Sequential Energy and Electron Transfer in Polyisocyanopeptide

ARTICLE pubs.acs.org/JPCB

Sequential Energy and Electron Transfer in PolyisocyanopeptideBased Multichromophoric Arrays Ya-Shih Huang,*,† Xudong Yang,† Erik Schwartz,‡ Li Ping Lu,†,‡ Sebastian Albert-Seifried,† Chris E. Finlayson,† Matthieu Koepf,‡ Heather J. Kitto,‡ Burak Ulgut,† Matthijs B. J. Otten,‡ Jeroen J. L. M. Cornelissen,‡ Roeland J. M. Nolte,‡ Alan E. Rowan,*,‡ and Richard H. Friend*,† † ‡

Optoelectronics Group, Cavendish Laboratory, JJ Thomson Avenue, Cambridge CB3 0HE, United Kingdom Institute for Molecules and Materials, Radboud University Nijmegen, Heyendaalseweg 135, 6525 AJ Nijmegen, The Netherlands

bS Supporting Information ABSTRACT: We report on the synthesis and detailed photophysical investigation of four model chromophore side chain polyisocyanopeptides: two homopolymers of platinum-porphyrin functionalized polyisocyanopeptides (Pt-porphyrin-PIC) and perylene-bis(dicarboximide) functionalized polyisocyanopeptides (PDI-PIC), and two statistical copolymers with different ratios of Pt-porphyrin and PDI molecules attached to a rigid, helical polyisocyanopeptide backbone. 1H NMR and circular dichroism measurements confirm that our model compounds retain a chiral architecture in the presence of the chromophores. The combination of Pt-porphyrin and PDI chromophores allows charge- and/or energy transfer to happen. We observe the excitation and relaxation pathways for selective excitation of the Pt-porphyrin and PDI chromophores. Studies of photoluminescence and transient absorption on nanosecond and picosecond scales upon excitation of Pt-porphyrin chromophores in our multichromophoric assemblies show similar photophysical features to those of the Pt-porphyrin monomers. In contrast, excitation of perylene chromophores results in a series of energy and charge transfer processes with the Pt-porphyrin group and forms additional charge-transfer states, which behave as an intermediate state that facilitates electronic coupling in these multichromophoric systems.

1. INTRODUCTION Photoinduced energy and charge transfer processes are at the heart of the operation of organic semiconductor devices, such as light-emitting diodes and solar cells. Inspired by many functional assemblies in nature, such as DNA and light-harvesting complexes in photosynthesis,1-3 great efforts have been devoted to creating artificial, highly ordered arrays of chromophores, with the aim of constructing molecular optoelectronics devices which facilitate efficient and directional energy and charge transfer.4-19 Self-assembly of materials using noncovalent interactions, termed as “supramolecular” chemistry,20 is one the most promising approaches for the preparation of functional building blocks. In comparison with covalent synthesis, the thermodynamically controlled self-assembly has the advantages of simplicity and the possibility of self-repair without undesired side products. The design of supramolecular materials usually involves interactions such as hydrogen-bonding, π-π stacking, van der Waals, dipole-dipole, and charge-transfer interactions.21 As a result of these noncovalent interactions, highly ordered supramolecular r 2011 American Chemical Society

structures with sizes of a few nanometers can be constructed. However, it is more challenging to create larger structures of interacting molecules up to the micrometer scale for use in optoelectronics devices. We have previously shown a novel approach to overcome this hurdle by introducing a scaffolding template.22 Utilizing an intrinsically rigid polyisocyanide that adopted a welldefined helical confirmation as a backbone with photoactive chromophores attached to it, we were able to control the spatial position and packing of the chromophoric blocks. It has been demonstrated that there are four repeat units per helical turn for the main carbon chain in polyisocyanides.23 The introduction of peptides brings about the formation of intramolecular hydrogen bonds between the nth and (n þ 4)th chromophores of the polymer to stabilize the scaffold.24 In this way, the adjacent chromophores are closely packed with only 0.4 nm between the nth and (n þ 4)th chromophores.25 Received: July 30, 2010 Revised: November 9, 2010 Published: January 31, 2011 1590

dx.doi.org/10.1021/jp1071605 | J. Phys. Chem. B 2011, 115, 1590–1600

The Journal of Physical Chemistry B

ARTICLE

Scheme 1. Molecular Structures of (a) the Pt-Porphyrin Isocyanide Monomer, (b) the PDI Isocyanide Monomer, (c) the Statistical Copolyisocyanides with Pt-Porphyrin and Perylene Side Groups,a and (d) Schematic Drawing of the Helical Structure of the Statistical Copolymer Pt-Porphyrin-PDI10-PIC, with the Backbone in Blue, the PDI Groups in Red, and the Pt-Porphyrin Groups in Purple

a

R = 1 (Pt-porphyrin-PDI-PIC) and R = 10 (Pt-porphyrin-PDI10-PIC) were used in our study.

Perylenes and porphyrins are interesting supramolecular building blocks. Perylenes and their derivatives are characterized as highly absorbing and photostable materials, the strong fluorescence of which has nearly unity of quantum yield when in dilute solution26 and a long, monoexponential lifetime of around 4 ns.27 Porphyrins are an important class of compounds, the photophysics of which have been extensively investigated.28,29 Metal atoms incorporated at the center of the porphyrin ring may also be used in order to promote intersystem crossing from singlet to triplet manifolds. Work on organic light-emitting diodes and photovoltaics has been exclusively focused on systems combining electron- and hole-accepting groups. The properties of the heterojunctions formed between the two groups are crucial to the device performance. Numerous organic donor-acceptor systems comprised of perylenes or/and porphyrins have been studied in terms of their photophysical properties. One elegant example is a molecule complex consisting of perylene derivatives and oligo(p-phenylene vinylene) (OPV), in which the intermolecular interaction facilitates highly efficient and fast photoinduced charge separation.30-34 Another example is the metalloporphyrin-fullerene ensemble.35-37 The interactions between metallophorphyrins and fullerenes have been shown to form exciplexes.38-43 Motifs combining both porphyrins and perylenes have been widely investigated in terms of their excited-state dynamics.2,12,16,44-56 Energy and charge transfer taking place in these arrays have been studied in light of porphyrin metalation states,47 the linkers,45,46 relative orientations of molecules,50,54 solvent effects,56 and so on. Van der Boom et al. have studied the system of zinc 5,10,15,20-tetrakis(perylenediimide)porphyrin,49 in which molecules were arranged such that

