Optical properties of the phthalocyanines

Pergamon Press 1963. Vol. 24, pp. 751457.

OPTICAL

Printed in Great Britain.

PROPERTIES OF THE PHTHALOCYANINES LEWIS T. CHALUERTON Physics and Chemistry

of Solids, Cavendish Laboratory,

Cambridge

(Received 12 December 1962)

AMra&--The

optical properties of some of the ~bth~~~an~e~ have been irwestigated for mofecuies in different degrees of associatian. If the spectraI shifts arising from temperature dependence and the broadening due to cry&a1 fteld effects are accotmted for there appears to be a one-to-one correspondence between peaks observed in vapour, sublimed film, and single crystal absorption spectra. It is proposed that the simple band model offers a reasonably good description of crystalline copper, platinum, and metal-free phthalocyanines. The sharp energy levels of the free molecule transform into broad quasi-continuous bands on crystallization.

Tm semiconduction properties of some of the phthalocyanine derivatives have stimulated interesto-14) in the electrical, photoconductive, and optical properties of the same compounds. ~m..rXNG and GUTMAN(I) have reported on the relation between absorption spectra of thin sublimed films and the temperature dependence of resistivity, while VARTANYAN and KARPOVICH(~-41 have been concerned with the spectral dependence of the photoconductivity and optical activation energy. The Russian authors T~NXN, PUTZEIKO and -~~I~~o~(~) point out the impo~t~ce of such studies because of the similarity of the phthalocyanine and chlorophyll molecules, and, more recently, TOLLIN, KEARNS and CALVIN@J) have published a detailed treatment of the kinetics of conductivity in metal-free phthalocyanine. These treatments generally deal with sublimed layers of the compound. The only investigation of the optical properties of single crystals is that reported by LYONS, WALSHand WHITE@) who give the reflection absorption spectra of phthalocyanine. This communication describes a number of features in the optical properties of the phthalocyanines which have been investigated in the course of a research programme concerned with radiation damage in the solid state.(r”Js” EXPERIMENTAL

Piatinum phth~ocy~ine is difIicult EQprepare in large quantities and metal-free phthalo751

cyanine(l7) itself exists in distinctly separate forms (a, ,$ and y) which are stable in different temperature regions. Hence the results which are reported mostly concern copper phthalocyanine, though some values have been obtained for the platinum and metal-free compounds. It will be evident, however, that there are close similarities in spectra obtained for the three compounds, so little is lost in generalizing the results obtained for the copper derivative. Absorption spectra were obtained for molecules in different degrees of association : (a) Vapour Crystals of copper phthalocyanine were obtained from the Imperial Chemical Industries Ltd. and these were purified by repeated vacuum sublimaton. A little of the purified material was vacuumsealed in a 1 -ft silica tube with optically flat quartz terminal faces and the latter was placed at the centre of a 3-ft cylindrical furnace. Light from a tungsten strip source was passed through the copper phthalocyanine vapour and focused into the slit of a Hilger D 187 wavelength spectrometer. The absorption spectrum was recorded photographically on Kodak 0.250 rapid ortha metallographic plates, and the procedure was repeated with purified metal-free phthalocyanine. Vapour absorption spectra for the two compounds are shown in Fig. 1. Densitometer traces are given in Fig. 2.

752

LEWIS

T.

CHADDERTON

Wavelength,

FIG. 2. Densitometer

traces

.8

of the absorption

These were prepared by direct sublimation of a very small quantity of the purified material onto quartz discs. A Unicam S.P.500 absorption spectrophotometer was used to determine the absorption spectra at room temperature. The results obtained for metal-free, copper and platinum phthalocyanines are shown in Fig. 3. (c) Single crystals Thin flat crystalline ribbons of copper phthalocyanine were grown by entrainer-gas vacuum sublimation in a sealed silica tube in a furnace. At temperatures between 600°C and 706°C and pressures of about 10 cm of argon or nitrogen beautiful thin crystals of copper phthalocyanine in the (001) orientation were formed. Thicknesses ranged from about one tenth of a micron up to several microns and the crystals were blue when observed in transmission. Specimens of phthafocyanine were prepared in a similar manner and two good crystals of the platinum derivative were obtained with an apparatus which has been described previously. (16)

spectra

in Figure

1.

