Spectroscopic study of pentacene assembled in …nopr.niscair.res.in/bitstream/123456789/25356/1/IJPAP 40...Langmuir-Blodgett (LB ) technique currently represents the most versatil

Indian Journ al of Pure & Applied Physics Vol. 40. January 2002, pp. 12- 16

Spectroscopic study of pentacene assembled in Langmuir-Blodgett film mixed with stearic acid

Somobrata Acharya", T apan Kr Parichhah & G 8 Talapatra"

"Department of spectroscopy, Indian Association fo r the Culti vat ion of Science. Jadavpur, Kolkata 700 832

11Dcpartment of Chemi stry, Suri Vidyasagar College. Su ri , Birbhum

Received 4 April 200 I; accepted 24 September 200 I

Langmuir Blodgett (LB) fi lms of non-amphiphilic pent accnc mi xed with stearic ac id have been prepared and its photo­phys ica l propert ies are reported here. Surface pressure versus area-per-molecul e isotherms (7t-A) at different composi ti ons arc measured. Spectroscopi c propert ies (UV-Vi sible absorption, emi ssion and Scanni ng Electron Mi crograph) of pcntaccne in LB film have been reponed. The blue-shifted absorpti on and red shift ed emi ssion suggest the formation of H-typc of aggregates.

1 Introduction

Since the interacti on that takes pl ace between two materi als forming a juncti on is essenti ally confined to the interface, it is important to uti lize process ing techniques that provide control over the supermolecular organi zati on of thin fi lms when one considers the fabrication of alternating layer super­lattices and related th in fi lm heterostructures. The Langmuir-Blodgett (LB ) technique current ly represents the most versatil e thin film-processing route ava il able fo r the purpose of fab ricating such st ructures because of their poss ible applicati ons in va ri ous optoelectronic devices, Bi osensing1 etc. Although, these studies have received considerable at tenti on, still the role of n-electron conjugati on, ultras tructure as well as domain structure on electrica l, optical and spectroscopi c propert ies is not very clearl y understood. Thus, a thorough understanding of bas ic phys ics in vo lved in those properti es of such fi lms is a topi c of fund amental importance. Typical LB-compatibl e materials are amphiphilic molecul es, however, non-amphiphili c molec ul es also form excellent LB fi lms when mi xed with long chain fa tty acids H>. The spectroscop ic and aggregating properties of such non-amphiphi lic molecul es in LB films is quite similar to their amphiphilic counter parts, which are usuall y expensive and di fficult to prepare2

•7

• These similariti es justi fy the study of non-amphiphili c compounds in mi xed LB layers. Due to the presence

of su itable spectral iocati on of its lowest energy electronic singlet-singlet transition, pentacene is ex tensively used to demonstrate novel experimental techniques and phenomenaR· 10

• It may also be used as a potential acceptor to study the energy transfer processes in bi-chromophoric sys tems. So it is very usefu l to study the spectroscopic properties of pentacene in LB fi lm, a two-dimensional surface where orientati on of the molecu les can be controlled prec isely at molecul ar level.

The preparati on of LB fil m of non-amphiphi lic pentacene mi xed with stea ric ac id (SA) has been reported. Studies on isotherm help u ~ to determi ne the optimum conditi on fo r LB lifting. The spectroscopic propert ies (UV -visible absorpti on, emiss ion) of such film have also been reported. Spectra l changes with respect to that in solut ion has been compared. Scanning Electron Microscop ic (SEM) study has been done to characteri ze the surface morphology of the film.

The studies on isotherms, absorption , emission and SEM sugges t formati on of aggregates. The blue-shifted absorpti on and red-shi fted emission sugges t the formation of H-type of aggregates.

