Materials for Nonlinear Optics - American Chemical Society

Chapter 42

Downloaded by UNIV OF CALIFORNIA SANTA BARBARA on March 1, 2018 | https://pubs.acs.org Publication Date: March 11, 1991 | doi: 10.1021/bk-1991-0455.ch042

Nonlinear Optical Properties of Substituted Phthalocyanines James S. Shirk, J. R. Lindle, F. J. Bartoli, Zakya H. Kafafi, and Arthur W. Snow Naval Research Laboratory, Washington, DC 20375

The third order optical susceptibility was measured for a series of transition metaltetrakis(cumylphenoxy)phthalocyanines at 1.064 μm. Metal substitution caused a dramatic variation in the third order susceptibility. The largest 's were found in the Co, Ni, and Pt complexes. Metal substitution introduces low lying electronic states which can enhance the susceptibility in these phthalocyanines. A strategy for enhancing the figure of merit, χ /α, of centrosymmetric nonlinear optical materials is suggested. xxxx

(3)

In a recent communication we reported that the third order nonlinear optical susceptibility of Pt, Pb, and H tetrakis(cumylphenoxy)phthalocyanines was large and varied substantially with the metal substituent. (1) The structure of these compounds is shown in Fig. 1. The susceptibility was measured by degenerate four-wave mixing at 1.064 μπι, a wavelength far from the main absorption bands of phthalocyanines near 650 nm. The nonlinear susceptibility of the Pt phthalocyanine was about a factor of 9 larger than that of the Pb phthalocyanine and a factor of 45 larger than the metal free compound. This paper is a more extensive survey of the influence of the metal on the hyperpolarizability of a series of the transition metal tetrakis(cumylphenoxy)phthalocyanines (MPcCP ). The compounds chosen were those most closely related to PtPcCP , the compound which showed the largest hypeφolarizibility in the previous study. Specifically, phthalocyanines substituted with the last four members of the first row transition metal series (Co, Ni, Cu, and Zn) and also with the Ni, Pd, Pt triad were prepared and studied. The near IR spectra of these tetrakis(cumylphenoxy)phthalocyanines are briefly discussed. Speculation on how metal substitution can influence the third order susceptibility of a near centrosymmetric structure, like that of the phthalocyanines, is presented. 2

4

4

Experimental The third order optical susceptibility was measured by degenerate four-wave mixing (DFWM). A single pulse at 1.064 μπι with a full width at half maximum of 35 ps was selected from the output of a passively mode locked N d / Y A G laser and split into three

This chapter not subject to U.S. copyright Published 1991 American Chemical Society Marder et al.; Materials for Nonlinear Optics ACS Symposium Series; American Chemical Society: Washington, DC, 1991.

42.

SHIRK E T A L .

Substituted

Ν


627

Phthalocyanines

W

Ν

Figure 1. The structure of the metal tetrakis(cumylphenoxy)phthalocyanine (MPcCP ). This is one resonance form; in the metal complexes, the phthalocyanine moiety has D symmetry. 4

4 h

beams. The beams were overlapped in the sample using a counter-propagating pump geometry. Time delays could be introduced into either the probe or the backward pump beam. The beams were weakly focussed onto the sample contained in a 0.2 mm thick glass or quartz cell. The laser intensities at the sample were ca 0.2 to 20 GW/cm in each of the pump beams and 0.05 to 5 GW/cm in the probe beam. The phase-conjugate reflection was detected with a Si photodiode. The temporal dependence of this signal was measured by delaying the arrival time of the back pump beam. All beams were polarized parallel to each other. The preparation of the tetrakis(cumylphenoxy)phthalocyanines has been described.(2) The cumylphenoxy derivative was chosen because of its solubility in common organic solvents. The four-wave mixing experiments were performed on CHC1 solutions of the phthalocyanines with concentrations in the range of 5 χ ΙΟ" M to 0.1 M . The concentration was chosen so that the solution χ was dominated by the phthalocyanine and the sample transmission was > 0.8. Most measurements were performed on solutions with concentrations near ΙΟ" M (~ 1% by weight). The measured transmission at 1.064 μπι of the samples used for the four-wave mixing experiments ranged from .8 (for 10" M NiPcCP ) to >0.99 for similar concentrations of PdPcCP , H PcCP and ZnPcCP . Absorption spectra were recorded on a Cary/Varian Model 2300 spectrophotometer. Path lengths of 5 mm or 1 cm were used when necessary to obtain accurate absorbance measurements for weak bands. 2

