How To Make Nitroaromatic Compounds Glow: Next‐Generation Large X‐Shaped, Centrosymmetric Diketopyrrolopyrroles

Abstract Red‐emissive π‐expanded diketopyrrolopyrroles (DPPs) with fluorescence reaching λ=750 nm can be easily synthesized by a three‐step strategy involving the preparation of diketopyrrolopyrrole followed by N‐arylation and subsequent intramolecular palladium‐catalyzed direct arylation. Comprehensive spectroscopic assays combined with first‐principles calculations corroborated that both N‐arylated and fused DPPs reach a locally excited (S1) state after excitation, followed by internal conversion to states with solvent and structural relaxation, before eventually undergoing intersystem crossing. Only the structurally relaxed state is fluorescent, with lifetimes in the range of several nanoseconds and tens of picoseconds in nonpolar and polar solvents, respectively. The lifetimes correlate with the fluorescence quantum yields, which range from 6 % to 88 % in nonpolar solvents and from 0.4 % and 3.2 % in polar solvents. A very inefficient (T1) population is responsible for fluorescence quantum yields as high as 88 % for the fully fused DPP in polar solvents.


General methods
All chemicals were used as received unless otherwise noted. All reported 1 H NMR and 13 C NMR spectra were recorded on 500 MHz spectrometer. Chemical shifts (δ ppm) were determined with TMS as the internal reference; J values are given in Hz. Mass spectra were obtained via EI MS. For HRMS measurements both quadruple and TOF mass analyzer types were used. UV-Vis and fluorescence spectra were recorded in toluene, chlorobenzene, anisole and benzonitrile. Chromatography was performed on silica (Kieselgel 60, 200-400 mesh). ps, maximum of repetition rate 100 kHz) was utilized for excitation.
For the differential pulse voltammetry (DPV) assays a three electrode cell configuration with a platinum working electrode, a platinum mesh counter electrode, and a silver wire quasi reference electrode were utilized. The measurements were performed in argon saturated dichloromethane at 0 °C, a respective DPP concentration of 1.5 x 10 −3 M, and using TBAPF6

Steady-State Absorption Spectroscopy
Vibrationally resolved spectra have been obtained using the FCclasses program. 7 The Franck-Condon (FC) approximation has been employed as we consider strongly dipole-allowed transitions (f>0.1), and we used the so-called Adiabatic Hessian (AH) 8  The number of integrals to be computed for each class was set to at least 10 6 , and it was tested that the FC factors exceeds 0.9 in all cases.

S42
To investigate the nature of the excited-states under relaxation of the solvent and of the geometry, in connection with the transient measurements, it is crucial to correctly describe states of various natures, as the second state presents a strong CT character, and we have therefore turned to the B97X-D functional, 10 using, as above the 6-31+G(d) basis set for the geometry and frequency calculations that have performed with the PCM model applying both toluene and benzonitrile as solvents. Both the ground and the two lowest singlet excited, and the lowest triplet states were optimized in this way, using the LR-PCM approach in equilibrium was applied for the TD-DFT calculations (S1 and S2) and applying TDA-DFT with the same functional for the vertical triplet state (after ISC) and U-DFT for the relaxed triplet state. The total energies were next computed on these structures with the same functional, and the LR+cLR solvent model 6 applying both the equilibrium and non-equilibrium effects for the singlet excited-state, so as to estimate the solvent reorganization energy.
For the sake of computational effort, the side t-Bu groups substituting the benzofuran side groups were modelled as methyl groups.