2 Experiment Details

Pentacene (98% pure) purchased fro m Al dri ch Chemical Company, USA was used as received. To check the purity of the sample, the absorpti on spectra of the sampl e is compared wi th the reported

ACHARYA et al.: SPECTROSCOPIC STUDY OF PENT ACENE 13

data 11'

12• SA (>99%), a Sigma product was used

without further purification. The solvents, ethanol (E Merck, Germany), chloroform (SRL, India) and benzene (SRL, India) used are of spectral grade and their emission spectra were checked before use. A commercially available alternate layer L-B trough (Joyce-Loeb! , model 4) was used for mono and multilayer film deposition onto solid substrate (thoroughly washed quartz and or glass). For the pressure-area (n-A) isotherm, measurement, about 80 Jll of the solution (either pure or mixture of pentacene and SA in chloroform at a pre-determined ratio) was spread on the air-water interface at a fixed temperature of 300 K. The barrier was compressed slowly at a rate of 2xi0·3 nm2

molecule·' s·'. Mono- or multilayer of such film was transferred onto the substrate at a constant surface pressure, 25 mN/m2 (corresponding to the solid phase as was observed in the isotherm). The transfer ratio was 0.93±0.02. Absorption spectra were recorded on a Shimadzu uvpc-2101 spectrophotometer. The steady state fluorescence and excitation spectra were measured with a Hitachi, model F-4500 fluorescence spectro­photometer. For recording the SEM micrographs of the films sputtered with gold, a Hitachi model S-415 electron microscope was used. The accelerating voltage of the electron beam was maintained at 25 kV.

....... 40 E .... z E

....... 30 ~ :::1 Ill Ill Q) 20 .. c. Q) 0

~ 10 :::1

1/)

OL_--~--~--~~~~d 0.0 0.1 0.2 0.3 0.4 0.5

Area per molecule (nm2)

Fig. I -Surface pressure versus area-per-molecule at different molar mixing ratio of pentacene and SA. (0.0 indicates that of pure SA and 1.0 is that of pure pentacene)

3 Results and Discussion

3.1 Behaviour of Langmuir film

After spreading the solution in air-water interface, sufficient time (-I 0 min) is allowed to evaporate the solvent and to form the stable monolayer. In Fig. I the surface pressure versus area/molecule isotherms at three different molar­mixing ratio (SA:pentacene) have been plotted. Pure pentacene at the air-water interface does not form a stable monolayer and the surface pressure hardly rises to - 15 mN/m2

• A large region in the isotherm covers the liquid-expanded to liquid­condensed state. However, when mixed with SA, it forms quite stable monolayer. At low molar-mixing ratio (mole fraction of pentacene -0.1 ), the isotherm is quite similar to that of pure SA. Surface pressure rises to about 40 mN/m2

• With the increase of mole fraction of pentacene, the area-per-molecule decreases and a plateau-like region develops. Thi s plateau-like region is perhaps due to some orientational change resulting in phase transition , and or arrangement of multilayers and aggregate formation . From the 'extrapolation to zero surface pressure', the area-per-molecule for pure SA (mole fraction 0.0) is found to be about 0.25 nm2 (which is slightly larger than the reported data of 0.23 nm2,

probably due to high temperature of the subphase) and for pure pentacene it is -0.12 nm2 which is much less than that expected from the space-filling model. The decrease in area-per-molecule may be due to the solvation of organic pentacene in subphase water, due to the submersing of surface molecules or due to the accommodation of pentacene molecules within the SA matrix and not in contact with water surface. The dissolution of pentacene in water is practically negligible and the second possibility may be ruled out, as we do not observe any appreciable emission from the aliquot collected (at different times) from the deep inside of the subphase 13

•

In the monolayer, there are three types of interaction, SA-SA, SA-pentacene and pentacene­pentacene. Among these three, SA-pentacene interaction helps in the formation and stability of monolayer. But in other two interactions, if strong, aggregation is formed and area-per-molecule is expected to decrease. The possibility of such aggregate formation increases as one goes from

14 INDIAN 1 PURE & APPL PHYS, VOL 40, JANUARY 2002

lower to hi gher mol e fracti on of pentacene in the mi xture as was indicated by the isotherm curves. From the isotherm, it is found that the isotherm curves during the cycle of barrier compress ion and expansion (not shown) do not foll ow the same path . Thi s type of hysteresis IS an indicati on of aggregati on.