2

3

3

(3)

2

2

4

2

4

4

4

Results Spectroscopy A spectrum of PtPcCP , which is typical of these phthalocyanines is shown in Fig. 2. The most intense band is the Q band which occurs between 640 nm and 680 nm for the different metal phthalocyanines. It is the lowest allowed π - π* transition of the phthalocyanine ring. In dilute solution, the Q band of the monomer typically had a molar extinction coefficient of 2 x 10 //mole-cm in agreement with previous reports. (2) Additional bands, which have been assigned to phthalocyanine aggregates (2)(3), were observed on the short wavelength side of the Q band in the relatively concentrated solutions used for the nonlinear optical studies. 4

5

Marder et al.; Materials for Nonlinear Optics ACS Symposium Series; American Chemical Society: Washington, DC, 1991.

628

MATERIALS FOR NONLINEAR OPTICS: CHEMICAL PERSPECTIVES


34

500

1000

1500

Wavelength (nanometers) Figure 2. The spectrum of Pt tetrakis(cumylphenoxy)phthalocyanine (PtPcCP ), 2.0 χ 10 M in CHC1 solution. The inset shows the near IR region on an expanded absorbance scale. (Reproduced with permission from reference 1) 4

3

3

Some of the metal tetrakis(cumylphenoxy)phthalocyanines were found to have weak absorptions in the region 1.1 - 1.5 μπι. The band for PtPcCP can be seen on an expanded scale in Fig. 2. The and the molar extinction coefficient, e, for the near IR band in each of the metal phthalocyanines studied here are given in Table 1. The near IR bands were very much weaker than the Q band. The strongest absorptions, in the Pt, Ni, and Co complexes, were more than 2 orders of magnitude weaker than the Q band. The Cu and Pb phthalocyanine absorptions were about four orders of magnitude weaker than the Q band. The Zn, Pd, and H (metal-free) complexes showed no distinct bands in this region. The nonlinear optics experiments were performed at 1.064 μπι, far from resonance with the Q band and except for CoPcCP , above the near IR band. 4

2

4

Nonlinear Optical Measurements The magnitude of the phase-conjugate reflection for each of the phthalocyanines in CHC1 solution was measured as a function of laser intensity. In each case the signal was much larger than that observed for pure solvent. For the Co, Ni, Cu, and Pt phthalocyanines the intensity of the phase-conjugate reflection was found to depend upon the laser intensity to the 3.0 ± 0.3 power with no evidence of saturation up to approximately 15 - 20 GW/cm in each pump beam. For Pd, Zn, and H PcCP , the slope was significantly greater than a pure cubic dependence. Such deviations from a cubic dependence might occur because at the highest powers 3

2

2

4


42.

Substituted Phthalocyanines

SHIRK ET AK

629

used in these experiments, the two photon transition probability can become significant. If two photon absorption produces a population grating in the sample, the signal arising from diffraction by such a population grating will depend upon the laser intensity to the fifth power. (4) In this paper, we are primarily interested in the third order response, so the signal for Pd, Zn and H PcCP was fit to a curve of the form a I + a I , and the cubic contribution was used to obtain χ . The phase-conjugate signal as a function of laser intensity for PdPcCP is shown in Fig. 3. The best fit to this data shows that the major part of the observed signal was due to the cubic term. In H PcCP the fifth power term was more significant. χ for each solution was obtained by comparison with a CS reference using(5): 3

2

4

5

3

5

(3)

4


2

4

(3)

2

s_

(3)

,0)

Y

"

r

\2

ref

\

al

ι)

V,

(1)

(l-e-")j

where S = a , the coefficient of the cubic term of a least squares fit of the phaseconjugate signal vs the laser intensity, 1 is the sample path length, η the refractive index, and a is the absorption coefficient at 1.064 μπι. The subscript " ref refers to CS , for which a value of x = 4 x 10" esu was used. (6)(7) 3

M

2

( 3 )

13

m

Table 1 Optical Properties of the metallo-phthalocyanines at 1.064 μπι (3) a Λ. xxxx γ

I xxxx

(esu)

(esu) 32

CoPcCP

4

NiPcCP

5 x 10"

32

4 x 10'

4

CuPcCP

4

3 x 10

32

ZnPcCP

4

5 x ΙΟ"

