Ligand-to-ligand Charge Transfer States Research Articles

Impact of ligand-centered excited states on luminescence sensitization in [formula omitted] complexes with [formula omitted]-diketones

In this study, two novel Pr3+ complexes with different 1,3-diketonate ligands were synthesized and investigated. To study the effect of the ancillary ligand on the energy transfer mechanisms in the complexes, a phenanthroline ligand was introduced. To take into account the influence of the ligand environment composed of different ligands on the energy transfer and relaxation processes, we compared the synthesized compounds with a similar complex containing the phenanthroline ligand. The spectroscopic studies in the visible and near-infrared spectral regions were supplemented with DFT and TD-DFT calculations. We found two ligand-to-ligand charge transfer (LLCT) states, with one state corresponding to energy transfer between 1,3-diketones and the other – to energy transfer from the 1,3-diketone to the phenanthroline motif. It was demonstrated that optical excitation via the latter channel leads to a fourfold increase in the luminescence quantum yield as compared with excitation via the π-π∗ transitions in 1,3-diketones. Moreover, both LLCT states provide sensitization of the Pr3+ luminescence involving the 3P0 and 3P1 levels.

Two Excited State Collaboration of Heteroleptic Ir(III)-Coumarin Complexes for H2 Evolution Dye-Sensitized Photocatalysts

Interfacial electron injection from a photoexcited surface-immobilized dye to a semiconductor substrate is a key reaction for dye-sensitized photocatalysts. We previously reported that the molecular orientation of heteroleptic Ir(III) photosensitizer on the TiO2 nanoparticle surface was important for efficient interfacial electron injection. In this work, to overcome the weak light absorption ability of heteroleptic Ir(III) photosensitizer and to improve the photoinduced charge-separation efficiency at the dye–semiconductor interface, we synthesized two heteroleptic Ir(III) complexes with different coumarin dyes, [Ir(C6)2(H4CPbpy)]Cl and [Ir(C30)2(H4CPbpy)]Cl [Ir-CX; X = 6 or 30; HC6 = 3-(2-enzothiazolyl)-7-(diethylamino)coumarin, HC30 = 3-(2-N-methylbenzimidazolyl)-7-N,N-diethylaminocoumarin, H4CPbpy = 4,4′-bis(methylphosphonic acid)-2,2′-bipyridine], as the cyclometalated ligands and immobilized them on the surface of Pt-cocatalyst-loaded TiO2 nanoparticles. Ultraviolet-visible absorption and emission spectroscopy revealed that the singlet ligand-centered (1LC) absorption and triplet 3LC emission bands of Ir-C30 occurred at shorter wavelengths than those of Ir-C6, while time-dependent density-functional-theory data suggested that the ligand-to-ligand charge transfer (LLCT) excited states of the two complexes were comparable. The photocatalytic H2 evolution activity of the Ir-C6-sensitized Pt-TiO2 nanoparticles (Ir-C6@Pt-TiO2) under visible light irradiation (λ > 420 nm) was higher than that of Ir-C30@Pt-TiO2. In contrast, their activities were comparable under irradiation with monochromatic light (λ = 450 ± 10 nm), which is absorbed comparably by both Ir-CX complexes. These results suggest that the internal conversion from the higher-lying LC state to the LLCT state effectively occurs in both Ir-CX complexes to trigger electron injection to TiO2.

DFT/TDDFT computational study of the structural, electronic and optical properties of rhodium (III) and iridium (III) complexes based on tris-picolinate bidentate ligands.

