Colloids and Surfaces

A: Physicochemical and Engineering Aspects 171 (2000) 3–12

p-Electron systems — building blocks of supramolecularmachines�

Hans KuhnRingoldswilstrasse 50, CH-3656 Tschingel, Switzerland

www.elsevier.nl/locate/colsurfa

Dietmar Mobius sixtieth birthday is a delightfuloccasion to recall the many years we both workedin close cooperation. This was a happy time forme. I always appreciated his original ideas, hisexcitement and his flair in approaching difficultexperimental problems and in successfully per-forming tricky experiments.

In 1962, when Dietmar had passed his exams atthe Philipps-Universitat Marburg to become achemist and started to do his diploma work (Fig.1) it was an absolutely exciting time in chemistryand the situation should be considered to under-stand Dietmars way to go. On the one hand,classical organic chemistry, developing the basicreactions of synthesis, had reached the goal. Onthe other hand, molecular biology had juststarted, showing that organisms are highly orga-nized at the molecular level. New play groundsfor physical chemists became visible, so Mobiusdecided to become a physical chemist. I was par-ticularly excited by the idea of having a molecularengineering besides molecular biology andthought that chemists should start with a newtask, constructing molecular functional units, i.e.synthesizing molecules that mutually interlock ina distinct purposely designed manner thus form-

ing a functional entity by selforganization assistedby appropriate environmental conditions. In otherwords, chemistry appeared to me to be in asituation ready to change its paradigm: the func-tional system, the machine of molecular size (in-stead of the isolated molecular species) being thegoal of synthesis.

Thus, the question was: how to try to catalysesuch a development? Trying to build a prototypeof a machine of molecular size appeared to be theway to go.

In our laboratory in Marburg we had experi-ence with organic dyes. Quantum mechanicalmodeling of the p-electron systems of dyemolecules and checking the theoretical resultswith spectroscopic findings was a main task.Thus, it was natural to use individual molecules ofdyes as functional components of proposed ma-chines. I thought we should try to get systems ofpurposefully arranged individual dye molecules byincorporating the molecules in monolayers offatty acids. Langmuir and Blodgett had demon-strated that monolayers of fatty acids can bemade and deposited on top of each other. Aprototype of a machine of molecular size ap-peared to be feasible by constructing an appropri-ate monolayer assembly, such that a molecule of adye D is fixed at a given distance from a moleculeof dye A (Fig. 2). Dye molecule D absorbs aquantum of light. Energy, according to Forster, istransferred to dye molecule A which emits a

� Some paragraphs are reproduced from the author’s articlein Comprehensive Biochemistry, Vol. 41. A history of bio-chemistry (G. Semenza, R. Jaenicke, Eds.), Elsevier Science, inprint.

0927-7757/00/$ - see front matter © 2000 Elsevier Science B.V. All rights reserved.

PII: S0927 -7757 (99 )00551 -8

H. Kuhn / Colloids and Surfaces A: Physicochem. Eng. Aspects 171 (2000) 3–124

quantum of light of its individual fluorescence.The two molecules are forming a functional unit,a machine where the dye molecules are the solidparts, the light quanta the movable parts of themachine.

When Dietmar Mobius started with hisdiploma work, Karl Heinz Drexhage and FritzPeter Schafer had done preliminary work show-ing, by interference experiments, the accuracy tobe reached when using fatty acid layers as spacers.They both supported Dietmar strongly in his at-tempts to realize arrangements for energy trans-fer. Mobius constructed such assemblies ofmonolayers of dyes D and A separated by fattyacid spacer layers and he measured the distancedependence of the energy transfer between twodye molecules and succeeded in building systemswith more complex architecture such as the ar-rangement in Fig. 3 where a second energy accep-tor A2 is added to the system of energy donor D

Fig. 2. Functional unit of a donor molecule D and an acceptormolecule A kept at well-defined positions and orientations. Dabsorbs a light quantum hn1, the energy is transferred to Awhich then emits a quantum of light hn2.

