The Role of Intermolecular Interactions in Molecular Electronics

9 The Role of Intermolecular Interactions in Molecular Electronics

Downloaded by UNIV OF SOUTHERN CALIFORNIA on June 18, 2016 | http://pubs.acs.org Publication Date: May 5, 1994 | doi: 10.1021/ba-1994-0240.ch009

Computational Design of Architectures with L a r g e S e c o n d - O r d e r O p t i c a l N o n l i n e a r i t i e s

1,2

Santo Di Bella, Ignazio L. Fragalá,

1,2

1

1

Mark A. Ratner, and Tobin J. Marks

1

Department of Chemistry and the Materials Research Center, Northwestern University, Evanston, IL 60208-3113

The role of intermolecular interactions in determining quadratic nonlinear optical macroscopic hyperpolarizabilities is investigated using the intermediate neglect of differential overlap (INDO/S) (ZINDO) sum-over-excited particle-hole-states (SOS) formalism on clusters (dimers and trimers) of archetypical donor-acceptor π-electron chromophores. The calculated aggregate hyperpolarizability strongly depends on relative molecular orientations, ex hibiting the largest values in slipped cofacial arrangements, where the donor substituent of one unit is in close proximity to the acceptor substituent of the nearest neighbor. These results argue that cofacial assembly of chromophores having low dipole moments should maximize molecular contributions to the macroscopic susceptibility. The classical "two-level" model is generally a good approximation for estimating hyperpolarizabilities in such systems. The second -order response of model molecular 1:1 and asymmetric 2:1 electron donor-acceptor (EDA) complexes is also investigated using the INDO/S formalism. Intermolecular charge-transfer (CT) transitions in EDA complexes represent a promising approach to achieving sizable second-order optical nonlinearities. Calculated hyperpolarizabilities reflect the strength of the donor-acceptor interaction in the complex, affording for a given acceptor, the largest values in the case of the strongest donors. The large change in dipole moment accompanying intermolecular CT transitions and low-lying *Corresponding authors. Permanent address: Dipartimento di Scienze Chimiche, Universitá di Catania, 95125 Catania, Italy

2

0065-2393/94/0240-0223$08.00/0 © 1994 American Chemical Society

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

224

MOLECULAR AND BIOMOLECULAR ELECTRONICS CT excitation frequencies are major sources of the large calculated second-order nonlinearities. The two-level model is again a useful first approximation for predicting the nonlinear response of such complexes.


THE DESIGN OF MOLECULEB -ASED MATERIALS exhibiting large secondorder nonlinear optical (NLO) responses (1-6) would greatly benefit from accurate and chemically oriented theoretical descriptions of the macroscopic nonlinear susceptibility, X . To date, however, quantum chemical approaches for calculating N L O properties (7-26) have been largely limited to the study of the molecular nonlinear responses of single chromophore units. Such chromophores are generally conjugated molecules having donor-acceptor substituents and possessing strong in tramolecular charge-transfer (CT) excitations. Importantly, the ultimate capability of such theoretical approaches only allows the optimization of the hyperpolarizability of single chromophore molecules (7-26). A l though such information is valuable, assembling molecules possessing large microscopic hyperpolarizahilities does not necessarily result in bulk materials having optimum N L O responses, because packing ge ometries and intermolecular interactions modulate the final N L O char acteristics of the material (27-31). In this perspective, it becomes rel evant to systematically investigate macroscopic nonlinearities as a function of various packing geometries as well as intermolecular inter actions that, of course, must play a major role in determining the mac roscopic N L O response (32-34). Regarding the latter topic, cofacial in teractions involving substituted aromatic rings of electron donoracceptor (EDA) complexes (35-37) provide an interesting case study. In E D A complexes, intermolecular C T transitions cause charge redis tribution between the electron donor molecule (D) and the electronacceptor counterpart (A) and thus might represent an alternative ap proach to achieving significant N L O responses. (2)

In this chapter, we present a theoretical analysis, using the inter mediate neglect of differential overlap-sum-over-excited particle-holestates (INDO-SOS) (ZINDO) formalism (22-24), of the subtle interre lations between effective quadratic hyperpolarizability β(—2ω; ω, ω) and the ^-determining architectural parameters of molecular clusters (dimers and trimers) containing archetypical chromophore molecules in various relative orientations. The N L O response can be related to various in termolecular interactions, and the possible packing geometries that result in the optimum macroscopic second-order nonlinear susceptibility are explored as well. Finally, the second-order nonlinear responses of 1:1 and asymmetric 2:1 E D A model complexes are studied, to develop new synthetic strategies for efficient N L O materials.


