Donor‐Appended N,C‐Chelate Organoboron Compounds: Influence of Donor Strength on Photochromic Behaviour

Abstract

Recently, four-coordinated N,C-chelate organoboron compounds have been found to show many interesting photochemical transformations depending on the nature of their chelating framework. As such, the effect of substitution on the chelate ligand has been well-established and understood, but the impact of the aryl groups attached to the boron atom remains less clear. To investigate the effect of enhanced charge-transfer character, a series of new N,C-chelate organoboron compounds with donor-functionalized aryl groups have been synthesized and characterized using NMR, UV/Vis, and electrochemical methods. These compounds were found to possess bright and tunable charge-transfer luminescence which is dependent on the donor strength of the amino substituent. In addition, some of these compounds undergo photochromic switching, producing dark isomers of various colors. This work establishes that donor-functionalization of the aryl groups in N,C-chelate boron compounds is an effective strategy for tuning both the photophysical and photochemical properties of such systems. The new findings also help elucidate the influence of electronic structure on the photoreactivity of N,C-chelate organoboron compounds which appears to be as important as steric crowding around the boron atom.

Regioselective Photoisomerization/C−C Bond Formation of Asymmetric B(ppy)(Mes)(Ar): The Role of the Aryl Groups on Boron

“…Compared to 1 ( Φ PI =0.39), this change in Φ PI confirms that the electron density on the aryl ring plays a pivotal role in the photoisomerization process. While the aforementioned CT to the first excited state was proposed to be partially responsible for the photoreactivity of this class of compounds, the behavior of 7 and 8 towards UV light provides the first direct experimental evidence supporting this hypothesis (see the Supporting Information for TD‐DFT calculations). Compound 8 a is yellow owing to the hypsochromic shift of its absorption spectrum relative to those of the other dark isomers.…”

“…Compared to 1 (F PI = 0.39), this change in F PI confirms that the electron density on the aryl ring plays apivotal role in the photoisomerization process. While the aforementioned CT to the first excited state was proposed to be partially responsible for the photoreactivity of this class of compounds, [6][7][8] the behavior of 7 and 8 towards UV light provides the first direct experimental evidence supporting this hypothesis (see the Supporting Information for TD-DFT calculations). Compound 8a is yellow owing to the hypsochromic shift of its absorption spectrum relative to those of the other dark isomers.The isomerization of 8ainto 8bis more difficult than that of 1a into 1b and only observed upon heating 8a at 110 8 8Cf or 24 hours;t his result constitutes an indication that the Hmigration is likely to be hydridic in nature.T hese findings demonstrate that the Hatom migration can be controlled and even completely shut down by tuning the electronic properties of the aryl group.C ompound 9 decomposes upon irradiation as observed previously with other C 6 F 5 -substituted boron compounds.…”

Regioselektive Photoisomerisierung/C‐C‐Bindungsbildung von asymmetrischem B(ppy)(Mes)(Ar): die Rolle von Arylgruppen am Boratom

“…In früheren Arbeiten entdeckten wir ein photochromes Phänomen bei vierfach koordinierten Organoborverbindungen wie A (Schema 1), die thermisch reversible intramolekulare C-C-Bindungsbildungen/-spaltungen unter Bildung "dunkler Isomere" (z. B. NMe 2 ) [8] führen zu photochemisch inerten Molekülen infolge der gestärkten B-C Ar -Bindung und der Einführung von Elektronenübergängen, die eine Isomerisierung unwahrscheinlicher werden lassen. B) eingehen.…”

“…Die Kristallstrukturen von 1, 4 und 6 wurden mittels Rçntgenstrukturanalyse bestimmt. [5][6][7][8] Das Fehlen jeglicher Singulettsignale in der Hochfeldregion des 1 H-NMR-Spektrums spricht dafür, dass der Mes-Substituent nicht an der Photoisomerisierung beteiligt ist und dass die Reaktion mit hoher Regioselektivitätv erläuft. Der Hauptunterschied bei diesen vier Molekülen liegt in den B-C Ar -Bindungslängen:S ie nehmen ab,w enn o-Methylgruppen von den Aryleinheiten am Boratom entfernt werden (durchschnittliche B-C Ar -Bindungslängen:1 .649, 1.638, 1.633, 1.614 für A, 4, 1 bzw.B -(ppy)Ph 2 ).…”

“…[14] Ein Vergleich der Festkçrperstrukturen von 1 und 4 mit denen von A und B(ppy)Ph 2 (ppy = 2-Phenylpyridin) zeigt, dass sie vergleichbare B-N/B-C ppy -Bindungslängen und C Ar -B-C Ar /N-B-C ppy -Bindungswinkel haben. [5][6][7][8] Weitere Bestrahlung führt zur kompletten Umwandlung in die endgültigen Isomere 1b/2b, die luftstabil sind und vollständig mittels NMR-Spektroskopie und Rçntgenstrukturanalyse charakterisiert wurden (Abbildung 1c). Die dichtefunktionaltheoretisch (DFT-)optimierten Strukturen von 1, 4 und 6 stimmen gut mit den Kristallstrukturdaten überein (siehe SI).…”