Single crystal absorption spectra were obtained with the Unicam S.P. 500 absorption spectrophotometer-the crystal being placed over a small hole in a piece of copper foil. Crystal thicknesses were measured in a high-powered optical projection microscope and the absorption coefficients were corrected for reflection losses. The liquid nitrogen Dewar flask which is shown in Fig. 4(a) was used to obtain the low temperature absorption spectrum of the copper derivative, and this was further investigated at higher resolution in the 4000 A to 6000 A range by using a Hilger D 187 wavelength spectrometer in conjunction with a quartz EMI 6255B photomultiplier and a Solartron D.C. AA900 amplifier. The results which have been obtained are illustrated in Figs. 5-X. In addition to the measurements of absorption reported in (a)-(c) above, the spectral dependence of photocondu~ti~~ity in the range 4000 A to IO,000 _& was investigated for single crystals of copper phthalocyanine. Long flat crystalline needles, up to 3 cm in length, were obtained from the vapour phase. The crystal, which was mounted on a quartz disc and secured at each end with

FIG, I.

Vapour

absorption

spectra

of:

(a)

Met&free

phthatocyanine

phtbafocyanine

(at 640°C).

(at

605°C).

fb)

Copper

OPTICAL

PROPERTIES

OF

platinum terminals, was bathed with radiation from a Hilger D 222 monochromator and chopped light source, and the spectral response was observed with a Vibron 33C electrometer. For experiments at liquid nitrogen temperatures the Dewar flask shown in Fig. 4(b) was assembled. The spectral response curve for the photocurrent in a single crystal of copper phthalocyanine surrounded by air at room temperature is given in Fig. 9. The curve obtained for a crystal in vacuum was identical but was considerably reduced in intensity, while at liquid nitrogen temperatures the photocurrent was only just detectable.

0 .E z P c

608~

Metal-free phtholocyanine

I

“‘0

2

3

I

I

I

I

I

I

4

5

6

7

6

9

IO

8

9

IO

THE

PHTHALOCYANINES

753

of phthalocyanine and its derivatives involves a consideration of the n-electrons in the conjugated double bond systems. If a crystal is illuminated, or if energy is supplied in some other manner, electrons are lifted to an excited level in the same molecule and the type of semiconductivity and photoconductivity observed is dependent on the mode of communication of the energy to the surrounding crystal environment. There is evidence that, in crystals composed of small molecules, the electron and resulting hole travel together in the form of an exciton and the theory of molecular crystals presented by DAVYDOV(~~)is then involved. For larger molecules the electron and hole which comprise the exciton may diffuse independently through the crystal and the wavelength dependences of the photoconductivity and optical absorption are then closely similar.

8x103

Wavelength,

C 20

I

40 60 80

Copper

phtholocyanine 100 LLVL 0 2

,

3

4

5

6

Wavelength,

FIG. 3. sublimed

7 8~10~

Absorption spectra in the visible films of metal-free, copper, phthalocyanines.

INTERPRRTATION

OF

THE

region for thin and platinum

RESULTS

A most reasonable explanation for the transmission absorption spectra of thin single crystals

FIG. 4(a). Diagrammatic representation of the holder used for the measurement of the absorption spectra of single crystals of copper, platinum, and metal-free phthalocyanines. The crystal is held over the aperture A. D isa Dewar flask. The Nicol prism P may be used to polarize the light in any required direction.

754

LEWIS

T.

CHADDERTON

L 4

/ 5

6

7

8

3-

8X103 'Wcwelength, FIG. 5. Absorption spectra for single crystal of metalfree phthalocyanine. Unpolarized light was used.

FIG. 4(b). Diagrammatic representation of the holder used for the measurement of the spectral response of photocurrent in single crystals of copper phthalocyanine. The crystal is secured on the quartz disc Q.by platinum terminals. 2’1 and Tz are thermocouple leads, and I.1 and L.2 are leads to the crystal, D is a Dewar flask. b

In the case of metal-free phthalocyanine we have a large conjugated molecule containing thirty-eight a-electrons. In the crystalline form the planar molecules lie at an angle of 44” to the n-c plane, alternate molecules pointing above and below the plane. ~tereochemist~(ls) suggests a possible overlap of n(pz> orbitals between one molecule and its neighbour along the c-axis, but BASU(~~) calculates that the extreme values of the n-electron density occur in the vicinity of the nitrogen atoms, in the central part of the molecule. It is therefore probable that r-electron overlap from molecule to molecule will occur between adjacent stacks, and that energy levels of 7~ electrons in the molecule will be important in considering interpretations of single crystal absorption spectra.

Ix1031 4

I 5

I 6

I 7

Wavelength,

8

9

0

%X103

FIG. 6. Absorption spectra for single crystal of platinum phthalocyanine. Unpolarized light was used.