and a first acceptor A1 [1]. Both acceptor dyes(acting as competitors) were fixed at various dis-tances (achieved by varying the number of fattyacid spacer layers between the dye layers). Thestrong distance dependence of energy transfer(easily checked by measuring the intensities of thefluorescence of donor and acceptor in the differ-ent sections of the plate, respectively) is importantto control the architecture of organized mono-layer assemblies and Dietmar demonstrated thisin many cases [2–7] This method of using theForster energy transfer as molecular ruler hasbeen applied later to determine the distance be-tween chromophores in biomolecules and is now apopular technique in biophysics [8,9].

Based on the experiments in his diploma workand in the work for his doctoral thesis Mobiushas constructed fascinating new systems. For ex-ample, he has developed methods to cleave simpleor complex monolayer assemblies precisely be-tween distinct monolayers and to contact eachpart with another assembly in almost atomic pre-cision [10–12]. The Forster energy transfer andelectron transfer from excited donors to acceptorswere important tools to control the precision of

Fig. 1. Dietmar Mobius as a student in Marburg/Lahn 1962.

Fig. 3. Monolayer assembly (schematic) with donor monolayer(D), acceptor monolayer (A1) and competing acceptor mono-layer (A2); analysis of the different fluorescence intensities ofD in the different sections of the assembly as a function of thedistances d1 and d2 provides information on the precision ofthe assembly technique.

H. Kuhn / Colloids and Surfaces A: Physicochem. Eng. Aspects 171 (2000) 3–12 5

Fig. 4. Checking the possible influence of defects; the relative fluorescence intensity I/I0 of the donor is independent of the donorsurface density at constant acceptor density, if the acceptor molecules are separated from the donor monolayer by a spacer layer of2.3 nm thickness (circles); when a small fraction of the acceptor molecules is placed at the same interface as the donor moleculesin order to mimic acceptor molecules reaching this interface via defects I/I0 depends on the donor density (bars).

the different manipulation techniques. The manip-ulation of monolayer assemblies was importantfor fixing dyes at precisely defined distances infront of a silver halide crystals, and this allowedto clarify a long standing problem in photo-graphic science (distinction between energy trans-fer and electron transfer as the basic mechanismin photographic sensitization) [13].

It is interesting to recall the situation in thosedays. We both, Dietmar and I, had been verymuch concerned to avoid artefacts and to becorrect in our conclusions, and we were happyand fascinated to have a useful new techniquethat allowed us to reach the goal of constructingprototypes of machines of molecular size. How-ever, other research groups were not able to re-produce our results, so we appeared as charlatans.

Dietmar’s way to go, his experimental skill, hisingenuity and care, his consequence and continu-ity in approaching important goals, were crucialin overcoming this draw back. Dietmar con-structed control systems demonstrating in manydifferent ways the absence of artefacts [12,14].The systems were designed in such a way that theeffect under consideration must be competed incontrast to any conceived artefact. An alternativestrategy was mimicking possible defects by assem-bling appropriate systems and compare the resultwith the correct structure. An example is shown inFig. 4. The relative fluorescence intensity I/I0 of adonor monolayer is independent of the donor

density for a given fixed acceptor density, if theacceptor molecules are separated from the donormonolayer by one monolayer of a fatty acid (Fig.4, circles). The fluorescence quenching observed inthis case is due to tunneling of an electron fromthe excited donor to acceptor molecules separatedby 2.3 nm. If, however, the acceptor moleculeshave reached the same interface as the donormolecules via defects in the system, I/I0 dependson the donor density as checked with monolayersystems mimicking such defects by placing a smallfraction of the acceptor molecules at the interfacewith the donor molecules (Fig. 4, bars). In addi-tion to this type of studies, a movie showing thedifferent assembly and manipulation techniques indetail that has been made with Paul Gilman at theresearch laboratories of the Eastman KodakCompany has assisted in regaining the confidenceof the scientific community in the new techniques(Fig. 5). Many other research groups entered thefield as demonstrated by the increasing number ofpublications and the growing participation in theInternational Conferences on Organized Molecu-lar Films.

