Molecules for plastic electronics: structure and electronic properties

Transcription

1 Molecules for plastic electronics: structure and electronic properties Beatrice aglio Dipartimento di Chimica, Materiali e Ing. Chimica G. Natta Politecnico di Milano beatrice.saglio@polimi.it tel

2 MART MATERIAL INPUT: Light Voltage Environment Pressure eat INPUT UTPUT UTPUT Electrical Light ptical Mechanical 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.

3 INPUT UTPUT 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.

4 Electrical ptical Chemical Mechanical INPUT Electrical ptical Chemical Mechanical UTPUT 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.

5 analyte ptical Electrochemical ensors/biosensors

6 Electrical Mechanical Piezoelectric materials

7 mart material modificationupon stimulus hν

8 mart material modificationupon stimulus Δn = F F F F C 3 7

9 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

10 Fundamental requirement: polarizable electrons ü basics on the C=C double bond ü delocalized chemical bond: conjugation ü aromatic rings: ü substituents: benzene benzenoid fused rings heterocycles alkyl chains electron-donors e electron-acceptors ü rganic compounds and solubility ü Aggregation phenomena ü (elf-)assembly

11 rganic functional materials: basic ingredients ü σ bonds constitute the skeleton of the molecules ü usually when σ bonds break, molecule gets degradation C C C C

12 rganic functional materials: basic ingredients ü the double bond consists of a part which arises from the formation of a σ bonds C C C C

13 rganic 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 p z (p y ) atomic orbitals C C C C

14 Alkenes: continuous chain of carbon atoms that contains the double bond. ügeneral formula: C n 2n (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. 2 C C 2 2 C C C 3 ethene propene C 2 C C C 3 1-butene 2 2 C C C C 2 2 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.

15 rganic functional materials: basic ingredients ü although Bond it is weaker Bond than energy σ bond, Calculated the π bond is strong enough Kcal/mol to allows the from Bond length Å From formation C-C of stable trans 80 and cis isomers C C 2 6 ü the relatively low bond energy makes π electrons C=Cless bound and 141more C C 2 4 polarizable C C 195 C C 2 2 E ΔE = 1Kcal/mol 3 C lefin Bond energy Kcal/mol 3 C C 3 π bond 65 σ bond Kcal/mol 27.6 Kcal/mol C 3

16 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. ün the contrary, it occurs more easily if the molecule is at the first singlet or triplet excited state. k tp 1 t * 1 c * 1 P * k cp hν k d α s (1 α s ) hν 3 C 1 t 1 c 3 C C 3 C 3

17 ne 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) C C C C 2 C 2 C C C 2 C 2 C C C 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 p z 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

18 ne double bond is not enough the simplest case of 1,3-butadiene Energy Ψ4 Ψ3 Ψ2 Ψ1

19 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. +

20 Giving some other π electrons more the oligoenes ethylene butadiene octatetraene 2 CB π LUM π LUM π LUM energy Eg π M π M VB π M

21 Giving some other π electrons more the oligoenes ethylene butadiene octatetraene polyacetylene 2 * n * CB π LUM π LUM π LUM π LUM energy Eg π M π M π M VB π M

22 Give me some other π electrons more the oligoenes ethylene butadiene octatetraene polyacetylene 2 * n * energy CB Eg VB π LUM π M π LUM π M π LUM π M π LUM π M üthe M increases in energy with increasing conjugation length üthe LUM 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

23 Peierls distortion polyacetylene * n * π LUM π M

24 Electronic levels and optical properties: the UV-vis electronic spectra antibonding M ΔE E = hν electronic excitation bonding M Ground state Excited state Electronic transitions E σ* π* n π σ Antibonding (single bonds) Antibonding (double bonds) Non bonding Bonding (doublebonds) Bonding (single bonds)

25 tructure Compound λ max (nm) ε Ethene ,4- pentadiene ,3-butadiene methyl-1,3- butadiene trans-1,3,5- hexatriene trans,trans- 1,3,5,7- octatetraene 222,

26 The UV-vis electronic spectra of oligoenes

27 Conjugation: not only linear double bonds ethene (alkene) acetylene (alkine) propylene (allyle) 1,3-butadiene BENZENE PLYCYCLIC BENZENID YDRCARBN CYCLIC PLYENE N N ETERCYCLE N 5-MEMBERED ETERCYCLE pyrrole furan tiophene

28 Conjugation: not only linear double bonds 1.09 A 1.39 A 120 BENZENE 120 PLYCYCLIC BENZENID YDRCARBN Resonance Energy= 29.6 kcal mol -1

29

30 Benzene: cyclic vs homologous linear compound benzene 1,3,5-hexatriene E

31 Conjugation: not only linear double bonds üisoelectronic as a benzene (aromatic compounds) ü thiophene: relative high chemical stability üthiophene: highly versatile in organic chemistry N N pyrrole furan N tiophene ETERCYCLE 5-MEMBERED ETERCYCLE

