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Block Building Agg NR 2008

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ISSN 1995-0780, Nanotechnologies in Russia, 2008, Vol. 3, Nos. 3–4, pp. 139–150. © Pleiades Publishing, Ltd., 2008. Original Russian Text © B.I. Shapiro, 2008, published in Rossiiskie nanotekhnologii, 2008, Vol. 3, Nos. 3–4. REVIEWS Block Building of Polymethine Dye Aggregates B. I. Shapiro NIIKhIMFOTOPROEKT Scientific Center, Leningradskii pr. 47, Moscow, 125167 Russia e-mail: bishapiro@mail.ru Received November 10, 2007; in final form, November 28, 2007 Abstract—Formation of J-aggregates of p
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   139   ISSN 1995-0780, Nanotechnologies in Russia, 2008, Vol. 3, Nos. 3–4, pp. 139–150. © Pleiades Publishing, Ltd., 2008.Original Russian Text © B.I. Shapiro, 2008, published in Rossiiskie nanotekhnologii, 2008, Vol. 3, Nos. 3–4.  STATEMENT OF THE PROBLEMThe substantial interest in complicated organizedstructures made of organic molecules, i.e., molecularensembles, stems from the fact that the use of thesetypes of structures is promising for storing and process-ing mass data. This is the way that nature uses. A tellingillustration is reliable data libraries stored in DNA andused in living organisms.At present, with the development of optical datarecording, processing, and presentation in the contextof nanotechnology, aggregates built of a limited amountof colored organic compounds, or dyes, take on specialsignificance. In particular, we are talking about meth-ods of the controlled formation of molecular ensemblesof a certain structure with desired optical and electronicproperties that can be of interest for systems of opticalinformation transformation, such as optical computers.Researchers have focused their attention on variousclasses of dyes. Polymethine dyes, however, are of spe-cial interest.POLYMETHINE DYESThe practical interest in polymethine (cyanine) dyesarose after their successful application as spectral sen-sitizers for silver halide photographic materials. Morethan 130 years have passed since Herman Vogel discov-ered the spectral sensitizing phenomenon [1]. Note thatit is Vogel who first demonstrated the uniqueness of polymethine dyes as spectral sensitizers. In spite of numerous attempts to use other class dyes, only cya-nines have actually been employed in photographictechnology.In the development of spectral sensitization, a sortof selection of polymethine dyes took place. Initially,the dyes were used mainly in molecular (M) form.However, dyes in aggregated form were preferred lateron. As shown in certain papers [2, 3], dyes capable of forming supramolecular structures, or J-aggregates,exhibit the greatest photographic efficiency. At present,most black-and-white and color photographic materialsare spectrally sensitized to the visible light by J-aggre-gates of polymethine dyes.Polymethine dyes stand out among other classes of organic dyes for both their light absorption and theircapacity to form polymolecular aggregates. Generally,the most frequently used amidinium ion-type dyes canbe presented as formula (I):(I)where Z = O, S, Se, NR, –CH=CH–, etc.; R–R    x  are var-ious substituents; n  = 0–7; and X   +–  is a counterion.Therefore, they are cationic chromophores, in which het-erocyclic rings of benzoxazole, benzothiazole, benzimida-zole, quinoline, etc., are linked with a polymethine chainof an odd number of methine groups (2  n  + 1) [2, 3].Aggregation of polymethine dyes is a striking exam-ple of self-organization in organic matter. The ability of polymethine dyes to J-aggregate is governed by therigid planar structure of the chromophore part of themolecule and alternation of the π  charges along thepolymethine chain of the dye [3]. What are the conse-quences of this electronic structure of polymethinedyes? Alternation of the π  charges along the chro-mophore chain and their inversion upon photoexcita-tion govern the high efficiency of absorption of visibleand infrared light owing to the interaction with thedipole of the light wave. Molar