The electronic structures and spectroscopic properties of two complexes [M(pic)3] (M=Ir, Rh) containing picolinate as bidentate ligands have been calculated by means density functional theory (DFT) and time-dependent DFT/TD-DFT using three hybrid functionals B3LYP, PBE0 and mPW1PW91. The PBE0 and mPW1PW91 functionals, which have the same HF exchange fraction (25%), give similar results and do not differ drastically from B3LYP results. Calculated geometric parameters of the complexes are in good agreement with the available experimental data. The UV absorptions observed in acetonitrile were assigned on the basis of singlet state transitions. The most intense band observed in the UV-C region corresponds to ligand-to-ligand charge transfer states (LLCT) in both complexes. The theoretical spectrum of the rhodium complex is characterized by a large degree of mixing between metal-to-ligand-charge-transfer (MLCT), LLCT and metal centered (MC) states in the UV-A region. The presence of low-lying excited states with MC character affects the absorption spectrum under spin-orbit coupling (SOC) effects and play important roles in the photochemical properties. Graphical abstract Frontier molecular orbital diagram of mer-M(pic)3 (M=Ir, Rh).

Electron transfer rate modulation in a compact Re(I) donor-acceptor complex.

Formation of the charge transfer state with the rate constant of (10 ps)(-1) has recently been reported for the complex fac-[Re(I)(CO)3(DCEB)(3DMABN)] (ReEBA); where 3DMABN is 3-dimethylaminobenzonitrile, serving as an electron donor, and DCEB is 4,4'-(dicarboxyethyl)-2,2-bipyridine, serving as an electron acceptor (Y. Yue et al., J. Phys. Chem. A, 118, 10407). 3-Pulse UV-pump - IR-pump - IR-probe spectroscopy is used in this work to study how the charge separation reaction in ReEBA to form a ligand to ligand charge transfer state (LLCT) can be modulated by vibrational excitation of various modes of the complex. While no significant rate modulation was found when the cyano group stretching mode of the 3DMABN donor was excited, a sizable effect was found when the ring stretching mode of the DCEB acceptor was excited. The accumulation of the charge separated state (LLCT state) in the 3-pulse experiment was observed as a sharp excited-state vibrational peak of the symmetric stretch of the three facial carbonyl groups, νSS(CO). Modeling indicates that the rate of charge separation is increased by ca. 28% when vibrational excitation is present. The vibronic coupling signal of the bpy ring mode and νSS(CO) as well as the energy transport dynamics from bpy to carbonyl contributed to the 3-pulse signal and was studied as well using the 3-pulse method. Energy transport between the same modes in the ground electronic state was measured by relaxation-assisted two-dimensional infrared (RA 2DIR) spectroscopy. The energy transport times of 4 ± 0.7 and 5 ± 1.5 ps were found for the ground and excited electronic states.

Full-electron ligand-to-ligand charge transfer in a compact Re(I) complex.

Ligand-to-ligand charge transfer (LLCT) states in transition metal complexes are often characterized by fractional electron transfer due to coupling of the LLCT state with many other states via the metal. We designed and characterized a compact Re(I) complex that displays essentially full-electron charge transfer in the LLCT excited state. The complex, [Re(DCEB)(CO)3(L)](+) (DCEB = 4,4'-dicarboxyethyl-2,2'-bipyridine), referred to as ReEBA, features two redox active ligands with strong electron accepting (DCEB) and electron donating (L is 3-dimethylaminobenzonitrile (3DMABN)) properties. The lowest energy excited state formed with a ca. 10 ps time constant and was characterized as the full-electron 3DMABN → DCEB LLCT state using time-resolved infrared spectroscopy (TRIR), transient absorption spectroscopy, and DFT computations. Analysis of a range of vibrational modes helped to assign the charge transfer characteristics of the complex. The LLCT state lifetime in ReEBA shows a strong dependence on the solvent polarity and features solvent dependent frequency shifts for several vibrational reporters. The formation of a full-electron LLCT state (∼92%) was enabled by tuning the redox properties of the electron accepting ligand (DCEB) and simultaneously decoupling the redox active group of the electron donating ligand (3DMABN) from the metal center. This strategy is generally applicable for designing compact transition metal complexes that have full-electron LLCT states.

