Abstract
The photophysical properties of the complex Bu4N[(4,4′-bpy)Re(CO)3(bpy-5,5′-diCOO)] were studied in protic and aprotic media with the aid of steady-state and time-resolved techniques and TD-DFT calculations. The absorption spectrum as well as the steady state and time resolved luminescence of the Re(I) complex display a marked solvent effect. The highest and lowest energy absorption bands experience a bathochromic shift as the polarity of the solvent decreases. In addition, the lowest energy band broadens. Two luminescence bands were observed around 430 and 600 nm in protic organic solvents like alcohols. The high energy emission is observed solely in aqueous solutions, while in aprotic solvents only the low energy luminescence is detected. TD-DFT calculations allowed us to identify the main electronic transitions in the low energy region as M1LLCTRe(CO)3→4,4′-bpy and M1LLCTRe(CO)3→bpy-5,5′-diCOO. The simulated absorption spectra of the Re(I) complex in H2O, protic (EtOH, MeOH) and aprotic (CHCl3, CH2Cl2, CH3CN) organic solvents follow the experimental absorption spectra with reasonable accuracy both in position and relative intensities. The magnitude of the calculated dipole moment (μ) increases with the dielectric constant of the solvent (εr). Besides, the energy of M1LLCTRe(CO)3→4,4′-bpy also increases with εr. However, the energy of the M1LLCTRe(CO)3→bpy-5,5′-diCOO transition is rather insensitive to εr. This disparity is attributed to the fact that the M1LLCTRe(CO)3→4,4′-bpy transition is nearly parallel to the orientation of μ while the M1LLCTRe(CO)3→bpy-5,5′-diCOO transition is almost perpendicular to it. Unrestricted TD-DFT calculations were successfully applied to the triplet species. It is observed that in the triplet state the ReN distances are shortened while ReC distances are elongated relative to the ground state. The calculated emission energy by TD-DFT and/or Δ(SCF) methods was compared to the experimental emission maximum in chloroform. All the experimental results as well as the theoretical calculations indicate that solvent effects on the steady state and time resolved luminescence of the Re(I) complex can be accounted by the coexistence of M3LLCTRe(CO)3→4,4′-bpy, M3LLCTRe(CO)3→bpy-5,5′-diCOO and 1IL excited states.
Highlights
The nature of the axial X ligand in fac-ReX(CO) (α-diimine) com3 plexes determines the fact that these compounds may or may not be strong luminophores, either in fluid solutions or at low-temperature glasses
Most of the fac-ReX(CO)3(α-diimine) complexes which annually appear on the literature reports are exclusively soluble in organic solvents while only a few of them can be managed in aqueous solutions at physiological pHs [14,15,16,17,18,19,20,21]
The electronic structure of the rhenium complex was studied by performing Density Functional Theory [29,30,31] (DFT) and Time Dependent Density Functional Theory [29–31] (DFT) [32,33,34] (TD-DFT) calculations using Gussian 09 software [35]
Summary
The nature of the axial X ligand in fac-ReX(CO) (α-diimine) com plexes determines the fact that these compounds may or may not be strong luminophores, either in fluid solutions or at low-temperature glasses. Most of the fac-ReX(CO)3(α-diimine) complexes which annually appear on the literature reports are exclusively soluble in organic solvents while only a few of them can be managed in aqueous solutions at physiological pHs [14,15,16,17,18,19,20,21]. In previous work we have synthesized and characterized a water soluble fac-ReX(CO)3(α-diimine) complex coordinating the ligands 2,2′-bipyridine-5,5′- dicarboxylate (bpy-5,5′-diCOO) and 4,4′-bipyridine (4,4′-bpy) [22].
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