extinction coefficients NZ(CC) n R 3 R 4 RR 1 CNZRR 2 R  x X ± ,  Block Building of Polymethine Dye Aggregates  B. I. Shapiro   NIIKhIMFOTOPROEKT Scientific Center, Leningradskii pr. 47, Moscow, 125167 Russiae-mail: bishapiro@mail.ru  Received November 10, 2007; in final form, November 28, 2007  Abstract  —Formation of J-aggregates of polymethine dyes in solutions and in a heterogeneous system of AgHalgrains is considered. A new mechanism of formation of polymethine dye J-aggregates from dimers is proposedon the basis of the equilibrium between dimers and J-aggregates in solution. It was concluded that “block build-ing” of J-aggregates from dimers is most preferable for constructing perfect J-aggregates of the same size (mono-structural J-aggregates). Mixed J-aggregates and “heterocontact dimers” of polymethine dyes with variouspolymethine chain lengths are obtained using the block building technique.  DOI: 10.1134/S1995078008030014  REVIEWS   140  NANOTECHNOLOGIES IN RUSSIA   Vol. 3   Nos. 3–4   2008  SHAPIRO  of cyanines reach the maximum values known for dyes,  ε  = 2–3 ×  10   5  L mol   –1  cm   –1  [3]. Additionally, chargealternation in the ground state causes substantialadsorbability of the dyes on the surfaces of ionic crys-tals, where charge alternation also takes place [3].The bond orders in the polymethine chain of thedyes are almost equal and close to one and a half, whichmakes the molecule rigid and planar. This is supportedby the X-ray diffraction data for a series of symmetricdyes [2]. Most typical cyanine molecules are almostplanar with angles between the heterocycle planes of less than 15  °  . The rigid planar structure of a cyaninefavors narrow and selective absorption bands. The pla-narity of polymethine dye molecules is a prerequisitefor the plane–plane aggregation of molecules with acertain shift angle for better π  –  π  charge interaction of oppositely charged methine groups of neighboringmolecules.On the basis of the X-ray diffraction and simulationdata for cyanine dye aggregates, it has been concludedthat the molecule packing angle in the aggregate is themost important for the spectral properties of aggregates[2, 3]. The simplest dye aggregate is a dimer D, whoseabsorption band is hypsochromically shifted relative tothe molecular band M.It is useful to consider the spectral manifestation of the molecular interaction in the aggregates within theMcRae–Kasha model [4, 5] for the simplest case of adimer. As shown schematically in Fig. 1, the probabilityof the electronic transition from the ground state (G) tothe excited state (E) split owing to the intermolecularinteraction depends on the angle α  between the transi-tion dipoles and the aggregate axis (the axis connectingthe molecule centers in the aggregate). When the angle  α  = 90  °  , the spectral transition to the upper split excitedstate is most probable, which corresponds to the sand-wich-type dimers of cyanine dyes. The transition to thelower split state is forbidden. On the contrary, for α  =0  °  , the transition to the lower split excited state is pref-erable, which corresponds to the bathochromic shift of the dimer absorption band. In general, when α  > 54  °  ,the transition to the higher split state with the hypsoch-romic shift of absorption is more probable, whereas for  α  < 54  °  , the transition to the lower split state with thebathochromic shift is preferable. This model wasextended for the case of larger aggregates, such as H-and J-aggregates.For clearness, these aggregates can be presented asa pack of cards, which is illustrated by Fig. 2. The shiftangle of the cards in a loose pack is the packing anglegoverning the light absorption of the aggregate.According to the simulation data, α  = 60  °  correspondsto H-aggregates, whereas more acute angles α  = 30  °  and α  = 19  °  correspond to J-aggregates. The smaller α  ,the greater the bathochromic shift of the J-aggregateabsorption band.At present, the two-dimensional structure consid-ered by Kuhn for J-aggregates, the so-called “brick-work” structure with a packing angle of  α  = 19  °  , is