Directional charge transfer and highly reducing and oxidizing excited states of new dirhodium(ii,ii) complexes: potential applications in solar energy conversion

Two new series of dirhodium(II,II) complexes cis-[Rh2(μ-DTolF)2(L)2][BF4]2 and cis-[Rh2(μ-F-form)2(L)2][BF4]2 were synthesized and fully characterized (DTolF = p-ditolylformamidinate, F-form = p-difluorobenzylformamidinate; L = the chelating diimine ligands dpq (dipyrido[3,2-f:2′,3′-h]quinoxaline), dppz (dipyrido[3,2-a:2′,3′-c]phenazine) and dppn (benzo[i]dipyrido[3,2-a:2′,3′-h]quinoxaline). The complexes undergo facile oxidation and exhibit directed ligand-to-ligand charge transfer (LLCT) excited states upon excitation from the corresponding formamidinate to the diimine ligand. Time-resolved studies reveal that the LLCT states decay with lifetimes that range from 16 to 100 ps and generate a longer-lived excited state. For complexes with dpq and dppz ligands, the longer-lived excited state, with lifetimes that range from 40 to 100 ns, has been assigned as 3MC (MC = metal-centered) arising from the Rh2(π*) → Rh2(σ*) transition. In the case of cis-[Rh2(μ-DTolF)2(dppn)2]2+ and cis-[Rh2(μ-F-form)2(dppn)2]2+, the dppn-centered 3ππ* excited state is observed from ∼10 ps to ∼2 ns, but following its decay, the 3MC state of each complex is observed with lifetimes of 2.4 and 3.0 μs, respectively. The long lifetimes observed for the dppn complexes is explained by a pre-equilibrium of the low-lying 3ππ* and 3MC states. The excited state oxidation potentials, E*ox(1LLCT), for the complexes are calculated to lie between −2.5 and −2.8 V vs. SCE and E*ox(3MC) ∼ −1.8 V vs. SCE. The 1LLCT excited states of the complexes are also good oxidizing agents, with E*red(1LLCT) ∼ +1.3 V vs. SCE, making them significantly better oxidizing and reducing agents than commonly used Ru(II) complexes.

Nonlinear Absorbing Cationic Iridium(III) Complexes Bearing Benzothiazolylfluorene Motif on the Bipyridine (N∧N) Ligand: Synthesis, Photophysics and Reverse Saturable Absorption

Four new heteroleptic cationic Ir(III) complexes bearing benzothiazolylfluorene motif on the bipyridine (N∧N) (1 and 2) and phenylpyridine (C∧N) (3 and 4) ligands are synthesized and characterized. The influence of the position of the substituent and the extent of π-conjugation on the photophysics of these complexes is systematically investigated by spectroscopic methods and simulated by time-dependent density functional theory (TDDFT). The complexes exhibit ligand-centered (1)π,π* transitions with admixtures of (1)ILCT (π(benzothiazolylfluorene) → π*(bpy)) and (1)MLCT (metal-to-ligand charge transfer) characters below 475 nm, and very weak (1,3)MLCT and (1,3)LLCT (ligand-to-ligand charge transfer) transitions above 475 nm. The emission of these complexes at room temperature in CH2Cl2 solutions is ascribed to be predominantly from the (3)MLCT/(3)LLCT states for 1 and from the (3)π,π* state for 2, while the emitting state of 3 and 4 are assigned to be an admixture of (3)MLCT, (3)LLCT, and (3)π,π* characters. The variations of the photophysical properties of 1-4 are attributed to different degrees of π-conjugation in the bipyridine and phenylpyridine ligands induced by different positions of the benzothiazolylfluorenyl substituents on the bipyridine ligand and different extents of π-conjugation in the phenylpyridine ligands, which alters the energy and lifetime of the lowest singlet and triplet excited states. 1-4 all possess broadband transient absorption (TA) upon nanosecond laser excitation, which extends from the visible to the NIR region. Therefore, 1-4 all exhibit strong reverse saturable absorption (RSA) at 532 nm for ns laser pulses. However, the TA of complexes 1, 2, and 3 are much stronger than that of 4. This feature, combined with the difference in ground-state absorption and triplet excited-state quantum yield, result in the difference in RSA strength, which follows this trend: 1 ≈ 2 ≈ 3 > 4. Therefore, complexes 1-3 are strong reverse saturable absorbers at 532 nm and could potentially be used as broadband nonlinear absorbing materials.