The goal of precisely joining modules — simpleor complex packages of monolayers — must bedistinguished from the synthesis of a ‘super’molecule (constituting a functional entity ob-tained from components by chemical bonding).An advantage of the modular assembly is thegreat variability in the possibilities to construct

H. Kuhn / Colloids and Surfaces A: Physicochem. Eng. Aspects 171 (2000) 3–126

Fig. 5. Dietmar Mobius with Paul Gilman (left), Mrs Gilman, and Tom Penner (right); photo taken in 1984.

organized systems. Dietmar Mobius has demon-strated the importance of the modular strategy inmany examples. He has constructed molecularmachines for photo-induced vectorial electrontransfer, arrangements acting as rectifires [15], asmolecular wires [16,17] and as systems for signalamplification by energy transfer [6].

Following this strategy he has designed excitingnew materials based on the idea that amphiphileswith bulky head groups form a matrix in whichnon-amphiphilic molecules with interesting prop-erties can be incorporated in a well organized way[18]. To obtain reversible photo-induced cis– transisomerization requires space, and by incorporat-ing such molecules in a monolayer matrix in acarefully designed way Dietmar obtained bistablesystems that can be used as memory with enor-mous storage capacity [19]. By incorporating non-polar molecules with p-electron system in anamphiphilic matrix Mobius and coworkers wereable to produce monolayer assemblies in whichthe p-electron system was strongly polarized bythe surrounding amphiphilic matrix [14,20]. Inthis way it was possible to obtain matrix inducedsecond order non-linear optical properties (Fig.6).

Of particular interest is the incorporation of‘super’ molecules in a monolayer matrix. DietmarMobius, in cooperation with J. Fraser Stoddartand Helmut Ringsdorf realized such an assemblyby a ‘super’ molecule that constitutes by itself aphoto-induced electron transfer system [21]. An

Fig. 6. Second harmonic generation in monolayers ofquinthiophene, QT, organized in various amphiphile matrices;due to the orientation of QT normal to the layer plane, themolecule is polarized, and, as expected, the square root of theobserved SHG intensity is proportional to the induced dipolemoment which was determined from surface potential mea-surements. QT, in the absence of the polarizing influence ofthe organized environment, shows no SHG due to its symme-try.

H. Kuhn / Colloids and Surfaces A: Physicochem. Eng. Aspects 171 (2000) 3–12 7

Fig. 7. Imprinting a complementary charge patterin (‘foot-print’) in a mixed monolayer of a neutral and a chargedamphiphile, respectively; by adsorption of a phthalocyanine-tetrasulfonate, Pc, the binding sites are organized, the patternis subsequently immobilized by transfer of the monolayer to ahydrophobic solid substrate, and the Pc is removed; thisorganized surface binds the Pc from a dilute solution muchstronger than a surface with the same density of binding sites,that have not been organized but assume a statistical distribtu-tion, as shown by the absorption spectra.

cally, showing that the footprints were stillpresent in the mixed monolayer [22].

I mentioned the importance of the Forster en-ergy transfer as a molecular ruler. Dietmar Mo-bius, in his doctoral thesis, made an interestingobservation. Using a thin gold layer as the accep-tor instead of a dye layer he found that theForster equation described the experiments per-fectly. But this was puzzeling. Forster had calledthe effect radiationless resonance energy transfer,but in gold, equally absorbing over a broad rangeof frequencies, there are essentially free electronsexposed to the field of the light-emitting dyemolecule. Indeed, the Forster equation can beeasily derived [23] when treating the donor asclassical oscillator, as Hertz antenna emitting lightin the absence of the acceptor, and the acceptor asunspecified absorber located in the near field ofthe radiating antenna. In the usual absorption,the situation is different only in that the absorberis in the far field of the dipole. Forster energytransfer is simply absorption in the near field. Inthis view the modern term fluorescence resonanceenergy transfer (FRET) reminding of Perrin’soriginal idea to describe energy transfer by reso-nance between coupled oscillators replacing thetwo molecules, is misleading.

