LMCT State Research Articles

Colloidal down-shifting Gd2O3:Eu3+/SiO2 phosphors nanocomposites using one-pot modified surfactants-based soft templates for catalytic applications

The application of rare earth (RE)-based phosphor materials faces a significant challenge in adopting strategies to bring down the cost without compromising their properties. The most straightforward approach is to minimize the use of RE elements. Herein, we demonstrate the use of SiO2 as an economical solid nano-matrix for lanthanides-based phosphor nanomaterials. Gd2O3:Eu3+(x%) and Gd2O3:Eu3+(x%)/SiO2 nanophosphor composites were prepared through a modified soft templating technique using cetyltrimethylammonium bromide (CTAB) as the surfactant. The photoluminescence properties were studied by excitation, emission, and luminescence decay investigations. Upon excitation in the O2−(2p)→Eu3+(4f6) LMCT state, these nanophosphors exhibited predominantly red emission from the 5D0→7FJ transition (where J = 0–4) of the Eu3+ ion. Moreover, the synthesized low-cost Gd2O3Eu3+(5 %)/SiO2 composites, with a small number of lanthanides, exhibited excellent photocatalytic degradation activity for organic dye (methylene blue) under visible light (λ ≥ 420 nm), showing promising potential for photocatalytic wastewater treatment.

Photophysics and photochemistry of (n-Bu4N)2[Pt(NO3)6] in acetonitrile: ultrafast pump-probe spectroscopy and quantum chemical insight.

The ultrafast processes caused by photoexcitation of (n-Bu4N)2[Pt(NO3)6] complex in acetonitrile were studied by means of transient absorption (TA) pump-probe spectroscopy and verified by quantum chemical calculations. The primary photochemical process was found to be an inner-sphere electron transfer followed by an escape of an •NO3 radical to the bulk solution. The reaction occurs via the dissociative triplet excited LMCT state of the initial complex. Based on the experimental data and quantum chemical calculations, the mechanism of ultrafast photophysical and photochemical processes is proposed.

Photoaquation of cis-[Rh(dppz)(phen)Cl2]Cl complex prospective as potential light-activated anti-cancer agent

The photoactivated drug usage in anti-cancer therapy, the working mechanism of which doesn’t rely on the formation of reactive oxygen species, allows reducing damage to healthy tissue by controlling toxicity. For instance, light-activated Rh(III) complexes show nuclease activity after irradiation with UV and visible light. However, literature analysis shows a lack of descriptions of basic photochemical mechanisms. Here we report the study of cis-[Rh(dppz)(phen)Cl2]Cl (DPPZPHEN, dppz is dipyrido[3,2-a:2′,3′-c]phenazine, phen is 1,10-phenanthroline) photochemistry in water solutions using stationary photolysis, nanosecond laser flash photolysis and ultrafast pump–probe spectroscopy. After the irradiation with 308 nm light, DPPZPHEN undergoes aquation, which results in the substitution of one chloride ligand by a water molecule and leads to the formation of [Rh(dppz)(phen)(H2O)Cl]Cl2. UV–vis spectra of complex and capillary electrophoresis data were collected and discussed. Laser flash photolysis (355 nm) demonstrates intermediate absorption attributed to the lowest electronic excited state (triplet) of the initial complex. The study of primary photophysical processes showed a rather complicated behavior of transient absorption, which could be described by a set of four exponential functions. The early photophysical processes were described by the scheme including the formation of two excited states of different natures, namely a dppz-based IL state and an LMCT state.

