Matrix Effects on Photoluminescence and Oxygen Sensitivity of a Molecular Ruby

Abstract

The molecular ruby analogue [Cr(ddpd)2]3+ (ddpd=N,N’‐dimethyl‐N,N’‐dipyridine‐2‐ylpyridine‐2,6‐diamine) exhibits near infrared (NIR) emission with a high photoluminescence (PL) quantum yield ΦPL of 11 % and a lifetime of 898 μs in deaerated water at room temperature. While ligand‐based control of the photophysical properties has received much attention, influences of the counter anions and microenvironment are still underexplored. In this study, the luminescence properties of the molecular ruby were systematically examined for the counter anions Cl−, Br−, [BF4]−, [PF6]−, [BPh4]−, and [BArF24]− in acetonitrile (MeCN) solution, in crystals, and embedded into polystyrene nanoparticles (PSNP). Stern‐Volmer analyses of the oxygen quenching studies in the intensity and lifetime domain showed the highest oxygen sensitivity of the complexes with the counter anions of [BF4]− and [BArF24]−, which also revealed the longest luminescence lifetimes. Embedding [Cr(ddpd)2][PF6]3 in PSNPs and shielding with poly(vinyl alcohol) yields a strongly NIR‐emissive oxygen‐insensitive material with a record ΦPL of 15.2 % under ambient conditions.

Molecular ruby: exploring the excited state landscape

“…3) with a lifetime of 1122 µs and quantum yields of 11% in aqueous and 13.7% in acetonitrile solution at room temperature. 7,15 The excitation spectra observed at 775 nm closely follow the absorption spectra in the region 300-500 nm demonstrating that all states efficiently evolve to the emissive states. Indeed, only a weak fluorescence ( 4 T 2 → 4 A 2 , blue area in Fig.…”

Spin-flip luminescence

“…The counter ions affected lifetime and quantum yield in parallel. Thus, only the nonradiative relaxation pathways from the long-lived 2 E/ 2 T 1 states were influenced by the anions, while the fast evolution from initially excited 4 T 2 or 4 LMCT states to the doublet manifold (ISC, VC, IC) remained unaffected [100].…”

“…Cr4[BF 4 ] 3 forms a contact ion pair with one [BF 4 ]anion on average in acetonitrile solution, due to the high charge of the complex cation [99]. In fact, the counter ions of Cr4 3+ affect the phosphorescence lifetime and quantum yield [100]. In acetonitrile solution, Φ increased from 5.2% with chloride anions to 13.6% with tetrakis(3,5-bis(trifluoromethyl)phenyl)borate [BArF 24 ] -.…”

Molecular Rubies in Photoredox Catalysis

“…General. The photocatalysts [Cr ( ddpd) 2 ][BF 4 ] 3 ( Otto, et al, 2015 ; Wang, et al, 2022 ), [Cr ( tpe) 2 ][BF 4 ] 3 ( Treiling, et al, 2019 ) and [Cr ( dmcbpy) 3 ][BF 4 ] 3 ( McDaniel, et al, 2010 ) were prepared according to literature procedures. 2,3-Dimethyl-1,3-butadiene ( DMB , 98%, Sigma-Aldrich with 100 ppm 3,5-di- tert -butyl-4-hydroxytoluene as stabilizer) was distilled prior to use.…”