the electron acceptor and photon collector, perylenes, are attached to a central electron donor, zinc porphyrin. Their ultrafast spectroscopy measurements demonstrated that radical ion pairs formed as a result of rapid charge separation and subsequently recombined into perylene triplet states. Work by Prodi et al. rationalized energy and charge transfer processes in detail by selectively exciting either the ruthenium porphyrins or perylene bisimides of the multichromophoric arrays.51 Their results also imply the occurrence of intramolecular sensitizations of perylene triplet states. Here, we report on the synthesis and characterization of four systems consisting of closely stacked arrays based on Ptporphyrins and PDI chromophores. Scheme 1a,b displays the molecular structures of the Pt-porphyrin-isocyanopeptide and perylene-isocyanopeptide monomers, respectively. These monomers were subsequently polymerized to the Pt-porphyrinpolyisocyanopeptide (Pt-porphyrin-PIC) and perylenepolyisocyanopeptide (PDI-PIC), respectively. Scheme 1c demonstrates the chemical structure of statistical copolymers in which Pt-porphyrin and perylene chromophores are randomly attached to the polyisocyanopeptide backbones. The ratios of Pt-porphyrin and PDI chromophores are 1:10 and 1:1 for copolymers Pt-porphyin-PDI10-PIC and Pt-porphyrin-PDI-PIC, respectively. In Scheme 1d, a schematic drawing of the Pt-porphyrin-PDI10-PIC is depicted with the backbone in blue, the perylene groups in red, and the Pt-porphyrin groups in purple. These large multichromophoric arrays can be regarded as alternatives to conjugated polymers and exhibit rich and complex photophysics. In addition, the interface between the multichromophores can be treated as “intramolecular heterojunctions”. We are able to rationalize complex photophysical behaviors 1591

dx.doi.org/10.1021/jp1071605 |J. Phys. Chem. B 2011, 115, 1590–1600


ARTICLE

taking place in these systems using a variety of spectroscopic techniques.

2. EXPERIMENTAL METHODS Materials. Syntheses of the homopolymers were prepared via a nickel-induced polymerization reaction of isocyanide monomers. The Pt-porphyrin appended isocyanide was synthesized by a slightly modified strategy of the synthesis of the free base isocyanoporphyrin.25 A monoamine Pt-porphyrin was coupled to Boc-L-alanine by using standard peptide coupling reagents. The resulting Pt-porphyrin-L-alanine product was formylated using 2,4,5-trichlorophenyl formate. The terminal N-formyl group was subsequently converted into an isocyanide. The synthesis and characterization of the PDI isocyanide monomer have been reported previously.57 The polymerization of isocyanide 1 and 2, Scheme 1a,b, resulted in homopolymers Ptporphyrin-PIC and PDI-PIC, respectively. During the polymerization of the Pt-porphyrin isocyanide 1 to form the polymer Pt-porphyrin-PIC, no clear change in the color was observed. Upon polymerization of the PDI isocyanide 2, however, the reaction mixture turned from yellow to red, which is typical for the aggregation of PDIs and in this particular case is indicative of the polymerization leading to intermolecular aggregation of PDIs. Statistical copolymers were obtained by the statistical copolymerization of monomers 1 and 2 using a feed ratio of 1:10 for Pt-porphytin-PDI10-PIC and of 1:1 for Pt-porphyrinPDI-PIC. The polymers were then purified by repetitive precipitation in methanol/water and subsequently subjected to sizeexclusion chromatography to remove nonreacted monomers from the solution.58 See the Supporting Information for the characterization of the Pt-porphyrin compounds. Optical Spectroscopy. All samples measured using optical spectroscopic techniques were prepared in a glovebox filled with nitrogen and dissolved in chloroform. Films were spin-coated from chloroform solution. UV-vis absorption was recorded by a Hewlett-Packard ultraviolet-visible spectrometer in ambient conditions. We note that, since a serious wetting problem occurred during the spin-coating process, a high extent of aggregates formed in all of the samples in polystyrene matrices and thin films. This led to strong background signals from the scattering detected in the associated absorption spectra. For measurements of integrated photoluminescence spectra and photoluminescence decay dynamics, either of the two Picoquant pulsed diode lasers, exciting at 407 and 470 nm, were used as the excitation source. Steady-state photoluminescence spectra signals were recorded using a Spectra Pro 2500i, a 500 mm focal length spectragraph, and a Pixis 100F CCD camera from Acton, Princeton Instruments. The photoluminescence decay dynamics were measured using a microchannel plate photomultiplier from Hamamatsu, coupled to a monochromator and time-correlated single photon counting (TCSPC) electronics from Edinburgh Instruments.59 The quantum efficiencies of the sample solutions were obtained by comparing the emission and optical density of an emitter with known efficiency, Rhodamine 6G in ethanol, with excitation at 470 nm.60 Quantum efficiencies obtained using this method are consistent with the result of measurements in nitrogen atmosphere using an integrating sphere setup in Cambridge Display Technology Ltd., Cambridge, with excitations of 325 and 440 nm.61 The setup of steady-state photoinduced absorption62 and femtosecond transient absorption63 have been described in detail elsewhere.

Figure 1. Normalized CD spectra of the Pt-porphyrin and PDI homopolymers and statistical copolymers (CHCl3; 10-5 M).

Figure 2. Visualization of HOMO and LUMO energies of Ptporphyrin-PIC and PDI-PIC polymers. The HOMO of Ptporphyrin-PIC and the LUMO of PDI-PIC were measured by cyclic voltammetry and their corresponding LUMO and HOMO are estimated with these values and the π-π* gap.

