Optical Kerr effect studies of octyl-cyanobiphenyl

Volume 59, number 1

OPFICALKERR

CHEMKAL

PHYSICS

LEXTERS

1 November 1978

EFFECT STUDIES OF OCPYWZYANOBIPHENYL

HJ, COLE+ PhysicsDepartmen&Btunel Unit

em?y. Oxbridge,MiddlessexUB8 3PH, UK

Received 5 June 1978 Revised manuscript receiwd 23 July X978 The

optid

and dc Kerr effects have been measwed for the nematogen oct+cyanobiphenylQ3

Ci3) as a function

of

temperaturein the isotropicphase- BY uskg p?llsedmethods dynamicpropertieshave been determinedand the pretransitiod behviour e__ ed in texms of the Landau-de Gemes model. The constants Q. Y. T* and (TC- T+) of this forma&m have been @ven_

The cyano-biphenyl f&y of liquid crystaIs [I] have evoked much interest lately because they form stable, colouriess mesophases at or near to room tcmpenture and therefore are extremely suitable for use in eiectro-optic devices [2] _ These devices rely on the dielectric and optical properties of the mesogens and their cooperative behaviour, Unfortunately measurement of such properties is difficuit in the mesomorphic phase due to its turbid nature. These materials may however be studied in the isotropic phase where they are transparent and exhibit marked pretrarktiona! behaviour, and where such behaviour may be related to that in the mesomorphic state [3] following the Landau-de Gennes phenomenological model [4]_ The pulsed field Kerr effect is particularly suitabIe for such measurements as it allows both the static uld the dynamic properties and behaviour to be studied, Using pulsed dc and Iaser fiehis it is possible to study permanent moment and opticd dipolar effects, and the pretransitional behatiour may be studied from the field free refaxation. We recently carried out such measurements for two of the cyano-biplxenyl famiIy% namely the pentyl and peniyloxy cyano-biphenyls, 5 CB f5 J and 5 OCB [6j _ Recent measurements [73 have shown that as the length of the alkcl chain is increased the mesogen forms, besides the nematic mesophase, a smectic phase for octyl cyano-bipheny1 (8 CB) and higher homologues. + Present address: C_N_R_S., 67083 Strasbcwg cedex, FE?IlU.Z.

168

The purpose of this note is to examine whether the change in the &y&tail length is reflected in the prctransitional behaviour, as a function of temperature, and if so whether the Izmdau-de Gennes phenomenoiogical theory [4] is applicable to this system. To our knowledge these are the first reporte& studies of 8 CB by either the dc or optical Kerr effects. The essential elements of the apparatus f5] are a few powered He-Cd laser @o = 441.6 mn) that probes the birefringence, induced by a pulsed dc (O-10 kV for puise durations up to 3 F with an electrode separation of 2 mm) or pulsed optical (O-9 MV m-l for a 50 ns, TEMou unfocussed Nd3* YAG laser pulse) field, between crossed polarisers (Glan-laser type), Recording was via a photomultiplier-100 MHz amplifier-storage oscilloscope recording system. The samples were a gift from B.D.H. Ltd. (Poole, Dorset, UK) to whom we express our gratitude. The thermostated cell had an optical path length of 49 mm and normai precautions were taken with respect to cleanliness of glassware etc. Infig_ I we have given the Iaser induced hirefhgence, for several temperatures, as a function of optical field strength o. From this it can be seen that the birefringence appears to saturate at An = 2.4 X lo-’ for a temperature 1°C from the nematic-isotropic transition tem@erature (Ic = 40.6OC) and a field strength Eo = 7 MV nr- l_ This critical field strength is laryr than that required for 5 CB (i.e. Eo = 5.8 MV m- ) under equivalent temperature conditions and is therefore an indication

Voiume 59, number 1

CHEMICAL

1 November 1978

PHYSICS LETTERS

0 Fig. 1. Optical field induced birefiingence at various temperatures.

of the decreased optical anisotropy of 8 CB. a decrease that was also noted in recent dielectric/ refractive index measurements [IS]. The optical birefringence depends only on the optical polarisability anisotropy of the material for a given temperature. Ihe samration effect, which should not be confused with the saturation of molecular orientation, arises we believe from the inbomogeneous or gaussian field distribution across the diameter of the inducing laser beam. This effect, which could be eliminated using a suitable homogeneous field, such as might be produced in the far field of a pulsed unstable resonator Nd3+ YAG laser [9], may however have possible commercial applications [S] _ From the initial slopes in fig. 1 it is clear that the birefringence for a given field strength is very temperature dependent and as this behavioclr arises from increasing molecuhu correlations with decreasing temperature it is likely to be reflected in the dynamic properties of the medium. Ibis can be seen to be the case from the field free relaxation (r), fig. 2. The relaxation curves were all mono-exponentiaI_ For the isotropic phase of nematogens the cooprative pretransitional effects, which are dominated by short range order, have been theoretically described in terms of a phenomenological model [4] based on the landau theory of phase transitions. in this model the excess free energy F is given as a series expansion of the scalar order parameter Q such that F=

