Hydrogen Bond-Mediated Conjugates Involving Lanthanide Diphthalocyanines and Trifluoroacetic Acid (Lnpc2@TFA): Structure, Photoactivity, and Stability

Abstract

The interaction between lanthanide diphthalocyanine complexes, LnPc2 (Ln = Nd, Sm, Eu, Gd, Yb, Lu; Pc = C32H16N8, phthalocyanine ligand) and trifluoroacetic acid (TFA) was investigated in benzene, and the stability of the resulting molecular system was assessed based on spectral (UV-Vis) and kinetic measurements. Structural Density Functional Theory (DFT) calculations provided interesting data regarding the nature of the bonding and allowed estimating the interaction energy between the LnPc2 and TFA species. Conjugates are created between the LnPc2 and TFA molecules via hydrogen bonds of moderate strength (>N∙∙H··) at the meso- -bridges of the Pc moieties, which renders the sandwich system to flatten. Attachment of TFA is followed by rearrangement of electronic density within the chromophore system of the macrocycles manifested in considerable changes in their UV-Vis spectra and consequently the color of the studied solutions (from green to orange). The LnPc2@TFA conjugates including Nd, Sm, Eu, and Gd appeared evidently less photostable when exposed to UV radiation than the related mother compounds, whereas in the case of Yb and Lu derivatives some TFA-prompted stabilizing effect was noticed. The conjugates displayed the capacity for singlet oxygen generation in contrast to the LnPc2s itself. Photon upconversion through sensitized triplet–triplet annihilation was demonstrated by the TFA conjugates of Nd, Sm, Eu, and Gd.

Photostability of indium phthalocyanines in organic solvents

“…To assess the photostability a similar procedure was applied, as in previously reported work [19]. The tested solutions were exposed to UV radiation (500 µW/cm 2 ) in a thermostated quartz cuvette (at 20° C).…”

“…The energy of the excited states in metal phthalocyanines is quite enough to sensitize other molecules like molecular oxygen [18]. Since excited phthalocyanine may sensitize O 2 species to give rise to the formation of singlet molecular oxygen ( 1 Δ g ) along with some different reactive oxygen species (ROS), it is frequently referred to as the principal reason for the photo-bleaching of phthalocyanines [19][20][21][22][23][24]. Photobleaching mechanism is usually related to the attack of 1 O 2 or other ROS to the ring.…”

Lanthanide–tetrapyrrole complexes: synthesis, redox chemistry, photophysical properties, and photonic applications

“…Excitation in the Q band at 650 nm results in ligand fluorescence with modest anti-Stokes shifts of about 13-15 nm for some lanthanides (Nd, Sm, Eu, Gd) but not for the conjugates of Yb and Lu. 330…”

Noncovalent Interactions at Lanthanide Complexes

“…promoted by the Shibasaki catalysts [Ln(binol) 3 (M) 3 ] (M=alkali metal, Ln=lanthanide, binol=1,1′‐ bi ‐2‐naphthol) [102] . Moreover, the roles of intermolecular hydrogen bonding in the improved quantum yield and excellent luminescence stability, [103] photoactivity, [104] proton conductivity, [105] extraction, [63] etc. of lanthanide complexes have also been demonstrated.…”