Mo₂Paddlewheel Complexes Functionalized with a Single MLCT, S₁Infrared-Active Carboxylate Reporter Ligand: Preparation and Studies of Ground and Photoexcited States

Abstract

From the reactions between Mo2(DAniF)3pivalate (DAniF = N,N'-di(p-anisyl)formamidinate) and the carboxylic acids LH, the title compounds Mo2(DAniF)3L have been prepared and characterized: compounds I (L = O2CC≡CPh), II (L = O2CC4H2SC≡CH), and III (L = O2CC6H4-p-CN). The new compounds have been characterized in their ground states by spectroscopy ((1)H NMR, ultraviolet-visible absorption, near-infrared absorption, and steady state emission), cyclic voltammetry, and density functional theory calculations. The compounds show strong metal Mo2 to ligand L δ-π* transitions in their visible spectra. The nature of the S1 (1)MLCT and T1 states has been probed by time-resolved (femtosecond and nanosecond) transient absorption and infrared spectroscopy. The observed shifts of the C≡C and C≡N vibrational modes are found to be consistent with the negative charge being localized on the single L in the S1 states, while the T1 states are (3)Mo2 δδ*. The present results are compared to earlier studies of the photoexcited states of trans-Mo2(2,4,6-triisopropylbenzoate)2L2 compounds that have been assigned as either localized or delocalized.

Synthesis, Structure, and Photophysical Properties of Mo2(NN)4 and Mo2(NN)2(TiPB)2, Where NN = N,N′-Diphenylphenylpropiolamidinate and TiPB = 2,4,6-Triisopropylbenzoate

“…The appearance of a single ν­(CC) band and the shift of the band 245 cm –1 to lower energy are indicative of a singly reduced ligand in the 1 MLCT S 1 state for I . The magnitude of this shift is very similar to the 240 cm –1 shift that is seen for the compound (DAniF) 3 Mo 2 (O 2 CCCPh), which only has one alkynyl group, and the charge has to be localized . In II , the appearance of two shifted ν­(CC) bands (70, 235 cm –1 ) is akin to that for the previously studied compound Mo 2 [( i PrN) 2 CCCPh] 2 (O 2 CMe) 2 (45, 241 cm –1 ) .…”

Carboxylate Anchors Act as Exciton Reporters in 1.3 nm Indium Phosphide Nanoclusters

“…For this to be the case, the exciton would have to induce a frequency shift in the carboxylate stretching vibrations of more than 100 cm –1 . Such a large shift seems unreasonable considering that typical carboxylate frequency shifts of similar ligand–nanoparticle systems are between 5 and 20 cm –1 , following photoinduced electronic structural rearrangement (e.g., electron transfer). , Additionally, this frequency shift argument would require the tIR signal in the region between 1350 and 1450 cm –1 to result from a convolution of the symmetric stretch GSB and the asymmetric stretch ESA, which is also unlikely given that the measured signals in the asymmetric and symmetric stretching regions are of similar intensities.…”

“…Such a large shift seems unreasonable considering that typical carboxylate frequency shifts of similar ligand−nanoparticle systems are between 5 and 20 cm −1 , following photoinduced electronic structural rearrangement (e.g., electron transfer). 35,45 Additionally, this frequency shift argument would require the tIR signal in the region between 1350 and 1450 cm −1 to result from a convolution of the symmetric stretch GSB and the asymmetric stretch ESA, which is also unlikely The measured % change is defined as (ε GS − ε ES )/ε GS . Excited state values were extracted assuming that 30.4% of the molecules were excited.…”

Probing Interligand Electron Transfer in the 1MLCT S1 Excited State of trans-Mo2L2L′2 Compounds: A Comparative Study of Auxiliary Ligands and Solvents

“…In IV , the single observed ν­(CC) band suggests that the 1 MLCT S 1 state is a class I type, and the electron density is present exclusively on one of the two NN ligands. We have previously studied the fs-TRIR spectra of (DAniF) 3 Mo 2 (O 2 CCCPh), which is a similar compound functionalized with a single MLCT ligand, and the transferred electron is necessarily localized. A single ν­(CC) band was observed in the 1 MLCT S 1 state with a shift of 240 cm –1 , which agrees well with the fs-TRIR spectra of IV .…”