Efficient Charge Separation in TiO2 Films Sensitized with Ruthenium(II)-Polypyridyl Complexes: Hole Stabilization by Ligand-Localized Charge-Transfer States

We have studied the interfacial electron-transfer dynamics on TiO(2) film sensitized with synthesized ruthenium(II)-polypyridyl complexes--[Ru(II)(bpy)(2)(L(1))] (1) and [Ru(II)(bpy)(L(1))(L(2))] (2), in which bpy=2,2'-bipyridyl, L(1)=4-[2-(4'-methyl-2,2'-bipyridinyl-4-yl)vinyl]benzene-1,2-diol, and L(2)=4-(N,N-dimethylaminophenyl)-2,2'-bipyridine-by using femtosecond transient absorption spectroscopy. The presence of electron-donor L(2) and electron-acceptor L(1) ligands in complex 2 introduces lower energetic ligand-to-ligand charge-transfer (LLCT) excited states in addition to metal-to-ligand (ML) CT manifolds of complex 2. On photoexcitation, a pulse-width-limited (<100 fs) electron injection from populating LLCT and MLCT states are observed on account of strong catecholate binding on the TiO(2) surface. The hole is transferred directly or stepwise to the electron-donor ligand (L(2)) as a consequence of electron injection from LLCT and MLCT states, respectively. This results an increased spatial charge separation between the hole residing at the electron-donor (L(2)) ligand and the electron injected in TiO(2) nanoparticles (NPs). Thus, we observed a significant slow back-electron-transfer (BET) process in the 2/TiO(2) system relative to the 1/TiO(2) system. Our results suggest that Ru(II) -polypyridyl complexes comprising LLCT states can be a better photosensitizer for improved electron injection yield and slow BET processes in comparison with Ru(II)-polypyridyl complexes comprising MLCT states only.

Molecular logic circuit based on a multi-state mononuclear platinum(ii) terpyridyl complex

A new type of multi-state platinum(ii) terpyridyl acetylide complex: platinum(ii) 4'-(4-dimethylaminophenyl)-2,2' : 6',2''-terpyridyl ferrocenyl acetylide (1), together with its reference complexes: platinum(ii) 4'-(4-methylphenyl)-2,2' : 6',2''-terpyridyl ferrocenyl acetylide (2), platinum(ii) 4'-(4-dimethylaminophenyl)-2,2' : 6',2''-terpyridyl phenyl acetylide (3) and platinum(ii) 4'-(4-methylphenyl)-2,2' : 6',2''-terpyridyl phenyl acetylide (4), are designed and synthesized. UV-vis-NIR absorption and electrochemical studies demonstrate that complex 1 possesses three ordered intraligand charge transfer (ILCT), ligand-to-ligand charge transfer (LLCT) and metal-to-ligand charge transfer (MLCT) states, and the successive modulation of the intramolecular ILCT, LLCT and MLCT states in a single mononuclear platinum(ii) complex 1 is well evidenced by their reference complexes 2-4. More interestingly, a near-infrared absorption band ranging from 720-1000 nm was clearly observed for complexes 1 and 2 upon oxidation of the σ-bonded ferrocene-ethynyl group by Fe(ClO(4))(3), which was tentatively assigned to platinum(ii)-disturbed ligand-to-metal charge transfer (LM'CT) of the ferrocenium group. These promising features allow us to integrate the multiple states to reproduce basic logic operations at the molecular level. A four-state (LLCT, ILCT, MLCT and LM'CT) molecular switch of complex 1 that responds to the stimulation of H(+), Fe(ClO(4))(3) and Zn producing four outputs has been encoded in binary digits. The logic function executed by complex 1 is equivalent to that of a combinational logic circuit integrating seven or eight logic functions as AND, OR, NOT. A three-input-four-output molecular logic circuit has been established.

