Molecular and Biomolecular Electronics - American Chemical Society

11

Downloaded by NORTH CAROLINA STATE UNIV on May 3, 2015 | http://pubs.acs.org Publication Date: May 5, 1994 | doi: 10.1021/ba-1994-0240.ch011

Molecular Optoelectronics Based on Phthalocyanine Tatsuo Wada, Masahiro Hosoda, and Hiroyuki Sasabe Frontier Research Program, RIKEN (The Institute of Physical and Chemical Research), Hirosawa, Wako, Saitama 351-01, Japan

The molecular design and assembly of metallophthalocyanine sys tems that have the enhanced macroscopic third-order nonlinear susceptibility χ(3) (—ω ;ω ,ω ,ω ) and show ultrafast responses are described. Enhancement of the third-harmonic susceptibil ity χ(3) (—3ω;ω,ω,ω) was observed in vanadylphthalocyanine vacuum-deposited films with the staggered stacking arrangement induced by thermal treatment. A processible polymeric system that was rich in tert-butyl monosubstitutions was developed, and the favorable staggered stacking arrangement was induced in a polymer matrix to enhance χ(3) (—3ω;ω,ω,ω). Femtosecond time-resolved spectroscopy was performed on vanadylphthalocyanine thin films with different morphological forms to elucidate the exciton dynamics. 4

ijkl

1

2

3

1111

1111

O R G A N I C P H O T O N I C S IS T H E F R O N T I E R of optoelectronic applications of organic materials, such as in optical computing, image processing, and communication systems. Fundamentally, the nonlinear optical (NLO) effects of materials are applicable for these purposes, for example, addition or subtraction of optical frequencies for computing, optical bistability for switching and memories, the optical Kerr effect for wave guides, and spatial light modulators. From the materials viewpoint, the intramolecular charge transfer through π-eleetron conjugation gives large optical nonlinearities on the molecular level, whereas the symmetry of the crystal structure determines the macroscopic second-order nonlinearity X yfc (~ω3;ω!,ω ); if the crystal is centrosymmetric, then X becomes zero. However, the third-order optical nonlinearity X % & (—ω ;ωι,ω ,ω ) does not depend on the crystal symmetry but on the microscopic third-order susceptibility yû (—ω ,ωι,ω ,ω ) of the con(2)

( 2 )

2

(

4

2

3

4

2

3

0065-2393/94/0240-0303$08.00/0 © 1994 American Chemical Society In Molecular and Biomolecular Electronics; Birge, R.; Advances in Chemistry; American Chemical Society: Washington, DC, 1994.


304

MOLECULAR AND BIOMOLECULAR ELECTRONICS

stituent molecular unit. Charge-transfer complexes and 7r-eonjugated compounds are the most promising materials for N L O applications, but the systematic investigation of features such as controls of conjugation length, intermolecular interaction, and packing structure is still occurring. The quantum field theory for low-dimensional systems (I) suggests that in conjugated linear chain structures, such as polyenes and polydiacetylenes, ττ-electrons are delocalized in their motion only in one dimension along the chain axis. The major contribution to yî is the dominant chain axis component y with all electric fields aligned along the chain axis (x-axis). That is, in the one-dimensional (ID) ^-conjugation systems, only one tensor component y contributes to the averaged susceptibility (7) in isotropic media; (7) is equal to ( l5)y . A power law dependence of yî on the number of carbon atom sites has been found, with exponents of 5.4 for the frans-polyene and 4.7 for the cispolyene conformer, and y is more sensitive to the physical length of the chain than to the conformation (J). When the dimensionality of the 7r-electron system is expanded from linear to cyclic chains, the theoretical results on cyclic structures such as cyclooctatetraene show a decrease of y i due to an actual reduction in the effective length available for the 7r-electron to respond to an optical electric field (2). In the widely spread two-dimensional (2D) ^-conjugation systems, on the contrary, other tensor components also contribute to (7), that is, xxxx

