Dynamics of the ³MLCT in Ru(II) Terpyridyl Complexes Probed by Ultrafast Spectroscopy: Evidence of Excited-State Equilibration and Interligand Electron Transfer

Abstract

Ground- and excited-state properties of [Ru(tpy)(2)](2+), [Ru(tpy)(ttpy)](2+), and [Ru(ttpy)(2)](2+) (where tpy = 2,2':6',2″-terpyridine and ttpy = 4'-(4-methylphenyl)-2,2':6',2″-terpyridine) in room temperature acetonitrile have been investigated using linear absorption, electrochemical, and ultrafast transient pump-probe techniques. Spectroelectrochemistry was used to assign features observed in the transient spectra while single wavelength kinetics collected at a variety of probe wavelengths were used to monitor temporal evolution of the MLCT excited state. From these data, the excited-state lifetime of each complex was recovered and the rate limiting decay step was identified. In the bis-heteroleptic complex [Ru(tpy)(ttpy)](2+), photoexcitation to the (1)MLCT manifold generates both tpy-localized and ttpy-localized excited states. Accordingly, interligand electron transfer (ILET) from tpy-localized to the ttpy-localized (3)MLCT excited states is observable and the time scale has been measured to be 3 ps. For the homoleptic complex [Ru(tpy)(2)](2+), evidence for equilibration of the (3)MLCT excited-state population with the (3)MC has been observed and the time scale is reported at 2 ps.

Probing excited state charge transfer dynamics in a heteroleptic ruthenium complex

“…18 In addition, a recent paper also reported the lifetime of the inter-ligand charge transfer process in the excited state of a ruthenium terpyridyl complex as about 2.5 ps. 28 Ultrafast localization of electronic charge in the excited state has also been observed for a DNA light switch complex, which however occurs on a relatively longer timescale. 25 The lifetime of the third component is wavelength dependent and becomes longer as the probe wavelength is tuned from 650 nm (lifetime is 15 ps) to 540 nm (lifetime is 35 ps).…”

“…22 Though ultrafast dynamics of the prototype [Ru(bpy) 3 ] 2+ have been extensively studied to discern ISC, vibrational relaxation and charge localization processes, real time dynamics studies of mixed ligand (heteroleptic) complexes have not been extensively undertaken and studies on such complexes has gained impetus in recent years. [23][24][25][26][27][28] As mentioned earlier, heteroleptic ruthenium complexes have gained a lot of interest in photovoltaic systems due to the broadened MLCT absorption envelope covering the solar spectrum due to multiple MLCT bands corresponding to different ligands. Effective use of the photoexcitation energy depends on the temporal kinetics of charge distribution around different ligands.…”

“…However, the real time dynamics of the charge localization processes with subpicosecond time resolution are still scarce and only a few studies are reported in literature. 18,[26][27][28] To explore the real time dynamics of charge transfer and vibrational relaxation of heteroleptic complexes, we have studied the excited state dynamics of [Ru(bpy) 2 (pap)] 2+ , (where bpy = 2,2 0 -bipyridine and pap = 2-(phenylazo)pyridine, Scheme 1) using subpicosecond transient absorption spectroscopy. This complex allowed us to monitor the charge transfer from the bpy to pap ligand because of the stronger p-acidity of the pap ligand in comparison to that of the bpy ligand and thus the lowest MLCT state (either in the singlet or in the triplet manifold) will be localized on the pap ligand, whereas, bpy-localized MLCT states will be the next higher energy state.…”

Photophysics of a Ruthenium 4H‐Imidazole Panchromatic Dye in Interaction with Titanium Dioxide

“…The ESA feature in the short-wavelength range is due to ligand-centered transitions in the formally reduced imidazole ligand, [14] and that in the long-wavelength range is due to ligand-to-metal charge transfer transitions. [37][38][39][40] Protonation causes a bathochromic shift of the ESA and GSB features. The minima of the GSB signals at 555 nm for Ru1(ImCOO), 563 nm for Ru1(ImCOOH), and 588 nm for Ru1(ImHCOOH) correspond to the steady-state long-wavelength absorption band of the respective protonation state of Ru1.…”

Experimental Evidence of Ultrafast Quenching of the 3MLCT Luminescence in Ruthenium(II) Tris-bipyridyl Complexes via a 3dd State

“…,16 to date there is no spectroscopic study of the corresponding ISC and internal conversion processes. In this Communication, we present ultrafast transient absorption spectra of [Ru(m-bpy)3 ] 2+ (m-bpy = 6-methyl-2,2′-bipyridine) and [Ru(tm-bpy) 3 ] 2+ (tm-bpy = 4,4′,6,6′-tetramethyl-2′,2′-bipyridine) and compare them to those of [Ru(bpy) 3 ] 2+ , thus identifying the 3 dd state in the two complexes as an intermediate state in the relaxation cascade.…”