Charge Separation in Donor-Chromophore-Acceptor Assemblies: Linkage and Driving Force Dependence of Photoinduced Electron Transfers

Abstract

A series of covalently linked Ru(bipyridine)3-donor-acceptor complexes was prepared where the donorto-chromophore and acceptor-to-chromophore methylene chain lengths were varied. Time-resolved absorption studies were performed to elucidate intramolecular electron transfer rates. The electron donor in the above series is a phenothiazine moiety linked to a bipyridine by a (-CH2-)P, p = 3-8 chain, and the electron acceptor is an MX-diquaternary-2,2'-bipyridinium moiety linked to a bipyridine by a (-CH2-)m, m -2, 3, 4 chain. Oxidative quenching of the Ru(bipyridine)3 metal-to-ligand charge transfer (MLCT) state followed by phenothiazine-to-ruthenium electron transfer resulted in a long-lived charge-separated state. A wavelengthdependent excitation resulted in a slowly decaying absorption which is assigned to the excited-state phenothiazine. The magnitude of this component in the transient absorption serves as an internal standard for determining relative quantum yield for formation of the charge-separated state. Marcus inverted region behavior was observed in back electron transfer. Rate constants for electron transfer from phenothiazine to Ru(ffl) decreased as the length of the bridging chain increased from p = 4 to 8. Chain length independence of the back electron transfer rate in the series of complexes with varied chromophore-acceptor distances (m = 2, 3, and 4) suggests the formation of an association complex during oxidative quenching of MLCT state and argues against a -bond superexchange pathway for back electron transfer.

Spin Relaxation in Ru‐Chromophore‐Linked Azine/Diquat Radical Pairs

“…11 Detailed models of the energetics and kinetics of the CSS formation and decay back to the ground state in these specific DCA triad systems have been provided earlier. [11][12][13][14][15][16][17][18][19][20][21] Ru II Of particular relevance to the present discussion is the observation that the CSS, which is a biradical cation, is formed with essentially pure triplet spin correlation. 12 For energetic reasons, this triplet radical pair cannot recombine to form the 3 MLCT state and can only form the singlet ground state.…”

Bifunctional Rhodium Intercalator Conjugates as Mismatch-Directing DNA Alkylating Agents

“…The mono-anion of 4,4'-dimethylbipyridine, obtained by deprotonation of 3 with one equivalent of freshly prepared lithium diisopropylamine, was reacted with an excess of 1,6-dibromohexane under carefully controlled conditions. [2,3] After aqueous workup and column chromatography on silica with a gradient of dichloromethane and diethylether, 4-(7-bromoheptyl)-4'-methylbipyridine 4 was obtained in 58 % yield. This was converted to phthalimide 5 in almost quantitative yield by reaction with potassium phthalimide followed by cleavage to 4-(7-aminoheptyl)-4'-methylbipyridine 6 with hydrazine monohydrate in methanol.…”

A donor–chromophore complex containing the polyazine bridging ligand 2,3-bis(2-pyridyl)pyrazine

“…Molecular diads and triads have been designed to covalently attach light absorbing chromophores, electron donors, and/or electron acceptors into a single molecular architecture to generate relatively long lived photoinduced charge separated states [1][2][3][4][5][6]. Triads are constructed to systematically promote sequential electron transfer events affording charge separated states which store light energy as chemical energy [1][2][3][4][5][6][7][8][9][10][11][12]. Diad systems support the study of electron transfer events leading to the charge separated states [11][12][13][14][15].…”

“…In these systems, the photochemistry is initiated by metal-to-ligand charge transfer (MLCT) excitation. Many of the electron transfer studies have focused on triad systems which incorporate covalently attached phenothiazine (PTZ) electron donors and various electron acceptors to ruthenium polypyridyl based chromophores [3][4][5][6]. In these systems, the MLCT state is initially generated and then quenched by electron transfer from the PTZ to the metal to form charge separated states.…”

“…1. Phenothiazine and its derivates are efficient intramolecular reductive quenchers [3][4][5][6]. The bridging ligand used in this study is dpp, Fig.…”