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A two-step route to strongly absorbing and efficiently orange to deep red fluorescent, doubly B/N-doped, ladder-type pyrrolo[3,2- b ]pyrroles has been developed. We synthesize and study a series of derivatives of these four-coordinate boron-containing, nominally quadrupolar materials, which mostly exhibit one-photon absorption in the 500–600 nm range with the peak molar extinction coefficients reaching 150 000, and emission in the 520–670 nm range with the fluorescence quantum yields reaching 0.90. Within the family of these ultrastable dyes even small structural changes lead to significant variations of the photophysical properties, in some cases attributed to reversal of energy ordering of alternate-parity excited electronic states. Effective preservation of ground-state inversion symmetry was evidenced by very weak two-photon absorption (2PA) at excitation wavelengths corresponding to the lowest-energy, strongly one-photon allowed purely electronic transition. π-Expanded derivatives and those possessing electron-donating groups showed the most red-shifted absorption- and emission spectra, while displaying remarkably high peak 2PA cross-section ( σ 2PA ) values reaching ∼2400 GM at around 760 nm, corresponding to a two-photon allowed higher-energy excited state. At the same time, derivatives lacking π-expansion were found to have a relatively weak 2PA peak centered at ca. 800–900 nm with the maximum σ 2PA ∼50–250 GM. Our findings are augmented by theoretical calculations performed using TD-DFT method, which reproduce the main experimental trends, including the 2PA, in a nearly quantitative manner. Electrochemical studies revealed that the HOMO of the new dyes is located at ca . −5.35 eV making them relatively electron rich in spite of the presence of two B − –N + dative bonds. These dyes undergo a fully reversible first oxidation, located on the diphenylpyrrolo[3,2- b ]pyrrole core, directly to the di(radical cation) stage.more » « less
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Abstract 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.
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Abstract 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.