PdPcCP

4

1 x ΙΟ"

PtPcCP

1 x ΙΟ"

4

11

8 x ΙΟ"

6 x 10" 11

4 x ΙΟ"

max 1

(M-W )

1.15

400

1.03

shoulder

1.20

1500

1.10

-40

(

X \xx/«

° 1.064

1.5 x 10

1 x 10

18

1 x 10

1.6 x 10 1.7 x 10

19

2 x 10"

31

2 x 10

10

1.38

1600

2.4 x 10

32

2 x 10"

1.23

10

4 x ΙΟ

2x 10

33

4 x 10

4x 10

630


The molecular hyperpolarizibilities, γ, given in Table 1 were derived from the measured χ of the solutions using the expression: (3)

= X

( 3 )

4

x r a

/L N

(2)

where Ν is the number density of the phthalocyanine and L is the local field factor. L is assumed to be a constant, in this case due to the solvent and given by: 2


L = (n + 2)/3

(3)

where η = 1.44 the refractive index of the solvent. Another procedure that has been used to compare the nonlinearities in new materials (8)(9) is to extrapolate to the χ of the pure solid material. Table 1 includes such an extrapolated χ * ^ that was calculated using: (3)

3

Λ.

XXXX 3

( Q / C ) χ
40 GW/cm implies a population grating χ substantially smaller than the observed χ . For PtPcCP , for example, the implied population grating χ < 3 x 10" esu, compared to the measured value of 2 x 10" esu. in table 1. We conclude that a simple population grating is probably not the predominant mechanism for the 35 psec component of the observed χ . Another potential source of the enhancement in the ηνρβφο^ΓίζίοΐΙ^ of phthalocyanines is the contributions of the new electronic states introduced by metal substitution. The usual theoretical expression for the η ν ρ β φ ο ^ π ζ ί ο ϋ ^ involves a sum of the contributions of each electronic state.(17) Introducing new electronic states expands the number of terms which may contribute to the hypeφolarizibility. A rigorous calculation of the contribution of the new states to the χ of these large molecules, or


β

(3)

(3),

2

(3)

s

2

s

2

(3)

(3)

(3)

11

4

10

(3)

(3)


42.

SHIRK ET A L

Substituted

Phthalocyanines

633

its variation with metal, would be difficult. It is interesting, however, that opening up the d shell occupancy in the transition metal phthalocyanines (as in the sequence: Zn, Cu, Ni, Co) increases the number of electronic states that may contribute to the hyper polarizibility and it also enhances the observed χ . The hyperpolarizibility of PdPcCP was anomalously low compared to the N i and Pt complexes. This low hyperpolarizibility may be related to the differences in the low lying electronic states. There is, for example, no near IR band in PdPcCP corresponding to those in the N i and Pt complexes. The details of and the reason for these differences in electronic structure among the d cumylphenoxy phthalocyanines will require further clarification. The contributions of optically forbidden electronic states to the χ of centrosymmetric structures are of particular interest.(18) Each of the terms in a sum-over-states calculation of χ involves the product of transition moments between a sequence of four states. There are symmetry selection rules that govern which states which can contribute to the individual terms. In a centrosymmetric molecule the symmetry of the contributing states must be in a sequence g --> u --> g - > u --> g.(19) This means that all the non-zero terms in the summation which determines the hyperpolarizibility must include an excited electronic state of g symmetry (or the ground state) as an intermediate state. The tetrakis(cumylphenoxy)phthalocyanines are approximately centrosymmetric and many of the new electronic states in a metal phthalocyanine will be of g symmetry. Such states may well contribute to the dependence of the hypeφolarizibility on metal substitution. If, in centrosymmetric molecules, states to which a transition is forbidden in the normal absoφtion spectrum can make important contributions to χ , this suggests a strategy for enhancing the figure of merit, χ / α , of such a nonlinear material. Chemically introducing low lying states with gerade symmetry and thus small or zero absoφtion cross sections has the potential to enhance the χ but not increase the absoφtion probability, a. (3)

4

4


8

(3)

(3)

(3)

(3)

(3)