9.

DIBELLA ET AL.

Role of Intermolecular

225

Interactions


Computational Methods The SOS formalism (38) has been used in connection with the all-valence INDO/S model (39-41). The details of this computationally efficient ZINDO-SOS-based method for accurately describing second-order mo lecular optical nonlinearities have been reported elsewhere (22-24). The ZINDO model Hamiltonian is based on a minimum basis set of Slater orbitals, with many matrix elements defined semiempirically. This approach yields rather good agreement with experimental dipole mo ments and, therefore, it should reliably describe the electrostatic inter actions among cluster dimers and trimer s. Moreover, the use of Slater orbitals renders the formalism appropriate for treatment of long-range interactions. The single-determinant, molecular orbital approximate ground state was used, and the monoexcited configuration interaction (MECI) ap proximation was employed to describe the excited states. In all of the present computations, the 130 lowest energy transitions between selfconsistent field (SCF) and M E C I electronic configurations were chosen to undergo configuration interaction (CI) mixing and were included in the SOS. This SOS truncation was found to be sufficient for a complete convergence of the second-order response in all the cases considered. The great advantage of the SOS method can be related to the sim plicity of the analysis of ft determining parameters. This simplicity allows both a chemical interpretation of molecular N L O properties as well as the analysis of the role and the nature of intermolecular interactions that modulate the N L O response. In addition, further simplification can be of value when the two-level approximation is applicable (42, 43). In these cases, a single C T transition determines the N L O response (eq 1). Here ft is the two-level hyperpolarizability term, Δμ is the difference between ββ

ft(-2a>; ω, ω) =

3e

ftajge/A^ge

2

2

- (M ]P"ge) "

[(hc* ) ge

2

2

2

(2M ] 2

(1)

excited- and ground-state dipole moments, Ηω is the incident radiation frequency, ho> is the energy, and / is the oscillator strength of the optical transition involved in the two-level process. In the case where the C T transition has a dominant contribution along one-molecular axis (e.g., the ζ axis), the ft term should be largely proportional to the β tensor, so that ft (—2ω; ω, ω) ^ ft, . Finally, it must be remembered that a number of important theo retical issues (neglect of multicenter exchange repulsion effects, possible role of basis set superposition error, and adequacy of the semiempirical parameterization at long distances) must and ultimately will be addressed further in the context of comparison with extensive ab initio studies. ge

ζζζ

zzz


226

MOLECULAR A N D BIOMOLECULAR

ELECTRONICS

Nevertheless, the results of the present study certainly are of value for understanding the effects of intermolecular interactions on N L O response.


Dimer and Trimer Clusters We first consider the simplest case represented by two monomeric units of a molecule having donor-acceptor substituents connected by a con jugated pathway such as p-nitroaniline (PNA, 1). The two units are brought together in a cofacial eclipsed conformation (2) with dipolar axes aligned parallel to the ζ axis of a reference coordinate system.

1

2

Figure 1A shows the dependence of β (the largest component of the β φ tensor) upon the interplanar separation (R). At interplanar dis tances near 3.5 À (typical mean van der Waals' interaction distances in organic crystals), the curve shows a flat minimum that increases asymptotically to a limiting value at larger distances (>9.0 À). For distances smaller than 3.0 Â, a sharp increase is observed. For each distance, the estimated two-level β^ terms (eq 1) approximate the SOS-derived β values reasonably well (Table I). This means that a single C T transition dominates the perturbation sum. Therefore, the simplified analysis of variables influencing the dependence of β is straightforward. In partic ular, the N L O response at various interplanar distances is largely de termined by the position of the lowest energy C T transition. Cofacial interactions occurring at both large and medium distances may be ra tionalized in terms of exciton theory of interacting monomers. As a mat ter of fact, the composition of the CI states is rather well described as a linear combination of monomer states, and, therefore, the low-energy iS-determining band remains unsplit although slightly shifted to higher energy compared to the monomer (Figures 2A and 2B) by the relative amount of the electrostatic interactions. This shift as well as the gradual decrease of both Δμ and/values, leads to a decrease of fi as the inter planar spacing is contracted (Table I). In contrast, at distances less than 3.3 À, the dipole-dipole interactions cause a strong energy splitting of each monomer-related molecular orbital (MO) with a resulting bathochromic shift of the lowest energy C T transition (Table I) and a consequent increase of β values. ζ