Role of LMCT States in Luminescence Excitation Processes in Europium Indolecarboxylates

Role of LMCT States in Luminescence Excitation Processes in Europium Indolecarboxylates

Europium(III) and gallic acid ligand coordination compounds: synthesis, characterization, photophysical processes, and optimization of luminescent properties

We obtained EuIII, GdIII, and GdIII doped with EuIII (1 at.% and 5 at.%) complexes with the ligand derived from 3,4,5-trihydroxybenzoic acid (H4Gal). Synthetic routes are proposed from acetate and chloride solutions of the trivalent lanthanides. We demonstrated the decomposition of the ligands 3,4,5-trihydroxybenzoate (H3Gal ≡ L–) and 3-hydroxo-4,5-dihydroxybenzoate (H2Gal2– ≡ L’2–) in an acetic solution medium. [LnLL’(H2O)2] complexes are obtained in a metal chloride solution medium. We studied the complexes in the solid state. We described the photophysical processes in the complexes, and they resulted in an energy diagram of the EuIII compounds in which the position of the states related to the LMCT transition are intermediate to the ligand's singlet (S1) and triplet (T1) states. We obtained the position of the triplet state (T1) from the luminescence of the GdIII complex. We performed the calculations of the EuIII complex's electronic structure using the Lumpac software to confirm the propositions obtained through experiments. The proposition is that the excitation in the LMCT state rapidly populates the T1 state, and the L–L’2–→EuIII back energy transfer may occur. One of the possible disadvantages of the complexes is the enormous non-radioactive deactivation due to the oscillators (OH) and (CH). The EuIII characteristic luminescence is of little intensity. We obtained GdIII compounds doped with EuIII at 1.0 and 5.0 at.% to optimize luminescent properties. We obtained the emission and excitation spectra of the complexes at 77 and 17 K, showing ligands' phosphorescence superimposed on the characteristic emissions 5D0 →7FJ (J = 0–4) of the EuIII ion.

(Invited) Ultrafast Solar Energy Conversion with Earth-Abundant Complexes

Transition-metal complexes are widely used as photosensitizers and photocatalysts, and in light-emitting devices.[1] We have studied ultrafast proceses in a variety of DSSC and photocatalytic aplications.[2,3] More recently we demonstrated the first iron complex [FeIII(btz)3]3+ exhibiting room temperature photoluminescence from a CT state and show that it has a 100 ps excited state CT lifetime which is unprecedented for any iron complex.[4] This was characterized as a rare low-spin FeIII d5 complex with emission from a long-lived doublet ligand-to-metal charge transfer (2LMCT) state. Absence of intersystem crossing (ISC) in this complex avoids significant excited state energy losses as encountered in the prevailing class of d6 TM complexes of e.g. FeII and RuII. Furthermore, we have also shown [FeII(btz)3]2+ low-spin complex [5], a FeII analogue to [FeIII(btz)3]3+. It exhibits strong metal-to-ligand charge transfer (MLCT) absorption bands throughout the visible spectrum, and excitation of these bands gives rise to a 3MLCT state with a 528 ps excited state lifetime in CH3CN solution that is more than one order of magnitude longer compared to the MLCT lifetime of any previously reported FeII complex. Together, these results show that the FeII and FeIII oxidation states of the same Fe(btz)3 complex feature long lived MLCT and LMCT states, respectively. This demonstrates the versatility of iron N-heterocyclic carbene complexes as promising light-harvesters for a broad range of photophysical and photochemical applications, and on-going efforts for further improvements of the excited state dynamics of such systems will be outlined, including the very latest development of an iron compound [Fe(phtmeimb)2]+ with an excited state stable enough to emit light for 2 ns and able to engage in bimolecular electron transfer [6].1 Ponseca, C. S., Jr.; Chabera, P.; Uhlig, J.; Persson, P.; Sundstrom, V. Ultrafast Electron Dynamics in Solar Energy Conversion. Chem Rev 2017, 117 (16), 10940.2 H. Imahori et al, Photoinduced Charge Carrier Dynamics of Zn−Porphyrin−TiO2 Electrodes: The Key Role of Charge Recombination for Solar Cell Performance, J. Phys. Chem. A 2011, 115, 16, 3679-36903 M. Yamamoto et al, Visible light-driven water oxidation with a subporphyrin sensitizer and a water oxidation catalyst, Chem. Commun. 2016,52, 13702-137054 Chabera, P. et al. A low-spin Fe(III) complex with 100-ps ligand-to-metal charge transfer photoluminescence. Nature 2017, 543 (7647), 695.5 Chabera, P. et al. Fe(II) Hexa N-Heterocyclic Carbene Complex with a 528 ps Metal-to-Ligand Charge-Transfer Excited-State Lifetime, J Phys Chem Lett 2018, 9, 459−463.6 Kjær, K. S. et al. Luminescence and reactivity of a charge-transfer excited iron complex with nanosecond lifetime. Science 2019, 363 (6424), 249-253. Figure 1