We were very much fascinated by the idea thatit should be possible to read and write with visiblelight with a resolution of 5 nm (critical distance ofForster transfer), that a near field optical mi-croscopy should be possible [24]. Peter Zingsheimand Ulrich Fischer in the lab were able to demon-strate experimentally imaging with visible lightwith a resolution below the wavelength (first bybleaching [25] an acceptor through Forster energytransfer, and later by scanning [26] using a smalllight source). Today, scanning near field opticalmicroscopy (SNOM) is an important tool withgreat future prospects as a possibility to addressmolecular devices.

Donor and acceptor in the Forster energytransfer are at a distance larger than the extensionof the p-electron system of the excited molecule.Thus, the molecule can be replaced by an oscillat-ing point dipole. In contrast, in all cases wherep-electron systems are in direct contact, e.g. in themany sandwich arrangements constructed by Mo-

integration of supramolecular chemistry and mod-ular strategy should be the leading way to futuredevelopments. This example is an exciting realiza-tion of the way to go traced in the early 1960swhen Dietmar started to work on monolayers.

The idea of a molecular information transfer bycontacting monolayers and separing them againhas been realized by Dietmar Mobius by formingsandwich pairs of dye molecules [6]. A rigidmonolayer with the first dye was superimposed bya soft monolayer with another dye. The moleculesof the second dye reached the molecules of thefirst dye by diffusion forming the dimer. Thesecond monolayer was rigidized and separatedfrom the first. In another nice experiment DietmarMobius has demonstrated the formation of foot-prints of molecules in a monolayer (Fig. 7): dyemolecules were adsorbed at a soft monolayer ofcharged and uncharged molecules. By diffusion anarrangement was reached where the charges inthis layer linked to the opposite charged sub-stituents of the dye. The monolayer was immobi-lized and the dye removed. Later on, themolecules of the particular dye readsorbed specifi-

H. Kuhn / Colloids and Surfaces A: Physicochem. Eng. Aspects 171 (2000) 3–128

bius, the point dipole model is no longer appro-priate. It even leads to the wrong sign in thepredicted shift of the absorption band in thegeometry of Fig. 8 (the Coulomb attraction forcesprevail, while in reality the Coulomb repulsionoutweighs attraction. The shifts calculated by thefree electron model agree with experimental shifts[27]. A quasi classical description (oscillatingdipoles extending over the p-electron systems)leads to the same result in contrast to the pointdipole approximation. It is surprising that thepoint dipole approximation is still widely used totreat aggregates. It would be so easy to replacethe point dipole by the extended dipole [28].

We used the extended dipole model to treatScheibe aggregates. The absorption band of aScheibe aggregate (J-aggregate), in contrast to theband of the dye monomer, is very narrow and isshifted toward the longer wavelengths [29]. Thefluorescence band almost coincides with the ab-sorption band. The spectral properties of theScheibe aggregates were explained by assuming atightly packed brickstone work like arrangementof the dye molecules using the extended dipolemodel [30].

Dietmar Mobius observed a very interestingeffect in a monomolecular layer of a Scheibeaggregate. Using the Scheibe aggregate of a oxa-cyanine, Dietmar added traces of thiacyaninewhich is sterically very similar to the oxacyanineand each molecule of the guest replaces amolecule of the host [31]. When exciting the hostwith UV radiation the sensitized green fluores-

cence of the guest appears instead of the bluefluorescence of the host observed in the absence ofthe guest. Mobius observed the green fluorescenceeven when only one out of 10 000 molecules of thehost was replaced by a molecule of the guest,constituting an extremely efficient light harvestingsystem. How can one understand this fascinatingeffect?

