Article pubs.acs.org/JPCA
Synthesis and Optical Properties of Pyrrolo[3,2‑b]pyrrole2,5(1H,4H)‑dione (iDPP)-Based Molecules Mindaugas Kirkus,† Stefan Knippenberg,‡ David Beljonne,‡ Jérôme Cornil,‡ René A. J. Janssen,† and Stefan C. J. Meskers*,† †
Molecular Materials and Nanosystems, Eindhoven University of Technology, P.O. Box 513, 5600 MB Eindhoven, The Netherlands Laboratory for Chemistry of Novel Materials, University of Mons, Place du Parc 20, B-7000 Mons, Belgium
‡
S Supporting Information *
ABSTRACT: We describe the synthesis and photophysical properties of a series of derivatives of pyrrolo[3,2-b]pyrrole2,5(1H,4H)-dione-3,6-diyl (iDPP) linked to two oligothiophenes of variable length (nT). The iso-DPP-oligothiophenes (iDPPnTs) differ from the common pyrrolo[3,4-c]pyrrole1,4(2H,5H)-dione-3,6-diyl-oligothiophene analogues (DPPnTs) by a different orientation of the two lactam rings in the bicyclic iDPP unit compared to DPP. In contrast to the highly fluorescent DPPnTs, the new isomeric iDPPnTs exhibit only very weak fluorescence. We demonstrate with the help of quantum-chemical calculations that this can be attributed to a different symmetry of the lowest excited state in iDPPnT (A in C2 symmetry) compared to DPPnTs (B) and the corresponding loss in oscillator strength of the lowest energy transition. Upon extending the oligothiophene moiety in the iDPPnTs molecules, the charge transfer character of the lowest A excited state becomes more pronounced. This tends to preclude high fluorescence quantum yields even in extended iDPPnTs systems.
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The first synthesis of the symmetrical and asymmetrical pyrrolo[3,2-b]pyrrole-2,5-diones (iDPPs, Ar1 = Ar2 = aryl and R1 = R2 = H, Figure 1) has been reported by Fürstenwerth,8 starting from pulvinic acid derivatives using harsh reaction conditions in low reaction yields due to formation of side products. A number of symmetrical N-arylated iDPPs (R1 = R2 = Ar and Ar1 = Ar2, Figure 1) have been synthesized by reacting aromatic ester carbanions with aromatic oxalic acid−bis(imidoyl)dichlorides9 that can be obtained in a one-pot reaction from aromatic amines, oxalyl chloride, and PCl5.10,11 Asymmetrical N-arylated iDPPs (R1, R2 = aryl with R1 ≠ R2 and Ar1 = Ar2) have been prepared by the same synthetic strategy12 but using asymmetrical oxalic acid−bis(imidoyl)dichlorides which can be prepared in a few steps starting from commercial ethyl-2-chloro-2-oxoacetate and different derivatives of aniline.13 The nature of the lowest excited state in DPP and iDPP derivatives is of importance for application in opto-electronics. While normal DPPs are renowned for their bright fluorescence in solution,14,15 many structurally related chromophores show hardly any photoluminescence activity. For example, recent studies on benzodipyrrolidones revealed that these molecules show essentially no fluorescence.16 Similarly low fluorescence quantum yields (from 0.3% to 2%) were reported for
INTRODUCTION The pyrrolo[3,4-c]pyrrole-1,4-dione-3,6-diyl (DPP, Figure 1) chromophore, first reported in 1974,1 has recently emerged as
Figure 1. Chemical structures of 3,6-diaryl-pyrrolo[3,4-c]pyrrole-1,4dione (DPP), 3,6-diaryl-pyrrolo[3,2-b]pyrrole-2,5-dione (iDPP), and pulvinic acid.
an important and versatile building block for organic πconjugated polymers and extended π-systems with interesting opto-electronic properties that find application in organic solar cells and thin-film transistors.2 DPP-based molecules have several desirable optical properties such as high extinction coefficients, readily tunable absorption wavelength, and bright fluorescence.3,4 The usefulness and versatility of the DPP unit has inspired the search for related chromophores. In this research, we focus on the isomeric pyrrolo[3,2-b]pyrrole-2,5dione-3,6-diyl (iDPP) which is a natural derivative of the pulvinic acid (Figure 1)5,6 and used as a pigment.7 © 2013 American Chemical Society
Received: January 9, 2013 Revised: March 4, 2013 Published: March 5, 2013 2782
dx.doi.org/10.1021/jp400256s | J. Phys. Chem. A 2013, 117, 2782−2789
The Journal of Physical Chemistry A
Article
Scheme 1. Synthesis of Pyrrolo[3,2-b]pyrrole-2,5-dione-Based N-Arylated Derivatives T1, T2, and T3a
a (a) K2CO3, 2-ethylhexyl bromide, 2-butanone, reflux, 12 h; (b) ethanol, HCl, reflux, 12 h; (c) oxalyl chloride, toluene, rt, 30 min; PCl5, reflux, 1 h; (d) diisopropylamine, 2.5 M BuLi in n-hexane, THF, −78 °C; then ethyl-2-thiopheneacetate, rt, 12 h; (e) 2.5 M BuLi in n-hexane, THF, −78 °C, 1. 5 h, −78 °C, rt, C12H25Br, reflux, 2 days; (f) NBS, AcOH/chloroform, 0 °C; then rt, 12 h; (g) Mg, 2-bromothiophene, Et2O, rt; then 50 °C, 4 h; (6), Pd(dppf)Cl2, Et2O, 0−5 °C, rt, overnight; (h) 2.5 M BuLi in n-hexane, THF, −78 °C, 30 min; then rt, 30 min; then −78 °C, trimethyltin chloride, rt, overnight; (i) NBS, 50 °C, 12 h; (j) 2-(tributylstannyl)thiophene or 8, Pd[PPh3]4, toluene, 100 °C, 36 h.
benzodifuranone derivatives17 as well as for iDPP polymers (