Conclusions The third order optical Ι^βφο^ΓίζΜΐίίββ of the tetrakis(cumylphenoxy)phthalocyanines substituted with four sequential members of the first transition series, Co, Ni, Cu, and Zn, and the Ni, Pd, Pt triad were measured at 1.064 μπι, a wavelength far from the main absoφtion band. The third order susceptibilities were remarkably large and varied dramatically with the metal. A monotonie variation of γ with the metal was observed in the series Co, Ni, Cu, Zn where substitution with metals with a more open d shell gave larger nonlinearities. There was a qualitative correlation between a large η ν ρ β φ ο ^ Γ ί ζ Μ ^ and the presence of a weak, near IR transition but the correlation between γ and the absoφtion coefficient was not linear. These absoφtions are a manifestation of the new low lying electronic states introduced into the phthalo cyanine electronic manifold by metal substitution. A population grating from optical pumping of the weak absoφtion at 1.064μπι in some of the tetrakis(cumylphenoxy)phthalocyanines does not easily account for the magnitude and time response of the observed χ . Contributions of these new electronic states to the electronic η>ρβφοΐ3ΓΪζibility may account for a variation in χ with the metal. A quantitative estimate of the magnitude of their contributions is difficult and requires a more complete knowledge of the electronic states than is currently available. This work suggests a way to influence the figure of merit, χ^χχχχ/α, of centrosymmetric or near centrosymmetric nonlinear materials like the phthalocyanines. (3)

(3)


634


Symmetry considerations show that chemical substitution to introduce low lying, one photon forbidden states into the molecule has the potential to enhance the χ without increasing the absorption probability, a. (3)

Acknowledgments


This work was supported by the Office of Naval Research, the Office of Naval Technology and the Strategic Defense Initiative Organization, Innovative Science and Technology Program. Literature Cited 1. J.S. Shirk, J.R. Lindle, F.J. Bartoli, C.A. Hoffman, Z.K. Kafafi and A.W. Snow, Appl. Phys. Lett. 55, 1287 (1989) 2. A.W. Snow and N.L. Jarvis, J. Am. Chem. Soc. 106, 4706, (1984) 3. M.J. Stillman and T.Nyokong, in "Phthalocyanines" ed. C.C. Leznoff and A.B.P. Lever, VCH, New York, p. 133-290 (1989) 4. G.C. Bjorkland, D.M. Burland, and D.C. Alvarez, J. Chem. Phys. 73, 4321 (1980) 5. R.G. Caro and M.C. Gower, IEEE J. Quant. Electr. QE-18, 1376 (1982) 6. M.J. Moran, C.S. She, R.L. Carman, IEEE J. Quant. Electr. QE-11, 259 (1975) 7. R.W. Hellwarth, Prog. Quant. Electr., 5, 1 (1977) 8. S.A.Jenekhe, S.K.Lo, S.R. Flom, Appl. Phys. Lett. 54, 2524 (1989) 9. D. Ricard, P. Roussignol, C. Flytzanis, Opt. lett. 10, 511 (1985); 10. Z.Z. Ho, C.Y. Ju, and W.M. Heatherington, J. Appl. Phys. 62, 716 (1987) 11. A.W. Snow and N.L. Jarvis, J. Am. Chem. Soc. 106, 4706 (1984) 12. A.M. Schaffer, M. Gouterman, and E.R. Davidson; Theor. Chim. Acta 30, 9 (1973) 13. for a recent review see: M.J. Stillman and T.Nyokong, in "Phthalocyanines" ed. C Leznoff and A.B.P. Lever, VCH, New York, p. 133-290 (1989) 14. A.B.P. Lever, S.R. Pickens, P.C. Minor, S.Licoccia, B.S. Ramaswamy, and K. Magnell; J. Am. Chem. Soc. 103, 6800 (1981) 15. P.S. Vincett, E.M. Voigt, K.E. Reickhoff, J. Chem. Phys. 55, 4131, (1971) 16. M.A. Kramer, W.R. Tompkin, and R.W. Boyd; Phys. Rev. A, 34, 2026 (1986) 17. J.F. Ward, Rev. Mod. Phys 37, 1 (1965) 18. M.G. Kuzyk and C.W. Dirk, Phys. Rev. A, 41, 5098 (1990) 19. J.W. Wu, J.R.Heflin, R.A. Norwood, K.Y. Wong, O. Zamani-Khamiri, A.F. Garito, P. Kalyanaraman and J. Sounik; J. Opt. Soc. B, 6, 707 (1989) RECEIVED September 4, 1990


Materials for Nonlinear Optics - American Chemical Society

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