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Intermediate donor–acceptor electronic coupling leads to a brilliant fluorescence behaviour.

Charge transfer (CT) is key for molecular photonics, governing the optical properties of chromophores comprising electron-rich and electron-deficient components. In photoexcited dyes with an acceptor– donor–acceptor or donor–acceptor–donor architecture, CT breaks their quadrupolar symmetry and yields dipolar structures manifesting pronounced solvatochromism. Herein, we explore the effects of electronic coupling through biaryl linkers on the excited-state symmetry breaking of such hybrid dyes composed of an electron-rich core, i.e., 1,4-dihydropyrrolo[3,2-b]pyrrole (DHPP), and pyrene substituents that can act as electron acceptors. Experimental and theoretical studies reveal that strengthening the donor–acceptor electronic coupling decreases the CT rates and the propensity for symmetry breaking. We ascribe this unexpected result to effects of electronic coupling on the CT thermodynamics, which in its turn affects the CT kinetics. In cases of intermediate electronic coupling, the pyrene-DHPP conjugates produce fluorescence spectra, spreading over the whole visible range, that in addition to the broad CT emission, show bands from the radiative deactivation of the locally excited states of the donor and the acceptors. Because the radiative deactivation of the low-lying CT states is distinctly slow, fluorescence from upper locally excited states emerge leading to the observed anti- Kasha behaviour. As a result, these dyes exhibit white fluorescence. In addition to demonstrating the multifaceted nature of the effects of electronic coupling on CT dynamics, these chromophores can act as broad-band light sources with practical importance for imaging and photonics. 

The Laporte rule dictates that one‐ and two‐photon absorption spectra of inversion‐symmetric molecules should display alternatively forbidden electronic transitions; however, for organic fluorophores, drawing clear distinction between the symmetric‐ and non‐inversion symmetric two‐photon spectra is often obscured due to prevalent vibronic interactions. We take advantage of consecutive single‐ and double‐protonation to break and then reconstitute inversion symmetry in a nominally symmetric diketopyrrolopyrrole, causing large changes in two‐photon absorption. By performing detailed one‐ and two‐photon titration experiments, with supporting quantum‐chemical model calculations, we explain how certain low‐frequency vibrational modes may lead to apparent deviations from the strict Laporte rule. As a result, the system may be indeed considered as an on‐off‐on inversion symmetry switch, opening new avenues for two‐photon sensing applications.

 

The Laporte rule dictates that one‐ and two‐photon absorption spectra of inversion‐symmetric molecules should display alternatively forbidden electronic transitions; however, for organic fluorophores, drawing clear distinction between the symmetric‐ and non‐inversion symmetric two‐photon spectra is often obscured due to prevalent vibronic interactions. We take advantage of consecutive single‐ and double‐protonation to break and then reconstitute inversion symmetry in a nominally symmetric diketopyrrolopyrrole, causing large changes in two‐photon absorption. By performing detailed one‐ and two‐photon titration experiments, with supporting quantum‐chemical model calculations, we explain how certain low‐frequency vibrational modes may lead to apparent deviations from the strict Laporte rule. As a result, the system may be indeed considered as an on‐off‐on inversion symmetry switch, opening new avenues for two‐photon sensing applications.