ζ

ζ

ζζζ

ζ ζ ζ

ί

t

ζ ζ ζ

The same trend in calculated β-values is observed (Figure 1A) in the case of the analogous P N A trimer having the same conformation



9.

D I B E L L A E T AL.

227

Role of Intermolecular Interactions

16.0 I 2.5

35

45

55

6.5

15

95

%5

Interplanar Distance R (Â)

B.

58.0

25

35

45

55

6.5

15

8.5

Interplanar Distance R (Â) Figure 1. Variation of calculated hyperpolarizability component β (h ω = 0.65 eV) with interplanar distance (R) in cofacial eclipsed PNA (A) and POM (B) clusters. ζζζ

(3), in the same range of interplanar distances. Nevertheless, a more pronounced (than found in the dimer) minimum is observed near the equilibrium interplanar distances. In particular, for R = 3.5 À, on passing from the dimer to the trimer, β values are 16 and 34% smaller than those estimated when considering the simple additivity of monomer β values (Table II). This observation indicates that, at equilibrium interplanar distances, the local field corrections (F) due to intermolecular interactions (27-31) clearly begin to play a role in determining the macroscopic quadratic susceptibility, X . In contrast, for larger interplanar distances (>12 Â), the magnitude of β is an additive quantity. ζζζ

ζζζ

(2)


228

M O L E C U L A R A N D BIOMOLECULAR ELECTRONICS

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9.

DIBELLA ET AL.

229



ΡΝΑ MONOMER

P N A D I M E R S L I P P E D C O N F . B Y 7JO À

f

R . 3 . 5 Â

R « 3.5 À 1.0-j

0.5:

0.0:

3.40

1 4.40

I 5.40

1

6.40

7.40

Energy (eV) Figure 2. Calculated optical absorption spectra of PNA monomer (A), PNA dimer eclipsed conformation (B), and PNA dimer slipped conformation (C).

Table II. Molecular Hyperpolarizability Values for PNA and P O M Monomers and for PNA and Dimers and Trimers in the Eclipsed Conformation Unit

Monomer

Dimer

Trimer

PNA POM

12.29 -17.23

20.54 (24.58) -31.77 (-34.46)

24.37 (36.87) -43.75 (-51.69)

NOTES: Values in parentheses are calculated by the simple addition of monomer β values; hyperpolarizability is β ( Ι Ο c m e s u ; Ηω = 0.65 eV). Interplanar separation is 3.5 Â. ζζζ

ζζζ

-30

5

-1


MOLECULAR AND BIOMOLECULAR ELECTRONICS

230

A

-/

/-

D


3

Turning now to a cofacial dimer constructed from molecular subunits with nearly vanishing ground-state dipole moments such as 3-methyl4-nitropyridine-l-oxide (44,45) (POM, 4), a rather different interplanar spacing-dependent trend in β is observed relative to the PNA dimer (Figure IB). In this case, the β versus R curve does not show a mini mum. On moving to larger distances, the β value increases slightly until ~6.0 À. At this point, the curve becomes nearly flat, reaching twice the calculated β monomer (Table I). The same trend is observed in the analogous P O M trimer (Figure IB), and for distances greater than 6.0 Â, β values are approximately three times the P O M monomer values. The N L O response at relatively larger distances may again be rationalized in terms of the two-level approach used for the PNA dimer (Table I), with weaker (than in the P N A dimer) electrostatic interactions and consequent unperturbed ground and excited states (Figures 3A and 3B). For distances less than 3.7 Â, the monomers interact strongly and behave as a supermolecule. In these cases, the CI states cannot be simply described as combinations of monomer states, and the optical spectrum shows new C T bands at lower energies (Figures 3C and 3D). As a consequence, for smaller interplanar distances, the two-level model breaks down (Table I). Furthermore, on passing from the monomer to the dimer and trimer (R = 3.5 À), β values are 8 and 15% smaller, respectively, than those estimated on the basis of the simple additivity of the monomer β values (Table II). Therefore, the deviation from the simple model of molecular jS-additivity in determining the hyperpolarizability in P O M clusters is smaller than in the P N A analogues. ζζζ

