Gas-phase Ion Spectroscopy of Flexible and Nonflexible Nitrophenolates: Effect of Locking the Two Phenyl Units in 4’-nitro-[1,1’-biphenyl]-4-olate by a Bridging Atom

Nitrophenolates (NPs) are molecular anions that can undergo charge-transfer(CT) transitions determined by the degree of electron delocalization betweenthe phenolate oxygen (donor group) and the nitro group (acceptor). Herewe have studied four different NPs: 4’-nitro-[1,1’-biphenyl]-4-olate (1),7-nitro-9H -carbazol-2-olate (NH linker, 2), 7-nitrodibenzo[b,d]furan-3-olate (oxygen linker, 3), and 7-nitrodibenzo[b,d]thiophen-3-olate (sulphurlinker, 4), and recorded their electronic absorption spectra when isolatedin vacuo to determine the effect of locking the biphenyl spacer group betweenthe donor and acceptor on transition energies. Absorption was identified fromion dissociation (action spectroscopy) using a homebuilt setup (sector massspectrometer combined with pulsed laser). We find that the absorption isbroad in the visible region for all four NPs with significant vibronic features.The lowest energy peak is at 601 ± 4 nm, 606 ± 4 nm, 615 ± 4 nm, and620 ± 4 nm, for 3, 4, 2, and 1, respectively. NP 1 is flexible, and its lowestenergy structure is nonplanar while the other three NPs are planar accordingto density functional theory calculations. Hence in the case of 1 the electronic transition has a higher degree of CT than for the other three, accounting for itsabsorption furthest to the red. Our work demonstrates that oxygen and sulphurare best at conveying the electronic coupling between the donor and acceptorsites as 3 and 4 absorb furthest to the blue (i.e., the degree of CT is lowestfor these two NPs). Based on the average spacing between the peaks in thevibrational progressions, coupling occurs to skeleton vibrational modes withfrequencies of 649 ± 69 cm−1 (3), 655 ± 49 cm−1 (4), and 697 ± 52 cm−1 (2).