The [Ru(phen)2(dppz)]2+ complex (1) is non-emissive in water but is highly luminescent in organic solvents or when bound to DNA, making it a useful probe for DNA binding. To date, a complete mechanistic explanation for this “light-switch” effect is still lacking. With this in mind we have undertaken an ultrafast time resolved infrared (TRIR) study of 1 and directly observe marker bands between 1280–1450 cm−1, which characterise both the emissive “bright” and the non-emissive “dark” excited states of the complex, in CD3CN and D2O respectively. These characteristic spectral features are present in the [Ru(dppz)3]2+ solvent light-switch complex but absent in [Ru(phen)3]2+, which is luminescent in both solvents. DFT calculations show that the vibrational modes responsible for these characteristic bands are predominantly localised on the dppz ligand. Moreover, they reveal that certain vibrational modes of the “dark” excited state couple with vibrational modes of two coordinating water molecules, and through these to the bulk solvent, thus providing a new insight into the mechanism of the light-switch effect. We also demonstrate that the marker bands for the “bright” state are observed for both Λ- and Δ-enantiomers of 1 when bound to DNA and that photo-excitation of the complex induces perturbation of the guanine and cytosine carbonyl bands. This perturbation is shown to be stronger for the Λ-enantiomer, demonstrating the different binding site properties of the two enantiomers and the ability of this technique to determine the identity and nature of the binding site of such intercalators.

The photochemistry and photophysics of three model “half-sandwich” complexes (η6-benzophenone)Cr(CO)3, (η6-styrene)Cr(CO)3, and (η6-allylbenzene)Cr(CO)3 were investigated using pico-second time-resolved infrared spectroscopy and time-dependent density functional theory methods. The (η6-benzophenone)Cr(CO)3 complex was studied using two excitation wavelengths (470 and 320 nm) while the remaining complexes were irradiated using 400 nm light. Two independent excited states were detected spectroscopically for each complex, one an unreactive excited state of metal-to-arene charge-transfer character and the other with metal-to-carbonyl charge transfer character. This second excited state leads to an arrested release of CO on the pico-second time-scale. Low-energy excitation (470 nm) of (η6-benzophenone)Cr(CO)3 populated only the unreactive excited state which simply relaxes to the parent complex. Higher energy irradiation (320 nm) induced CO-loss. Irradiation of (η6-styrene)Cr(CO)3, or (η6-allylbenzene)Cr(CO)3 at 400 nm provided evidence for the simultaneous population of both the reactive and unreactive excited states. The efficiency at which the unreactive excited state is populated depends on the degree of conjugation of the substituent with the arene π-system and this affects the efficiency of the CO-loss process. The quantum yield of CO-loss is 0.50 for (η6-allylbenzene)Cr(CO)3 and 0.43 for (η6-styrene)Cr(CO)3. These studies provide evidence for the existence of two photophysical routes to CO loss, a minor ultrafast route and an arrested mechanism involving the intermediate population of a reactive excited state. This reactive excited state either relaxes to reform the parent species or eject CO. Thus the quantum yield of the CO-loss is strongly dependent on the excitation wavelength. Time-dependent density functional theory calculations confirm that the state responsible for ultrafast CO-loss has significant metal-centred character while the reactive state responsible for the arrested CO-loss has significant metal-to-carbonyl charge-transfer character. The CO-loss product (η6-allylbenzene)Cr(CO)2 formed following irradiation of (η6-allylbenzene)Cr(CO)3 reacts further with the pendent alkenyl group to form the chelate product (η6,η2-allylbenzene)Cr(CO)2.

The potential energy profiles of the optically accessible excited states of two model (η6-arene)Cr(CO)3 systems were explored using Time-Dependent Density Functional Theory. Two photochemical reactions were investigated, CO-loss and the haptotropic or ring-slip of the arene ligand. In both cases the photochemical reaction requires the surmounting of a small thermal barrier in the lowest energy excited state. In the case of (η6-benzene)Cr(CO)3 only one excited state is populated following 400 nm excitation and this leads to the release of CO. The calculated energy barrier to this process is 13 kJ mol–1. In the case of (η6-thiophenol)Cr(CO)3 two excited states are accessible one leading to CO-loss while the other results in the ring-slip process. The calculated barrier to the ring-slip process is 11 kJ mol–1. The calculations are consistent with the results of picosecond time-resolved infrared studies.