ζζζ

ζζζ

ζζζ

ζζζ

ζζζ

ζζζ

4

The observed differences in the N L O response of P N A and P O M clusters is a consequence of different intermolecular interactions (46, 47). In the PNA clusters, intermolecular interactions are largely elec trostatic (dipole-dipole) in nature and dominate the other contributing


9.

DIBELLAETAL.


A

231

l.o f

POM MONOMER


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0.01.0

1 • , , Β , • , Π. R Β 2.9 Â

0.74

0.3

0.0 3.2

4 ^

4.6

6.0

Energy (eV) Figure 3. Calculated optical absorption spectra of POM monomer (A) and POM dimer (B-D) in the eclipsed conformation at various interplanar distances (R).



232

MOLECULAR A N D BIOMOLECULAR ELECTRONICS

interactions. In contrast, in the P O M clusters, derealization or inter molecular C T interactions operating only at short distances, dominate the intermolecular interactions, whereas for relatively large distances each molecule may be considered to be independent of the surrounding ones, and the aggregate hyperpolarizability is essentially a sum of those of the individual molecular components. That is, local field corrections are relatively unimportant in this regime. This analysis conveys interesting consequences for the molecular engineering of high-efficiency second-harmonic generation (SHG) thinfilm materials (48) such as self-assembled or Langmuir-Blodgett films characterized by relatively low chromophore packing densities (N ). Us ing POM-like molecular precursors having small dipole moments, it should in principle be possible to cofacially assemble chromophoric lay ers having high packing densities without appreciable reduction in mo lecular susceptibility. In fact, assuming equation 2 for the SHG response, where (T y is a transformation s

IJK

XIJK

2

=

(2)

N F& s

i/K

matrix and β is the dominant component of the molecular hyperpo larizability, the X value becomes proportional to N . Therefore, the optimization of packing density is clearly crucial for obtaining highefficiency SHG materials. Among the various possible distortions of idealized molecular arrays, the most interesting case occurs in slipped arrangements (5), generated from the eclipsed conformation (R = 3.5 Â) by parallel translations (R') ζζζ

(2)

s

5

along the ζ axis. In fact, the increase in slip distances can be correlated with enhanced β-responses (Figure 4). A ^-maximum is found when the donor substituent of a PNA unit lies approximately above the acceptor substituent of the other molecule. In this arrangement, all the parameters that give rise to large β values are optimized (compared to the eclipsed conformation). In particular, the low-energy C T transition is red-shifted, and the oscillator strength is increased in comparison to those of the cofacial dimers (Figures 2B and 2C). This indicates small stabilizing dipole-dipole interactions, with CI states simply characterized as linear combinations of monomer states. It can therefore be concluded that bulk materials having slipped (strongly dipolar) chromophore compoΓ


9.

233


DIBELLA ET AL.

PNA DIMER SLIPPED CONF.

R = 3.5À


27.04

17.0

1 • » « t •••»••• 0.0

ι •

2.0

ι . . . 4.0

6.0

ι . . .

ι . . .

i

8.0

Slip Distance R' (A) Figure 4. Variation of calculated hyperpolarizability component 0 (hœ = 0.65 eV) with slip distance (R!) along the ζ axis in the PNA dimer. vec

nents would be expected to exhibit an optimum second-order nonlinearity. As a possible confirmatory example, 3-methyl-4-methoxy-4nitrostilbene (NMONS), the crystal structure of which consists of stacked π-π planar molecules in a slipped cofacial donor-acceptor con figuration (5), has one of the largest powder SHG efficiencies reported to date (49, 50).

