on the electronic structure of barrelene-based rigid organic donor-acceptor systems. an indo model...

On the Electronic Structure of Barrelene-Based Rigid Organic Donor-Acceptor Systems. An INDO Model

Study Including Solvent Effects

THOMAS FOX, MANFRED KOTZIAN, and NOTKER ROSCH* Lehrstuhl fur Theoretische Chemie, Technische Universitat Munchen, W-8046 Garching, Germany

Abstract

We present an INDO/S molecular-orbital investigation of organic molecules containing a barrelene moiety that provides a rigid link between an aromatic donor and a maleic ester acceptor group. Molecules of this type have recently been synthesized and characterized spectroscopically. We discuss the ground state and various excited states both in vamo and in solution. Solvent effects are incorporated by use of an electrostatic cavity model which is not restricted to a spherical cavity, but allows for a cavity shape that is adapted to the solute molecule. The calculations indicate low-lying charge-transfer (CT) excitations in the region of the first aromatic transitions, even in the gas phase. 0 1992 John Wiley & Sons, Inc.

Introduction

Photoinduced intramolecular charge-transfer ( CT ) and electron-transfer (ET ) reactions have recently been studied intensively [ 1-41. Much attention is focused on understanding the various factors that influence the rate of ET reactions, for example, the nature of the thermodynamic driving force and its modification [ 51, the effects of the surrounding solvent [ 1,6,7], and the spatial relationship between donor and acceptor moieties [ 2,8-lo]. In early experimental investigations, the donor and acceptor units were separated by a flexible spacer which exerted only a limited control over the spatial relationship between donor and acceptor groups [4,1 I]. Detailed investigations of orientational effects were not feasible since this type of mediating bridge allowed for a variation both in the actual donor-acceptor distance as well as their relative orientation. In recent years, new insight into the nature of the donor-acceptor interaction was gained by connecting donor and acceptor groups via a rigid bridge [ 12- 141. In this way, both groups are not only held at a fixed separation, but also in a definite relative orientation. These well-defined donor-acceptor systems also made it possible to study effects of the bridge that go beyond pure geometrical implications, but comprise specific (“through bond”) modifications of the electronic interaction between donor and acceptor groups [1,8,15].

* Author to whom correspondence should be addressed.

International Journal of Quantum Chemistry: Quantum Chemistry Symposium 26, 55 1-56 1 (1992) 0 1992 John Wiley & Sons, Inc. CCC OO20-?608/92/0 1055 1 - 1 I

552 FOX, KOTZIAN, AND ROSCH

Since most previous investigations have been carried out in a condensed medium, it was rather difficult to achieve a separation of the solvent relaxation and its influence on the ET process from the effects of intramolecular degrees of freedom. Particularly the relative importance of the intramolecular and the solvent reorganization energy, crucial variables in ET theory [ 21, could not be established reliably. Investigations of ET processes in the gas phase would provide an opportunity to overcome these difficulties since, under such circumstances, the reorganization energy is only caused by intramolecular rearrangements. Unfortunately, molecules exhibiting ET characteristics are normally rather large and involatile. However, recently a large family of moderately sized donor-acceptor molecules has been synthesized and characterized [ 161 which holds promise of providing a breakthrough in studying this problem. These molecules consist of a central barrelene unit (Fig. 1 ) which is substituted by annelated aromatic compounds as the donor unit and by two carboxymethylester groups forming the acceptor unit.

In this article we shall report electronic structure investigations of dibenzo-bar- relenedicarboxydimethylester ( DBBD) , the prototypical compound of this family (Fig. 2). We will discuss the electronic structure of DBBD by building this molecule formally from barrelene and by successively adding donor and acceptor groups. We will focus our attention on the role of the rather short barrelene “bridge” in mediating the interaction of the donor and acceptor substituents. In addition, we will employ an electrostatic cavity model [17,18] to investigate the influence of a surrounding solvent on the transition energies of the charge-transfer excitations. The results of the extensive spectroscopic investigations as well as further theoretical discussions will be published elsewhere [ 191.