xxxx

l

xxxx

xxxx

ijk

~~ /δ [Ύχχχχ +

yyyyy ^~ A (^xxyy

*Yxyxy

Jxyyx

7yxyx

Ύι/t/xx

"Vyxxyft

The off-resonant third-order optical susceptibility X \ (—3ω;ω,ω,ω) (abbreviated X hereafter) for the macrocyclic conjugated structural class of annulenes with an 18-28-membered ring size—tetradehydromethano-[18]-, -[22]-, -[24]-, and -[28]-annulene—was determined sys tematically by optical third-harmonic generation (THG) measurements at 1907 nm (3). The X value was found to increase with increased size of the macrocyclic conjugated structure in a manner analogous to the behavior of conjugated linear chains. Thus, we focused our research on the development of macrocyclic compounds consisting of 2D conjugated τΓ-eleetron systems, especially metallophthaloeyanine (MPc) derivatives, which exhibit linear and nonlinear optoelectronic responses such as photoconductive, photovoltaic, and photocatalytic behavior. These ma terials also have attractive physical properties such as large absorption in the visible region, thermal and chemical stability, and thin-film for mation. This chapter describes our material research approach (4-8) to developing MPc thin films that enhance the macroscopic nonlinear sus ceptibility, ultrafast N L O response, and optical waveguide application. (

3

m

( 3 )

( 3 )

In Molecular and Biomolecular Electronics; Birge, R.; Advances in Chemistry; American Chemical Society: Washington, DC, 1994.

11.

W A D A ET AL.

Molecular Optoelectronics

305


Thin Films of Metallophthalocyanines Vacuum-Deposited F i l m s . For MPcs, several approaches exist for fabricating a thickness-controlled thin film, as shown in Scheme I. Unsubstituted MPcs show exceptionally high thermal stability. There fore, vacuum deposition techniques have been widely used to obtain thin films 10 nm to several micrometers thick by controlling the de position rate and time. In the electronic spectra of MPcs, two charac teristic absorption bands are well established, for example, the Soret band (300-400 nm) and the Q-band (the ττ-ττ* transition in 600-800nm region). The Q-band is sensitive to the environment, such as the orientation and packing of MPc rings. In general, the central metal has little effect on the electronic state of phthalocyanine but a strong influ ence on the packing arrangement of the phthalocyanine molecules in the condensed state. Therefore, depending on the central metal, features of the Q-band changed remarkably in the condensed state. This Q-band has been widely studied as a probe of the phase transitions induced by thermal or solvent vapor treatment (9). Our previous studies on T H G of MPc vacuum-deposited (VD) films showed that vanadylphthalocyanine (VOPc) has a large X , of the order (3)

Enhancement o f γ • Symmetry • Metallation

Scheme I.

Enhancement of χ · Molecular Stacking (

Assembly

Thin-film fabrication of phthalocyanines.


306


of 10~ electrostatic units (esu) at a fundamental wavelength of 1907 nm (10). Figure 1 shows typical absorption spectra of VOPc V D thin films. The absorption maximum of an as-prepared VOPc film is located at 740 nm, with a shoulder near 680 nm in the Q-band. Heating the VOPc film at 125 °C in the air produced a new near-IR absorption at 820 nm in the spectrum and decreased the intensities of 740- and 680nm peaks, as shown in Figure 1. Spectral change ceases after 15 h under these conditions. A similar absorption change was observed by thermal annealing of TiOPc V D thin films. According to Griffiths et al. (9), the phase with peaks at 680 and 740 nm is attributed to the cofacial packing of VOPc molecules (phase I). The as-prepared film seems to have a cofacially stacked arrangement (phase I); that is, it is aligned linearly along the metal-oxo bond. The packing arrangement that leads to a bathochromic shift in the Q-band is called phase II (9). An X-ray diffraction study showed that the phase II introduced by heating has a triclinic crystal structure with the space group P i and is identified as a slipped-stack arrangement, that is, a staggered assembly on adjacent molecules in the thin films (JJ). Figure 2 shows the molecular packing of phase II of VOPc. In our preliminary X-ray diffraction study, a significant difference in those thin films was observed before and after heating. The peak intensity at 20 = 12.7° and 20 = 25.5°, corresponding respectively to interplanar distances of 6.96 and 3.48 attributable to the phase II of VOPc, gradually increased with the thermal treatment.


