Molecules for plastic electronics: structure and electronic properties Beatrice Saglio Dipartimento di Chimica, Materiali e Ing. Chimica “G. Natta” Politecnico di Milano e-mail: [email protected] tel. 02.2399.3226 SMART MATERIAL INPUT OUTPUT ¤ INPUT: ¤ OUTPUT Light Electrical Voltage Light Environment Optical Pressure Mechanical Heat Electrical ü The change of properties in the bulk reflects a modification at the molecular level. ü The output has to be easily detected ü Reversibility to feature a working function. ü Fatigue resistance → device lifetime. INPUT OUTPUT ü The change of properties in the bulk reflects a modification at the molecular level. ü The output has to be easily detected ü Reversibility to feature a working function. ü Fatigue resistance → device lifetime. Electrical Optical Electrical Chemical Optical Mechanical Chemical … Mechanical INPUT … OUTPUT ü The change of properties in the bulk reflects a modification at the molecular level. ü The output has to be easily detected ü Reversibility to feature a working function. ü Fatigue resistance → device lifetime. analyte Optical Electrochemical Sensors/Biosensors Electrical Mechanical Piezoelectric materials Smart material modification upon stimulus hν S S S S Smart material modification upon stimulus F F Δn = 0.127 O F O O F C3H7 Fundamentals of molecular design of materials for molecular electronics/organic optoelectronics ü Fundamental ingredients: polarizable electrons ü Molecular design → main units → substituents (EA, ED) ü Lego-chemistry as a tool = tuning of properties ü solubility → material characterization processing ü L stability → device lifetime Fundamental requirement: polarizable electrons ü basics on the C=C double bond ü delocalized chemical bond: conjugation ü aromatic rings: benzene benzenoid fused rings heterocycles ü substituents: alkyl chains electron-donors e electron-acceptors ü Organic compounds and solubility ü Aggregation phenomena ü (Self-)Assembly Organic functional materials: basic ingredients ü σ bonds constitute the skeleton of the molecules ü usually when σ bonds break, molecule gets degradation C C H H H C C H H H Organic functional materials: basic ingredients ü the double bond consists of a part which arises from the formation of a σ bonds… C C H H C C H H Organic functional materials: basic ingredients ü the double bond consists of a part which arises from the formation of a σ bonds… …. and a part due to the overlapping of the pz (py) atomic orbitals C C H H C C H H Alkenes: continuous chain of carbon atoms that contains the double bond. üGeneral formula: CnH2n (homologous of cycloalkanes) üThe name given to the chain is obtained from the name of the corresponding alkane by changing the ending from -ane to -ene. H H H2C CH2 H2C C H2C C CH3 CH3 C ethene propene H H 1-butene H2 H2C C C CH2 H2 cyclobutane üThe location of the double bond along an alkene chain is indicated by a prefix number that designates the number of the carbon atom that is part of the double bond and is nearest an end of the chain. The chain is always numbered from the end that brings us to the double bond sooner and hence gives the smallest number prefix. üIn propene the only possible location for the double bond is between the first and second carbons; thus, a prefix indicating its location is unnecessary. Organic functional materials: basic ingredients ü although it is weaker than Bond Bond energyσ bond, the Calculated Bond length From π bond is strong enough to allows the Kcal/mol from Å formation of stable C-C trans80and cis isomersC H 1.53 C H ü the relatively low bond energy makes 2 6 2 6 π electrons less bound and more C=C 141 C2H4 1.34 C2H4 polarizableC≡C 195 C2H2 1.20 C2H2 Olefin Bond energy E Kcal/mol H3C CH3 π bond −27.665 Kcal/mol ΔE = 1Kcal/mol σ bondH H 108 H3C H −28.6 Kcal/mol H CH3 ü The cis-trans isomerisation of olefin involves 180° rotation about a C,C double bond. ü Except in strained cyclic olefins, the reaction is usually highly activated as a thermal process in the absence of catalysts. üOn the contrary, it occurs more easily if the molecule is at the first singlet or triplet excited state. 