Method The geometry of the molecules under investigation was derived from molecular

mechanics calculations [ 201. Common bond distances were used for symmetry-

Figure 1. Sketch of bicyclo- [ 2.2.2]-2,5,7-octatriene (barrelene).

DONOR-ACCEPTOR SYSTEMS 553

Me0 OMe

Figure 2. Sketch of dibenzo-barrelene-dicarboxymethylester ( DBBD).

equivalent bonds. The carbon atoms of the carboxymethylester groups were found to lie in one plane with the carbon atoms of the ethylene moiety. Relative to this plane, the 0 - C - 0 moieties were found to be rotated about the C - C axis by an angle of about 40” with the carbonyl oxygens pointing “outward”. To preserve the C2 symmetry of the acceptor-substituted barrelene unit (see Fig. 2), in the subsequent calculations a common angle of 45” was used.

INDOIS calculations [21,22] were performed to obtain the electronic structure of the ground state and of the excited states. To incorporate solvent effects in the electronic structure we used an electrostatic cavity model with a cavity surface that is adapted to the molecular shape [ 181. The model takes the polarization of a dielectric medium into account in a self-consistent fashion. Details concerning this electrostatic cavity model may be found elsewhere [ 17,18 1.

Results

The electronic structure of barrelene (bicyclo- [ 2.2.21 -2,5,7-octatriene, see Fig. 1 ) has been the subject of several studies [23-281. The order of the frontier ( T )

orbitals, a textbook example for simple group theory arguments, is determined by the high symmetry ( D3h) of the molecule: e’ < a; (HOMO) < e” ( LUMO) < a‘;. The a molecular-orbital spectrum is displayed in the middle column of Figure 3, also depicted are sketches of these MOS as viewed along one of the three twofold axes. In the following we will use the a molecular orbitals of barrelene as a reference for the discussion of the electronic structure of various substituted compounds.

The substitution of the barrelene “bridge” by two adjacent electron withdrawing carboxymethyl groups reduces the symmetry at least to C2 and leads to a rearrange-


-8 --

-9 - -

Figure 3. Frontier orbitals of dibenzo-barrelene (PB), barrelene (B), and barrelenedi- carboxomethylester ( B-A) as viewed from a vertical C, axis.


ment of the A molecular orbitals (see Fig. 3). The orbitals of this acceptor-substituted barrelene (B-A) are sketched on the right-hand side of Figure 3. In this figure the view is along the unique C2 axis of the substituted barrelene compound (vertical in Fig. 1 ) . However, all contributions from the substituents have been omitted for clarity. An INDO calculation finds only a small energetic relaxation of the occupied orbitals, whereas a strong interaction of the virtual A orbitals of barrelene with the electron withdrawing substituents is observed. Two acceptorderived orbitals are found in the frontier orbital energy region of B-A, one symmetric, the other one antisymmetric with respect to the vertical plane that contains the substituted C - C double bond. (This mirror plane, although no longer a symmetry element of the substituted compound, may still be invoked to a rather good approximation.) The antisymmetric acceptor-derived virtual orbital interacts strongly with that orbital of the barrelene LUMO e” set which has the same symmetry characteristic. The bonding combination thereof, the LUMO of B-A, is lowered in energy by about 1.5 eV. This orbital exhibits an acceptor contribution ( COOMe) of 52% and a C = C ?r bond contribution of 40%. There is almost no A interaction with the second partner of the e” set which originally was symmetric with respect to the nodal plane at the substituted double bond. The A orbital a’{ interacts only weakly with the acceptor orbitals. The two further virtual orbitals of B-A drawn in Figure 3 (the second and third from the top in the right-most column) have dominant acceptor contributions. From these findings a distinct energy reduction of the HOMO-LUMO excitation is expected for the acceptor-substituted compound. This is one of the key features of the family of donor-acceptor molecules under discussion. Based on the composition of the corresponding molecular orbitals, this HOMO-LUMO transition exhibits charge-transfer character as charge density is shifted “upward” from the unsubstituted “ethylene” moieties to the acceptor-substituted “ethylene” moiety and to the dicarboxymethylester units.