3. STRUCTURAL CHARACTERIZATION 1 H NMR spectroscopy showed strong broadening of all of the proton signals after polymerization of the isocyanide monomers, which suggests that the polymers have a rigid structure. In the infrared spectrum, the signal of the isocyanide peak at 2140 cm-1 had disappeared. Analogues to the related isocyanopeptides,25,57,64 the NH stretch and the amide I vibration, had shifted to lower wavenumbers upon polymerization of the isocyanide monomers, indicating the presence of a hydrogen bonding network in the side chains of the polymers. The L-alanine unit in the side chains can induce a helical conformation of the polymers and therefore circular dichroism (CD) serves as a powerful tool to analyze the chromophoric arrangement of the polymers. In Figure 1, the CD spectrum of Pt-porphyrin-PIC resembles that of the free base porphyrin functionalized polyisocyanide.25 The intense bisignate signal at 402 nm is, however, blue-shifted as compared to the free base analogue (431 nm) and is ascribed to the presence of the platinum. This bisignate signal stems from coupling interactions between the first and fifth Pt-porphyrin molecules in the 1592



ARTICLE

Figure 3. (a) Normalized absorption (solid lines) and emission (dashed lines) spectra of Pt-porphyrin monomer in chloroform solutions (blue), and Pt-porphyrin-PIC polymer in chloroform solutions (red) and films (green). (b) Normalized phosphorescence decays of Pt-porphyrin-PIC in polystyrene matrices upon excitation at 407 nm. Multiexponential fits are shown in red solid and dashed lines for triplet aggregates and triplet excitons, respectively. (c) Photoinduced absorption spectra of Pt-porphyrin-PIC films recorded at a modulation frequency of 224 Hz at 28 K upon excitation at 514 nm. The black circles correspond to the signal out-of-phase with the pump light, and the red circles correspond to the in-phase signal.81 (d) Femtosecond transient absorption spectrum of Pt-porphyrin-PIC in polystyrene matrices. (e) Absorption transients of Pt-porphyrin-PIC probed at 730 nm measured at different pump fluences indicated in the graph upon excitation at 400 nm. Curves are guides to the eye.

polymer backbone, which are located on top of each other. The unique CD spectrum of PDI-PIC displays exciton coupled vibrations between 420 and 600 nm and is different from any other signals observed for chiral PDI stacks.57 The small Cotton effect at 310 nm originates from the imine n-π* group of the backbone and reflects the robustness of the isocyano-Lalanine backbone, which is not influenced by the introduction of chromophores. We note that no CD signal was detected for either of the monomers. For Pt-porphyrin-PDI10-PIC, a similar spectrum was observed as for PDI-PIC. The incorporation of the small amount of Pt-porphyrin apparently does not influence the helical arrangement of the PDIs and might indicate that the chirality is primarily expressed in homoblocks. The characteristic CD signals of both the PDIs and Pt-porphyrin functions were observed in the CD

spectrum of Pt-porphyrin-PDI-PIC, although the signals in the 350-450 nm regions were somewhat shifted and slightly different in shape.

4. CHARACTERIZATION OF PHOTOPHYSICAL PROPERTIES 4.1. Energy Levels. In Figure 2, we present the relative positions of HOMO and LUMO levels of the Pt-porphyrinPIC and PDI-PIC homopolymers. Both the HOMO and LUMO levels of Pt-porphyrin-PIC are higher than those of PDIPIC, leading to an interface that may support charge-transfer excitation. In addition, the band gap of Pt-porphyrinPIC is greater than that of PDI-PIC. We therefore expect 1593



ARTICLE

Figure 4. Photophysical processes of (a) Pt-porphyrin-PIC and (b) PDI-PIC.

energy transfer from Pt-porphyrin to PDI molecules to occur in the copolymers. 4.2. Photophysics of Homopolymers. 4.2.1. PtPorphyrin-PIC Homopolymer. As shown in Figure 3a, a sharp, intense Soret absorption at 405 nm together with a weak Q-band at 513 nm were observed for both monomer and polymer solutions. A phosphorescence emission, induced by the strong spin-orbit coupling present in Pt-porphyrin-PIC, was detected at 680 nm with a shoulder at around 745 nm. The phosphorescence band peaking at the higher energy (∼680 nm) is commonly observed for conventional Pt-porphyrin derivatives and is due to triplet excitons, while the additional phosphorescence shoulder (peaking at 745 nm) is likely due to the formation of triplet aggregates.29,65-69 Positions of absorption and emission bands remain the same from the monomer to the polymer with only slight changes in the band widths and relative intensities, suggesting that the Pt-porphyrin-PIC in diluted solution arranges in a conformation in which exciton coupling is very weak between chromophores. The exciton coupling in Pt-porphyrin-PIC is enhanced by interchain interactions as can be concluded from the comparison of spectra of diluted solution and spun-cast thin films. In the case of thin films, the Soret absorption is slightly red-shifted and the Q-band is broadened with respect to the solution. The emission of the films is also characterized by phosphorescence from excitons and aggregates, similar to the molecularly dissolved case. However, the triplet aggregation band turns out to be the most prominent emission in the films while the triplet excitons dominate the emission spectrum in the case of solutions. The phosphorescence of triplet excitons and aggregates both show multiexponential decay dynamics, albeit with different lifetime components, Figure 3b. The faster decay of triplet excitons at short times could be an indication of energy migration between the two emissive states. A broad triplet absorption band extending to 1000 nm is detected in the semisteady-state photoinduced absorption

spectrum at low temperatures, Figure 3c. The linear excitation intensity dependence of the photoinduced absorption indicates that the decay process is monomolecular. Figure 3d shows the femto second transient absorption spectrum of the Ptporphyrin-PIC in polystyrene matrices. The broad band observed here is essentially a combination of S1 and T1 excited states absorption. Both of the species have lifetimes longer than we were able to record with this setup (∼2 ns). The intensity dependence of the absorption transients probed at 730 nm, Figure 3e, demonstrates that dynamics at different pump fluences are all similar, suggesting that no annihilation phenomenon takes place in Pt-porphyrin-PIC on picosecond time scales. The photophysical deactivation pathways in the Ptporphyrin-PIC homopolymer are summarized in Figure 4a. Upon excitation of the Pt-porphyrin-PIC, Pt-porphyrin singlet excitons undergo rapid intersystem crossing due to spin-orbit coupling induced by the presence of the heavy metal platinum. They subsequently result in the triplet excitons, 3 *Ptexciton, and triplet aggregates, 3*Ptagg. Both the triplet excitons and triplet aggregates can decay radiatively and give rise to phosphorescence. Since triplet aggregates are lower in energy with respect to triplet excitons, energy transfer can occur from the latter to the former. 4.2.2. PDI-PIC Homopolymer. The photophysical and electrical studies of PDI-PIC have been reported previously.57,70-72 It has been suggested from absorption spectra that transition dipole moments of perylene chromophores in the PDI-PIC stack in a face-to-face orientation and the molecular-dynamics studies showed efficient intramolecular electron transport along the polymer direction in comparison to single crystals of small molecules such as the oligoacenes. Its PL spectrum shows a broad and structureless aggregation band centered at 625 nm, which has a long lifetime of 25 ns. While the perylene monomer has a photoluminescence quantum yield of nearly 100% in solution, the photoluminescence quantum yield of PDI-PIC was measured to be 57%, strongly quenched due to the formation of the 1594