The photochemistry of (η6-anisole)Cr(CO)3 and (η6-thioanisole)Cr(CO)3 was investigated by picosecond time-resolved infrared spectroscopy in n-heptane solution at 298 K. Two independent excited states are populated following 400 nm excitation of each of these complexes. An excited state with some metal-to-CO charge-transfer character is responsible for the CO-loss process, which is slow compared to CO-loss from Cr(CO)6. Observed first order rate constants of 1.8 × 1010 s–1 and 2.5 × 1010 s–1 were obtained for the anisole and thioanisole complexes, respectively. The second excited state has metal-to-arene charge transfer character and results in a haptotropic shift of the thioanisole ligand. DFT calculations characterized the excited states involved and the nature of the haptotropic shift intermediate observed for the thioanisole species.

The photochemistry of (η6-benzene)Mo(CO)3 was investigated by picosecond time-resolved infrared spectroscopy in n-heptane solution at room temperature. Two excitation wavelengths, 400 and 266 nm, were used. The photophysical processes of this system depend on the excitation wavelength. Excitation with 400 nm light populates predominantly a metal-to-arene charge-transfer excited state which relaxes to the ground state within 150 ps of excitation. In addition, (η6-benzene)Mo(CO)2(n-heptane) is produced in low yield within the rise time of these experiments (500 fs). Excitation with 266 nm radiation results in a depletion of (η6-benzene)Mo(CO)3 absorptions but no excited state or product νCO absorptions are evident at short time scales (<10 ps). The CO-loss process proceeds over the subsequent 100 ps with concomitant formation of a metal-to-CO charge-transfer excited state and parent recovery. An excited-state model involving excited-state crossing is proposed to explain these observations.

The nature of the lowest energy optical transition for the complexes (η6-naphthalene)Cr(CO)3 and (η6-phenanthrene)Cr(CO)3 in the solid state has been investigated by Raman spectroscopy using a range of different excitation wavelengths progressively approaching the resonant condition. Examination of the resonantly enhanced Raman modes confirms that the first absorption is attributed predominantly to a metal-to-arene charge transfer transition for both complexes. A notable difference in the photochemistry of the two complexes was observed. In the case of the phenanthrene complex, population of the lowest energy excited state leads to a photochemical process which resulted in the loss of the arene ligand and formation of Cr(CO)6.

The photochemistry of (η6-methylbenzoate)Cr(CO)3, (η6-naphthalene)Cr(CO)3, and (η6-phenanthrene)Cr(CO)3 in n-heptane solution was investigated by picosecond time-resolved infrared spectroscopy (TRIR). The observation of two transient IR features in the organic carbonyl region at 1681 and 1724 cm−1 following 400 nm excitation of (η6-methylbenzoate)Cr(CO)3 confirms formation of two excited states which are classified as metal-to-arene charge transfer (MACT) and metal-to-CO charge transfer (MCCT), respectively. Time-dependent density functional theory calculations have been used to support these assignments. Population of the MCCT excited state results in a slow (150 ps) expulsion of one CO ligand. Excitation of (η6-naphthalene)Cr(CO)3 or (η6-phenanthrene)Cr(CO)3 at either 400 or 345 nm produced two excited states: the MCCT state results in CO loss, while the MACT excited state results in a change to the coordination mode of the polyaromatic ligands before relaxing to the parent complex. A comparison of the infrared absorptions observed following the population of the MACT excited state with those calculated for nonplanar polyaromatic intermediates provides a model for the reduced hapticity species.

