Abstract
The objective of this study was to investigate the influence the introduction of a nitrogen containing heterocyclic spacer unit between the pyridinium termini of the viologen (1,1'-dialkyl(aryl)-[4,4'- bipyridine]-1,1'-diium) unit upon the electrochromic properties (reduction potential, reversibility, radical cation colour and background colour). The central heterocyclic spacer unit was selected from either a pyridine, quinoline, isoquinoline or imidazole ring or their respective N-quaternised salts.A series of isomeric terpyridines was synthesised by Suzuki-Miyaura cross-coupling reactions between various dibromopyridines and 4-(4,4,5,5-tetramethyl-1,3,2- dioxaborolan-2-yl)pyridine. Tris- N-alkylation was readily accomplished for the terpyridine isomers in which the central pyridine ring was sterically unencumbered, otherwise, only alkylation of the terminal 4-pyridyl units was observed with n-hexyl iodide. N-Methylation of the hindered central pyridine ring could be accomplished in some instances using methyl tosylate at high temperature. The target pyridyl-spaced viologen tetrafluoroborate salts were obtained by a simple tetrafluoroborate for iodide counterion exchange.
The behaviour of bromo-2,4'-bipyridines towards functionalisation by Negishi cross-coupling with 2-pyridylzinc bromide met with mixed fortune depending upon the location of the bromine substituent. Interestingly the 3-bromo-2,4'-bipyridine underwent a homocoupling reaction to afford the 4,2':3',3'':2'',4'''-quaterpyridine from which a novel electochrome was obtained. The structure of the 4,2':3',3'':2'',4'''-quaterpyridine and the derived tetramethylated tetrakis(tetrafluoroborate) was established by single crystal X-ray determination.
The electrochromic response of the novel pyridyl spaced viologens were characterised by the reversible generation, from clear unactivated states, of intensely coloured, orange through to blue, radical cations with reduction potentials in the range -0.70 – -1.24 V versus Ag/Ag+. The 1,1',1''-trihexyl-[4,3':5',4''-terpyridine]-1,1',1''-triium tris(tetrafluoroborate) was unique amongst the series of pyridyl separated viologens due to inherent fluorescence which could be modulated through chemical and electrochemical redox cycling.
A selection of mono-halo substituted 4-chloroquinolines were synthesised by thermal cyclisation of 2,2-dimethyl-5-[(arylamino)methylene]-1,3-dioxane-4,6-dione, derived from the condensation of Meldrum’s acid with a halo-aniline and trimethyl orthoformate, followed by chlorination with phosphorus oxychloride. Alternatively, iodination of (bromo)quinolin-4(1H)-one and subsequent chlorination afforded (bromo)-4-chloro-3-iodo-quinolines. Suzuki-Miyaura cross-coupling between the various halogeno- quinolines and -isoquinolines and 4-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2- yl)pyridine gave (iso)quinoline spaced viologen tetrafluoroborates after alkylation with either n-hexyl iodide or methyl tosylate and counter ion exchange. The electrochromic response of these (iso)quinoline containing viologens exhibited, upon reduction, a colour gamut from orange to green / blue hues with similar reduction potentials to the pyridine separated analogues but often with either poor reversibility or coloured ground states.
The acid-catalysed multi-component reaction between either ammonium acetate, 4-t-butylaniline, (hetero)aromatic aldehydes and either a dibromophenanthrene-9,10-dione or 4,4'-dibromobenzil gave the dibromo-1-[4-(t-butyl)phenyl]-2(hetero)aryl-1H-phenanthro[9,10-d]imidazoles or 4,5-bis(4-bromophenyl)-1-[4-(t-butyl)phenyl]-2-aryl-1H-imidazoles, respectively in generally good yield. Multi-site Suzuki-Miyaura cross-coupling of the foregoing (phenanthro)imidazoles with 4-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)pyridine gave poly(4-pyridyl) substituted 1H-phenanthro[9,10-d]imidazoles and poly(4-pyridyl) substituted 1H-imidazoles, respectively. Poly(Nmethylation) of the pyridyl and imidazolyl N-atoms using methyl tosylate at 180 oC followed by conversion of the tosylate counterion to tetrafluoroborate gave novel (phenanthro)imidazole spaced viologens with reduction potentials in the range -0.98 – -1.53 V versus Ag/Ag+ which developed green / brown hues.
| Date of Award | 31 Jul 2019 |
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| Original language | English |
| Supervisor | Mark Heron (Main Supervisor), Christopher Gabbutt (Co-Supervisor) & Stuart Aiken (Co-Supervisor) |