Time-Resolved study in nitro-dpp compounds: the role of environmental polarity and stiffness.

We investigated how structural rigidity and nitro group position influence the emission properties and excited-state dynamics of four diketopyrrolopyrrole (DPP)-based molecules: two structurally rigid derivatives (K133 and K187) that confer a planar structure at the molecules and two flexible ones (K132 and K186). The fluorescence properties of these molecules are controlled by shifting the nitro group on the diketopyrrolopyrrole unit from the para to the ortho position. All molecules exhibit a quadrupolar charge distribution in both the ground and first excited state and they show poor solvatochromism. The excited-state dynamics of K132 and K186 are governed by the presence of an excited charge-transfer (CT) state. K132 is fluorescent in toluene but becomes non-emissive in more polar solvents, where CT state stabilization promotes efficient non-radiative decay. In contrast, K186 is non-fluorescent in all solvents due to the rapid and efficient population of the CT state. The structurally more rigid molecules (K133 and K187) evidenced a strictly similar behavior, their fluorescent properties being influenced by the polarity of the solvent due to the presence of an excited charge transfer state. However, no direct spectroscopic markers of the CT state were observed, either in visible pump-probe experiments or in transient infrared measurements; only DFT calculations suggested the presence of the CT state. The most important indication of a thermally activated population of the CT state was obtained from temperature dependent (RT 77K) fluorescence lifetime measurements in 2Me-THF. In contrast, the structural flexibility of K132 and K186 allowed the CT state to stabilize more effectively due to interactions with the surrounding chemical environment. This enabled the direct observation of spectral markers of the CT state: an excited state absorption (ESA) exhibiting a progressive blue shift with increasing solvent polarity in the visible and the growth of an excited state absorption band in the mid-IR spectral range (observed with TRIR measurements). To assess the impact of solvent polarity and molecular rigidity, we conducted spectroscopic measurements in various solvents, ranging from highly polar ones such as benzonitrile (BZN) and dimethyl sulfoxide (DMSO) to less polar solvents like toluene. To disentangle the synergic effects of polarity and rigidity of the environment, transient absorption measurements were conducted in solid state matrices possessing different polarity (PMMA and DPEPO) at room temperature. Both measurements demonstrate that rigidity plays a minor role in the excited state relaxation: CT state is populated in both matrices, and only a slight slowing-down of the ground state recovery is observed. Finally, the presence of Br atoms directly attached to the DPP core of the flexible molecules open and minor additional deactivation channel towards the population of a long lifetime excited state, ascribed as an excited triplet state. By combining experimental data with theoretical calculations, we developed a kinetic model describing the excited-state dynamics of a series of DPP derivatives, Global analysis of transient measurements enabled us to extract kinetic constants and state lifetimes, offering a comprehensive understanding of their photophysical behavior. These findings demonstrate how molecular rigidity and nitro group positioning influence excited-state relaxation pathways, providing valuable insights into the rational design of DPP-based materials with tailored optical properties for optoelectronic applications.