Photophysics of α-furil at room temperature and 77 K: Spectroscopic and quantum chemical studies

Abstract

Steady state and time resolved spectroscopic measurements have been exploited to assign the emissions from different conformations of α-furil (2, 2'-furil) in solution phase at room temperature as well as cryogen (liquid nitrogen, LN2) frozen matrices of ethanol and methylcyclohexane. Room temperature studies reveal a single fluorescence from the trans-planar conformer of the fluorophore or two fluorescence bands coming from the trans-planar and the relaxed skew forms depending on excitation at the nπ(∗) or the ππ(∗) absorption band, respectively. Together with the fluorescence bands, the LN2 studies in both the solvents unambiguously ascertain two phosphorescence emissions with lifetimes 5 ± 0.3 ms (trans-planar triplet) and 81 ± 3 ms (relaxed skew triplet). Quantum chemical calculations have been performed using density functional theory at CAM-B3LYP/6-311++G(∗∗) level to prop up the spectroscopic surveillance. The simulated potential energy curves (PECs) illustrate that α-furil is capable of giving two emissions from each of the S1 and the T1 states-one corresponding to the trans-planar and the other to the relaxed skew conformation. Contrary to the other 1,2-dicarbonyl molecular systems like benzil and α-naphthil, α-furil does not exhibit any fluorescence from its second excited singlet (S2) state. This is ascribed to the proximity of the minimum of the PEC of the S2 state and the hill-top of the PEC of the S1 state.

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 fluorophore, 9,9′-anthril (Scheme 1), was synthesized in the laboratory by using the standard method of Becker et al 23 The compound was purified by using silica gel column chromatography followed by repeated recrystallization with minimum amounts of ethanol. The sample was characterized by using traditional spectroscopic techniques like IR (KBr), 1 H NMR (500 MHz, CDCl 3 ), 13 C NMR (500 MHz, CDCl 3 ), and UV−vis absorption. The position of the >CO signal in the IR and the 1 H NMR and UV−vis absorption spectral patterns were consistent with the ones reported by Becker et al 23 13 C NMR showed two close lying signature peaks of the two carbonyl groups at around 190 ppm.…”

“…The molecule approaching toward the trans conformation (i.e., ϕ = 180°) leads to the red shift of the nπ* absorption band maximum due to the greater conjugation between the two aromatic rings leading to a reduction in the dipole moment and consequently solvent stabilization of the nπ* band. 7,13,14,32 The quantum chemical calculations give the intercarbonyl dihedral angles to be 126.6°, 113°, 156.1°, and 80.48°for benzil, α-naphthil, α-furil, and 2,2′-pyridil, respectively. 11,13,14 Figure 1B depicts the linear correlation between the intercarbonyl dihedral angle and the nπ* absorption band maxima for a series of 1,2-dicarbonyl compounds in MCH solvent.…”

“…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.…”