In the Scheibe aggregate of the pure host, whenexciting with UV radiation, a group of aboutN=9 neighboring molecules is excited at roomtemperature. This is due to the tight packing ofmolecules in the Scheibe aggregate. The distribu-tion of the excitation energy over severalmolecules leads to a small change in electrondensity only, thus no appreciable change in bondlengths, and, therefore, to the observed narrowabsorption band and to the almost coincidingfluorescence band. Each dye molecule, in a sim-plified description, can be replaced by a classicaloscillator, and the N oscillators representing theexcited state, oscillate in phase (coherent exciton).The oscillating extended dipoles, being in phase,attract each other by Coulomb forces [32]. Theexcitation energy (E−E %), as compared with theexcitation energy E of an isolated molecule, isdiminished by the Coulomb energy correspondingto this attraction (E %=0.18 eV). The absorptionband of the Scheibe aggregate is shifted to longerwavelengths, in agreement with the experiment.

N is given by the relation N=E %/kT. The oscil-lators oscillating in phase are knocked out ofphase by thermal collisions as soon as the Cou-lomb forces keeping the oscillators in phase aregetting too weak by distributing over too manyoscillators. Thus N is given by equalizing E %/Nwith kT. Obviously, N is increasing with decreas-ing temperature.

By the attraction of the oscillators oscillating inphase the excited domain is compressed. Thus asound wave is assumed to be produced moving inthe upward direction (Fig. 9) with a speed of 2km/s along with a corresponding sound wavemoving in the downward direction. One wavecarries the exciton (symmetry breaking). The exci-ton holds the sound wave together, so the exciteddomain constitutes a soliton, a confined exciteddomain that moves along the aggregate until it

Fig. 8. Extended and point dipole models to describe theinteraction between two dye molecules in a sandwich dimer;according to the point dipole model, an attraction is expectedin contrast to the observation.

H. Kuhn / Colloids and Surfaces A: Physicochem. Eng. Aspects 171 (2000) 3–12 9

Fig. 9. Snapshot of a coherent exciton in a Scheibe aggregate.The number N of dipoles oscillating in phase in the domain isestimated from the shift of the absorption band in comparisonto the thermal energy: N=10 at 300 K and N=100 at 30 K.In the classical picture, the lifetime tExciton is shortened ascompared to that of the monomer due to the fact that theoscillators replacing the dye molecules are moving in phase.

Scheibe aggregates are interesting componentsfor supramolecular machines, and understandingtheir strange properties with simple model consid-erations should be helpful in attempts to constructnew and increasingly sophisticated supramolecu-lar machines.

The dream of a molecular engineering, of tai-lor-made molecules that are driven to self-orga-nize by interlocking forming a machine hasstrongly motivated Dietmars work. Has thisdream of a new paradigm in chemistry becomereality or is it still a dream? This change inparadigm is clearly taking place — supramolecu-lar chemistry has become a powerful, dramaticallydeveloping and most exciting subject in chemistry[33]. Chemical synthesis in combination withmodular strategy is progressing.

But constructing complex artificial supramolec-ular machines that serve human needs is still adream. I think it is important for catalyzing sucha development to reflect on increasingly complexsupramolecular machines, to invent simple theo-retical models, and to start with their experimen-tal realization.

Approaching problems by inventing simple the-oretical models is a success story in physicalchemistry in the past. The computer has partlyreplaced this attitude. Simple theoretical modelsfor complex situations are often considered to besuspicious and results of computer calculationsare frequently accepted without asking for what isphysically relevant and what is an unnecessarycomplication that is veiling insight.

We must take care that the fascination in seeingwhat is essential in a complex phenomenon, theflair for reasonable simplification, is kept alive.The traditional approach, inventing simple, intel-ligent theoretical models is of interest in futureattempts inspite of the marvellous possibilitiescreated by the computer.

It seems interesting to reflect on this in a casewhere the approach with the computer has beenfundamental: The fascinating work of Michel andDeisenhofer in 1984 leading to the detailed struc-ture of the reaction center of photosynthesis ofpurple bacteria acting as electron pumps. DietmarMobius and myself, in the early seventies, havebeen strongly engaged in trying to find a possibil-

disappears forming a quantum of blue fluores-cence light.