1t* ktp kcp 1c* 1P* k hν d hν αs (1−αs) 1t 1c H3C H H3C CH3 H CH3 H H One double bond is not enough… …the simplest case of 1,3-butadiene It cannot be represented by a single Lewis formula There are some possible canonical forms (#3) + - - + H C C C CH CH C C CH CH C C CH 2 H H 2 2 H H 2 2 H H 2 The double bonds are not localized between the C1-C2 and C3-C4 carbon atoms, but π molecular orbitals arise from the linear combination of the four pz orbital, one belonging to each carbon atom. It follows that: the double bond (C1-C2) and (C3-C4) is longer than a typical double bond the single bond is longer than a typical single bond (C2-C3) Nomeclature: conjugated olefines, oligoenes, polyenes One double bond is not enough… …the simplest case of 1,3-butadiene Energy Ψ4 Ψ3 Ψ2 Ψ1 Conjugated systems Conjugated molecules undergo not only typical reactions of olefins, possibly modified by the extended p overlapping, but also peculiar cycloaddition thermally or photochemically driven. + Giving some other π electrons more… …the oligoenes ethylene butadiene octatetraene 2 CB π∗LUMO π∗LUMO π∗LUMO energy Eg πHOMO πHOMO π VB HOMO Giving some other π electrons more… …the oligoenes ethylene butadiene octatetraene polyacetylene * 2 n * CB π∗LUMO π∗LUMO π∗LUMO π∗LUMO energy Eg πHOMO πHOMO πHOMO π VB HOMO Give me some other π electrons more… …the oligoenes ethylene butadiene octatetraene polyacetylene * 2 n * CB π∗LUMO π∗LUMO π∗LUMO π∗LUMO energy Eg πHOMO πHOMO πHOMO VB πHOMO üThe HOMO increases in energy with increasing conjugation length üThe LUMO decreases in energy with increasing conjugation length üThe band gap (Eg) is decreases with increasing conjugation length Properties: üIonization potential gets lower ü material gets more susceptible to electrophiles (more reactive, in general) ü spontaneous oxidations Peierls distortion polyacetylene * n * π∗LUMO πHOMO Electronic levels and optical properties: the UV-vis electronic spectra antibonding MO E = hν ΔE electronic excitation bonding MO Ground state Excited state Electronic transitions σ* Antibonding (single bonds) π* Antibonding (double bonds) E n Non bonding π Bonding (double bonds) σ Bonding (single bonds) Structure Compound λmax (nm) ε Ethene 171 15500 1,4- 178 --- pentadiene 1,3-butadiene 217 21000 2-methyl-1,3- 222,5 --- butadiene trans-1,3,5- 268 36300 hexatriene trans,trans- 330 ---- 1,3,5,7- octatetraene The UV-vis electronic spectra of oligoenes Conjugation: ethene acetylene propylene 1,3-butadiene not only linear double bonds… (alkene) (alkine) (allyle) BENZENE POLYCYCLIC BENZENOID HYDROCARBONS CYCLIC POLYENES HETEROCYCLES N N 5-MEMBERED HETEROCYCLES N O S H furan tiophene pyrrole Conjugation: not only linear double bonds… 1.09 A BENZENE 1.39 A 120° H H 120° POLYCYCLIC BENZENOID HYDROCARBONS Resonance Energy= 29.6 kcal mol-1 Benzene: cyclic vs homologous linear compound benzene 1,3,5-hexatriene E Conjugation: not only linear double bonds… üIsoelectronic as a benzene (aromatic HETEROCYCLES compounds) N N ü thiophene: relative high chemical stability 5-MEMBERED HETEROCYCLES üthiophene: highly versatile in organic N O S H furan tiophene chemistry pyrrole Electronic levels and optical properties: the UV-vis electronic spectra of oligo-/poly-thiophene Electronic levels and optical properties: the UV-vis electronic spectra of oligothiophene S S S S S S S S S S T 2T 3T 4T S S S S S S S S S S S 5T 6T T2 T3 T4 T5 T6 absorbance 300 400 500 wavelength (nm) Electronic levels and optical properties: the UV-vis electronic spectra of oligothiophene S S S S S S S S S S T 2T 3T 4T S S S S S S S S S S S 5T 6T T2 T3 T4 S T5 S λmax = 305 nm T6 2T absorbance λmax = 245 nm Thiophene: smaller resonance energy 300 400 500 wavelength (nm) smaller steric hindrance H2-H2’ Steric Hindrance Courtesy of J. M. Rujas Electronic spectra (UV-vis) t-Bu H C t-Bu 3 H3C t-Bu O O S t-Bu O S S S CH3 t-Bu S t-Bu O t-Bu CH3 O t-Bu t-Bu t-Bu S S t-Bu t-Bu O t-Bu S t-Bu O S t-Bu O t-Bu absorbance t-Bu H3C CH O t-Bu 3 S O t-Bu S t-Bu O t-Bu CH t-Bu 3 S S t-Bu t-Bu O CH 3 400 600 800 wavelength (nm) Ladder polymers polyacenes n n n Egap = 0 eV (Bredas, Pomerantz) nmax= 7 Egap = 0.5 eV (Yamabe) Egap = 0.2 eV (Kao and Lilly) polyphenantrene n n pentacene (OFET) The electronic spectra (UV-vis): Ladder polymers O O O O R N N R R N R n n O O O O Branched oligoenes Branched oligoenes Dendrimers The attractive properties of multi-chromophores dendrimers suggested a new approach towards blue light-emitting materials with the following characters: (i) blue light emission is brought about by the presence of electronically decoupled polycyclic aromatic hydrocarbon (PAH) units; (ii) the units are incorporated into a rigid polyphenylene dendritic structure and thus adopt sterically defined positions and disallow intra-dendrimer chromophore-chromophore interactions; (iii) amorphous films are obtained due to the lack of intermolecular interactions; (iv) the amount of “useless” substituent and coupling units is kept at a minimum Electronic spectra: electrocyclization Stilbene: hν H O2 hν, Δ H electrocyclisation Analogous dithienylethenes 1.4 1.2 1.0 0.8 Abs 0.6 0.4 0.2 0.0 300 400 500 600 700 800 nm The side groups Side groups are usually introduced onto the main molecular skeleton : ü solubility ü conjugation ü HOMO/LUMO levels and bandgap ü intermolecular interactions: solid state packing/self-assembly #1: alkyl chains its main role is to increase