SP = 5 mN/m

0.0 0.2 0.4 0.6 0.8 1.0

Mole fraction

Fig. 2 - Plol of area-per-molecule versus mole fracti on (values arc taken from isotherm plot): (a) al surface pressure 5 mN/m

2:

(h) at surface pressure IS mN/nl

For a non-interacting two component system, the average area-per-molecul e according to the addi ti vity rul e '~ is given by A 12 = N,A ,+N2A2, where N, and N1 are the mole frac ti ons and A, and A2 are the area-per-molecule of component I an d 2 respectively. In Fig. 2, it is shown, a plot of the average area-per-molecul e versus mole fraction at constant surface pressure 5 mN/m2 and I 5 mN/m2

•

From the curve it is observed that in case of low surface pressure, area/molecul e shows a positive deviati on from the idea l curve (shown by----- line). Thi s indicates that there is a repul sive interacti on between the components, which support the fo rmati on of aggregates of pentacene molecules in the mi xed films even at low surface pressure, and low mole frac tion. Thi s is consistent with the behav iour of other non-amphiphiles2

·6

•

However, at re latively high surface pressure, a negative deviati on is observed indicating an attracti ve interacti on. whi ch helps in the formJ. ti on of molecul ar film. The reason is not very clear, but possibly due to the fac t that, though nature of

stearic acid and pentacene is quite different, their hydrophobicity (large acene and -CH2 groups of SA) are comparable and pl ay a major role.

~ .0 .. 0 VI .0 ~ '0 Q)

.t:! IV E .. 0 z

300

z. :0 ... 0 Ill JJ <1:

400 500 600

1--in benzene solution

2--in LB film

400 500 600 Wavelength (nm)

700

700

Fig. 3 - Plot of absorpti on versus wavelength of light in soluti on and 10 bi-l ayer LB film. The inset shows the same in the range (400-700) nm of ( I) in benzene solmion ( I o·o M); (2) in I 0 bi-layer LB film

3.2 Absorption and emission spectra

The UV -visib le absorption spectra of pentacene in benzene solution (concentration -I Q·" M), and in I 0 bi-layer LB film (mixed with SA) are shown in Fig. 3. The absorption spectrum of pentacene (benzene solution) consists of three d is tinct band regions. The low-energy structured band from -480 to -600 nm (' A~ 'L..) , middl e porti on in the region of -400 nm (' A~'Lh) and a high energy strong band at -300 nm (' A~'Bh) transition 12

• In LB film , these band s are somewhat blue-shifted and low-energy band (' A~ 'L.,) is masked by t e ( ' A~'Lh)

transi ti on. The bands are also fo und to be broadenr d. Thi s broadenin g is perhaps due to the closure associat ion of molecules i the film, suggesting aggregati on. Although, small shi ft may ari se due to different micro-environ ment , in less polar stearic ac id , the shift and broaden ing observed in the mi xed LB films is likely to ori ginate fro m large exciton sp litting due to strong dipole-d ipole interacti on between the pentacene molecules. The change in energy due to such an in teracti on can be accoun ted by considering McRay and Kasha's 15

·16

intermedi ate strength exc iton coupling theory. It is

A CHARY A et al.: SPECTROSCOPIC STUDY OF PENT ACENE 15

mathematically expressed as ilE=21J.2(1-3 cos2 8)( I-1/N)Ir\ where 11 is the dipole moment, N is the number of monomer in the aggregate, e is the angle made by the dipole moment with the r vector, joining the centers of the two dipoles. When the alignment of the dipole moments in the aggregates

is such that 54.7° < e < 90°, the created new exciton band will be located above the monomeric band , which gives a blue-shifted absorption and the aggregates are referred to as H-type aggregates .

. ... .. ' ... .. ... "•

:.· .... :· .. • .. •.'

450 500 550 600

Wavelength (nm)

Fig. 4 - Emi ss ion spectra of pentacene (excitation is at 400 nm); ( I) in ch loroform (298 K); (2) in I 0 bi- layer LB film (SA :pentacene :: 8:2) (298 K) ; (3) in chloroform (77 K); (4) in 10 bi-layer LB film (SA :pentacene :: 8:2) (77 K)

In Fi g. 4, the emission spectra of pentacene in chloroform solution (I 0 " M) and in LB fi lm both at room temperature and at 77 K have been plotted . In CHCI 1 so luti on at room temperature, the emission curve .shows two peaks, one at -485 nm and another at -525 nm. In the low temperature (77K) chloroform solution (not g lass , so lidificati on occurs), the re are three bands, sharp and prominent peaks appear at -480, -525 and -565 nrn. In LB film , at room temperature the spectrum broadens and the peaks are slightly red-shifted , peaks appear at - 490, - 528 and -575 nm . However, the rel ative intensity of the highe r ene rgy peak at - 480 nm is more than the other band . The broadening and stokes shi fted em iss ion is perhaps due to the close proximity of molecules in the film . In the low temperature (77K) LB film , in stead of the sharpness

in the peaks as generally expected, the bands are more broadened. The reason is not very c lear.

