Side Chain Effects on Solution

HIGHLY FLUORESCENT OLIGOTHIOPHENES INCORPORATING CENTRAL DITHIENO[3,2-b:2′,3′-d]PYRROLES: SIDE CHAIN EFFECTS ON SOLUTION AND SOLID-STATE PROPERTIES Hong Mo and Seth C. Rasmussen Department of Chemistry and Molecular Biology North Dakota State University Fargo, ND 58105 Introduction Over the last couple of decades there has been considerable fundamental and technological interest in the development of conjugated organic materials. These materials combine the electronic and optical properties typical of semiconductors with many of the desirable properties of plastics, including the ability to tune their electronic properties at the molecular level via synthetic modification.1-4 Because of their environmental stability and ease of synthetic modification, thiophene-based materials are thought to be one of the most versatile and promising classes of conjugated systems. In addition to the well-studied polythiophenes, the analogous oligothiophenes have received particular attention as their controllable and rigorously defined structure allows the correlation of the physical properties with the chain and conjugation length.1,5-12 In addition, their lower molecular weights allow deposition via vacuum sublimation and, providing that they have significant solubility; can also be solvent cast in the same manner as the polymeric analogues.1 Applications demonstrated for oligothiophenes include their use in field effect transistors (FETs),6 light-emitting diodes (LEDs),7,8 photovoltaic cells,9 and spintronics.10 A recent approach to the design of improved oligothiophenes is the combination of the stability of the thiophene ring with the planarity of linear acenes such as pentacene.5,12 Our group has previously shown that incorporation of the fused-ring dithieno[3,2-b:2′,3′-d]pyrrole13 (DTP, Scheme 1) unit into oligothiophenes retains the desirable properties of the oligomers, while limiting deviations from planarity that disrupt conjugation and potentially affect the band gap in the solid state. The N-alkyl side chain of the DTP also allows for greater solubility without additional steric interactions that cause deviations from planarity.3 The combination of these effects leads to DTPbased oligothiophenes exhibiting significantly enhanced fluorescence.12 In order to further study the structure−property relationships within these DTP-based oligothiophenes, we have prepared a series of quaterthiophene analogues as illustrated in Scheme 1. Through the use of linear and branched N-functionalized side chains, the influence of the side chains on the optical properties in both solution and the solid-state have been investigated. As far as we are aware, the fluoresence efficiency reported here for 2c (65%) is the highest reported quaterthiophene analogue and constitutes one of the highest reported for any oligothiophene (Roncali and co-workers11 have reported 92% for a terthiophene analogue and Bäuerle and co-workers8b have reported 62% for a quinquethiophene analogue). R N

R t

1) 2 eq. BuLi S S DTP (1a-c)

2) 2 eq. Me3SnCl

S Br Pd

a: R = C8H17 b: R = 2-ethylhexyl c: R = tert-butyl

N S

S S

S 2a-c

Pd = PdCl2(PPh3)2

Scheme 1. Synthesis of dithieno[3,2-b:2′,3′-d]pyrrole-based oligothiophenes Experimental Materials. Materials were reagent grade and used without further purification. DTPs 1a-c were prepared as previously reported.13,14 Chromatographic separations were performed using standard column methods with basic aluminum oxide or silica gel (230−400 mesh). All glassware was ovendried, assembled hot, and cooled under a dry nitrogen stream before use, and all reactions were performed under nitrogen. General procedure for synthesis of DTP quaterthiophene analogues: This is a modification of a previously reported procedure.12 The desired DTP (2.0 mmol) was dissolved in dry THF (450 mL) and cooled in an acetone/dry ice bath for 20 minutes. Tert-butyllithium (2.4 mL, 4.0 mmol) was then added dropwise and the mixture was allowed to stir for 1 h. The mixture was then removed from the ice bath, allowed to warm to room temperature, and stirring

continued for 2 h. The mixture was returned to the ice bath and (CH3)3SnCl (1.0 M in THF) (4.0 mL, 4.0 mmol) added dropwise. The mixture was allowed to stir for 0.5 h before warming to room temperature and stirred for another 3 h. In a separate flask, PdCl2(PPh3)2 (0.077g, 0.11mmol) and 2-bromothiophene (4.0 mmol) were combined in dry THF (20 mL). The 2,6-bis(trimethylstannyl)dithieno[3,2-b:2′,3′-d]pyrrole solution was transferred to the catalyst/bromothiophene solution and heated at reflux for 20 h. The solution was allowed to cool, quenched with water, and extracted with Et2O. The combined ether washes were concentrated and purified by column chromatography (basic alumina and silica; 2% EtOAc in hexane). Recrystallization was achieved using isopropyl alcohol. N-Octyl-2,6-bis(2'-thienyl)dithieno[3,2-b:2',3'-d]pyrrole (2a). 70-75% yield; mp 67.5-68.5 ºC; 1H NMR: δ 7.20 (m, 4H), 7.07 (s, 2H), 7.03 (dd, J = 3.6, 5.1 Hz, 2H), 4.15 (t, J = 7.2 Hz, 2H), 1.87 (m, 2H), 1.27 (m, 12H), 0.87 (t, J = 7.2 Hz, 3H); 13C NMR: δ 144.70, 139.09, 135.14, 128.14, 124.09, 123.20, 114.14, 107.69, 47.60, 32.06, 30.60, 29.47, 29.42, 27.22, 22.88, 14.36. N-(2-Ethylhexyl)-2,6-bis(2'-thienyl)dithieno[3,2-b:2',3'-d]pyrrole (2b). 55-60 % yield; mp 78.1-79.1 ºC; 1H NMR: δ 7.21 (dd, J = 1.2, 5.2 Hz, 2H) 7.19 (dd, J = 1.2, 3.6 Hz, 2H), 7.04 (s, 2H), 7.03 (dd, J = 3.6, 5.2 Hz, 2H), 4.06 (m, J = 21.2, 7.6 Hz, 1H), 4.02 (m, J = 21.2, 7.6 Hz, 1H), 1.97 (h, J = 6.4 Hz, 1H), 1.33 (m, 8H), 0.93 (t, J = 7.2, 3H), 0.90 (t, J = 7.6, 3H); 13C NMR: δ 145.1, 139.1, 135.1, 128.1, 124.1, 123.2, 114.1, 107.8, 51.5, 40.6, 30.8, 28.8, 24.3, 23.3, 14.3, 11.0. N-tert-Butyl-2,6-bis(2'-thienyl)dithieno[3,2-b:2',3'-d]pyrrole (2c). 4045 % yield; mp 167.9-168.9 ºC; 1H NMR: δ 7.30 (s, 2H), 7.20 (dd, J = 1.2, 7.2 Hz, 2H), 7.19 (dd, J = 1.2, 5.6 Hz, 2H), 7.03 (dd, J = 3.6, 5.2 Hz, 2H), 1.84 (s, 9H); 13C NMR (500 MHz): δ 143.1, 139.2, 134.2, 128.0, 124.1, 123.3, 115.7, 111.7, 58.4, 30.9. Instrumentation. Unless noted otherwise, NMR spectra were obtained in CDCl3 on a Varian 400 MHz spectrometer and referenced to the chloroform signal. UV-visible spectra were measured on a dual beam scanning spectrophotometer using samples prepared as dilute solutions in 1 cm quartz cuvettes. Emission spectra were carried out using dilute CH3CN or cyclohexane solutions (