10

The typical results of rotational T H G Maker fringes are shown in Figure 3. Because of the large X value of VOPc, the contribution from the fused silica substrate is insignificant. The enhancement of third( 3 )

Figure 1. Absorption spectra of VOPc thin films: (solid line) as-prepared film, (line with long dashes) film annealed at 125°C for 1.5 h, and (line with short dashes)filmannealed at 125°C for 1.5 h. a.u. (Reproduced with permission from reference 7. Copyright 1991.)

500

600

900 700 800 WAVELENGTH (nm)


1000


11.

W A D A ET AL.

307


Space group:Ρ1 a = 12.027 Â, b = 12.571 À, e = 8.690 A α = 96.04°, β = 94.80°, γ = 68.20° Figure 2. b-axis.

Projection of molecular packing of phase II of VOPc along the

harmonic (TH) intensity is observed in the sample after thermal an nealing. The X value of fused silica at 1543 nm was calculated from Miller's rule by using X at 1907 nm determined by Heflin et al. (12) (X = 1.40 X 10~ esu), as 1.47 Χ 1 0 " esu. The X values of the phthalocyanine thin films were estimated with reference to the X of fused silica. In a nonabsorbing medium, the T H intensity is given as (13) ( 3 )

( 3 )

(3)

14

14

( 3 )

i 3 )

γ

2567Γ

4

(AX&X

. /Δ^\ 2

where c is the velocity of light, Ac the dielectric constant dispersion (and equals η - η , where η is the refractive index), A the factor arising from transmission and boundary conditions, and Αφ the phase mismatch between the fundamental and harmonic frequencies. In an absorbing medium, on the other hand, the T H intensity is given by ω

2

3ω

2


308


too

>

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Γ

(a)

b 03 80 Ζ

LU

H

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> LU


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-30

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J

-10

10

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30

40

50

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INCIDENT ANGLE Ί

J

20

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1

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ι

LU CE

»

-50

-40

-30

-20

-10

0

10

20

INCIDENT ANGLE

30

40

50

/DEG.

Figure 3. Rotational THG Maker fringe patterns: (a) as-prepared VOPc (thickness 178 nm) on fused silica, and (h) a similar film annealed at 125° C for 15 h. (Reproduced with permission from reference 7. Copyright 1991.)

equation 2 under the condition of I (the sample thickness) « coherence length) (13), h„ = (^r)

(AX

( 3

l (the c

W/.

(2)

where Ι and ί are the light intensity for fundamental and T H fre quencies, respectively, a n d / is the absorption-dependent factor, ω

3ω

a

, U

=

[1 - exp(-« (η

3ω

2

3M

1

/2)] + ( A f l exp(«3. /2) 2

2

- η J - k Jf 3

x

+ (2n J; „) 3

3

2

K

'

where η and k are the real and imaginary parts of refractive indices, respectively, and a is the linear absorption coefficient. The results of


11.

W A D A ET AL.

309


T H G measurement are summarized in Table I, together with the values of refractive indices of phthalocyanine thin films obtained by KramersKronig analysis. In spite of the small changes in refractive indices at fundamental and T H wavelengths, the macroscopic X values in VOPc and TiOPc thin films increased by thermal treatment to about 2-5 times those in untreated films. These results indicate that the macroscopic X* values are larger in phase II than in the other phase. ( 3 )


3)