ARTICLE

Figure 5. Time-resolved transient absorption spectra of PDI-PIC in chloroform solution at an excitation wavelength of 492 nm.

aggregates. As is shown in Figure 5, the transient absorption spectrum of PDI-PIC in chloroform solutions can be characterized by three bands: (1) the ground state bleach below 560 nm, (2) a broad excited-state photoinduced absorption band extending from 560 to 800 nm due to strong interactions between adjacent excited perylene molecules on the stack, and (3) a typical absorption band of perylene radical anion peaking at 767 nm.73 The perylene anion band in our PDI-PIC may be attributed to charges that are trapped in some low-energy sites of the aggregates or due to an intramolecular PDIþ-PDI- state as a result of charge transfer between two cofacial PDI stacks via excited state symmetry breaking.74,75 The similar time evolution of the perylene anion band and the broad excited-state absorption is an implication of the correlation of aggregate excitons and the charge-transfer band. The anisotropy data reported previously reveal fast energy transfer and depolarization processes in PDI-PIC due to its compact helical structure, which promotes rapid exciton migration along the chiral arrangement of perylene stacks.57 We illustrate the photophysical pathways in the PDI-PIC homopolymer in Figure 4b: the perylene aggregates form as a result of photoexcitation and give rise to fluorescence at 625 nm. We note that the dense stack induced by the polyisocyanopeptides appears to have greater influence on the photophysical properties of PDI-PIC than on the Pt-porphyrin-PIC. Enhanced electronic coupling in the Pt-porphyrin-PIC is only observed in films, where additional interchain interactions are possible. 4.3. Photophysics of Statistical Copolymers. 4.3.1. Absorption of Statistical Copolymers. In Figure 6b,c, we show that the absorption characteristics of the statistical copolymers are essentially mixtures of those of homopolymeric materials, Figure 6a,b. The Soret band absorption originating from Pt-porphyrin molecules red shifts with increasing ratios of perylene chromophores in the copolymers, while those bands due to perylene molecules remain at the same position. Furthermore, we show that the Pt-porphyrin molecules primarily absorb at around 410 nm, where only a small fraction of this excitation goes to perylene molecules. On the other hand, the majority of excitation at around 490 nm will be of the perylene molecules. Thus, we are able to make selective excitations directly to either the Pt-porphyrin or the perylene molecules, while keeping the electronic subsystems almost unperturbed unless through charge

Figure 6. Normalized absorption (solid-dot lines) and emission spectra upon excitation at 407 nm (solid lines) and 470 nm (circles) for (a) Pt-porphyrin-PIC, (b) Pt-porphyrin-PDI-PIC, (c) Ptporphyrin-PDI10-PIC, and (d) PDI-PIC in polystyrene matrices.

Figure 7. Relative phosphorescence of Pt-porphyrin-PIC and the two copolymers in polystyrene matrices probed at 675 nm as a function of 1000/T upon excitation at 407 nm. The phosphorescence maxima of each spectrum at various temperatures are normalized to that of corresponding materials at 292 K.

or/and energy transfer. The emission spectra and excited-state dynamics of the copolymers are thus investigated with excitations at 407 and 470 nm. 4.3.2. Direct Excitation of Pt-Porphyrin. As indicated by the solid lines in Figure 6b,c, emission spectra of both of the copolymers upon excitation of Pt-porphyrin chromophores exhibit similar phosphorescence features to that of the Ptporphyrin-PIC, while the fluorescence character arising from PDI molecules is negligible. The identical emission feature of the copolymers and the Pt-porphyrin-PIC homopolymer suggests that insignificant electronic coupling takes place between Ptporphyrin and PDI in their excited states when Pt-porphyrin molecules are directly excited. Similar phosphorescence decay 1595



ARTICLE

Figure 8. Time-resolved transient absorption spectra of Pt-porphyrin-PDI-PIC (upper panels) and of Pt-porphyrin-PDI10-PIC (lower panels) in polystyrene matrices upon excitations at (a) 400 nm and (b) 492 nm. The spectra of (a) are smoothed using a fast Fourier transform filter (OriginPro 8 SR2, Originlabs Inc.).

kinetics are observed in the two copolymers: a rapid decay at short times followed by a long-lived component, which can be fitted by a monoexponential model as 21 μs for triplet excitons and as 24 μs for triplet aggregates. In comparison with the Pt-porphyrin-PIC homopolymer, whose phosphorescence lifetime is measured as 19 μs, we find that the presence of PDI chromophores appears to have an insignificant influence in the evolution of Pt-porphyrin phosphorescence. Figure 7 compares the relative phosphorescence of the Ptporphyrin-PIC and copolymers under an excitation at 407 nm as a function of the inverse temperature. It is surprising to see that phosphorescence of these three materials reduces with decreasing temperature. In addition, phosphorescence intensities of these three materials reveal similar activation energies of tens of meV for temperatures above 150 K. Furthermore, they exhibit identical phosphorescence lifetimes, regardless of temperatures. These behaviors can be explained by a two-level model:76 (1) a lowerenergy, nonemissive level with a short lifetime and (2) a higherenergy, emissive level with a long lifetime. At a low temperature, the excitation can only produce nonemissive excitons with a short lifetime at the lower-energy level. Upon increasing the temperature, thermal activation could promote excitons to a higher level, giving rise to a higher phosphorescence intensity with a long lifetime. This mechanism can account for the increasing intensity of phosphorescence while remaining the lifetimes the same with increasing temperature. The similar activation energy and identical phosphorescence lifetime observed for the two copolymers and the Pt-porphyrin homopolymer again confirm that the phosphorescence emission in the copolymers is not disturbed by the presence of PDI molecules. The femtosecond transient absorption spectra of the copolymers in the solid state solutions upon excitation of the Pt-porphyrin,

