Structural Versatility of Pyrene-2-(4,4,5,5-tetramethyl-[1,3,2]dioxaborolane) and Pyrene-2,7-bis(4,4,5,5-tetramethyl-[1,3,2]dioxaborolane)

Three polymorphs of pyrene-2,7-bis­(Bpin)₂ (<b>1</b>) and two of pyrene-2-(Bpin) (<b>2</b>), where Bpin = 4,4,5,5-tetramethyl-[1,3,2]­dioxaborolane, two different 1:1 co-crystals of <b>1</b> with toluene, and co-crystals of hexafluorobenzene (HFB) with <b>1</b> (of highly unusual 2:1 composition) and <b>2</b> (of usual 1:1 composition) were isolated, studied by X-ray diffraction and differential scanning calorimetry, and described using Hirshfeld surfaces and two-dimensional fingerprint plots. Centrosymmetric phases β- and γ-<b>1</b> have densities respectively lower and higher than the chiral α-<b>1</b>; α- and β-<b>2</b> have different packing modes, both with <i>Z</i>′ = 3. Compound <b>1</b> is prone to form channel host–guest structures, for example, α- and β-<b>1·</b>PhMe and <b>1</b>·2HFB. The drastically different stabilities of α- and β-<b>1·</b>PhMe are discussed. The complex <b>2·</b>HFB has a mixed-stack packing motif. The structural versatility of <b>1</b> and <b>2</b> is explained by synthon frustration between structurally incongruent pyrene and Bpin moieties

Fluorescence in Rhoda- and Iridacyclopentadienes Neglecting the Spin–Orbit Coupling of the Heavy Atom: The Ligand Dominates

We present a detailed photophysical study and theoretical analysis of 2,5-bis­(arylethynyl)­rhodacyclopenta-2,4-dienes (<b>1a</b>–<b>c</b> and <b>2a</b>–<b>c</b>) and a 2,5-bis­(arylethynyl)­iridacyclopenta-2,4-diene (<b>3</b>). Despite the presence of heavy atoms, these systems display unusually intense fluorescence from the S₁ excited state and no phosphorescence from T₁. The S₁ → T₁ intersystem crossing (ISC) is remarkably slow with a rate constant of 10⁸ s^–1 (i.e., on the nanosecond time scale). Traditionally, for organometallic systems bearing 4d or 5d metals, ISC is 2–3 orders of magnitude faster. Emission lifetime measurements suggest that the title compounds undergo S₁ → T₁ interconversion mainly via a thermally activated ISC channel above 233 K. The associated experimental activation energy is found to be Δ<i>H</i>_ISC^⧧ = 28 kJ mol^–1 (2340 cm^–1) for <b>1a</b>, which is supported by density functional theory (DFT) and time-dependent DFT calculations [Δ<i>H</i>_ISC^⧧(calc.) = 11 kJ mol^–1 (920 cm^–1) for <b>1a-H</b>]. However, below 233 K a second, temperature-independent ISC process via spin–orbit coupling occurs. The calculated lifetime for this S₁ → T₁ ISC process is 1.1 s, indicating that although this is the main path for triplet state formation upon photoexcitation in common organometallic luminophores, it plays a minor role in our Rh compounds. Thus, the organic π-chromophore ligand seems to neglect the presence of the heavy rhodium or iridium atom, winning control over the excited-state photophysical behavior. This is attributed to a large energy separation of the ligand-centered highest occupied molecular orbital (HOMO) and lowest unoccupied MO (LUMO) from the metal-centered orbitals. The lowest excited states S₁ and T₁ arise exclusively from a HOMO-to-LUMO transition. The weak metal participation and the cumulenic distortion of the T₁ state associated with a large S₁–T₁ energy separation favor an “organic-like” photophysical behavior