Spin-Coated D o p e d Polymer F i l m s . The major problem encountered with unsubstituted MPc systems is poor processibility, particularly poor solubility in organic solvents. For the waveguide application, the materials should be designed to meet several fabrication requirements. Metallation and chemical modification, such as introduction of functional groups, can be used to improve the physical properties of MPc molecules. For example, several functional groups (e.g., alkyl, alkoxy, trimethylsilyl, and sulfamide groups) introduced into the peripheral position of each of the benzo rings give excellent solubility in common organic solvents (14). A polymer [polystyrene, poly(methyl methacrylate), and so on] doped with tetrakis(tert-butyl)metallophthalocyanines [MPc(f-bu) s] can be formed easily on various substrates by a conventional spin-coating technique, and these doped polymer (DP) films show excellent optical quality. Figure 4 shows the absorption spectra of spin-coated pure MPc(ibu) films [M is H (metal free), V O , and Ni). In these MPc(£-bu) films, the absorption peaks of the Q-band were observed at 615, 704, and 618 nm, respectively. In comparison with those of corresponding unsubstituted MPcs (15, 16), no remarkable changes of peak position were observed, except for a low-energy peak in VOPc. In poly(methyl methacrylate) (PMMA) doped with the MPc(t-bu) , the significant peak shift was not observed in the concentration range from 5 to 100 wt%. The relationship between a at each peak wavelength and the weight ratio MPe(f-bu) :PMMA is linear. Therefore, the dispersed state of MPc(fbu) does not vary under these conditions. The T H G measurements show the X value of VOPc(i-bu) is smaller by one order of magnitude than that of a VOPc V D film. On the other hand, other MPc(f-bu) films with M as Ni or H , for example, have the same value of X as corresponding unsubstituted MPcs (17). The differences between V D and spin-coated films can be observed in optical spectra and X-ray diffraction patterns. In the Q-band of pure VOPc(t-bu) film prepared by spin-coating or V D , two peaks similar to those of phase I in VOPc V D film were observed. Unlike the VOPc V D film, the VOPc(f-bu) spin-coated film showed no evidence of a crystalline structure, even after thermal aging treatment, presumably because of a steric hindrance of the bulky tert-butyl groups. Again we realized 4

4

2

4

4

4

4

( 3 )

4

4

( 3 )

2

4

4



2.7 ± 0 . 4 14 ± 2

TiOPc

as-prepared annealed

12

at 1543 nm (10~ esu)

4.1 ± 0 . 5 9.0 ± 1.1

(3)

as-prepared annealed

VOPc

Film

Values of X

Χ (-3ω;ω,ω,ω)

Table I.

(3)

(3)

10 ± 0 . 6 46 ± 6

38 ± 5 81 ± 8

12

at 1907 nm (10~ esu)

Χ (-3ω;ω,ω,ω)

1.93 1.92

2.38 2.52

3ω

1.76 1.74

1.84 1.80

η

1543 nm

0.33 0.29

0.44 0.48

1^3ω

ω

1.81 1.77

2.17 2.25

η

3ω

1.57 1.64

1.36 1.34

η

1907 nm

and Refractives Indices in Tetravalent Metallophthallocyanine Thin Films


3a>

0.87 0.85

1.04 1.03

k

Ο

1

t—

Co


11.

W A D A ET AL.

311


4 0 0

6 0 0 W A V E L E N G T H

8 0 0

1000 /nm

Figure 4. Absorption spectra of the tetrakis (tert-butyl) phtha locyanine films obtained by spin-coating of chloroform solutions: (a) H Pc(t-bu) , (b) VOPc(t-bu) , and (c) NiPc(t-bu) . (Reproduced with permission from reference 6. Copyright 1991.) 2

4

4

4

that the phase II type stacking of VOPc molecules is important to enhance the macroscopic third-order N L O susceptibility. Thus, VOPc(i-bu)i.i was designed to reduce the steric hindrance of bulky substituents. After dichloroethane vapor treatment, the similar phase transition was observed in both pure and polymer thin films doped with VOPc(fb u ) i . The X values at fundamental wavelengths of 1543 and 1907 nm increased after the crystalline phase transition in various doping concentrations (IS). The qualitative evaluation through the deconvolution of electronic spectra indicates that the low-energy band in phase II plays an important role in enhancing both the resonant and off-resonant X values (J9). Optical quality and mechanical strength of VOPc(t-bu)i.i: P M M A are adequate to make a slab optical waveguide by a conventional polymer process. Even after solvent vapor treatment, no significant propagation loss in guided-wave geometry was observed that was due to the scattering from the aggregated particles, and the X-ray diffraction study suggested that the domain size of associated VOPc(i-bu)i.i molecules was on the order of 10 nm. L