shown in Figure 8a, are dominated by a broad band ranging from 600 to 800 nm, resulting from a combination of the Pt-porphyrin triplet and PDI excited-state absorption. 4.3.3. Direct Excitation of PDI. In the case of the Ptporphyrin-PDI-PIC, circles in Figure 6b, the emission spectrum upon excitation at 470 nm is characterized with the monomer emission at 537 and 580 nm77 together with perylene aggregate fluorescence at 630 nm. In addition, a broad phosphorescence band with a shoulder at red wavelengths confirms the occurrence of porphyrin triplets. In the case of the Ptporphyrin-PDI10-PIC, Figure 6c, the emission spectrum is dominated by the perylene aggregates. Although it is not clearly identified in the steady-state emission spectrum, the Pt-porphyrin phosphorescence is overlapped with the perylene aggregation band in the time-resolved emission spectra. The presence of Ptporphyrin phosphorescence upon direct excitation of the perylene chromophores may be due either to some transfer mechanism from perylene to Pt-porphyrin molecules or to a fraction of absorption by isolated Pt-porphyrin molecules. Figure 9 shows that the decays of the perylene aggregate emission in the two copolymers become multiexponential and much shorter-lived with respect to their counterpart in the PDI-PIC homopolymer, which has shown to decay monoexponentially with a lifetime of 25 ns. We consider the process involved to account for the rapid, multiexponential decays of the aggregate emission and the presence of the Ptporphyrin phosphorescence upon excitation of PDIs to be charge transfer since energy transfer from PDI to Pt-porphyrin is not energetically favorable, as is suggested from the energy diagram in Figure 2. Figure 8b displays the transient absorption spectra evolution of the copolymers upon excitation at 470 nm. In comparison with the spectra obtained upon excitation at 407 nm, Figure 8a, 1596



Figure 9. Comparisons of photoluminescence decays detected at 625 nm for model compounds indicated in the graph in polystyrene matrices. The excitation was 470 nm.

we observe similar features of transient absorption spectra characteristics and dynamics of the copolymers regardless of which type of the chromophores is photoexcited. However, a distinct perylene radical anion band centered at 775 nm is only observed when the perylene chromophores are mainly excited. In the case of the Pt-porphyrin-PDI10-PIC, this radical anion band is likely due to PDI aggregates,57,78 and/or to chargetransfer states Ptþ-PDI-, with cations on the Pt-porphyrins and anions on the perylenes, induced by interactions between multichromophores. In the case of the Pt-porphyrin-PDIPIC, it is likely that the interactions between multichromophores are primarily responsible for this charge-transfer band, Ptþ-PDI-, since although the perylene aggregation is weaker in this case because of a lower ratio of perylene chromophores, the relative intensity of this perylene anion band is as strong as that observed in the Pt-porphyrin-PDI10-PIC. Figure 10 shows the transient absorption dynamics of the radical anion bands of the PDI-PIC and of the copolymers in chloroform solutions. The absorption of perylene anions in the Pt-porphyrin-PDI10-PIC exhibits a lifetime similar to that of the PDI-PIC. This indicates that the radical anions observed in the copolymer Pt-porphyrin-PDI10-PIC solution are very similar to those localized charges observed in the PDI-PIC homopolymer solutions, which are caused by ionization of excitons as a result of perylene aggregations. The radical anions in the copolymer Pt-porphyrin-PDI-PIC, which has a more balanced ratio of Pt-porphyrin and perylene chromophores, are much longer-lived in comparison with respect to the PDI-PIC and Pt-porphyrin-PDI10-PIC. The presence of much longer-lived charges in the Pt-porphyrin-PDI-PIC confirms that the formation of charge-transfer states associated with the interaction between multichromophores, Ptþ-PDI-, is significantly stabilized with respect to anions induced only by perylene aggregates.

5. DISCUSSIONS The photophysical processes taking place in our multichromophoric arrays are summarized in Figure 11. Excited states found in the copolymers are the combinations of those

ARTICLE

Figure 10. Decay kinetics of the radical anion band at 770 nm of PDI-PIC, Pt-porphyrin-PDI10-PIC, and Pt-porphyrin-PDIPIC in chloroform solutions. The excitation was at 532 nm with a fluence of 2 1015 photons/cm2.

present in the homopolymers with an additional state of the Ptþ-PDI- charge-transfer states. The energy of this chargetransfer state, Ptþ-PDI-, is roughly estimated by the offset of the HOMO level of Pt-porphyrin-PIC and LUMO level of PDI-PIC. Upon excitation of Pt-porphyrin chromophores, the deactivation pathways of the copolymers are similar to those of the Pt-porphyrin-PIC homopolymer and the presence of perylene molecules does not seem to disturb the photophysics in this case. The reason for the absence of the charge-transfer states is that the energy difference between the charge-transfer states Ptþ-PDIand the Pt-porphyrin triplet states is not large enough to overcome the binding energy and thus the excitons are stabilized in Pt-porphyrin triplet states and do not form the chargetransfer states in this case. Direct excitations of PDI chromophores result in more complex sequential energy and charge transfer in the copolymers. The excitation absorbed by perylene chromophores leads to singlet fluorescence of isolated perylene molecules at 535 nm and perylene aggregates at 625 nm. These singlet excitons may also form charge-transfer states. The charge-transfer state, PtþPDI-, can either deactivate to the ground state or go further to yield Pt-porphyrin triplets, which are at a similar energy level to Ptþ-PDI-, via intersystem crossing.55,79,80 The triplet states of the PDIs have been reported to have an energy as low as 1.2 eV,27 much lower than the energy of the charge-transfer state Ptþ-PDI-. The Ptþ-PDI- state would then be expected to decay to the perylene triplet states;63 however, we have observed this charge-transfer state to be very long-lived, beyond 10 μs, as is shown in Figure 10. We would also expect such a low-lying PDI triplet to quench the phosphorescence from nearby Pt-porphyrin units.81 For these reasons, we consider that the PDI triplet may have a higher energy, and if lower than the Ptþ-PDI- charge-transfer state and Pt-porphyrin triplet states, by no more than a few kBT so that endothermic back-transfer would occur. We note that we have some evidence for the presence of the phosphorescence from the Pt-porphyrin upon excitation of PDIs (Figure 6) that may indicate that a decay mechanism for the Ptþ-PDI- charge-transfer state is by conversion to the Pt-porphyrin triplet (noting that this has a similar lifetime as the charge-transfer state). 1597



ARTICLE

Figure 11. Photophysical pathways in copolymers. Blue, direct excitation of Pt-porphyrin chromophores; green, direct excitation of PDI chromophores.