&(T-

T*)Q2

f $bQ3 + $cQ4 + . . . ,

Fig. 2. Temperature dependence of r and r-’ fields.

for puked laser

where a, b, c are phenomenoIogicd

constants characteristic of the transition. T* is the second order transition temperature that the system would have in the absence of the intervening weak first order transition that occurs at the clearing temperature T,. For the case of orientation induced in an electric field the amount of order produced is small and terms in Q 3 and higher order may be neglected, and it has been shown [IO] that B dc = A~QAQ~~Q~o(T

-

T*),

where &jc is the dc field Kerr constant (Bdc = AIZ/ &-JE&), &-Jis the probe wavelength and AIZ~ and Aeo are the refractive index and dielectric anisotropies respectively. For the same model it has also been shown [I I] that the field free relaxation (7) is given .by T = vla(T - T*)r, where v is a weakly temperature dependent viscosity coefficient and 7 is an exponent that is unity in any mean field theory. From fig. 2 it can be seen that r-l (z:(T - T*) and thus the Landau-de Gennes theory is upheld for 8 CB. Further, from this graph we find T* = 39.4 (* 0.2)“C and v = 0.46 X 10A6a J m-’ s-l (including 169

Volume 59. number 1

~EMICAL

PHYSICS LETl%RS

the units of a). It should be noted that within x0.7% of the transition the experbnental points fall consistently Maw the line of r-l versus Tand this prbbably marks the lower limit of applicability of the---&an field approximation used in & theory, For the case of dc field induced birefriqmce, strong pretran.sitionaIbebaviour is also observed as a functionof temperature, figs. 3 and 4_ in this case when the inducing dc Eeld across the measuring beam is uniform no samration of the effect is observed even though the birefringence corresponds tc optical phase retardations of several71,fig. 3, From fig 4 it can be seen that B-l a (T - T*); thus the model is again upheid and further 3’” = 39-l (+o,i)*C, ando=6_3X laf5AnoA~~3m-3K-2_U~gthe vahresof Aq = 0.37 c&&ted from ref_ f12] and Aeu =8.5 IS] wefinda=O,lSX lo6 fmW3 K-“,and Y = 0.83 Poise from above. The comparative values for 5 CB under identical conditions were a = 0.15 X lo6 Jmm3 K-‘,andv=0_73Po~.Thusbothnandv are increased by 2G% and 14% respectivelyby increasing the t&i fen$b by (CJ&h_ From the op%zal Kerr effect, the optical anisotropy appears to decrease V&A the increased tail length, a result also recen@ observed from dielectric and refractive index data 173_ Fkaily the Zandarr-de Cermes theory Is seen to be upheld aver a temperaturerange of ~15% above Tc and leads to values of T* = 39.3 (&03)‘C and (Tc - T*) of 1.2 (kO3)“C. In conch&on the parametersa. Y, and the optical anisotropy are different for 5 CD and 8 CB and as the latter mesogen is the first of the cyano-biphenyls to

Fii. 3. dc fidd induced birefk&ence at various teznpeatrares. T&e@bt hand ordinategivestheoptic&pba3e reisr&tion-

170

exhibit a smectic phase it will be interesting to Observewhether this behatiour is continued with the bigber homoiogues of the series. ‘Ihe author thanks professor DR. Jennings for the kind loan of the apparatus,Dr. D. Drmmur for the communication of his resultsprior to publication and the S.R.C. for a fellow&p,

Refen3xes [Xl G.-W_ Gray, KJ. -II

and J.&A, Nash, Electron.

Lerters 9 (X973) 130. PI A. Asbford, J. Canstast, 3. H;irronand EP. Raynes, Eksctron-Letters 9 (1973) 118133 Y. PO@ P. Atten and J.C- Fi&pini, Mol. Cryst_ Uquid Cry.% 37 (1976) f. i41 PG. de Gmr~es,Pbys. Letters 30A (1969) 454. f51 NJ. Coks, Mel- Phys- (:978), to be published. f6f HJ. COICZ,MoL Clyst- L+id Crya <1978). to be pU=ed. g71 G.W. Gray, 3, Rhyr @%ris)Ci ff975) 337. C81 D.A, Dursmur,M.RR-MantexficId,Wfi. Miller and XX, Dunkavy. Mol. Cryst. Liquid Cxyst.45 (1978) 127. [91 D-R. Hanna,L.C. Iaycock, PALf&skiesandJJK. Wright, Papex 24 of the 3rd National Quantum Electronicscorlfkseuce*universityof Southampton, 14-16 Septernbr 1977, Y.. PO&. J.C. Filippini asd K. Aleonard, Phys. Letten 57A (197Q 53. J.FL L&me and R. Lefevre,3. Chim.l?hys_ 73 fF976)3s7. i?.P.Karat and N-V. Ma&usudana, Mol. Cry&. Liquid Cryst. 36 (1976) St.-