Fig. 5 - Scanning electron micrograph of mi xed LB film (SA:pentacene :: 8:2), I 0 bi-layer, printed length is 30* I 03 nm wi th magnification 1000 times

3.3 Scanning electron micrograph

The formation of aggregates is further supported from the observation of Scanning Electron Microscopi c pictures of the LB su rface , shown in Fig. 5 . As the nature of the two mixed components is quite different, their complete miscibility is hinde red and is expected to form aggregate . However, as was di scussed in case of Fig. 2, there is somewhat attractive nature a lso. Here, in comparison to other non-amphiphi I ic compounds and acenes2

·", the s ize o f aggregates is found to be re latively smal l which supports our earli er observation .

4 Conclusion

In the prl~ Sent work, non-amphiphili c pentacene may be incorporated in LB film , by mixing with SA . The mean molecular area is found to decrease with increas ing pentacene percentage in the mi xture. Studies of Langmuir monolayer and spectroscopic properti es suggest the formation of microcrystalli ne

16 INDIAN J PURE & APPL PHYS, VOL 40, JANUARY 2002

aggregates m film. In LB film, the blue-shifted absorption and broadening in the absorpti on spectrum is perhaps due to fo rmation of H- type of aggregates . In the emission spectra, the stokes­shifted broad emiss ion in LB film with peak at -575 nm supports the formation of aggregat ion . The aggregation may be due to dimer or excimer. The time resolved decay ana lysis might provide the right information. At this time, there is no provision to measure the same. However, efforts will be made to measure the data and the results will be communicated later on.

Acknowledgement

This research was partially supported by DST, Government of India, Grant No. SP\S2\M-I I \94.

References

Ulman A, An introduction to ultra-thin organic films: From Langmuir-Blodgell films to self-assemblies, (Academic Press, New York), 1991.

2 Parichha T K & Tal apatra G B. Optical Mater, II ( 1998) 9.

3 Parichha T K & Talapatra G B, 1 Phys Chem Solids, 60 ( 1999) 111.

4 Somobrata Acharya, Parichha T K & Talapatra G B, Mol

Mater, 12 (2000) 91.

5 Kuhn H. Pure & Appl Chem, 53 ( 1981 ) 21 OS.

6 Baker S, Petty M C, Roberts G G & Twigg M V, Thin Solid Films, 99 ( 1983) 53 .

7 Angelova A, Van der Auweraer M & de Schryver F C. Langmuir, II (1995)3167 .

8 Hesselink W H & Wiersma D A, Spectroscopy and excitation dynamics of condensed molecular systems, (Eel ) V M Agranovich & R M Hochstresser. (North-Holland, Amsterdam), 1983 , p. 249.

9 Decola P L, Andrews J R, Hochstrasser R M & Trommsdorff H P, 1 Chern Phys. 73 ( 1980) 4695 .

I 0 Orrit M & Bernard J, 1 Phys Rev Lells , 65 ( 1990) 2716.

II . Friedel R A & Orchin M, Ultra violet spectra of aromatic compounds, (John Wiley & Sons, New York), 195 1.

12 Jaffe H H & Orchin M, Theory and applications of ultraviolet spectroscopy, (John Wiley & Sons, New York), 1962.

13 Dutta A K, Misra TN & Pal A J, Langmuir, 12 ( 1996) 459.

14 Adamson A W. Phystcal chemistt)' of in;erfaces. (Wiley lnterscience, New York), 1990.

15 McRae E G & Kasha M, 1 Chem Phys, 28 ( 1958 ) 721 .

16 Kasha M, Rawals A R & El-Bayaumi , Pure Appl Chem, II (1965) 37.