1:1 EDA Complexes Among the known E D A complexes, those formed by cofacial interaction of substituted aromatic rings seem the most attractive, because they might be readily incorporated into functionalized polymers (51) or into self-assembled architectures. We focus on archetypical 7r-acceptor mol ecules such as 7,7,8,8-tetracyanoquinodimethane (TCNQ, 6), tetracyanoethylene (TCNE, 7), and 1,3,5-trinitrobenzene (TNB, 8), interacting with arene molecules having various electron-donating groups (methyl, methoxy, and amine) as substituents. The structures (52, 53) used for calculations on 1:1 E D A complexes are depicted in Scheme I.

Ν

Ν 6

ϊΓ

Ν 7

λ

Ν

0

2

8

Table III summarizes calculated linear optical, dipole moment, and second-order hyperpolarizability data for selected 1:1 cofacial E D A



234


C

D Scheme I

complexes. Calculated dipole moments range from 0.5 to 1.5 Debyes, and their magnitude reflects the degree of C T interaction in the ground state, that is, the strength of the E D A complex (35-37). As expected, the dipole moment increases with increasing donor strength and, in each series, is maximum in the case of the aminoarene donor. An anal ogous trend is observed in the lowest energy C T transition, which be comes stronger (increasing oscillator strength) and is shifted to lower energy as the donor strength increases. Calculated β values (Table III) may be roughly related to the strength of the E D A complexes and may range from ~ 8 X 10~ cm esu" (hœ = 0.65 eV), similar to that reported for PNA (Table I), to - 7 0 Χ 1 0 " c m e s u (Ηω = 0.65 ζζζ

30

5

1

30

5

-1


9.

235


DIBELLA ET AL.

Table III. Calculated Dipole Moment, Linear Optical Spectroscopic, and Molecular Hyperpolarizability Data for Cofacial 1:1 Electron DonorAcceptor Complexes Involving Various Electron Acceptor Molecules Donor (D)

a

μ (debyes)

b

hœ (eV) >° b

ge

f

d

β Hzzz

β Hzzz

14.43 14.46 14.29 13.75 14.04

68.36 39.04 8.39 9.08 11.08

69.96 38.50 7.77 8.04 10.23

14.54 14.94 14.98 14.95 15.65

68.51 23.22 8.23 7.28 9.34

69.08 22.78 7.79 6.97 8.95

14.46 15.03

21.95 11.18

18.97 7.10

^ (debyes)

h

ge


Acceptor is TCNQ TMDP PD DMB PX HMB

1.07 1.04 0.40 0.60 0.68

2.04 2.33 3.02 3.27 3.03

0.14 0.14 0.08 0.11 0.11

d

b

Acceptor is TCNE TTAB PD DMB PX HMB

1.33 0.83 0.54 0.62 0.59

2.04 2.45 3.25 3.48 3.37

0.14 0.10 0.09 0.11 0.12

Acceptor is TNB TAB TRIAZ

0.59 0.36

2.29 3.00

0.06 0.06

NOTES: For definition of parameters see text; hyperpolarizability is β(—2α>,ω,ω)(10 c m esiT ; h;œ = 0.65 eV). T M P D is N,N,N',N'-tetramethyl-p-phenylenediamine; P D is p-phenylenediamine; D M B is 1,4-dimethoxybenzene; P X is p-xylene; H M B is hexamethylbenzene; TTAB is 1,2,4,5tetraaminobenzene; T A B is 1,3,5-triaminobenzene; and T R I A Z is 2,4,6tris(dimethylamino)-l,3,5-triazine. Calculated using the INDO/S SOS formalism. Lowest energy C T transition. Calculated using the simple two-level model of equation 1. 3 0

5

1

a

h

c

d

eV), exceeding those reported for highly efficient second-order N L O chromophores such as (4-N,N-dimethylamino-4 -nitrostilbene)stilbone (DANS). As expected, the N L O response is largely determined by the lowest energy C T transition, so that the two-level model (eq 1) is a suitable guideline for rationalizing the N L O response of E D A complexes (Table HI). All of the present E D A complexes are characterized by large, almost constant changes in the dipole moment between the first excited and ground state (Δμ ), so that the ft-values may be related to hœ a n d / , the optimizations of which are crucial for the design of efficient N L O E D A complexes. For a given acceptor, the energy of the lowest energy C T transition may be simply related to the donor strength, affording ,