The orbitals of the donor-substituted compound dibenzobarrelene (D-B) may be viewed either as orbitals of a bent (9,lO)-dihydroanthracene perturbed by a bridging ethylene unit or as perturbed barrelene orbitals. Using the latter scheme, one may relate three occupied (b2 , a l , and b2) and three virtual orbitals ( a l , a2, and az) to the barrelene orbitals e’, a;, e” and a‘:, respectively (Fig. 3). The additional orbitals of D-B in this relevant energy window, two occupied and two virtual orbitals, may be rationalized by reference to the fact that the frontier orbitals of benzene are actually comprised of degenerate pairs, e lg and e2,. A significant interaction of the two benzene fragments in a bent dihydroanthracene leads to a bonding and antibonding linear combination of each of these four benzene A orbitals. For symmetry reasons, the A and ?r* orbitals of the bridging ethylene moiety can only mix with one orbital in each of these submanifolds, the one with proper nodal structure. Therefore, we arrive at five occupied and five virtual orbitals as sketched in Figure 3. Since the HOMO of benzene is delocalized over the ring one finds the antibonding interaction of the various C - C A units reduced in the HOMO of D-B compared to DBBD and, consequently, the HOMO energy is lowered by 0.4 eV compared to barrelene. The virtual orbitals of the benzo-substituted compound are shifted to lower energy by about 0.5 eV with respect to the e” level of barrelene for analogous


reasons. Therefore, similar to the situation in the acceptor-substituted barrelene, but not quite to the same extent, the gap between HOMO and LUMO becomes smaller.

From these results one would expect for both derivatives of barrelene a decrease of the energy of the corresponding HOMO-LUMO transition as is indeed the case for B-A where the lowering of the HOMO-LUMO gap by about 0.6 eV with respect to the value in barrelene is accompanied by a decrease of the energy of the HOMO- LUMO transition by 6500 cm-' . However, for the donor-substituted compound the simple picture of the one-electron levels fails as a guide for the transition energy. Although a small decrease of the HOMO-LUMO gap is found, the INDO-CI model yields a value for the HOMO-LUMO transition which is about 3000 cm-' larger than the corresponding value calculated for barrelene. As will be shown later, the transition energy of the donor-acceptor substituted barrelene results from a combination of both effects which leads to an overall decrease of the transition energy with respect to the unsubstituted barrelene.

The one-electron levels of DBBD, where both the benzene and the acceptor groups are attached to the central barrelene unit, are shown in Table I. The contributions from the various fragments of the molecule are listed in percent; C-H denotes the contributions of the two CH fragments connecting the two benzene rings (Benz) and the bridging ethylene unit (C = C). In accordance with the acceptor-substituted barrelene compound B-A, the LUMO (25a) of DBBD is located on the bridging ethylene and on the acceptor. It consists of 54% C=C and 34% COOMe contri-

TABLE I. Energy and Mulliken populations of pertinent molecular orbitals of DBBD (in C, symmetry). C-H denotes the populations of the fragments that connect the two benzene rings (Benz) and the bridging ethylene (C=C); COOMe denotes the contribution of the acceptor groups. The orbital 24a is

the HOMO of DBBD.

Orbital Energy C-H c=c COOMe Benz c2 tev1 9a % % 9a

20b 2la 21b 22a 226 23b 23a 24a 24b

- 1 1.55 -11.55 -10.51 - 10.29

-9.17 -9.02 -8.92 -8.9 1 -7.37

1 0

12 8

42 5 0 3

24

98 96 81 88 12 5 0 3 4

1 3 5 3

40 87 99 90 72

25a -0.09 1 54 34 1 1 25b 0.65 1 2 1 96 26n 0.74 0 1 1 98 26b 1.16 0 0 14 86 2 7a 1.21 0 0 17 83 27b 1.52 0 5 81 14 28a 2.05 1 37 48 14 28b 2.77 21 43 21 15


butions. Compared to D-B, the LUMO energy is lowered by 0.9 eV through the interaction with the acceptor. The higher lying virtual orbitals are located on the aromatic systems, acceptor orbitals follow at even higher energy. None of the four highest occupied molecular orbitals carries any significant contributions from the acceptor. At lower energy one finds acceptor orbitals, essentially of oxygen lone pair character.

