Ground and Excited States of Bis‐4‐Methoxybenzyl‐Substituted Diketopyrrolopyrroles: Spectroscopic and Electrochemical Studies

Abstract

A series of symmetrically bis‐4‐methoxybenzyl (4MB) N‐substituted 1,4‐diketopyrrolo[3,4‐c]pyrrole (DPP) derivatives have been synthesized. The 4MB unit makes the DPP core soluble, and shows subtle modification of up to 0.2 eV in ground and excited states of the core when compared with related alkyl derivatives. Absorption and emission spectroscopy, as well as electrochemical and computational methods have been employed to prove the importance of the peripheral aryl units on the donor/ acceptor properties of the molecules. The 4MB products are highly fluorescent (quantum yields approaching 100 % in solution), with a unique distribution of frontier states shown by spectroelectrochemistry. The solid‐state fluorescence correlates with the X‐ray crystal structures of the compounds, a Stokes shift of approximately 80 nm is seen for two of the compounds. The frontier energy levels show that this subtle substitutional change could be of future use in molecular energy level tailoring in these, and related, materials for organic (opto)electronics.

Solid state structure and properties of phenyl diketopyrrolopyrrole derivatives

“…For PhOMeDPP N-Boc interactions between the carbonyl oxygen and C-H of the t-butyl unit, as well as interactions between the C-H's of the aryl unit with the methoxy unit are noted. For PhOBocDPP N-Boc interactions between the carbonyl oxygen and C-H of the N-t-butyl unit as well as additional interactions between the carbonyl of the N-Boc unit with C-H of the aryl unit, the carbonyl of the OBoc unit with C-H of the aryl unit and interaction of the OBoc and N-Boc units gives a cross shaped packing structure, previously seen for PhOMeDPP N-PMB 81 and attributed to the twisting of the flanking phenyls.…”

“…This large contribution from J-aggregation to the absorption band maximum in PhOMeDPP derivatives correlates well with literature. 81 For PhOMeDPP N-Boc (ESI † Fig. S24), the absorption band is split and is either blue shifted (THF) or red shifted (CH 2 Cl 2 ) depending on the solvent.…”

Photostable orange-red fluorescent unsymmetrical diketopyrrolopyrrole–BF2 hybrids

“…The molar absorption coefficients for these bands are in the range 15000-27100 M -1 cm -1 , consistent with other DPP derivatives. 56 While the two maxima have similar intensities, the redder band is more intense for dyes 3a-3d, which contain smaller aryl substituents, whereas the bluer is stronger for dyes 3e and 3f, which contain larger π-systems. Expansion of the imidazopyridyl substituent red-shifts the absorption maxima by 10-21 nm per additional benzannulation (Figure 2a).…”

Straightforward N-alkylation of diketopyrrolopyrroles through the Mitsunobu reaction with benzyl, α-branched, and chiral alcohols

“…17 The N-alkyl groups on DPP derivatives play a crucial role not only in the improvement of the solubility but, depending on their nature, they can also affect the structural, conformational, and solid-state organization through intra-and intermolecular interactions with strong consequences on the electronic and photophysical properties. [18][19][20][21][22] For instance, the - stacking interactions (which often determine a quenching of the photoluminescence in the solid state for DPP-based fluorophores) can be considerably reduced by using sterically demanding side chains such as benzyl groups [23][24][25] or -branched alkyl groups. 19,26 This latter class of N-alkyl substituents should be regarded as a practical expedient for introducing stereogenic centres close to the DPP core, opening the way towards a cutting-edge class of chiral fluorophores with enhanced chiroptical properties.…”