But now, when a tiny fraction of the moleculesof oxacyanine is replaced by thiacyanine, the exci-ton, on its path through the aggregate, may meeta thiacyanine; then the excitation energy will betransferred to the thiacyanine molecule which,later, will be emitting a quantum of green fluores-cence light. The effect, depending on the tempera-ture and the density of the guest in the aggregateis found to be in quantitative agreement with theexperiment [32,40]. In essence, the exciton, on itsway through the aggregate, covers a sufficientarea to meet a molecule of the guest even at adilution of 1:10 000.

It should be emphasized that the decay time ofthe fluorescence of the host in the absence of theguest is strongly shortened in comparison with thefluorescence of the isolated molecule. The effecthas been explained as a very particular quantummechanical phenomenon, called super radiation,and this was a sensation. In the simple classicalpicture of N dipoles oscillating in phase, it iseasily seen that the emission of fluorescent light isN times faster than in the case of an isolatedmolecule, the lifetime of the excited state accord-ingly shorter. In this description the effect israther trivial.

H. Kuhn / Colloids and Surfaces A: Physicochem. Eng. Aspects 171 (2000) 3–1210

ity to construct a photo-induced electron pump[34,40]. The first question of course was finding abasic mechanism. Structure and mechanism of thereaction center were a puzzle at that time. Theidea of our approach was this (Fig. 10): considera dye F which is excited, a high potential barrieron the left, preventing motion of the excited elec-tron in this direction, a low barrier on the rightwith several sites of lower energy (molecular wireW). Finally an electron acceptor A at a distanceof about 3 nm preventing the electron, now cap-tured by the acceptor, to tunnel back to theoxidized dye F+. On the other hand, the energybarrier on the left is sufficiently narrow to allowtunneling of an electron from the donor D to F+.Then F is in the original state ready to be excitedagain.

Using the quantum mechanical tunneling, itsstrong dependence on the thickness of the energybarrier, appeared to us as nature’s trick to solvethe problem that had been called ‘horrible para-dox’ in those days: the experimental finding in thebacterial reaction center that the electron, afterexcitation, needs 10 ps to move from the exciteddye to the acceptor, but, on the other hand, needs10 ms to move from the acceptor back to theoxidized dye when it has not been used up before.

We used the data on quantum mechanical tun-neling resulting from experiments with monolayer

assemblies to determine optimal distances betweenD, F, W, and A as well as optimal positions ofenergy levels [34,35]. The geometric and energeticdata thus obtained turned out to agree, in theessence, with the actual data resulting from thework of Michel and Deisenhofer [36]. Knowingthe actual situation the model consideration is oflittle interest. But the role of this kind of theoret-ical modeling in the search for possibilities torealize entirely new supramolecular machinesshould be appreciated. The development of so-phisticated machines is crucial for real progress insupramolecular engineering.

I mentioned the importance of keeping alive aflair for reasonable simplification. Let me illus-trate this in the case of the two dye molecules 1and 2, see Scheme 1. Surprisingly, 1 absorbs atlonger wavelengths than 2 inspite of the smallersize of the p-electron system [37]. The effect wasexplained [37] by performing a standard quantummechanical calculation considering the interactionof 50 configurations, using the usual values of theadjusted parameters. A reasonable agreement ofthe wavelength of the absorption maximum(lmax=649 nm) with the experiment (lmax=630nm) was found, but the oscillator strength of theband of dye 1, f=0.05 was 4 times smaller thanin the experiment, f=0.2. We have obtained thesef-values from the o-values (24 100 mol−1 Lcm−1

(experimental) and 6800 mol−1 Lcm−1 (theory)given in [37] by assuming that o=105 mol−1

Lcm−1 corresponds to f=0.8).Using the simple free electron model approxi-

mation — de Broglie waves extending over themolecular skeleton considering heteroatoms bypotential wells — the position and oscillatorstrength of the absorption band is found to be inagreement with the experiment in both dyes [38].