gen-erally accepted [6]. It is shown in Fig. 3. MO calcula-tions show that only three to five cyanine molecules inthe brickwork structure are sufficient for the absorptionJ-band to appear [7].BLOCK FORMATIONOF MOLECULAR ENSEMBLESAs a rule, the aggregation of cationic polymethinedyes in solutions occurs from individual molecules andis a chaotic and uncontrolled process resulting in aggre-gates consisting of a different number of moleculesand, subsequently, in colloid particles [8]. Our studiesshow that anionic dyes with disulfoalkyl substituents atthe nitrogen atoms of the heterocyclic rings (formula (I),R = (CH   2  )  n  ) easily form dimers in aqueous solu-tions [9, 10]. The stability of dimers of anionic polyme-thine dyes is possibly governed by the substantial inter-molecular Coulomb interaction between the sulfogroup of one molecule and the positive charge localizedon the chromophore of the other one. Dimer is a sim-plest aggregate. The formation of dimers is described ina number of earlier papers [8]. Dimers are formed bySO 3–   MonomerDimersEG  αα  0  °  54  °  90  °  Fig. 1.  Energy level diagram of the dye monomer and its dimers for different angles α  between the transition dipoles and the aggre-gate axis [4, 5]. G is the ground state; E are the excited states.   NANOTECHNOLOGIES IN RUSSIA   Vol. 3   Nos. 3–4   2008  BLOCK BUILDING OF POLYMETHINE DYE AGGREGATES141  cyanines with various polymethine chain length (dyesof structure (I), n  = 0–3). Figure 4 shows the equilibriumbetween the monomers and dimers with the example of thiamonomethine cyanine D-1 (3,3'-di-(  γ   -sulfopropyl)-4,5-benzo-5'-chloro-thiamonomethine cyanine betainetriethylammonium salt, a dye of structure (I), n  = 0) [11].According to Fig. 4, there is a thermodynamic equilib-rium between M and D forms. Decrease in the dye con-centration causes a rise in the intensity of the M bandand a decrease in the absorbance of the D band; thecurves exhibit an isosbestic point. It is shown [9, 11]that, as the dye concentration increaes and the temper-ature decreases, the dye dimers form larger J-aggre-gates, which is illustrated in Fig. 5. These processes areequilibrium and reversible. A new block mechanism of J-aggregate formation from dimers was proposed onthe basis of the equilibrium between the dimers and J-aggregates of anionic dyes [9].A method has been developed for constructing verysmall monosized J-aggregates possessing new elec-tronic and optical properties from dimers. Not only J-aggregates of thiamonomethine cyanines are obtainedby this method, but also J-aggregates of trimethine cya-nines, pentamethine cyanines, and heptamethine cya-nines. The thermodynamics of dimer J-aggregateequilibria in aqueous solutions has been studied. Figure 6shows the equilibrium between the dimers and J-aggre-gates of pyridinium salt of 3,3'-di-(  γ   -sulfopropyl)-4,5,4',5'-dibenzo-9-ethyl-thiatrimethine cyanine betaine(D-2) as an example [10]. It follows from the figure thatthe equilibrium is thermodynamically reversible; thespectra exhibit an isosbestic point. As the temperaturedecreases and the concentration increases, the dyes tendto aggregate. The data agree well with the regularities of aggregation of thiamonomethine cyanines [9, 11].Figure 7 shows the equilibrium between the dimersand J-aggregates for a longer-chain dye, triethylammo-nium salt of 3,3'-di-(  γ   -sulfopropyl)-5,5'-dichloro-9,11-(  β  ,  β  -dimethyltrimethylene)-10-methyl-thiapentamethinecyanine betaine (D-3) [10]. One can see that the equi-librium is thermodynamically reversible, and the spec-tra also exhibit an isosbestic point. The size of J-aggregates in the mD  J equilibriawas determined graphically by constructing a plot of versus [10], where C    J  and C    D  are theconcentrations of J-aggregates and dimers in solutionand n  is the number of molecules in the J-aggregate.The slope of the line is equal to the number of dimersin the J-aggregate. As follows from Fig. 8, m  = 2 for thi-atrimethine cyanine D-2; that is, the J-aggregate con- nC  J () log C  D log   NNNNNNNNNNNNNNNNNN  α  = 30  °α  = 19  °α  = 60  °α  = 19  °α  = 30  °α  = 60  °  H-aggregateJ-aggregates  Fig. 2.  