Re(I) complexes of substituted dppz: a computational and spectroscopic study

Area of Science:

• Coordination Chemistry
• Spectroscopy
• Computational Chemistry
• Photophysics

Background:

• Dipyrido[3,2-a:2',3'-c]phenazine (dppz)-based ligands are crucial in developing functional metal complexes.
• Understanding the excited-state properties of rhenium(III) complexes is vital for applications in photochemistry and photophysics.
• Electron-withdrawing substituents significantly influence the electronic and photophysical behavior of dppz ligands and their metal complexes.

Purpose of the Study:

• To synthesize and characterize novel dppz-based ligands and their rhenium(III) complexes.
• To investigate the electronic structure and excited-state dynamics of these complexes using a combination of spectroscopic and computational methods.
• To elucidate the nature of excited states and energy transfer pathways in these rhenium complexes.

Main Methods:

• Synthesis and X-ray crystallography of dipyrido[3,2-a:2',3'-c]phenazine (dppz) ligands and their [Re(CO)3(L)Cl] and [Re(CO)3(L)(py)]PF6 complexes.
• Spectroscopic techniques including Raman, resonance Raman, transient resonance Raman (TR2), and time-resolved infrared (TRIR) spectroscopy.
• Computational chemistry (DFT B3LYP) for frequency calculations and analysis of electronic structures.
• Electrochemical studies (cyclic voltammetry) and emission/absorption spectroscopy of ground and excited states.

Main Results:

• DFT calculations accurately assigned vibrational modes in Raman spectra, aiding chromophore identification in resonance Raman studies.
• Electrochemical studies revealed that substituents strongly influence the phenazine molecular orbital, altering reduction potentials by up to 200 mV.
• Transient absorption and TRIR spectroscopy identified distinct excited-state properties: intraligand (IL) π,π* states for [Re(CO)3(L)(py)]+ complexes and mixed IL/MLCT states for [Re(CO)3(L)Cl] complexes.

Conclusions:

• The study successfully characterized novel dppz-based rhenium(III) complexes, correlating structural modifications with electronic and photophysical properties.
• Spectroscopic and computational data provide a detailed understanding of the excited-state behavior, distinguishing between IL and MLCT states.
• The findings offer insights into the design of rhenium complexes with tailored photophysical properties for potential applications.