It is evident that, for copper phthalocyanine, the photoconductivity response curve closely follows the optical absorption spectrum for a single crystal. This also suggests that, for molecular crystals like copper phth~o~yanine, the energy required to jump the n-electron across an intermolecular barrier is negligible compared with the energy required to lift the electron, to the excited state. The quasi-continuous bands shown in the single crystal absorption spectra of metal-free and

OPTICAL

PROPERTIES

OF

THE

PHTHALOCYANINES

755

7

;o

4

Wavelength,

FIG. 7. Absorption phthalocyanine.

4.0

4.2

6

7

6

5

9

8~10~

spectra for single crystal of copper Unpolarized light was used.

4.4

4.6

4.8

Wavelength,

5.0

52

5.4

A x IO3

FIG. 8. Absorption

spectrum for a single crystal of copper phthalocyanine in the region 4000 A to 6000 A. Phonon assisted transitions introduce subsidiary absorption peaks which become sharper as the temperature falls.

copper phthalocyanines can be correlated with sharper peaks observed in the transmission absorption spectra of vapour, solution(l), and sublimed films. If the spectral shifts arising from temperature dependence and the broadening due

FIG. 9. The spectral dependence of photocurrent in the range 4000 A to 1,000 A for a single crystal of copper phthalocyanine at room temperature and in air.

to crystal field interactions are accounted for there appears to be a one-to-one correspondence between peaks observed in the absorption spectra for molecules in different degrees of association. This makes possible the interpretation of the optical properties in terms of the simple band model. In particular it would appear that a doubly degenerate Bl,-E, transition which has been predicted by BASU(~~) in a molecular orbital calculation on phthalocyanine is present in the sublimed film spectrum for this compound. Moreover, this peak, at a wave number of 14720 cm-1 (1.82 eV), has a close parallel in the platinum and copper derivatives. There is little evidence of absorption at 21,620 cm-r (2.68 eV), the other peak which was predicted by BASU(~~) (Table l), although there is a slight increase in intensity in this region for the vapour absorption spectrum. Calculations of optical activation energies have been made for copper, platinum and metal-free phthalocyanines on the basis of two arbitrary methods. In the first method (A) the central point on the linear part of the low energy absorption edge was taken as the characteristic wavelength for calculating the single crystal optical activation energy. In the second method (B) that point on the top of the band at which absorption begins to fall was used in the calculations. Such arbitrary methods only yield approximate results but the agreement with values

LEWIS

756

T.

obtained by the Russian authors(ah4) (Table from work with sublimed films is encouraging.

CHADDERTON

2)

Table 1

~. ~~~-~..-..~__--__..II” .“._ --~~~~~ .~

_- ._.__ “.”.._..- ----

Calculated Frequency Transition

Degeneracy Uncorrected (cm- ‘> Double DoubIe

Bau-q BWE, __._...”

.“.._ _---

20,470 (898) 13,570 (598)

Corrected (cm-‘) ___~ 21,620 (94/3> 14,720 (6413)

..~__

-

The fine structure which is revealed in Fig. S for the single crystal absorption spectrum of copper phthalocyanine is to be attributed to the stimulation of lattice vibrations. Energy absorbed from the incident radiation may be used in the energizing of electrons rmd in the creation of On other occasions the energy for phonons. electron excitation derives from both the external radiation and internal lattice vibrations, Phononassisted transitions have been observed by other

--____---

which can be identified in the crystal absorption spectra. The low energy absorption occurs always as a doublet, usually with one branch much more intense than the other.(t4) From observations of solution absorption spectra@s) it has been suggested that, for phthalocyanine, two electronic transitions are present with origins at 14,290 cm-1 and 15,060 cm-l. There is no evidence, however, that this explanation is more feasible than the suggestion that a 770 cm-1 vibrational interval is responsible. First-order calculations of factor group splittings have been made by X;YONS(~~~ who, on the basis of molecular symmetry, suggests tentatively that the 14,290 and 15,060 cm-1 bands are components of a single transition which is terminated by a degenerate level. These conclusions are consistent with the prediction by BASU(~~) of a doubly degenerate &--& transition from the ground state to the first excited state of a n-electron on the phthalocyanine ring. The higher energy absorption does not correlate very well with a doublv degenerate Bs&$ transition at 21,620 cm-1 which is also predicted by Ifasu (Table 1).