ββ


eg

MOLECULAR AND BIOMOLECULAR ELECTRONICS


236

lowerftojegvalues, thus larger hyperpolarizabilities, in the case of aminesubstituted donors. The relative orientation of donor-acceptor components is the most important feature, leading to stabilization of the ground state as well as to maximization of the oscillator strength of the lowest energy C T transition (35-37). This requires that the highest occupied molecular orbital (HOMO) (for the donor) and the lowest occupied molecular orbital (LUMO) (for the acceptor) can be mixed by the dipole in the point group of the complex, and fis maximized when these MOs are strongly overlapping. In the case of the T C N Q complexes, the L U M O , independent of the donor molecule, is of b symmetry (in the local D h point group) and the charge distribution is largely localized at the 1, 4, 7, and 8 atomic positions (Figure 5A). Consequently, an aromatic donor molecule having a H O M O of the same symmetry and having two electron-donating substituents in 1 and 4 positions is expected to have an optimum overlap (Figure 5C), thus a large/. In the case of T C N E complexes, the same arguments indicate that overlap maximization may be achieved in the same fashion as the T C N Q complexes or, better, with an arene donor molecule having four substituents in the 1, 2, 4, 5 positions such as the 1, 2, 4, 5-tetraminobenzene (Figures 5B and D). In each case, the trend of calculated/values (Table III) accords well with these qualitative symmetry considerations. 2g

2

C

D

Figure 5. Frontier orbitals of selected EDA complexes: LUMO of TCNQ (A) and TCNE (B) complexes; HOMO of TCNQ-N,N,N\N'-tetramethyl-pphenylenediamine (C), and TCNE-1,2,4,5-tetraminobenzene (D) complexes.


9.

DIBELLA ET AL.

237



Asymmetric 2:1 EDA Complexes The formation of asymmetric acceptor, donor, donor ( A D D ) - E D A complexes has been achieved in cyclophane molecules containing rigidly constrained donor-acceptor moieties (54). Formation of A D D or acceptor, acceptor, donor (AAD) complexes has also been reported in functionalized E D A polymeric structures (51). Formation of 2:1 A D D (9) or A A D (10) complexes, the geometries of which are constructed from those of 1:1 analogues (R = R = 3.5 Â), leads to increasing calculated dipole moments (Table IV) that can be related to more extensive C T interactions in the ground state. The lowest energy C T transitions are red-shifted compared to those of the 1:1 analogues, as found experimentally (55-57). In A D D complexes, the lowest energy C T transition in the CI expansion involves, to various extents, a combination of H O M O and second H O M O (SHOMO), both localized in the D D moiety, and the L U M O . The L U M O has essentially the same energy and atomic population found in 1:1 complexes. The red-shift of the C T transition may be simply related to the energy splitting of the H O M O and S H O M O due to the D D interaction. An analogous situation occurs in A A D complexes, where the lowest energy C T transition involves the H O M O and a combination of L U M O and SLUMO.

10

9

For the present 2:1 E D A complexes, an increase of β compared with that of the 1:1 analogues is observed (Table IV). The two-level model is still a good approximation in predicting the N L O response. Therefore, the β enhancement in asymmetric 2:1 complexes may be essentially related to the red-shift of the lowest energy C T transition and, to a smaller extent, to the increase of Δμ . ζ ζ ζ

ββ

Conclusions The present results indicate that intermolecular interactions represent a major factor in determining macroscopic second-order susceptibilities of molecular materials. The N L O response depends critically not only on molecular packing geometries, but also on the characteristics of mol ecule constituents. At equilibrium intermolecular distances, the N L O response is a strong function of the relative molecular orientations, ex hibiting the largest values in slipped cofacial arrangements, that is, in a



1..63 0.,10 0..64 0..74 0..80

0,.71 (0.71)

