The crystals have a deep blue-black colour so that IR transmission experiments were impossible. Rarnan spectra
P3TI 45795.
_i_A_i_
235K
Fig. 2. Raman and_Fcence spectra as a function of temperature - emission WISobserved from yz surfaces with .z polarized excitation at 2 cm slit width. (a) P3Tl scale: 1.5 X IO4 cps at 234 K, lOi cps for other temperatures: (II) Pd3Tl scale: 0.5 x lo4 cps at 295 K, IO’ cps for other ?emperatures.
A.D..Bandrauk eral./Raman specna of (pperytene_jsTCNQ complex
_
295
PST1
A complete Run2n spectrum at room temperature was obtained for various polarkation for P3Tl and Pd3T1 single crystals [4] _ SeIective resonance enhance-
ment of f&e perylene vibrations was observed with no fIuorescence evident at the perylene resows wave length at 4572 mu. This is to be contrasted with the usual strong fluorescence of perylene at such excitation wavelength even at 10~ temperature [3]. In fig. 2, we present the resonant F?2m2n spectr2 of our P3Tl and the deuterated perylene Pd3T1 caystak at various temperatures with excitation along the z axis of theyz surface (fig. 1). This axis 2nd surface were determined by the total absence of TCNQ viirations for such scattering conditions. This choice of excitation axis therefore excites mainly the intmmolecular modes of the isolated perylene, thus reducing the number of lines and dimhnishing overlaps especialIy in the high frequency overtone region. This incident pokization also favors coupling of librations of the complexed perylene and TCNQ to the charge transfer bands which are pohu-ized parallel to rhe stacking axis z [4]. We observed a strong fluorescence which vases at low temperatures and at room temperature in both complexes. Temperature dependent measurements of the appearance of this fhrorescence leads us to con&de that in both complexes this fluorescence exists over a range of about SC%! between 200 and 250 K.
This fluorescence interferes with any careful intensity and linewidth measurements for vibrations above 1000 cm-l _For this reason, it was possible to put accent only on intmmolecuhu modes below this frequency and in particular on IO-Wfrequency modes. Only in the case of the deuterated compound did any noticeable intensity changes occur io the fluorescence region. Thus in fig. Zb, the relative intensity of the most intense 1357 cm-l line versus the doublet at 1550 cm-l in Pd3T1 seems to change in the fluorescence region when compared at other temperatures_ The undeuterated complex showed littie re!ati\q ‘mtensity variation, fig. 2a. Previous fluorescent and simdtaneous Raman spectral cku~ges have been obse-ed in the mthrace2e-
Fig. 3. Temperature dependence of linewidths of nerylene vibmtioos - experimental conditions as in fg. 22.
changes, it is likely that the transitions are of continuous type in this complex and our perylene TCNQ CryStalL% In the Mse of P3Tl crystals, only the low frequency mode at 354 cm-l was measurable with any precision_ In fig. 3 we present the temperature width J? of that perylene mode and its overtone at 708 cm-l in addition to the 1367 cm-l fundamental. In fig. 4 we present the widths for the corresponding mode at 340 cm-l and its overtone at 680 cm-’ for the deuterated perylene P, . In addition we were able to measlnre the Ividth of the deuterated 1357 cm-1 vibra-
red lot 9.
8. 7.
I$3Ti x
34omr
2 1957
I 2x540
0 2x1357 a 340.1357
trinitrobenzene complex [lo]. The most likely explanation forsuch temperature dependent changes is thought to be a smwal phase tmnsition causing an alteration of the excited charge tmnsfer geometry [IO]. Due to the gradual nature of these spectral
Fis. 4. Tempenture dependence of liriewidrhs of deutenred perykne vibr2tions - experimental conditions 2s irr fir. 2b.