32 Electronic levels and optical properties: the UV-vis electronic spectra of oligo-/poly-thiophene

33 Electronic levels and optical properties: the UV-vis electronic spectra of oligothiophene T 2T 3T 4T 5T 6T T2 T3 T4 T5 T6 absorbance wavelength (nm)

34 Electronic levels and optical properties: the UV-vis electronic spectra of oligothiophene T 2T 3T 4T 5T 6T T2 T3 T4 T5 T6 λ max = 305 nm absorbance 2T λ max = 245 nm wavelength (nm) Thiophene: smaller resonance energy smaller steric hindrance 2-2

35 teric indrance Courtesy of J. M. Rujas

36 Electronic spectra (UV-vis) t-bu 3 C t-bu t-bu C 3 t-bu t-bu 3 C t-bu t-bu t-bu C t-bu 3 t-bu t-bu t-bu t-bu t-bu t-bu absorbance 3 C t-bu t-bu C 3 t-bu t-bu t-bu t-bu C t-bu 3 t-bu t-bu C wavelength (nm)

37 Ladder polymers polyacenes n n n E gap = 0 ev (Bredas, Pomerantz) E gap = 0.5 ev (Yamabe) E gap = 0.2 ev (Kao and Lilly) n max = 7 polyphenantrene n n pentacene (FET)

38 The electronic spectra (UV-vis): Ladder polymers R N N R R N R n n

39 Branched oligoenes

40 Branched oligoenes

41 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 (PA) 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

42 Electronic spectra: electrocyclization tilbene: hν hν, Δ 2 electrocyclisation Analogous dithienylethenes Abs nm

43 The side groups ide groups are usually introduced onto the main molecular skeleton : ü solubility ü conjugation ü M/LUM levels and bandgap ü intermolecular interactions: solid state packing/self-assembly #1: alkyl chains its main role is to increase solubility cooperate to the intermolecular packing through Van der Waals interactions C 3 methyl hexyl decyl dodecyl #2: functional groups selective/specific interactions post-functionalization electronic effect

44 teric effect of side groups: distortion of the skeleton θ = 45 C 3 λ max = 245 nm (ε max = 19000) λ max = 236 nm (ε max = 10000)

45 Electronic effect of side groups: inductive effect Inductive effect: polarization of a bond caused by the polarization of an adjacent bond δδ+ ü trongly related to electronegativity ü the effect is greatest for adjacent bonds but may be felt far away ü The only effect in saturated hydrocarbons δ+ δ- 3 C C 2 Cl - I Electron-withdrawing + I Electron-donating The analogous effect which does not operate through bonds, but directly through space (or solvent molecules) is named field effect. It is often very difficult to separate these two effects.

46 Electroactive substituents onto aromatic rings Inductive effect donor acceptor C 3 CF 3 Ring relatively rich of electrons (more reactive) Ring relatively poor of electrons (less reactive) Resonance effect (mesomeric effect) Decrease in electron density in one position (and corresponding increase elsewhere) due to the presence of unshared electrons pair :N 2 N 2 N 2 N 2 + M - - -

47 Electroactive substituents onto aromatic rings C - Inductive effect -M examples -M: N 2 C C CN

48 Electroactive substituents onto aromatic rings : N 2 - I Inductive effect Mesomeric effect :N 2 N 2 N 2 N 2 + M examples +M: Me N 2 Alogeni

49 Electronic effect of side groups: C 3 + I + mesomeric and inductive effects + I + - I - ime 3 + I + N 2 - I + M + N 2 - I - M - - I + M + Me - I + M + C - I - M - C - I - M - CN - I - M - F - I + M - Cl - I + M - Br - I + M -

50 Effect of electo-active substituents on the electronic levels of the skeleton LUM M D A Introduction of electro-active groups: ü Affects the ionization potential and electron affinity, that is M/LUM ü Affects E GAP üelectively enhances electron vs hole transport ü Tunes the barrier org/mt

51 Effect of electo-active substituents on the electronic levels of the skeleton 3 C N N Good hole transporter C 3 C * CN CN n * C igh electron affinity Good electron transporter

52 ubstituted benzene: Electronic spectra R R Band at 200 nm Band at 256 nm λ (nm) ε λ (nm) ε C N C 2 =C

53 donor-acceptor systems D A Donor-acceptor hν I D -E A 2 N N N N - Charge-transfer 2 N C λ max = 289 nm (ε max = 18600) λ max = 245 nm (ε max = 19000)

54 donor-acceptorsystems: effect on the electronic levels E gap (polymer) = 1.2 ev M.C. Gallazzi et al., Macromol. Chem. Phys. 2001, 202, 2074