TTAB PD DMB PX HMB

TAB

b

c

1.93 (2.05)

2.73 (3.13)

3.01 (3.22)

1.85 (1.81) 2.19 (2.20) 2.99 (2.99)

2.78 (2.94) 2.58 (2.70)

1.77 (1.77) 2.02 (2.05) 2.64 (2.69)

ge

ha> (eV) ββ

18.29 18.63 18.62 18.91 19.04

(16.91) (16.84) (16.66) (16.63)

18.57

(15.42)

19.74 (15.70)

17.29 17.88 16.81 19.20

Acceptor is T N B

(0.45) (0.10) (0.09) (0.10) (0.12)

(17.35) (17.24) (16.27) (16.03) (16.67)

b

(debyes)

Acceptor is T C N E

(0.16) (0.15) (0.07) (0.09) (0.09)

0.06 (0.05)

0.16 0.11 0.11 0.11 0.12

0.16 0.15 0.08 0.11 0.11

Δμ

Acceptor is T C N Q

b

f

b

57, .32

(38.50)

148. 25 (150.73) 49. ,03 (40.24) 13. ,75 (11.38) 14. ,76 (9.87) 22. ,34 (12.16)

188, .44 (187.92) 89, .85 (80.68) 16, .78 (13.09) 20. .82 (13.25) 26. 29 (17.28)

zzz

ft

45.26

5

esu

- 1

;

hw = 0.65

eV).

d

c

h

a

T M P D is N,N,N',N'-tetramethyl-p-phenylenediamine; P D is p-phenylenediamine; D M B is 1,4-dimethoxybenzene; P X is p-xylene; H M B is hexamethylbenzene; TTAB is 1,2,4,5-tetraaminobenzene; and T A B is 1,3,5-triaminobenzene. Calculated using the INDO/S SOS formalism. Lowest energy C T transition. Calculated using the simple two-level model of equation 1.

cm

3 0

(22.80)

152.75 (157.88) 50.17 (41.58) 13.26 (11.30) 15.40 (9.86) 23.27 (11.71)

199.52 (194.86) 95.92 (83.54) 17.38 (11.47) 20.80 (11.59) 27.25 (16.37)

A,, ζ ζ ζ d

NOTES: Values for A A D complexes are reported in parentheses. For definition of parameters see text; hyperpolarizability is β(—2ω;ω,ω); 10

(1.63) (0.96) (0.61) (0.69) (0.71)

1 .42 (1.44) 1 .25 (1.26) 0 .50 (0.47) 0,.75 (0.67) 0..91 (0.88)

TMDP PD DMB PX HMB

b

(debyes)

Donor" (D)

T a b l e I V . C a l c u l a t e d D i p o l e M o m e n t , L i n e a r O p t i c a l Spectroscopic, a n d M o l e c u l a r H y p e r p o l a r i z a b i l i t y D a t a for C o f a c i a l Asymmetric A D D and A A D Electron D o n o r - A c c e p t o r Complexes Involving Various Electron Acceptor Molecules



9.

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regime of smaller, stabilizing dipole-dipole interactions. Furthermore, for relatively large intermolecular distances, particularly for component molecules having a small ground-state dipole moment, the aggregate hyperpolarizability is essentially a sum of those of the individual mo lecular components. That is, the hyperpolarizability at these distances is essentially dominated by weak long-range dipole-dipole interactions. Finally, these results indicate that intermolecular C T transitions in E D A complexes can, under favorable conditions, lead to sizable secondorder N L O responses. Therefore, these results suggest promising alter native approaches to the synthesis of materials having large secondorder nonlinearities.

Acknowledgments This research was supported by the National Science FoundationMaterials Research Laboratory (MRL) program through the Material Research Center of Northwestern University (Grant DMR9120521) and by the Air Force Office of Scientific Research (Contract 93-1-0114). S. D i Bella thanks the Italian Consiglio Nazionale delle Ricerche (CNR, Rome) for a postdoctoral fellowship. We thank D. Kanis for ongoing collaboration.

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RECEIVED for review May 22, 1992. ACCEPTED revised manuscript October 21,

1992.


The Role of Intermolecular Interactions in Molecular Electronics

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