296
A.D. Bandnnrk eI aL/Raman specfra of @?ylene);
r’
F& 5. Temperature dependence of Lheshpe of the overtone (2 x 1367) cm-’ and the combination (1367 + 1376) cm-* - see fg. 2 for details.
tion, its overtone and combination with the 340 cm-l mode. In the fluorescence region, the fluorescent background was subtracted to obtain the ‘true lines. Tbis was possible for Pi3T1 crystals but not for P3Tl crystals where shoulders were always observed in the line structure of the corresponding overtones and combinations of the 1367 cm-l mode (see fig. 5j. We remark fustly that the wid-hs of the two fundamentals illustrated in tigs. 3 and 4 vary slightly through the fluorescence temperature region, wheras the -widths of the overtones and combinations tend to increase exponentially in and past that region, except for the P3T1 overtone at 708 cm-1 which would seem to peak in the fluorescence region. These two strong fundamentals are attributed to an $ vibration comprised of planar C-C and C-H vi’srations [11], in agreement with our isotope shifts. @f note is that the widths of these fundamentals (354 and 1367 cm-l) of perylene, P, are about mice the widths of the corresponding vibrations (340 and 1357 cm-l) of the deuterated perylene, Pd. The same seems to be true for the overtone of the 354 (340) cm-l vibration. We note that the excitation profde of this 354 CR-I-~
TGVQ complex
vibration has been studied carefully by Hochstrasser and Nyi [3] at 77 K. What we are observing here are isotopic effects on the temperature dependence of the widths of this and other vibrations in the ground state of the perylene molecule when complexed with TCNQ. The single crystals which we obtained enabled us to Iook carefully at very low frequency modes which are usually classified as iibrations, i.e. hindered rotations. Temperature measurements on such lattice modes have helped identify orientational transitions in crystals of anthmccne [12] and naphthalene [13] with tetracyanobenzene (TCNB). These transitions have been discussed in connection with Peierls transitions in organic radical ion salts [14], and are therefore indicative of lattice dynamics. In fig. 6 we show the temperature dependence of the low frequency modes in P3Tl and P,3Tl. For this first compound, we show spectra at two different excitation wavelengths, 457.9 and 514.5 MI. Thus the three intense lines at 38,6.5 and 113 cm-l (attributed to TCNQ) at 514.5 nm excitation become weaker at 4.579 nm excitation where now modes at 45,50 ?, 75 and 84 cm-l (attributed to perylene) become prominent. The attribution of these lines is based on the selective enhancement of perylene vibrations at 457.9 nm 1143. Of note is that at 5 14.5 nm excitation, TCNQ intramolecular modes are very weak whereas the three low frequency modes at 38,65 and 113 cm-1 are prominent. Due to the selective enhancement of perylene vibrations at 457.9 nm observed in the present and previous work [2,4], we attribute the fust three lines at 37,65 and 113 cm-l to TCNQ and the last four to perylene. Gas phase Raman spectra of TCNQ show low lying internal modes at 40,75 and 145 cm-1 [15]. We thus assign the 37 and 6.5 cm-l modes as internal modes of TCNQ. The 113 cm-l line broadens very rapidly and is therefore assigned as a libration. A mode at 110 cm-l has been observed in solid perylene [3] and solid TCNQ [16]. We are witnessing therefore the overlap of modes from different molecules. This could explain the rapid temperature dependence of this mode (fig. 6). The pair of modes of roughly equal intensity at 75 and 84 cm-l as mentioned above is assigned to peryfene and broadens also rapidly with tem7 This mode is masked by a phma line at 457.9 run excitation
only
[4] _
A.D. Bandmuk et al./Rmnm specua of (peiyIem& TCiVQ complex
’i
I
P3TI 5145 A” P3T! 4529A’
k Z
F -Z
L
i-
237%
i43’K
4 k!
50
rm
150
2aJ
250
cn
;
Fe_ 6. TemperaturedependentspectrumofP3Tl low frequency modes from the yz surfaces,= polarized excitation, 2 cm-r speo tral slit, 10 s time constant, 2 cm-t/min scan: (a) 457.9 mn excitation, (b) 514.5 nm excitation.
perature. We tentatively assign these two as perylene librations. The disappearance of the 45 cm-l mode at higher temperature wo?lld indicate it is also a liiration of perylene. Finally, the strong temperature dependent
feature at 189 cm-l (disappearing at 37 K) in both complexes is attributed to a difference band [548 (523)354(340) cm-l] of perylene due to its selectivti enhancement by the 457.9 nm laser line.
