Exploration of photophysics of 2,2’-pyridil at room temperature and 77 K: a combined spectroscopic and quantum chemical approach

Abstract

The photophysics of 2,2'-pyridil has been explored thoroughly using steady state and time resolved fluorometric techniques at room temperature (RT) in liquid media as well as in glassy matrices at cryogenic temperature (77 K). Ethanol and methylcyclohexane are exploited for this purpose, as polar and non-polar media respectively. Notwithstanding the observation of multiple emissions from the fluorophore, the experiments unequivocally rule out emission from excited singlet states other than the S state, consistent with Kasha's rule. Among 1,2-dicarbonyl molecular systems, this behavior resembles that of α-furil, while it contradicts that of benzil and α-naphthil which exhibit S emissions. The dual fluorescence and dual phosphorescence of the fluorophore are ascribed to the emissions originating from the two conformers, namely near-trans and relaxed skew. Coexistence of the two conformers is substantiated by time resolved area normalized emission spectroscopy (TRANES) at both RT and 77 K. The potential energy curves (PECs) simulated from calculations based on density functional theory and its time dependent extension provide adequate support to the experimental observations.

Room-temperature phosphorescence of a supercooled liquid: kinetic stabilisation by desymmetrisation

“…In solution, 3 was virtually non-emissive, even under Ar ( Φ p = 0.04%, 2.2 × 10 −4 M in cyclohexane). 9,12 The PL spectrum of solution-phase 3 is in good agreement with that of the solvent-free liquid, which indicates that the emissive conformers are the same ( i.e. , the TP conformer) for both states (Fig.…”

Photophysics of Anthril in Fluids and Glassy Matrixes

“…The details of construction of such PECs in the ground state (S 0 ), excited singlet states (S 1 , S 2 , S 3 ), and the first excited triplet state (T 1 ) are available in several of our publications. 11,13,14,31 The constructed PECs of anthril in the excited singlet and triplet states provide a qualitative theoretical support for the assignment of the multiple emissions originating from various singlet and triplet state geometries.…”

“…The detailed technical methodology for TRES and TRANES studies at room temperature and 77 K have already been illustrated in one of our recent publications. 14 The quantum chemical calculations on the ground and excited states of anthril were performed by using the Gaussian 09 program. 26 For the global ground state energy minimized geometry of the probe, we have opted density functional theory (DFT) employing Becke's three-parameter hybrid functional B3 with the nonlocal correlation of Lee−Yang− Parr (B3LYP) functional using the 6-311++G** basis set.…”

“…For the assignment of the fluorescence and phosphorescence emission bands of the fluorophore, we have selected different time windows (depending on the fluorescence and phosphorescence lifetimes) at 77 K frozen matrixes to segregate both these bands precisely. [11][12][13][14]17 Importantly, coexistence of the two isomeric fluorescences and phosphorescences from the same electronic states (S 1 and T 1 respectively) are confirmed by time-resolved emission spectroscopy (TRES) and its extension, namely, time-resolved area normalized emission spectroscopy (TRANES). TRES provides useful information on the heterogeneity of the emissive species and the excited state kinetics.…”

“…11,12 However, α-furil and 2,2′-pyridil do not show any emission from the higher excited state due to the energy proximity between the S 2 and S 1 states and thus the molecules can effortlessly move from the S 2 state to the S 1 state. 13,14 In the present work, we have performed exhaustive spectroscopic studies of another 1,2-dicarbonyl compound, namely, anthril, in both polar and nonpolar solvents at room temperature and liquid nitrogen temperature (77 K) to assign the multiple emissions.…”