On the basis of the above discussion of the electronic structure of DBBD one expects several low-lying excitations of some charge transfer character from the highest occupied orbitals into the LUMO. The calculated excitation energies, the nonzero Cartesian component of the state dipole as well as the corresponding oscillator strength are shown in Table 11. The lowest transition at 30700 cm-' results from the HOMO-LUMO excitation and is classified as a D + A transition. The amount of charge transfer is measured by the change of the Mulliken population of the acceptor moiety (C = C plus COOMe; also shown in Table 11). About 0.4 atomic units of charge are transferred to the acceptor during this transition. The next higher states up to about 42500 cm-' are intra-aromatic transitions (D + D) that exhibit very little charge transfer. The admixture of A + A type to the state 2B is an artifact of the present INDO parameterization which places n + ?r* transitions too low in energy [ 171. The next higher transitions beyond 42600 cm-' are excitations into the orbital 27b which is an almost pure acceptor orbital. These excitations may therefore be viewed as "classical" donor-acceptor transitions. The nonzero component of the state dipole also shows the largest change for these charge-transfer excitations. From these findings, it becomes clear that the goal of designing a relatively small compound that exhibits a low-lying electron transfer excitation may be within reach by using members of the barrelene based donor- acceptor complexes [ 19 ]. First experimental evidence supporting this statement has been obtained from fluorescence spectra of DBBD in various solvents [ 161.

TABLE 11. Calculated excited states of DBBD (in C, symmetry) using INDO single excitations. The charge- transfer is measured by the change of the Mulliken populations of the C=C and the COOMe moieties.

Energy State dipole Oscillator Charge transfer State [cm-'1 [Debye] strength [a.u.] Character

I A 0 1.551 - I B 30694 -5.392 0.039 0.422 D + A 2,4 34062 1.175 0.0 14 0.0 18 D + D 2B 34509 1.744 0.079 -0.004 D + D,

A + A 3B 34127 1.015 0.033 0.045 D + D 3.4 36356 2.434 0.0 17 -0.06 1 D + D 4A 36773 4.79 I 0.000 -0.250 D + D 4B 42516 1.197 0.133 -0.054 D - + D 5A 426 I3 -8.080 0.0 I7 0.562 D + A 6A 43192 -8.160 0.067 0.529 D - * A


However, close inspection of the theoretical results presented here makes it evident that the lowest excitation of DBBD, although of definite CT character, does not provide an example for a photoinduced electron transfer excitation. In such a photoinduced ET process, an intra-donor excitation is followed by a configurational change of molecular (and solvent) degrees of freedom which entails a stabilization of a charge-separated state that would otherwise lie too high in energy [1,2,29]. The barrelene-derived ethylene bridge in DBBD plays a dual role in that it provides a close spatial arrangement of donor and acceptor unit. But it also entails quite a strong direct electronic coupling of these two moieties. The ethylene bridge (C = C) contributes substantially both to the donor HOMO (24% see Table I) and to the acceptor LUMO (54%). Therefore, the lowest excitation is no longer of pure donor- donor character from which an electron transfer process evolves, but exhibits direct CT character. This may be contrasted with the nature of some higher lying excitations of type D + A which, after suitable geometrical relaxations, could evolve into typical final states of an electron transfer process that originated from suitable D * D excitations. Examples for such “precursors” Of ET final states are provided by the excited states 5A and 6A (see Table 11) which may be described as a mixture of the excitations from the MO 24a into 25a and from MO 24b into 27b (see Table I). In these cases one finds the acceptor orbitals spatially well separated from the donor orbitals which allows classification of the transitions IA + 5A and IA * 6A as D + A CT excitations.