4. i.Xsction 4.1. High frequency modes The most striking features of this frequency regime as illukated in figs. 3-5 are: (i) An important isotope effect on the linewidths of
the fundamentals of perylene in P3Tl as compared to deuterated perylene in Pd3T1.
(ii) An exponential increase .of the linewidths of combinations and overtones in the fluorescence region. Since the fundamentals at 354(340) cm-l and
1367(1357) cm-l (deuterated frequencies in parentheses) contain some C-H vibration, the amplitude of that vibration will decrease upon deuteration. Tem-
perature effects arise from anharmonic terms corresponding to thermal expansion as well as phonon-phonon scattering [17-181. One can therefore expect a reduction in coupling with the environment~in the case of the deuterated molectde. The linewidths for the 354 cm-! fundamental and its overtone decrease by about a factor of two in going from the perylene to deuterated perylene complex. This wo-uldsuggest that it is maidy the C-H component of the vibration wbicb is
298
A.D. Bmahu.k et aL/Raman spectra of (psylene)s TC2VQcomplex
responsiile for the width. This same decrease in linewidtfr is observed for the fundamental at 1367 cm-l _ Unfortunately, due to overlapping bands, fig. 5, accurate lintxvidt&s of the overtone at 2730 cm-l (2 X 1367) and the combination of 2743 cm-l (1367 + 1376) were not measurable.
Table 1 Activation
enei%es Ei obtained
from tin- 8 (accuracy *20 cm-l)
Ei (cm-' j w (cm-lj
IilAr
(2 x 340) 340 c 1357
212
hAw
NormaBy, C-H stretch&g
modes are nearly as sensitive to lattice perturbations as are lattice modes whereas carbon skeletal modes are Ie& SeIlS3ii~ [19]. We thus see fiom the &ovc temperature belxkour of the linewidth?s that a small C-H bond component can lead to important isotope effects as it is probing tie emironment external to the molecule.
More sign&ant is the iinewidtb bebaviour of the overtones and combinations in bo& deuterated and undeuterated compound. Althou& the measure widths are not exceptionaliy accurate due to the fkorescence backgouad which bad to be subtracted in ‘Lhe200250 K region, nevertheless a noticeable increase in bnewidth was observed, figs. 3-5. As the beha%jour appeared exTonentiai Gke, the temperature dependent increments of widths, At?, and of frequency shifts, Aw, were obtained from the experimental data by subtracting the essentially temperature independent values at 37 k. This corresponds to the assumption that Ar and Aw can be represented as the exponential forms 217,201
where 4, b are temperature independent constants and Ei is m acthation energy. In fig. 8 we present plots (least-squares fits) of In Aw and in AI’ versus i/T for the complex Pd3T1. The activation energies obtained by this method are tabuiated in table 1 (no due is given from mzvurements of Aw for the overtone at 2 X 340 cm-l s&e shSs were smali and therefore inaccurate). From tabie 3 we conciude that the actirrltion energies obtained from two different physical measurements (AI’ and Aw) coneiate rather well, thus leading credence to the applicability of eq.
(2 x 1357)
coupled to a 136 cm-l mode {close to the TCNQ mode at 144 cm-l ar?d the perylene mode at 135 cm-l, fig. 6) and the 1357 cm-1 mode k coupled to a 207 cm-l mode (fig. 7) we can expJ,zin the activetion energy Ei as the combination of these two low frequency modes (339 = 136 f 297, table 1). This would imply that the A overtones and combinations mainly are coupled to B overtones and combinations. This is reasonable since in some cases even Raman fundamentals have been observed to be coupled to low frequency combination bands [20]. Furthermore, the above hot band coupEng model tiplies an anharmonic coupiing of the form &p2 f20]. Thus broadening wiu be proportional to (eA? > &!A being
1
25
50
Ido
ax,
150
LL 76ii
(1). Although various models have been proposed to explain such experimental temperature dependence for fundamentais [17,20], the present data are more consistent with the model of interactions of high frequency modes A and hot bands of low frequency modes B. Thus assuming that the 340 cm-l mode is
329 426
339 418
25
53
co
150
ma
car’
Fig. 7. Teqemtwe dependent spectrum Of Pd3Tl Iow frequency modes - experimenta: dttails as in fg_ 6 (50 cm-’ - p&ma line).