This shows that the power of simple modelsshould be appreciated, and that the possibility ofdeficiencies in complicated approaches, not easyto localize, should be seen.

I remember with a feeling of delight the timeover 50 years ago when I tried to treat p-electronsas freely moving along the molecular skeleton[39,40]. Discussion on p-electrons was full of ex-citement at that time and the scientific communitywas open to new ideas while today everything is

Fig. 10. Model of a light driven elcetron pump mimicking thebacterial photosynthetic reaction center; after excitation of thedye F (step 1) the electron proceeds to the right via molecularwire to the acceptor A (step 2), since the barrier to the leftblocks that way; this barrier, however, is thin enough (2 nm)to permit fast tunneling of an electron from the donor D toF+ (step 3), whereas tunneling of the electron from A− to F+

is prevented by the thick barrier (3 nm); the large difference ofthe relaxation times of the forward process (10−11 s) and thatof the ‘back transfer’ (10−2 s) has been considered a horribleparadox, however, is easily undestood within the frame of sucha spatial and energetic structure.

H. Kuhn / Colloids and Surfaces A: Physicochem. Eng. Aspects 171 (2000) 3–12 11

Scheme 1. Structures of two dye molecules with dye 1 surprisingly absorbing at longer wavelengths than dye 2; comparison ofexperimental values of wavelength of the absorption maximum and of the oscillator strength of the transition with those calculatedaccording to usual methods (configuration interactions) and with the Free Electron Model.

considered to be settled. It was a great adventureto me to see that the wavelengths of the absorp-tion maxima and oscillator strengths of the cya-nine dyes can be given by a simple relation, noadjustable parameter had to be used. In contrast,the Huckel approximation, with the adjustedvalue of b used at that time, gave values for thewavelengths of the absorption maxima whichwere 4 times larger than the experimental values,predicting an absorption far in IR. Later on, bwas adjusted (a terrible ambiguity in my view ofthat time) and the discrepancy disappeared. Ithink we should not fully neglect the problemwhen using semi-empirical methods.

In reflecting on ways to approach new impor-tant tasks I recall enthusiastically the time when Idid my PhD work with Werner Kuhn. He wasone of the great pioneers in colloid science. Hisidea to understand the properties of polymers bydescribing the molecules as statistical coils chang-ing shape by external forces, was a breakthroughallowing the first quantitative theoretical descrip-tion of a macromolecule and its properties. Theimportance of his work in paving the way tomolecular biology and in providing a quantitativebasis to colloid science should be much more

appreciated. His 100th birthday this year shouldbe remembered.

He was a master in inventing simple and ex-tremely powerful theoretical models and simpleexperiments. To see what is of physical impor-tance was crucial to him. He has shown again andagain how to approach a problem with estimateby the eye, by extracting what is essential. Hisway to handle a complex problem was inventing astrongly simplifying model and to investigate themodel very carefully by studying all its conse-quences. He did not believe in a formalism with-out reaching a detailed understanding of eachstep. What he approached was based on deepreflection.

I think it is important to transmit to the younggeneration these traditional values: aiming attransparency, searching for simple models tograsp the essence, attempting experimental real-ization of important ideas.

Dietmar Mobius, with this focus in mind insetting goals, has achieved exciting and importantexperiments, he has constructed fascinatingsupramolecular systems. I sincerely wish him forthe coming decade much excitement and happi-ness in realizing his ideas.

H. Kuhn / Colloids and Surfaces A: Physicochem. Eng. Aspects 171 (2000) 3–1212

References

[1] D. Mobius, PhD Thesis, Philipps-Universitat Marburg/Lahn, Germany, 1965.

[2] D. Mobius, Z. Naturforsch. 24a (1969) 251–253.[3] H. Bucher, O.V. Elsner, D. Mobius, P. Tillmann, J.