Ground- and Excited-State Infrared Spectra of an Azacrown-Substituted [(bpy)Re(CO)3L]+ Complex: Structure and Bonding in Ground and Excited States and Effects of Ba2+ Binding

Ground- and excited-state infrared spectra are reported for a [(bpy)ReI(CO)3L]+ complex (bpy = 2,2'-bipyridine) in which L contains an azacrown ether that is linked to Re via an amidopyridyl group. Ground-state band assignments are made with the aid of spectra from model complexes in which a similar electron-donating dimethylamino group replaces the azacrown, in which an electron-donor group is absent, and from the L ligands, in conjunction with DFT calculations. Picosecond time-resolved IR (TRIR) spectra in the nu(CO) region show bands characteristic of a metal-to-ligand charge-transfer (MLCT) excited state, [(bpy*-)ReII(CO)3L]+, from the complex in which an electron-donor group is absent, whereas those from the azacrown complex show bands of an MLCT state evolving into those characteristic of a ligand-to-ligand charge-transfer (LLCT) excited state, [(bpy*-)ReI(CO)3(L*+)]+, formed upon intramolecular electron transfer. Picosecond TRIR spectra of the azacrown complex in the fingerprint region show strong L ligand bands that indicate that significant charge redistribution occurs within this ligand in the MLCT state and that decay as the LLCT state forms. Picosecond TRIR spectra obtained when Ba2+ was complexed to the azacrown show bands of only an MLCT state at all times up to 2 ns, consistent with the presence of Ba2+ inhibiting electron transfer from the azacrown N atom to form the LLCT state, and the positions of the bands in the fingerprint region provide direct evidence for the proposal that charge redistribution within the L ligand induces Ba2+ release from the azacrown in the MLCT state.

Switching between Ligand‐to‐Ligand Charge‐Transfer, Intraligand Charge‐Transfer, and Metal‐to‐Ligand Charge‐Transfer Excited States in Platinum(II) Terpyridyl Acetylide Complexes Induced by pH Change and Metal Ions

A series of platinum(II) terpyridyl alkynyl complexes, [Pt{4'-(4-R1-C6H4)terpy}(C[triple chemical bond]C-C6H4-R(2)-4)]ClO4 (terpy=2,2':6',2''-terpyridyl; R1=R2=N(CH3)2 (1); R1=N(CH3)2, R2=N-[15]monoazacrown-5 (2); R1=CH3, R2=N(CH3)2 (3); R1=N(CH3)2, R2=H (4); R1=CH3, R2=H (5)), has been synthesized and the photophysical properties of the complexes have been examined through measurement of their UV/Vis absorption spectra, photoluminescence spectra, and transient absorptions. Complex 3 shows a lowest-energy absorption corresponding to a ligand-to-ligand charge-transfer (LLCT) transition from the acetylide to the terpyridyl ligand, whereas 4 shows an intraligand charge-transfer (ILCT) transition from the pi orbital of the 4'-phenyl group to the pi* orbital of the terpyridyl. Upon protonation of the amino groups in 3 and 4, their lowest-energy excited states are switched to dpi(Pt)-->pi*(terpy) metal-to-ligand charge-transfer (MLCT) states. The lowest-energy absorption for 1 and 2 may be attributed to an LLCT transition from the acetylide to the terpyridyl. Upon addition of an acid to a solution of 1 or 2, the amino group on the acetylide is protonated first, followed by the amino group on the terpyridyl. Thus, the lowest excited state of 1 and 2 can be successively switched from the LLCT state to the ILCT state and then to the MLCT state by controlling the amount of the acid added. Such switches in the excited state are fully reversible upon subsequent addition of a base to the solution. Sequential addition of alkali metal or alkaline earth metal ions and then an acid to a solution of 2 also leads to switching of its lowest excited state from the LLCT state, first to the ILCT state and then to the MLCT state. All of the complexes exhibit a transient absorption of the terpyridyl anion radical, which is present in all of the LLCT, ILCT, and MLCT states. However, the shape of the transient absorption spectrum depends on both the substitution pattern on the terpyridyl moiety and the nature of the excited state.