A.D. Ban&auk et al_,fRamzm spectra of @rylene)3
290
TCNQ complex
fiequencymodes.
Theseactivation energiesarethere-
fore .larSer ‘&XI for fimdamentals and are therefore more readily measurable as they lead to stronger temperature dependences of A.o and AI’. 4.2. Low jiequency
n?odes
Interpretation of low frequency modes is usually based on the assumption of separation of external (translations, hbrations) from the internal molecular modes. Kecent measures of Grtineisen parameters in molecular [21] and quaai-one&nenaionai crystals [IS] indicate this distinction is blurred in complex
-1.5 t
Fig_ 8. Temperature dependence of lo&t.hmic increments in(a) frequency-Awj(b)tieWidth -Al-.
the mode amplitude) which is proportional to 11~ where JI is the reduced mass of the mode. Since the two modes studied in table I con’Z+.inconsiderable C-H bond component as witnessed by the isotopic shift of the fundamentals, we can thus explain the j&or of two enhancement of width of the fundamentals and overtones in table I for the perylene as compared to the deutemted perylene (fig. 4) as arising from the C-H bond component. Activation energies were not obtainable from the shift Ao of the fundamentals & these shifts were q---1cm-1 in general and difficult to measure v&h accuracy. What is interesting, is that for overtones and combinations of these Jndmentak, the activation energies correspond toexcitation of overtones and combiiations of low
loo
b)
2po
3GQK
3% r 120
t
pb
1
100
-1
’
3&K
Fig: 9. Temperature dependence of low frequencies at 514.5 nm excitation df (a) P3T1, (bj i’d3n
300
AD_ Eandmk et aL/Ramanspectraof [per/he)3
crystals with different bonding strengths due to the
strong dependence of the Griineisen parameters on anhronicity effects [21]. The temperature dependence of mode frequencies arises also from thermal expansion and anharmonicity effects [17,18]. Thus in fig. 9 we illustrate the temperature dependence of the fundamental at 354(340) cm-1 and the low freqttency modes observed below 120 cm-l in both complexes. The fundamental is only slightly down shifted (up to 1 crnl) by the thermal expansion and phononphonon scattering. As discussed above, it is the iinewidth which is a more sensitive measure of coupling of fundamentals to the lattice. All low frequency modes show a normal linear decrease in frequency with increasing temperature [17,18], including the intenzaI low frequency modes assigned at 65 and 37 cm-1 to TCXQ_ (This assignment was made on the resonance behaviour of their intensities, previous jettion.) S&e the thermal expansion coefficient and compressibility are not known for P3T1, no sepaation of the thermal expansion and anharmonicity contributions is feasible [ 17, IS] _ The present temperature measurements do not permit a clear distinction between internal and external modes; the similarity in temperature dependence of their frequer.zies woirid imply that these are mixed. On the other hand, the similar resonance behaviour of the intensities of the modes at 35,65 and 113 cm-l implies they are activated by the same excited election&z state, so that they manifest vestiges of TCNQ intiaamotecdar motion (see fig. 6). ln addition, due to the z axis polsrizstion of the excitation, the coupling would be to the charge transfer states appearing at 410 and 960 nm in ihe optical absorption spectra of PlTl and P3Tl I4 j _The rapid broadening of the 75-84 CXII-~doubler and the 113 cm_1 line sugests dramatic changes in coupling 2nd therefore in structure in tie region above 15O.K where fluorescence begins. Observed silangn in fluorescence with temperature are usually indicative of a structural phase transition [10,22] _ Sirxc the law temperature Raman spectrum is sharp and consistent with an ordered stmcture, the rapid broadening of the above low frequency mode in the korescence region (above 150 K) suggests a gradual order-disorder transition as in naphthalene-TCNB [2Zj. The curious disappearance of the fluorescence above 250 K and the complete collapse of most low frequency modes and some Kigh frequency modes