The electronic structure of (η6-benzene)Cr(CO)3 has been calculated using density functional theory and a molecular orbital interaction diagram constructed based on the Cr(CO)3 and benzene fragments. The highest occupied molecular orbitals are mainly metal based. The nature of the lowest energy excited states were determined by time-dependent density functional theory, and the lowest energy excited state was found to have significant metal to carbonyl charge transfer character. The photochemistry of (η6-benzene)Cr(CO)3 was investigated by time-resolved infrared spectroscopy with picosecond time resolution. The low energy excited state was detected following irradiation at 400 nm, and this exhibited νCO bands at lower energy than the equivalent νCO bands of (η6-benzene)Cr(CO)3, consistent with metal to carbonyl charge transfer character, and is formed with excess vibrational energy, relaxing to the v = 0 vibrational state within 3 ps. The resulting “cold” excited state decays to form the CO-loss species (η6-benzene)Cr(CO)2 in approximately 70% yield and to reform (η6-benzene)Cr(CO)3 within 150 ps. The rates of relaxation from the vibrationally hot state to the cold excited state and its subsequent reaction to yield (η6-benzene)Cr(CO)2 were measured over a range of temperatures from 274 to 320 K, and the activation parameters for both processes were obtained from Eyring plots. The vibrational relaxation exhibits a negative activation enthalpy ΔH‡ (−10 (±4) kJ mol−1) and a negative activation entropy ΔS‡ (−50 (±16) J mol−1 K−1). A significant barrier (ΔH‡ = +12 (±4) kJ mol−1) was obtained for the formation of (η6-benzene)Cr(CO)2 with a ΔS‡ value close to zero. These data are used to propose a model for the CO-loss process to yield (η6-benzene)Cr(CO)2 and to explain why low temperature irradiation of (η6-benzene)Cr(CO)3 with light of wavelengths greater than 400 nm produced relatively minor amounts of (η6-benzene)Cr(CO)2.

The singlet and triplet surfaces for the interaction of thiophene or selenophene with a chromium tricarbonyl unit were calculated using the B3LYP/LanL2DZ+p model chemistry. The singlet surfaces confirm that the (η5-C4H4E)Cr(CO)3 (E = S or Se) are the lowest energy species. The (η1(C2)-C4H4E)Cr(CO)3 and (κ2(E,C)-C4H4E)Cr(CO)3 species were also located in shallow energy minima. Reaction path modeling on the singlet surfaces provided activation energies for the endothermic insertion process of 172 and 160 kJ mol−1 for E = S or Se, respectively. The activation energy for the insertion process on the triplet surfaces is on the order of 70 kJ mol−1. A novel η3(E,C,C)-η1(C)-coordinated species was located as a transition state between the insertion species and (η5-C4H4E)Cr(CO)3 on the singlet surfaces and an intermediate on the triplet surface for E = S.

The photochemistry of (η6-C6H6)M(CO)3 (M = Cr or Mo) is described. Photolysis with λexc. > 300 nm of (η6-C6H6)Cr(CO)3 in low-temperature matrixes containing CO produced the CO-loss product, while lower energy photolysis (λexc. > 400 nm) produced Cr(CO)6. Pulsed photolysis (λexc. = 400 nm) of (η6-C6H6)Cr(CO)3 in n-heptane solution at room temperature produced an excited-state species (1966 and 1888 cm−1) that decays over 150 ps to (η6-C6H6)Cr(CO)2(n-heptane) (70%) and (η6-C6H6)Cr(CO)3 (30%). Pulsed photolysis (λexc. = 266 nm) of (η6-C6H6)Cr(CO)3 in n-heptane produced bands assigned to (η6-C6H6)Cr(CO)2(n-heptane) (1930 and 1870 cm−1) within 1 ps. These bands increase with a rate identical to the rate of decay of the excited-state species and the rate of recovery of (η6-C6H6)Cr(CO)3. Photolysis of (η6-C6H6)Mo(CO)3 at 400 nm produced an excited-state species (1996 and 1898 cm−1) and traces of (η6-C6H6)Mo(CO)2(n-heptane) within 1 ps. For the chromium system CO-loss can occur following excitation at both 400 and 266 nm via an avoided crossing of a MACT (metal-to-arene charge transfer) and MCCT/LF (metal-to-carbonyl charge transfer/ligand field) states. This leads to an unusually slow CO-loss following excitation with 400 nm light. Rapid CO-loss is observed following 266 nm excitation because of direct population of the MCCT/LF state. The quantum yield for CO-loss in the chromium system decreases with increasing excitation energy because of the competing population of a high-energy unreactive MACT state. For the molydenum system CO-loss is a minor process for 400 nm excitation, and an unreactive MACT state is evident from the TRIR spectra. A higher quantum yield for CO-loss is observed following 266 nm excitation through both direct population of the MCCT/LF state and production of a vibrationally excited reactive MACT state. This results in the quantum yield for CO-loss increasing with increasing excitation energy.