The different nature of the various CT transitions is also reflected in the calculated solvatochromic shifts. Typically, such shifts of CT excitations may range up to 4500 cm-’ in a polar solvent [ 301. Calculated values for the solvent-induced shift of excitations of DBBD in an unpolar (cyclo-hexane) and in a polar solvent (water) are collected in Table 111. The transition energies of the three lowest-lying excitations with significant charge transfer are shown. The HOMO-LUMO transition 1B under- goes a solvent shift of about 1000 cm-’ , a moderate value which reflects the rather low degree of charge separation. For the higher lying transitions with a more pro- nounced CT character the red shift amounts to 1600 cm-’. As noted previously [ 17,18 1, the difference of the shifts in the two solvents investigated is rather small compared to the calculated shift on going from vacuum to an unpolar solvent like cyclo-hexane. Here, it is important to note that the cavity model employed only includes electrostatic contributions, but does not take dispersion interaction into account [ 181. A rough estimate of the dispersion contribution [ 17,3 I ] yields an

TABLE 111. Transition energies (in cm-’) of the lowest- lying charge-transfer excitations of DBBD in vacuo and

solvated in cyclo-hexane and in H20.

State In vacuu cyc-hexane H 2 0

IB 30695 29650 29565 5A 426 15 40970 40855 6A 43795 42 190 42 180


additional red shift for all transitions of about 230 cm-' in cyclo-hexane and 190 cm-' in water.

It is interesting to investigate the effect of the surrounding solvent on the charge density in the ground state and in the lowest lying CT states of DBBD. We will use the total Mulliken charges on the acceptor group (A) and the donor group (D) as a rough, but informative measure for such solvent-induced changes (see Table IV) . While the ground-state charge distribution remains nearly unchanged upon sol- vation, one finds the amount of charge separation remarkably increased in the D - A excited states, an immediate reflection of the stabilization provided by the surrounding solvent. The charge transfer from the donor to the acceptor unit in- creases by 30%, but again this effect is significantly larger for the proper CT states 5A and 6A compared to the first excited state IB.

Conclusions

We have analyzed the electronic structure of dibenzo-barrelene-dicarboxydi- methyl-ester ( DBBD) and we have compared the donor-acceptor substituted barrelene with both the donor- and the acceptor-substituted compound. The INDO model calculations yield a first excited state with definite CT characteristics, in agreement with experiment [ 161. We were able to identify the structural features which are responsible for the fact that this molecule has such a low-lying CT state, even in the gas phase. However, this transition, although accompanied by a sub-

TABLE IV. Solvent effect on the Mulliken charges of the donor (D) and the acceptor (A) unit of DBBD for the ground state (1A) and for various excited states (IB, 5A, and 6 4 . The following quantities are displayed: A, Mulliken population of the C=C-bridge and the COOMe groups; D, Mulliken population of the benzene rings; D +

A, charge transfer from the donor to the acceptor monitored by the change in the population of the acceptor unit A.

State Vacuum cyc-hexane H2O

1A A -0.1 12 -0.1 15 -0.119 D -0.060 -0.058 -0.057

1B A -0.534 -0.666 -0.678 D 0.410 0.506 0.515

D + A 0.422 0.55 1 0.559

5A A -0.674 -0.768 -0.721 D 0.515 0.575 0.533

D + A 0.562 0.653 0.608

6A A -0.64 I -0.7 13 -0.74 I D 0.483 0.523 0.545

D + A 0.529 0.598 0.622


stantial charge separation of about 0.4 a.u., does not exhibit the characteristics that are typical for a photoinduced ET process since the ethylene bridge between donor and acceptor groups contributes both to the initial and final state. Therefore it remains to be seen to what extent barrelene based compounds will be able to serve as models for the investigation of photoinduced electron transfer phenomena [ 191.

In addition we have applied an electrostatic cavity model to investigate the effect of a surrounding solvent on the energy of various electronic excitations and the corresponding final state charge distribution. We showed that the electrostatic interaction leads to a considerable increase of the charge separation for the CT states. Concomitantly their excitation energy is lowered, but this shift is only of moderate size due to the short distance over which the charge separation occurs.

Acknowledgment

We thank Prof. M. E. Michel-Beyerle and Dr. H. Heitele for many stimulating discussions. This work has been supported by the Bundesministerium fiir Forschung und Technologie, Germany.

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Received May 18, 1992

on the electronic structure of barrelene-based rigid organic donor-acceptor systems. an indo model...

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