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 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 3), 7-nitrodibenzo[b,d]thiophen-3-olate (sulphurlinker, 4), recorded their electronic absorption spectra when isolatedin vacuo to determine effect locking biphenyl spacer donor acceptor on transition energies. Absorption was identified fromion dissociation (action spectroscopy) using a homebuilt setup (sector massspectrometer combined with pulsed laser). We find isbroad in visible region for all NPs significant vibronic features.The lowest energy peak is at 601 ± 4 nm, 606 615 and620 3, 4, 2, 1, respectively. NP 1 flexible, its lowestenergy structure nonplanar while other three planar accordingto density functional theory calculations. Hence case has higher CT than three, accounting itsabsorption furthest red. Our work demonstrates sulphurare best conveying coupling between acceptorsites as 3 absorb blue (i.e., lowestfor these two NPs). Based average spacing peaks thevibrational progressions, occurs skeleton vibrational modes withfrequencies 649 69 cm−1 (3), 655 49 (4), 697 52 (2).