6. CONCLUSIONS We have studied engineered arrays of chromophores in supramolecular assemblies with the potential for significant control over the energy and charge transfer processes in these systems. We have synthesized multichromophoric systems consisting of the Pt-porphyrins and PDIs, which are closely packed in a well-defined manner. Isocyanides exposing perylene and Pt-porphyrin functionalities could be prepared by alanine coupling to a chromophore derivatized amine and subsequent conversion into an isocyanide. The homo- and statistical copolymers used in this study were obtained after nickel induced polymerization of the chromophoric appended isocyanides. The resulting polymers are closely packed in a well-defined conformation, with CD studies revealing that the chromophores are in a helical arrangement. We have shown previously that such an architecture could enable strong electronic coupling and promote rapid transfer and depolarization mechanisms.57 Upon excitation of Pt-porphyrin molecules, rapid intersystem crossing induced by spin-orbit coupling gives rise to phosphorescence. On the other hand, excitation of the perylene chromophores leads to the formation of Ptþ-PDI- charge-transfer states, which facilitates a series of complex processes and enables the multichromophoric arrays to be electronically coupled. The absence of this chargetransfer state is not observed upon excitation of Pt-porphyrin molecules, which is due to energetic constraints. Our studies highlight the importance of the charge-transfer states in conjugated polymers, such as our polyisocyanopeptides based multichromophores or polyfluorenes, which may have both advantageous and detrimental consequences to device performance. A full understanding of these interfacial states will enable us to manipulate these electronic excitations and design advanced materials for novel device applications. ’ ASSOCIATED CONTENT

bS

Supporting Information. Full description of the syntheses and atomic force microscopy image of the materials.

This material is available free of charge via the Internet at http://pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected] (Y.S.H.); [email protected] (R.H.F.); [email protected] (A.E.R.).

’ ACKNOWLEDGMENT This work is supported by ESF-SONS-BIONICS and SUPERMATES. We thank Prof. Neil C. Greenham, Dr. Jessica M. Winfield, and Dr. Sebastian Westenhoff for valuable discussions. EPSRC, the Technology Foundation STW, the council for the Chemical Sciences of The Netherlands Organization for Scientific Research, and the Royal Academy for Arts and Sciences are acknowledged for financial support. ’ REFERENCES (1) K€uhlbrandt, W.; Wang, D. N. Nature 1991, 350, 130–134. (2) O’Neil, M. P.; Niemczyk, M. P.; Svec, W. A.; Gosztola, D.; Gaines, G. L.; Wasielewski, M. R. Science 1992, 257, 63–65. (3) Pullerits, T.; Sundstr€om, V. Acc. Chem. Res. 1996, 29, 381– 389. (4) Hoeben, F. J. M.; Jonkheijm, P.; Meijer, E. W.; Schenning, A. P. H. J. Chem. Rev. 2005, 105, 1491–1546. (5) Lawrence, D. S.; Jiang, T.; Levett, M. Chem. Rev. 1995, 95, 2229–2260. (6) Burrell, A. K.; Officer, D. L.; Plieger, P. G.; Reid, D. C. W. Chem. Rev. 2001, 101, 2751–2796. (7) Greig, L. M.; Philp, D. Chem. Soc. Rev. 2001, 30, 287–302. (8) Elemans, J. A. A. W.; van Hameren, R.; Nolte, R. J. M.; Rowan, A. E. Adv. Mater. 2006, 18, 1251–1266. (9) Li, X.; Sinks, L. E.; Rybtchinski, B.; Wasielewski, M. R. J. Am. Chem. Soc. 2004, 126, 10810–10811. (10) Ahrens, M. J.; Sinks, L. E.; Rybtchinski, B.; Liu, W.; Jones, B. A.; Giaimo, J. M.; Gusev, A. V.; Goshe, A. J.; Tiede, D. M.; Wasielewski, M. R. J. Am. Chem. Soc. 2004, 126, 8284–8294. 1598