-.~-.

l-l-____

Metal-free

Copper

Platinum

Single crystal (Method

A)

1.55 4 0.01 eV 2.61 r?r:0.03 eV

1.58 + O*Ol cv 2.81 & 0.04 eV

x.57 If: 0.02eV 2.81 k 0.03 eV

Single crystal (Method

B)

I.64 + 0.03 eV 2.76 + 0.06 eV

l-67 + 0.03 ev 3.10 f 0.15 ev

1.70 If: 0.05 eV 2.95 + 0.07 et’

1.64 ev

I.64 ev -

Sublimed films (flussian work(3,4,) -..__II -.-

-.---.--. _^---._-_--

workers in silver cyanamide and in certain of the azides. (21) There is therefore distinct evidence that the simple band model offers a reasonably good description of the behaviour of crystalline copper, platinum, and metal-free phthalocyanines. The low values of absorption coefficient in the single crystal spectra give cause for some concern, though it is likely that indirect transitions are involved. The energy levels for rr-electrons in the isolated molecule are sharp and well-defined, but interactions in the crystal smear out the levels into bands. For the three compounds investigated there appear to be two distinct strong transitions

._..- _~_-~_.._

___.

_II_..-._--

The conduction and valence band ideas which apply to inorganic semiconductors must not be extrapolated to organic crystals without careful consideration. It is possible that, for the phthalocyanines,(” the absorption of light might be accompanied by the formation of excitons which are capable of migrating in the crystal. These excitons, moreover, could be thermally dissociated by absorbing additioaal energy from the lattice and might disintegrate at impurity centres with the conseqnent formation of free charges. This would explain the increase of the photosensitivity of copper phthalocyanine crystals in the presence of oxygen and the close similarity of the

OPTICAL

PROPERTIES

OF

photoconductive and absorption responses. Similar effects have been observed in anthracene.@s) CONCLUSION

The transmission absorption spectra of some of the phthalocyanines have been investigated for molecules in different degrees of association. The close similarities which are obtained for vapour, sublimed film, and single crystal spectra suggest that the simple band model offers a reasonably good description of the features observed. In particular, the single crystal copper and platinum phthalocyanine spectra may be explained in terms of two strong r-electron transitions to a metastable excited state. The sharp energy levels of the free molecule evidently transform into broad quasicontinuous bands on crystallization.

Acknowledgements-I encouragement discussion.

and

thank Dr. F. P. BOWDEN for his Dr. A. D. YOFFE for helpful

REFERENCES 1. FIELDING P. E. and GUTMAN F., J. them. Pkys. 26, 411 (1957). 2. VARTANYAN A. T., Zh. fiz. khim. 1028 (1956). 3. VARTANYAN A. T. and KARPOVICH I. A., Zh. jiz. khim. 178 (1958).

D

THE

PHTHALOCYANINES

757

4. VARTANYAN A. T. and KARPOVICH I. A., Z/z. Jiz. khim. 274 (1958). c TERENIN A. N., PUTZEIKO E. and AKIMOV T., 3. I. them. Phvs. 26. 716 (1957). 6. T~LLIN G., K’EARN~ D. R.-and CALVIN bI., J. them. Phys. 32, 1013 (1960). 7. TOLLIN G., KEARNS D. R. and CALVIN \I., J. them. Phys. 34, 2022 (1961). 8. LYONS L. E., WALSH J. and WHITE J. W., J. them. Sot. 167 (1960). 9. ELEY D. D., Research 12, 293 (1959). 10. BORNMANN J. A., J. them. Phys. 27, 604 (1957). 11. MANY A., HARNIK E. and GERLICH D., J. them. Pkys. 23,1733 (1955). 12. NEIMAN R. and KIVELSON D., J. them. Phys. 35, 162 (1961). 13. LYONS L. E., J. them. Sot. 1347 (1958). 14. RICGLEMAN B. M. and DRICKAMER H. G., J. them, Phys. 35, 1343 (1961). 15. BOWDEN F. P. and CHADDERTON I,. T., Narzrre Land. 192, 31 (1961). 16. BOWDEN F. P. and CHADDERTON L. T., Proc. Roy. Sot. A269, 143 (1962). 17. ELEY D. D. and PARFITT G. D., Trans. Faraday Sot. 51, 1529 (1955). 18, 18. DAVMOV A. S., Zh. eksp. tear. fix. S.S.S.R. 210 (1948). 19. ELEY D. D., PARFITT G. O., PARRY M. J. and Trans. Faraday Sot. 49, 79 TAYSUM D. H., (1953). 20. BASU S., Indian J. Phys. 37, 511 (1954). 21. SOLE M. J., Private communication. 22. ANDERSON J. S., BRADBROOKE. F., COOK A. H. and LINSTEAD R. P. J. them. Sot. 1152 (1938). 23. CARSWELL D.,J. them. Phys. 21, 1890 (1953).