In-Depth Studies of Ground- and Excited-State Properties of Re(I) Carbonyl Complexes Bearing 2,2′:6′,2″-Terpyridine and 2,6-Bis(pyrazin-2-yl)pyridine Coupled with π-Conjugated Aryl Chromophores

Abstract

In the current work, comprehensive photophysical and electrochemical studies were performed for eight rhenium(I) complexes incorporating 2,2′:6′,2″-terpyridine (terpy) and 2,6-bis(pyrazin-2-yl)pyridine (dppy) with appended 1-naphthyl-, 2-naphthyl-, 9-phenanthrenyl, and 1-pyrenyl groups. Naphthyl and phenanthrenyl substituents marginally affected the energy of the MLCT absorption and emission bands, signaling a weak electronic coupling of the appended aryl group with the Re(I) center. The triplet MLCT state in these complexes is so low lying relative to the triplet 3 IL aryl that the thermal population of the triplet excited state delocalized on the organic chromophore is ineffective. The attachment of the electron-rich pyrenyl group resulted in a noticeable red shift and a significant increase in molar absorption coefficients of the lowest energy absorption of the resulting Re(I) complexes due to the contribution of intraligand charge-transfer (ILCT) transitions occurring from the pyrenyl substituent to the terpy/dppy core. At 77 K, the excited states of [ReCl(CO) 3 (L n -κ 2 N )] with 1-pyrenyl-functionalized ligands were found to have predominant 3 IL pyrene / 3 ILCT pyrene→terpy character. The 3 IL/ 3 ILCT nature of the lowest energy excited state of [ReCl(CO) 3 (4′-(1-pyrenyl)-terpy-κ 2 N )] was also evidenced by nanosecond transient absorption and time-resolved emission spectroscopy. Enhanced room-temperature emission lifetimes of the complexes [ReCl(CO) 3 (L n -κ 2 N )] with 1-pyrenyl-substituted ligands are indicative of the thermal activation between 3 MLCT and 3 IL/ 3 ILCT excited states. Deactivation pathways occurring upon light excitation in [ReCl(CO) 3 (4′-(1-naphthyl)-terpy-κ 2 N )] and [ReCl(CO) 3 (4′-(1-pyrenyl)-terpy-κ 2 N )] were determined by femtosecond transient absorption studies.

Impact of the Anthryl Linking Mode on the Photophysics and Excited-State Dynamics of Re(I) Complexes [ReCl(CO)3(4′-An-terpy-κ2N)]

“…Most importantly, the HOMO–LUMO energy gaps of compounds 1 and 2 become noticeably lower in comparison to the previously reported [ReCl(CO) 3 (4′-Ar-terpy-κ 2 N)] complex bearing the structural isomer 4′-(phenanthren-9-yl)-2,2′:6′,2″-terpyridine ( Figure S11 ). In contrast to 1 and 2 , the HOMO of [ReCl(CO) 3 (4′-Ar-terpy-κ 2 N)] with 9-phenanthryl-terpy is distributed over the {ReCl(CO) 3 } unit, 35 which also confirms a pivotal role of the molecular configuration of the appended aryl group. In contrast to linear anthracene, phenanthrene is nonlinear.…”

“…The geometry optimization shows good agreement with previously reported experimental X-ray data for structurally related Re(I) complexes. 35 , 68 , 70 − 73 The 9-anthryl and 2-anthryl groups introduced in the terpy ligand do not generate any noticeable structural changes in the {ReClN 2 C 3 } coordination core. The bond lengths and bond angles around the Re(I) ion are almost the same for 1 and 2 ( Table S2 ).…”

“… 48 , 64 , 65 A bathochromic shift of the lowest energy absorption of 1 and 2 caused by replacing acetonitrile with less polar chloroform is due to negative solvatochromism, which is well recognized for rhenium(I) tricarbonyl diimine complexes. 35 , 66 − 70 Intensive bands in the high-energy region of 1 and 2 are attributed to π–π*(terpy) and π–π*(anthracene) transitions ( Figure S9 ).…”

“…Femtosecond TA spectra were recorded using a pump–probe transient absorption spectroscopy system (Ultrafast Systems, Helios) described in our previous work. 35 TA experiments were carried out for the solution samples (in CHCl 3 , MeCN, and DMSO), stirred during the experiments to avoid photoproduct interference. The absorbance range was equal to 0.20–0.70 in the pumping wavelengths (corresponding to a concentration of 2.5 × 10 –4 mol·dm –3 ).…”

A fundamental role of the solvent polarity and remote substitution of the 2-(4-R-phenyl)-1H-imidazo[4,5-f][1,10]phenanthroline framework in controlling the ground- and excited-state properties of Re(i) chromophores [ReCl(CO)3(R-C6H4-imphen)]

“…107–109 The chosen functional and basis sets were successfully verified in our previous work for Re( i ) carbonyl complexes. 78,87,88,110,111 In all calculations, the chloroform solvent environment was simulated using the polarizable continuum model (PCM) correction. 112–114…”