TCNQ mptex
(fig. 5) is however diffic*ult tc reconcLle w&b the above interpretation_
5. CoilcllLsion
A study of the temperature dependence of widths and frequency shifts of the 354(340) cm-l and 1367(1357) cm-l fundarrentals, overtones and their combinations in crystals of P3Tl show a strong isotope’ effect when compared to Pd3Tl. Thus the linewidth of perylene vibrations was found to be usually twice that of undeuterated perylenes. A detailed temperature study of the increase of the widths and shifts of the overtones and combinations of these two viirations gave estimates of activation energies for these temperature effects. These activation energies were found to correlate well with observed low frequency modes which are coupling strongly to the above high frequency modes. Of note is that these activation energies involve overtones and combinations of the low frequency modes, rendering temperature increases in widths and shifts of high frequency overtones and combinations more significant than for fundamentals. An onset of fhrorescence was found at ~200 K which disappeared above ~250 K. An attempt was made to correlate changes in the low frequency (librations) spectrum to the onset of the fluorescence. Most low frequency modes broaden continuously, becoming extremely dampened at room temperature. Hence no light is shed on the onset and disappearance of this fluorescence, in terms of dynamics of the lattice as a function of temperature. A measure of the decay time as done in pure perylene [3] would be interesting to elucidate the drastic effects on the relaxation times in the excited states of this complex.
AcknGwIedgement
We thank the National Research Council of Canada for equipment‘grants and the Defense Research Board of Canada for grants supporting this research.
Refererxes [l J K_D_Tmorz and ADD.Bandrauk, G-mn Phys Lettexo 44 (1976) 232.
A.D. Bdndrouk et al/Roman spectra of (pegfenej3 TCNQ complex [2] A-D. Bandrauk, K.D. Truong, V.R. Salves and H.I. Bemsteh, I. Raman Spestry. 8 (1979) 5. [3] RX. Hochstrasser and CA. Nyi, J. Chem. Phys 72 (1980) 2591. [4] AD. Bandrauk, K-D. Truong and C. Carlone, submitted to Car-i.J. Chem (1981); K-D_Truong, Ph_DD. Thesis, to be submitted, Universit& de Sherbrooke (1981). [S] .JB. Torxaoce, AccountsChem. Res 12 (1973) 7s. 161 J.B. Torrance, AM. Acad. Sciences N.Y. 313 (1978) 210. 171 Jd. Torrance, JE.Va.zquez, JJ. Mayerle and V.Y. Lee, Phys. Rev. Letters46 (1981) 253. 181 S.P. Cramer, B. Hudson and D&l. Burland, J. Chem. Phys. 64 (1976) 1140. [9 ] A.W. Hanson, Acfa Cryst. B34 (1978) 2339. [IO] .R.L. Beckma;l, J-M. Hayes and G.J. Small, Chem. Phys 21 (1977) 135. [ll] M.A. Kovper, A.A. Terekhov and L.M. Babkov, Opt. Spectry. 33 (1972) 38.
301
[ 121 H. hfiihwald, E. Erdle and A. Thaer, Chem. Phys. 27 (1978) 79. [13] HJ. Bernstein, NS. DaIal, W.F. Murphy, AH. Reddoch, S. Sunder and D.F. Warns, Chem. Phys. Letters 57 (1978) 159. [14] H. Morawitz, Phys. Rev. Letters 34 (1975) 1096. [15] C. Carlone et al., 3. Chem. Phys. 75 (1981) 3220. [16] A. Girlando and C. PeciIe, Spectrochim. Acta 29A (1973) 1859. [17] J.C. Bellows and P-N. Prasad, J. Chem. Phys. 70 (1979) 1864. [lS] F.J. Owens and 2. Iqbal, J. Chem Phys. 74 (1981) 4242. [19] M. Ni&ol, hf. Vernon and J-T. Woo, I. Chem Phys. 63 (1975) 1992.
[20] S. Marks, P.A. Cornelius and C.B. Harris, J. Chem. Phys. ?3 (1980) 3069. [21] R. Zallen, Phys Rev. B9 (1974) 4485. [223 R.M. Macfarlane and S. Ushioda, J. Chem. Phys. 67 (1977) 3214.