Electronic Structure and Excited States of Rhenium(I) Amido and Phosphido Carbonyl−Bipyridine Complexes Studied by Picosecond Time-Resolved IR Spectroscopy and DFT Calculations

UV-vis absorption and picosecond time-resolved IR (TRIR) spectra of amido and phosphido complexes fac-[Re(ER2)(CO)3(bpy)] (ER2 = NHPh, NTol2, PPh2, bpy = 2,2'-bipyridine, Tol = 4-methylphenyl) were investigated in conjunction with DFT and TD-DFT calculations in order to understand their ground-state electronic structure, low-lying electronic transitions and excited-state character and dynamics. The HOMO is localized at the amido/phosphido ligand. Amide and phosphide ligands are sigma-bonded to Re, the pi interaction being negligible. Absorption spectra show a weak band at low energies (1.7-2.1 eV) that arises from essentially pure ER(2) --> bpy ligand-to-ligand charge transfer (LLCT). The lowest excited state is the corresponding triplet, (3)LLCT. Low triplet energies and large distortions diminish the excited-state lifetimes to 85 and 270 ps for NHPh and NTol(2), respectively, and to ca. 30 ps for PPh2. nu(CO) vibrations undergo only very small (<or=10 cm(-1)) shifts upon excitation, attesting to its LLCT character, which hardly affects the electron-density distribution on the Re(CO)3 moiety. Relaxation of the (3)LLCT state occurs with complex dynamics ranging from units to tens of picoseconds. The "pure" LLCT excitation, which does not mix with the Re --> bpy MLCT character, is a unique feature of the amido/phoshido complexes, whose lowest excited state can be viewed as containing a highly unusual aminyl/phosphinyl radical-cationic ligand. For comparison, the amino and phosphino complexes fac-[Re(NHPh(2))(CO)3(bpy)]+ and fac-[Re(PPh3)(CO)3(bpy)]+ are shown to have the usual Re --> bpy (3)MLCT lowest excited states, characterized by upshifted nu(CO) bands.

Picosecond Forward Electron Transfer and Nanosecond Back Electron Transfer in an Azacrown-Substituted [(bpy)Re(CO)3(L)]+ Complex: Direct Observation by Time-Resolved UV−Visible Absorption Spectroscopy

Intramolecular electron transfer in the excited state of a [(bpy)ReI(CO)3(L)]+ complex (bpy = 2,2‘-bipyridine), in which L contains a pendant azacrown ether that acts as an electron donor (L = N-[4-(4,7,10,13-tetraoxa-1-azacyclopentadecyl)benzoyl]-4-aminopyridine), has been studied directly using picosecond and nanosecond time-resolved UV−visible absorption spectroscopy. Picosecond studies show that the metal-to-ligand charge-transfer (MLCT) state produced on excitation, [(bpy•-)ReII(CO)3(L)]+, undergoes forward electron transfer with a rate constant of kFET = 2.0 × 109 s-1 to generate a ligand-to-ligand charge-transfer (LLCT) state, [(bpy•-)ReI(CO)3(L•+)]+, in which the metal has been reduced back to Re(I) and charge separation has been effected between the bipyridine and azacrown ligands. Nanosecond studies show that the LLCT state returns to the ground state by back electron transfer from the bipyridine to azacrown ligand, with a rate constant of kBET = 5.3 × 107 s-1. Studies of complexes in which the azacrown complex is protonated, or is absent, demonstrate that intramolecular electron transfer to form the LLCT state does not occur in these cases. Forward electron transfer in the azacrown complex takes place on the picosecond time scale: it is weakly exoergonic and occurs in the Marcus normal region, with electronic coupling between the azacrown ligand and the rhenium metal center of ca. 100 cm-1. Back electron transfer takes place on the nanosecond time scale: it is strongly exoergonic and occurs in the Marcus inverted region, with much weaker electronic coupling between the bipyridine and azacrown ligands. The rapid formation of a long-lived charge-separated state indicates that this molecule has a suitable design for a photochemical device.

