Near‐Infrared Dyes and Fluorophores Based on Diketopyrrolopyrroles

Abstract

Our research activities over the last few decades have concerned correlations between molecular structure and fluorescence. 2] A central theme of the research has been the synthesis and spectroscopic investigations of near-infrared (NIR) dyes to get insights into what degree the obtainable fluorescence quantum yields are restricted by the S0$S1 energy gap. Our results on the condensation of 1,4phthalazinediones with heteroarylacetonitriles in POCl3 prompted us to carry out the reaction with diketopyrrolopyrroles (DPPs) 1. Herein we report the results of the investigations. To date, attempts to activate the carbonyl group of DPPs with POCl3, and to subsequently convert the intermediates with nucleophiles, only led to monosubstitution or to ring opening. 10] The reaction of 1 and 2 in refluxing toluene with an excess of POCl3 according to Scheme 1 afforded disubstituted NIR dyes 3. The progress of the reaction was controlled by recording absorption spectra and stopping the heating as soon as the DPP was used up and/or products absorbing at short wavelengths appeared. Purification was carried out by digesting the product in acetone and subsequent flash chromatography (silica gel/CHCl3 or CH2Cl2). The NIR dyes 3a–3h (the structures and spectroscopic data of 3b–3h can be found in the Supporting Information) were synthesized according to this procedure (Scheme 1) from the reaction of 1 and 2. The course of the reactions was the same in all cases and side-products with strong absorptions below 350 nm were observed. These side-products are most likely compounds formed by the opening of the DPPs pentalene ring system. Prerequisite for the reactions is a certain solubility of the DPPs. This is a feature of all of the DPPs used here with the exception of the 4-methoxy derivative 1b. Remarkably, the 4(N-methyl-N-octyl amino) derivative 1e has a much better solubility than all the other DPPs used. Only in the case of the reaction of 1e with 2a were we able to isolate the 1:1 condensation product (13% yield). The heteroarylacetonitriles 2a and 2b substituted with a tert-butyl group were used to improve the solubility of the condensation products 3. In general, satisfying yields were obtained only if the solubility of both condensation partners was improved by having longer alkyl groups (Table 1). From these observations we draw the following conclusion concerning the reaction pathway: DPP reacts with POCl3 to form a monophosphorylated intermediate, which reacts with the Scheme 1. Reagents and conditions: a) absolute toluene/POCl3, reflux; b) 1,2-dichlorobenzene/BF3·Et2O, reflux, diisopropylethylamine; c) xylene/chlorodiphenylborane, reflux; DPPs 1: R=4-octyloxy (1a), R=4-methoxy (1b), R=4-butyloxy (1c), R=4-(hex-5-enyloxy) (1d), R=4-(N-methyl-N-octylamino) (1e); heteroarylacetonitriles 2 : 2-(6tert-butylquinolin-2-yl)acetonitrile (2a), 2-(6-tert-butylbenzothiazol-2yl)acetonitrile (2b), 2-(quinoxalin-2-yl)acetonitrile (2c), 2-(6-methylpyridin-2-yl)acetonitrile (2d). A: aromatic ring.