The Journal of Physical Chemistry B (11) Sautter, A.; Kaleta, B. K.; Schmid, D. G.; Dobrawa, R.; Zimine, M.; Jung, G.; van Stokkum, I. H. M.; Cola, L. D.; Williams, R. M.; W€urthner, F. J. Am. Chem. Soc. 2005, 127, 6719–6729. (12) Hasselman, G. M.; Watson, D. F.; Stromberg, J. R.; Bocian, D. F.; Holten, D.; Lindsey, J. S.; Meyer, G. J. J. Phys. Chem. B 2006, 110, 25430–25440. (13) Kobuke, Y. Eur. J. Inorg. Chem. 2006, 2006, 2333–2351. (14) Muthukumaran, K.; Loewe, R. S.; Kirmaier, C.; Hindin, E.; Schwartz, J. K.; Sazanovich, I. V.; Diers, J. R.; Bocian, D. F.; Holten, D.; Lindsey, J. S. J. Phys. Chem. B 2003, 107, 3431–3442. (15) Loewe, R. S.; Tomizaki, K.; Youngblood, W. J.; Bo, Z. S.; Lindsey, J. S. J. Mater. Chem. 2002, 12, 3438–3451. (16) Tomizaki, K.-Y.; Loewe, R. S.; Kirmaier, C.; Schwartz, J. K.; Retsek, J. L.; Bocian, D. F.; Holten, D.; Lindsey, J. S. J. Org. Chem. 2002, 67, 6519–6534. (17) W€urthner, F.; Thalacker, C.; Sautter, A.; Sch€artl, W.; Ibach, W.; Hollricher, O. Chem.—Eur. J. 2000, 6, 3871–3886. (18) Kim, D.; Osuka, A. Acc. Chem. Res. 2004, 37, 735–745. (19) Schwartz, E.; Le Gac, S.; Cornelissen, J. J. L. M.; Nolte, R. J. M.; Rowan, A. E. Chem. Soc. Rev. 2010, 39, 1576–1599. (20) Lehn, J.-M. Angew. Chem., Int. Ed. 1988, 27, 89–112. (21) Pollino, J. M.; Weck, M. Chem. Soc. Rev. 2005, 34, 193–207. (22) Hernando, J.; de Witte, P. A. H.; van Dijk, E. M. H. P.; Korterik, J.; Nolte, R. J. M.; Rowan, A. E.; García-Parajo, M. F.; van Hulst, N. F. Angew. Chem., Int. Ed. 2004, 116, 4137–4141. (23) Millich, F. Chem. Rev. 1972, 72, 101–113. (24) Cornelissen, J. J. L. M.; Graswinckel, W. S.; Adams, P. J. H. M.; Nachtegaal, G. H.; Kentgens, A. P. M.; Sommerdijk, N. A. J. M.; Nolte, R. J. M. J. Polym. Sci., Part A: Polym. Chem. 2001, 39, 4255– 4264. (25) de Witte, P. A. J.; Castriciano, M.; Cornelissen, J. J. L. M.; Monsu-Scolaro, L.; Nolte, R. J. M.; Rowan, A. E. Chem.—Eur. J. 2003, 9, 1775–1781. (26) Ebeid, E. Z. M.; El-Daly, S. A.; Langhals, H. J. Phys. Chem. 1988, 92, 4565–4568. (27) Ford, W. E.; Kamat, P. V. J. Phys. Chem. 1987, 91, 6373–6380. (28) Gouterman, M. J. Mol. Spectrosc. 1961, 6, 138–163. (29) Gouterman, M. The Porphyrins; Academic Press: New York, 1978. (30) W€urthner, F.; Chen, Z.; Hoeben, F. J. M.; Osswald, P.; You, C.-C.; Jonkheijm, P.; v. Herrikhuyzen, J.; Schenning, A. P. H. J.; van der Schoot, P. P. A. M.; Meijer, E. W.; Beckers, E. H. A.; Meskers, S. C. J.; Janssen, R. A. J. J. Am. Chem. Soc. 2004, 126, 10611–10618. (31) Ramos, A. M.; Meskers, S. C. J.; Beckers, E. H. A.; Prince, R. B.; Brunsveld, L.; Janssen, R. A. J. J. Am. Chem. Soc. 2004, 126, 9630–9644. (32) Peeters, E.; van Hal, P. A.; Meskers, S. C. J.; Janssen, R. A. J.; Meijer, E. W. Chem.—Eur. J. 2002, 8, 4470–4474. (33) Neuteboom, E. E.; Meskers, S. C. J.; van Hal, P. A.; van Duren, J. K. J.; Meijer, E. W.; Janssen, R. A. J.; Dupin, H.; Pourtois, G.; Cornil, J.; Lazzaroni, R.; Bredas, J.-L.; Beljonne, D. J. Am. Chem. Soc. 2002, 125, 8625–8638. (34) Beckers, E. H. A.; Meskers, S. C. J.; Schenning, A. P. H. J.; Chen, Z.; W€urthner, F.; Marsal, P.; Beljonne, D.; Cornil, J.; Janssen, R. A. J. J. Am. Chem. Soc. 2006, 128, 649–657. (35) Boyd, P. D. W.; Reed, C. A. Acc. Chem. Res. 2005, 38, 235–242. (36) Tashiro, K.; Aida, T. Chem. Soc. Rev. 2007, 36, 189–197. (37) Bonifazi, D.; Enger, O.; Diederich, F. Chem. Soc. Rev. 2007, 36, 390–414. (38) Kesti, T. J.; Tkachenko, N. V.; Vehmanen, V.; Yamada, H.; Imahori, H.; Fukuzumi, S.; Lemmetyinen, H. J. Am. Chem. Soc. 2002, 124, 8067–8077. (39) Sutton, L. R.; Scheloske, M.; Pirner, K. S.; Hirsch, A.; Guldi, D. M.; Gisselbrecht, J.-P. J. Am. Chem. Soc. 2004, 126, 10370–10381. (40) Chukharev, V.; Tkachenko, N. V.; Efimov, A.; Guldi, D. M.; Hirsch, A.; Scheloske, M.; Lemmetyinen, H. J. Phys. Chem. B 2004, 108, 16377–16385. .; Echegoyen, (41) Li, K.; Schuster, D. I.; Guldi, D. M.; Herranz, M. A L. J. Am. Chem. Soc. 2004, 126, 3388–3389.