Wiegand, Z. Phys. Chem. Neue Folge 65 (1969) 152–169.[4] D. Mobius, G. Dreizler, Photochem. Photobiol. 17 (1973)

225–231.[5] D. Mobius, G. Debuch, Chem. Phys. Lett. 28 (1974)

17–21.[6] D. Mobius, in: M. Hair, M.D. Croucher (Eds.), Colloids

and Surfaces in Reprographic Technology, vol. 200,American Chemical Society, Washington, DC, 1982, pp.93–110.

[7] T. Kaercher, D. Mobius, W. Jaeger, Fortschr. Ophthal-mol. 83 (1986) 90–94.

[8] J. Eisinger, Q. Rev. Biophys. 9 (1976) 21–33.[9] J. Yguerabide, M.C. Foster, in: E. Grell (Ed.), Membrane

Spectroscopy, Springer–Verlag, Berlin, 1981.[10] D. Mobius, Chem. i. u. Z. 9 (1975) 173–182.[11] O. Inacker, H. Kuhn, D. Mobius, G. Debuch, Z. Phys.

Chem. Neue Folge 101 (1976) 337–360.[12] D. Mobius, Photogr. Sci. Eng. 18 (1974) 413–418.[13] R. Steiger, H. Hediger, P. Junod, H. Kuhn, D. Mobius,

Photogr. Sci. Eng. 24 (1980) 185–195.[14] H. Kuhn, D. Mobius, in: B.W. Rossiter, R.C. Baetzold

(Eds.), Investigations of Surfaces and Interfaces, vol. IXB,Wiley, New York, 1993, pp. 375–542.

[15] E.E. Polymeropoulos, D. Mobius, H. Kuhn, J. Chem.Phys. 68 (1978) 3918–3931.

[16] E.E. Polymeropoulos, D. Mobius, H. Kuhn, Thin SolidFilms 68 (1980) 173–190.

[17] J. Cunningham, E.E. Polymeropoulos, D. Mobius, F.Baer, in: J.P. Fraissard, H.A. Resing (Eds.), MagneticResonance in Colloid and Interface Science, D. Reidel,Dordrecht, Netherlands, 1980, pp. 603–608.

[18] W. Cordroch, D. Mobius, Thin Solid Films 210/211(1992) 135–137.

[19] J. Maack, R.C. Ahuja, D. Mobius, H. Tachibana, M.Matsumoto, Thin Solid Films 242 (1994) 122–126.

[20] L.F. Chi, PhD Thesis, Georg-August-Universitat Gottin-gen, 1989.

[21] R.C. Ahuja, P.-L. Caruso, D. Mobius, D. Philp, J.A.Preece, H. Ringsdorf, J.F. Stoddart, G. Wildburg, ThinSolid Films 284–285 (1996) 671–677.

[22] D. Mobius, Z. Phys. Chem. Neue Folge 154 (1987) 121–144.

[23] H. Kuhn, J. Chem. Phys. 53 (1970) 101–108.[24] H. Kuhn, in: A. Rich, N. Davidson (Eds.), Structural

Chemistry and Molecular Biology, W.H. Freemann, SanFrancisco, 1968, pp. 566–572.

[25] U.C. Fischer, H.P. Zingsheim, J. Vac. Sci. Technol. 19(1981) 881–885.

[26] U.C. Fischer, J. Vac. Sci. Technol. B3 (1) (1985) 386–390.[27] V. Czikkely, G. Dreizler, H.D. Forsterling, H. Kuhn, J.

Sondermann, P. Tillmann, J. Wiegand, Z. Naturforsch.24a (1969) 1821–1830.

[28] V. Czikkely, H.D. Forsterling, H. Kuhn, Chem. Phys.Lett. 6 (1970) 207–210.

[29] G. Scheibe, Angew. Chem. 50 (1937) 51.[30] V. Czikkely, H.D. Forsterling, H. Kuhn, Chem. Phys.

Lett. 6 (1970) 11–14.[31] D. Mobius, Ber. Bunsenges. Phys. Chem. 82 (1978) 848–

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