( 3 )

( 3 )

M o l e c u l a r B e a m E p i t a x y F i l m s . Most organic materials crystallize because of weak van der Waals interaction, and hence the crystal structure is governed by several factors, such as hydrogen bonding and steric hindrance. In MPcs, polymorphism was observed, hence the packing structure of the film grown on the substrate can be expected to be easily affected by the interaction with a substrate. Molecular beam




312

epitaxy (MBE) is a sophisticated vacuum deposition technique in which the lattice matching of deposited molecules to the substrate surface takes place. This technique has been applied to epitaxial growth of MPc films (20). Tada et al. (22) reported the initial crystal structure of the VOPc thin film grown on alkali halide substrates with the M B E technique by monitoring the reflection high-energy electron diffraction (RHEED) patterns. From the R H E E D observation, a square lattice with a fourfold symmetry of VOPc and molecular planes of VOPc parallel to the cleaved (001) face of a KBr substrate were proposed. Compared to V D films, VOPc films grown by M B E show a narrow Q-band with a different peak position. In the condensed state, the broadening in the Q-band was observed as a result of exciton splitting in the Q-band due to transition dipole interactions between adjacent MPc molecules in aggregates. Op tical absorption spectra of various VOPc(t-bu) (n = 0 and 1.1) thin films are summarized in Figure 5. Because of the narrow Q-band in the M B E VOPc film, a highly ordered packing can be expected. n

Femtosecond Responses in VOPc(t-bu)

Thin Films

n

Another important characteristic of third-order N L O materials is the time response of their nonlinearity. In the resonant region, a resonant enhancement of N L O responses and a relatively slow decay can be expected. In MPc thin films made by various techniques, these com pounds can exist in several morphological forms with different stack ing arrangements of the disklike phthalocyanine molecules. These structural variations cause a modification of their electronic properties in the condensed state. Neher et al. (22) reported the influence of film 0.6

ι

500

1

600

1

I

Γ

700 800 900 W A V E L E N G T H /nm

1000

Figure 5. Absorption spectra in polystyrene thin films doped with 10 wt% VOPc(t-bu) ι j.* (a) phase I rich film, (b) phase II rich film, and (c) VOPc thin film grown by MBE on KBr.


11.

W A D A ET AL.

313


preparation—doped polymer and Langmuir-Blodgett films, for exam ple—on the N L O properties of phthalocyanine films measured by de generate four-wave mixing. In highly associated VOPc(i-bu)„, a resonant enhancement of N L O response might be expected without any bottle neck optical process. For this reason, we investigated the exciton decay dynamics in VOPe(t-bu) thin films with femtosecond pump-probe spec troscopy. Ultrashort pulses generated from a colliding-pulse modelocked (CPM) ring dye laser were amplified up to 2 μJ per pulse by a multipass amplifier pumped by a copper vapor laser (CVL) operating at 10 kHz. The 620-nm amplified pulses (50-60 fs in duration) were di vided into two beams: One was focused onto the sample as a pump with a variable delay time, and the other was focused into an ethylene glycol jet to generate continuum pulses, which were used as a broadband probe. The optical layout and a detailed description of the detection of transmittance changes are described elsewhere (23).


n

The differential transmission spectra (DTS) of phase II films of VOPc(i-bu) doped in polystyrene (10 wt%) are shown in Figure 6. The key feature of DTS is that the significant bleaching appears at the lowest energy absorption peak (centered around 810 nm), whereas the L1

0.2 Η

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