ARTICLE

(42) D’Souza, F.; Maligaspe, E.; Karr, P. A.; Schumacher, A. L.; Ojaimi, M. E.; Gros, C. P.; Barbe, J.-M.; Ohkubo, K.; Fukuzumi, S. Chem.—Eur. J. 2008, 14, 674–681. (43) Hosseini, A.; Taylor, S.; Accorsi, G.; Armaroli, N.; Reed, C. A.; Boyd, P. D. W. J. Am. Chem. Soc. 2006, 128, 15903–15913. (44) Kerp, H. R.; Donker, H.; Koehorst, R. B. M.; Schaafsma, T. J.; van Faassen, E. E. Chem. Phys. Lett. 1998, 298, 302–308. (45) Yang, S. I.; Lammi, R. K.; Prathapan, S.; Miller, M. A.; Seth, J.; Diers, J. R.; Bocian, D. F.; Lindsey, J. S.; Holten, D. J. Mater. Chem. 2001, 11, 2420–2430. (46) Prathapan, S.; Yang, S. I.; Seth, J.; Miller, M. A.; Bocian, D. F.; Holten, D.; Lindsey, J. S. J. Phys. Chem. B 2001, 105, 8237–8248. (47) Yang, S. I.; Prathapan, S.; Miller, M. A.; Seth, J.; Bocian, D. F.; Lindsey, J. S.; Holten, D. J. Phys. Chem. B 2001, 105, 8249–8258. (48) Ambroise, A.; Kirmaier, C.; Wagner, R. W.; Loewe, R. S.; Bocian, D. F.; D, D. H.; Lindsey, J. S. J. Org. Chem. 2002, 67, 3811–3826. (49) van der Boom, T.; Hayes, R. T.; Zhao, Y.; Bushard, P. J.; Weiss, E. A.; Wasielewski, M. R. J. Am. Chem. Soc. 2002, 124, 9582–9590. (50) Hayes, R. T.; Walsh, C. J.; Wasielewski, M. R. J. Phys. Chem. A 2004, 108, 3253–3260. (51) Prodi, A.; Chiorboli, C.; Scandola, F.; Iengo, E.; Alessio, E.; Dobrawa, R.; W€urthner, F. J. Am. Chem. Soc. 2005, 127, 1454–1462. (52) You, C.-C.; W€urthner, F. Org. Lett. 2004, 6, 2401–2404. (53) Xiao, S.; El-Khouly, M. E.; Li, Y.; Gan, Z.; Liu, H.; Jiang, L.; Araki, Y.; Ito, O.; Zhu, D. J. Phys. Chem. B 2005, 109, 3658–3667. (54) Kelley, R. F.; Shin, W. S.; Rybtchinski, B.; Sinks, L. E.; Wasielewski, M. R. J. Am. Chem. Soc. 2007, 129, 3173–3181. (55) Ahrens, M. J.; Kelley, R. F.; Dance, Z. E. X.; Wasielewski, M. R. Phys. Chem. Chem. Phys. 2007, 9, 1469–1478. (56) Ghirotti, M.; Chiorboli, C.; You, C.-C.; W€urthner, F.; Scandola, F. J. Phys. Chem. A 2008, 115, 3376–3385. (57) Schwartz, E.; Palermo, V.; Finlayson, C. E.; Huang, Y.-S.; Otten, M. B. J.; Liscio, A.; Trapani, S.; Gonzalez-Valls, I.; Brocorens, P.; Cornelissen, J. J. L. M.; Peneva, K.; M€ullen, K.; Spano, F.; Yartsev, A.; Westenhoff, S.; Friend, R. H.; Beljonne, D.; Nolte, R. J. M.; Samori, P.; Rowan, A. E. Chem.—Eur. J. 2009, 15, 2536–2547. (58) The rod-like nature of the purified polymers prevented us from obtaining information about their molecular weight and polydispersity by means of the usual techniques, such as mass or GPC. For an estimation of the molecular weight, see ref 57 as well as the SI for AFM images of the polymers. (59) Silva, C.; Russell, D. M.; Dhoot, A. S.; Herz, L. M.; Daniel, C.; Greenham, N. C.; Arias, A. C.; Setayesh, S.; M€ullen, K.; Friend, R. H. J. Phys.-Condens. Matter 2002, 14, 9803–9824. (60) Lackowicz, J. R. Principles of Fluorescence Spectroscopy; Kluwer Academic/Plenum Publishers: New York, 1999. (61) de Mello, J. C.; Wittmann, H. F.; Friend, R. H. Adv. Mater. 1997, 9, 230–232. (62) Ford, T. A.; Avilov, I.; Beljonne, D.; Greenham, N. C. Phys. Rev. B 2005, 71, 125212. (63) Westenhoff, S.; Howard, I. A.; Hodgkiss, J. M.; Kirov, K. R.; Bronstein, H. A.; Williams, C. K.; Greenham, N. C.; Friend, R. H. J. Am. Chem. Soc. 2008, 130, 13653–13658. (64) Cornelissen, J. J. L. M.; Donners, J. J. J. M.; de Gelder, R.; Graswinckel, W. S.; Metselaar, G. A.; Rowan, A. E.; Sommerdijk, N. A. J. M.; Nolte, R. J. M. Science 2001, 293, 676–680. (65) Kalinowski, J.; Stampor, W.; Szmytkowski, J.; Cocchi, M.; Virgili, D.; Fattori, V.; Marco, P. D. J. Chem. Phys. 2005, 122, 154710. (66) Mezyk, J.; Kalinowski, J.; Meinardi, F.; Tubino, R. Appl. Phys. Lett. 2005, 86, 111916. (67) Dienel, T.; Proehl, H.; Fritz, T.; Leo, K. J. Lumin. 2004, 110, 253–257. (68) Bansal, A. K.; Holzera, W.; Penzkofera, A.; Tsuboi, T. Chem. Phys. 2006, 330, 118–129. (69) Khairutdinov, R. F.; Serpone, N. J. Phys. Chem. B 1999, 103, 761–769. (70) Finlayson, C. E.; Friend, R. H.; Otten, M. B. J.; Schwartz, E.; Cornelissen, J. J. L. M.; Nolte, R. L. M.; Rowan, A. E.; Samori, P.; 1599



ARTICLE

Palermo, V.; Liscio, A.; Peneva, K.; M€ullen, K.; Trapani, S.; Beljonne, D. Adv. Func. Mater. 2008, 18, 3947–3955. (71) Foster, S.; Finlayson, C. E.; Keivanidis, P. E.; Huang, Y.-S.; Hwang, I.; Friend, R. H.; Otten, M. B. J.; Lu, L.-P.; Schwartz, E.; Nolte, R. J. M.; Rowan, A. E. Macromolecules 2009, 42, 2023–2030. (72) Dabirian, R.; Palermo, V.; Liscio, A.; Schwartz, E.; Otten, M. B. J.; Finlayson, C. E.; Treossi, E.; Friend, R. H.; Calestani, G.; M€ullen, K.; Nolte, R. J. M.; Rowan, A. E.; Samori, P. J. Am. Chem. Soc. 2009, 131, 7055–7063. (73) Salbeck, J. J. Electroanal. Chem. 1992, 340, 169–195. (74) Giaimo, J. M.; Lockard, J. V.; Sinks, L. E.; Scott, A. M.; Wilson, T. M.; Wasielewski, M. R. J. Phys Chem. A 2008, 112, 2322–2330. (75) Fuller, M. J.; Sinks, L. E.; Rybtchinski, B.; Giaimo, J. M.; Li, X.; Wasielewski, M. R. J. Phys Chem. A 2005, 109, 970–975. (76) Aartsma, T. J.; Gouterman, M.; Jochum, C.; Kwiram, A. L.; Pepich, B. V.; Williams, L. D. J. Am. Chem. Soc. 1982, 104, 6278–6283. (77) Kaletas€u, B. K.; Dobrawa, R.; Sautter, A.; W€urthner, F.; Zimine, M.; Cola, L. D.; Williams, R. M. J. Phys. Chem. A 2004, 108, 1900–1909. (78) Bulovic, V.; Burrows, P. E.; Forrest, S. R.; Cronin, J. A.; Thompson, M. E. Chem. Phys. 1996, 210, 1–12. (79) Chernick, E. T.; Mi, Q.; Kelley, R. F.; Weiss, E. A.; Jones, B. A.; Marks, T. J.; Ratner, M. A.; Wasielewski, M. R. J. Am. Chem. Soc. 2006, 128, 4356–4364. (80) Dance, Z. E. X.; Mi, Q.; McCamant, D. W.; Ahrens, M. J.; Ratner, M. A.; Wasielewski, M. R. J. Phys. Chem. B 2006, 110, 25163– 25173. (81) Cleave, V.; Yahioglu, G.; Barny, P. L.; Friend, R. H.; Tessler, N. Adv. Mater. 1999, 11, 285–288.

1600


Sequential Energy and Electron Transfer in Polyisocyanopeptide

Recommend Documents