The photochemistry of (μ2-RC2H)Co2(CO)6 (R=H or C6H5) has been investigated by both time-resolved and steady-state techniques. Pulsed excitation in cyclohexane solution with λexc=355 nm causes CoCo bond homolysis while photolysis at λexc=532 nm results in CO loss. Steady-state photolysis (λexc>500 nm) in the presence of suitable trapping ligands (L) produced the monosubstituted complexes (μ2-RC2H)Co2(CO)5(L) (L=C5H5N or PPh3) in high yields.

The [Ru(phen)2(dppz)]2+ complex (1) is non-emissive in water but is highly luminescent in organic solvents or when bound to DNA, making it a useful probe for DNA binding. To date, a complete mechanistic explanation for this “light-switch” effect is still lacking. With this in mind we have undertaken an ultrafast time resolved infrared (TRIR) study of 1 and directly observe marker bands between 1280–1450 cm−1, which characterise both the emissive “bright” and the non-emissive “dark” excited states of the complex, in CD3CN and D2O respectively. These characteristic spectral features are present in the [Ru(dppz)3]2+ solvent light-switch complex but absent in [Ru(phen)3]2+, which is luminescent in both solvents. DFT calculations show that the vibrational modes responsible for these characteristic bands are predominantly localised on the dppz ligand. Moreover, they reveal that certain vibrational modes of the “dark” excited state couple with vibrational modes of two coordinating water molecules, and through these to the bulk solvent, thus providing a new insight into the mechanism of the light-switch effect. We also demonstrate that the marker bands for the “bright” state are observed for both Λ- and Δ-enantiomers of 1 when bound to DNA and that photo-excitation of the complex induces perturbation of the guanine and cytosine carbonyl bands. This perturbation is shown to be stronger for the Λ-enantiomer, demonstrating the different binding site properties of the two enantiomers and the ability of this technique to determine the identity and nature of the binding site of such intercalators.

The photochemistry and photophysics of three model “half-sandwich” complexes (η6-benzophenone)Cr(CO)3, (η6-styrene)Cr(CO)3, and (η6-allylbenzene)Cr(CO)3 were investigated using pico-second time-resolved infrared spectroscopy and time-dependent density functional theory methods. The (η6-benzophenone)Cr(CO)3 complex was studied using two excitation wavelengths (470 and 320 nm) while the remaining complexes were irradiated using 400 nm light. Two independent excited states were detected spectroscopically for each complex, one an unreactive excited state of metal-to-arene charge-transfer character and the other with metal-to-carbonyl charge transfer character. This second excited state leads to an arrested release of CO on the pico-second time-scale. Low-energy excitation (470 nm) of (η6-benzophenone)Cr(CO)3 populated only the unreactive excited state which simply relaxes to the parent complex. Higher energy irradiation (320 nm) induced CO-loss. Irradiation of (η6-styrene)Cr(CO)3, or (η6-allylbenzene)Cr(CO)3 at 400 nm provided evidence for the simultaneous population of both the reactive and unreactive excited states. The efficiency at which the unreactive excited state is populated depends on the degree of conjugation of the substituent with the arene π-system and this affects the efficiency of the CO-loss process. The quantum yield of CO-loss is 0.50 for (η6-allylbenzene)Cr(CO)3 and 0.43 for (η6-styrene)Cr(CO)3. These studies provide evidence for the existence of two photophysical routes to CO loss, a minor ultrafast route and an arrested mechanism involving the intermediate population of a reactive excited state. This reactive excited state either relaxes to reform the parent species or eject CO. Thus the quantum yield of the CO-loss is strongly dependent on the excitation wavelength. Time-dependent density functional theory calculations confirm that the state responsible for ultrafast CO-loss has significant metal-centred character while the reactive state responsible for the arrested CO-loss has significant metal-to-carbonyl charge-transfer character. The CO-loss product (η6-allylbenzene)Cr(CO)2 formed following irradiation of (η6-allylbenzene)Cr(CO)3 reacts further with the pendent alkenyl group to form the chelate product (η6,η2-allylbenzene)Cr(CO)2.