Photoreactivity of (2,9-dimethyl-1,10-phenanthroline) copper(I) borohydride

The complex (dmp)Cu I(BH 4) with dmp=2,9-dimethyl-1,10-phenanthroline displays a long-wavelength absorption at λ max =465 nm which is assigned to a BH 4 −→dmp ligand-to-ligand charge transfer (LLCT) transition. In the solid state the LLCT state is emissive (λ max =626 nm) . In non-coordinating solvents such as CH 2Cl 2,CHCl 3 or toluene the complex undergoes a photoredox decomposition leading to the generation of metallic copper. At λ irr =436 nm ( dmp) Cu I ( BH 4) disappears with φ=1.2×10 −3.

Direct Observation of Ultrafast C-C Bond Fragmentation in a Diamine Radical Cation

Nanosecond and picosecond transient absorption studies were carried out on fac-(bpy)Re{sup 1}(CO){sub 3}(DA){sup +} (1), where bpy is 2,2{prime}-bipyridine and DA is the 1,2-diamine-substituted pyridine erythro-l-[p-[4-(pyridyl)methyl]-anilino]-2piperidino-1, 2-piperidino-1,2-diphenylethane. Photoexcitation of the d{pi} (Re) {pi}(bpy) metal-to-ligand charge-transfer excited state in 1 initiates the following sequence: (1) forward electron transfer from the 1,2-diamine unit to the photoexcited (bpy)Re(CO){sub 3}{sup -} chromophore to produce the ligand-to-ligand charge transfer (LLCT) state, (bpy{sup -})Re{sup 1}(CO){sub 3}(DA{sup +}); (2) C-C bond fragmentation of the diamine radical cation unit. Nanosecond transient absorption spectral data indicate that the {alpha}-amino radical which is the primary product of the C-C bond fragmentation is produced during the 10 ns laser excitation pulse. Picosecond pump-probe studies of 1 allow direct determination of the kinetics of back electron transfer in the LLCT state (1 x 10{sup 8} s{sup -1}) and for C-C bond fragmentation in the diamine radical cation (3 x 10{sup 8} s{sup -1}). 36 refs., 2 figs.

C-C Bond Fragmentation as a Probe for Photoinduced Intramolecular Electron Transfer

Photochemical and photophysical studies are reported for the ~omplex,fac-(bpy)Re’(CO)3(DA)~+ (e-l), where bpy is 2,2’-bipyridine and DA is a “reactive donor ligand” that contains a vicinal diamine functionality. Photoexcitation of e-1 into the dn (Re) -. n* (bpy) metal-to-ligand charge transfer (MLCT) excited state leads to formation of the ligand-to-ligand charge transfer (LLCT) state, *(bpy’-)Re1(CO)3(DA’+)]+, via diamine ligand-to-Re electron transfer. In the LLCT state, the reactive donor ligand radical cation undergoes an exceedingly rapid heterolytic C-C bond fragmentation reaction to produce an iminium ion and an a-amino radical fragment. Steady-state photochemical studies reveal that in air-saturated solution the only products observed arise from C-C bond fragmentation, which implies that under these conditions bond fragmentation is irreversible. Laser flash photolysis studies indicate that the a-amino radical that is produced by bond fragmentation in the LLCT state absorbs strongly in the near-UV region. A kinetic analysis is carried out under the assumptions that: (1) the quantum yield for formation the LLCT state can be derived from MLCT emission lifetime data on the diamine complex and a suitable non-donor-substituted model complex and (2) C-C bond fragmentation is irreversible. This analysis affords lower limits for the rates of back electron transfer and bond fragmentation (1.5 x lo8 and 1.0 x lo8 s-l, respectively).