Schematic drawing of the structure of H- and J-aggregates of cyanine dyes; α  is the aggregate packing angle [3].  α  = 19  °  Fig. 3.  Structure of the brickwork-type J-aggregate [6].   500400  λ   , nm0.80.4   A 3 21  Fig. 4.  Absorption spectra of aqueous solutions of thia-monomethine cyanine D-1 at 20  °  C and (  1  ) C   = 1 ×  10   –5  M and  l = 0.1 cm; (  2  ) 1 ×  10   –6  M and l  = 1.0 cm; and (  3  ) 1 ×  10   –7  Mand l = 10 cm [9].   142  NANOTECHNOLOGIES IN RUSSIA   Vol. 3   Nos. 3–4   2008  SHAPIRO  sists of two dimers, or four molecules. Similarly, it wasshown that J-aggregates of thiamonomethine cyanineD-1 and thiapentamethine cyanine D-3 also consist of two dimers, or four dye molecules [12].According to the architecture of J-aggregates con-sidered in the literature [6], Fig. 9 shows possible J-aggregate structures resulting from two dimers. Struc-ture a  corresponds to the staircase-type packing, struc-ture b  corresponds to the ladder-type packing, and,finally, structure c  is the most probable for the brick-work-type packing. In all cases, formation of J-aggre-gates from dimers requires that the molecules in thedimers shift relative to each other.Studying the equilibrium between different forms of anionic cyanine dyes in aqueous solutions made it pos-sible to make a sort of paradoxical conclusion thatblock building of J-aggregates from dimers is mostpreferable for constructing perfect J-aggregates of thesame size (monostructural J-aggregates). Therefore, weare speaking of the nanotechnology of polymethine dyeaggregates.MIXED AND HETEROCONTACTAGGREGATESThe above-mentioned method for constructing J-aggregates from dimers was used for producing mixedJ-aggregates from the dyes with the same or differentpolymethine chain length [10, 13]. Mainly, the processof block building of J-aggregates from different dyemolecules in water includes preformation of dye dimersas a result of shifting the J-aggregate dimer equi-librium towards dimers by heating their solutions andfurther formation of the composite aggregates on cool-ing [10, 13]. When thiatrimethine cyanine D-2 and thi-apentamethine cyanine D-3 solutions are mixed, theabsorption spectrum of the resulting mixture is a sum of the spectra of the parent dyes (Fig. 10, curve 1  ). Whenthe solutions are heated to 50–80  ° C, J-aggregates of theindividual dyes decompose, and the bands appear thatcorrespond to the absorption of the dye dimers (Fig. 10,curve 2 ). On further cooling the solutions, new J-aggre-gated forms involving different molecules are formedfrom dimers (Fig. 10, curve 3 ).The absorption band of the mixed J-aggregates liesin a longer wavelength region as compared to the J-aggregate band of thiatrimethine cyanine and is hypso-chromically shifted relative to the thiapentamethinecyanine J-aggregate absorption. The phenomenondescribed is typical for thiapolymethine cyanines capa-ble of J-aggregation in aqueous solutions and having anextensive π -conjugated system in the heterocyclicrings. Therefore, one can obtain mixed aggregates withthe desired spectral properties by matching dye pairs.Let us mention an important feature of the block mech-anism of constructing mixed J-aggregates. It is knownthat building a mixed J-aggregate from different dyemolecules is extremely difficult, because each dyeforms its own J-aggregate. The dimers acquire a sort of a new property, namely, tolerance of dyes to each other; 500400 λ  , nm2.01.61.20.80.4 1 2 3 4 56 7 7 6  5 4 3 21 A Fig. 5. Absorption spectra of aqueous solutions of thia-monomethine cyanine D-1 ( C  = 10 –5 M) at ( 1 ) T  = 10,( 2 ) 20, ( 3 ) 23, ( 4 ) 25, ( 5 ) 28, ( 6  ) 30, and ( 7  ) 37 ° C [9]. 700600500 λ  , nm1.61.20.80.4  A1 2 3 4 4 3 21 DJ Fig. 6. Absorption spectra of thiatrimethine cyanine D-2 inaqueous solutions at ( 1 ) C  = 1 × 10 –4 M and l = 0.1 cm;( 2 ) C  = 3.3 × 10 –5 M and l = 0.3 cm; ( 3 ) C  = 1 × 10 –5 M and l = 1.0 cm; and ( 4 ) C  = 0.5 × 10 –5 M and l = 2.0 cm [10].
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