Coordination-promoted photoluminescence induced by configuration twisting regulation

In this work, we reported a coordination-promoted photoluminescence (CPPL) phenomenon of a new organometallic complex tBu(ONO)-W formed by ligand tBu(ONO) and metal tungsten for the first time. At the same time, the photoluminescence mechanisms of both the ligand tBu(ONO) and organometallic complex tBu(ONO)-W were investigated by using the steady-state and time-resolved spectroscopies as well as the time-dependent density functional theory (TDDFT) method. It was demonstrated that the weak photoluminescence of ligand tBu(ONO) could be ascribed as the destroyed planarity after significant configuration twisting in the excited state. However, the formation of the coordination bond between tBu(ONO) and the metal tungsten in the complex changes the two twisted parts of tBu(ONO) into a nearly planar configuration. The excited state configuration has a slight configuration change while the coplanarity of tBu(ONO)-W has not been destroyed. Therefore, the photoluminescence of tBu(ONO)-W is significantly enhanced compared to the ligand tBu(ONO). It can be proved that the coordination effect between the ligand and the metal inhibits the configuration twisting of tBu(ONO) and then promotes light emission. Moreover, it was also found that the emission peaks of tBu(ONO)-W are mainly originated from the relaxation of the ILCT and LMCT states. The experimental and theoretical results clearly show that the regulation of metal-ligand coordination bonds on the photoluminescence performance provides a new strategy for designing new high-performance luminescent materials.

A lanthanide doped metal-organic framework demonstrated as naked eye detector of a trace of water in organic solvents including alcohols by monitoring the turn-on of luminescence

It is very crucial to have a very simple, instant and low-cost detection and estimation of a trace of water in common organic solvents including alcohols. To obtain the sensor material we have synthesized a metal-organic framework (MOF) {[Y1.0Mn1.5(PDA)3(H2O)3]·3.5H2O}, 1, (PDA = 2,6-pyridinedicarboxylic acid) through hydrothermal process. A doping of compound 1 by 10 % terbium {[Y0.9Tb0.1Mn1.5(PDA)3(H2O)3]·3.5H2O}, 1:Tb, was done through an isomorphous substitution technique. The most advantageous point about 1:Tb was the metal centre luminescence, which was largely stokes shifted from the excitation light making possible the naked eye observation. The weak metal centre luminescence intensity of dehydrated 1:Tb (excluding lattice and coordinated water molecules) in organic solvents EtOH, CH3OH, CH3CN, THF and n-heptane showed huge turn-on in presence of trace amounts of H2O in the said solvents. The luminescence intensity of Tb3+ centre was enhanced by several folds with a limit of detection 1.12 %(v/v), 0.47 %(v/v), 0.04 %(v/v), 0.13 %(v/v) and 0.53 %(v/v) respectively. The coordinated water molecules as well as the lattice water molecules in 1:Tb play a vital role during sensitization of the Tb3+ centre by enhancing the rigidity of the structure and facilitating the formation of LMCT state which ultimately results in a huge turn-on of metal centre luminescence.

(Invited) Ultrafast Solar Energy Conversion with Earth-Abundant Complexes

Transition-metal complexes are widely used as photosensitizers and photocatalysts, and in light-emitting devices.[1] We have studied ultra-fast processes in a variety of DSSC and photocatalytic applications.[2,3] More recently we demonstrated the first iron complex [FeIII(btz)3]3+ exhibiting room temperature photoluminescence from a CT state and show that it has a 100 ps excited state CT lifetime which is unprecedented for any iron complex.[4] This was characterized as a rare low-spin FeIII d5 complex with emission from a long-lived doublet ligand-to-metal charge transfer (2LMCT) state. Absence of intersystem crossing (ISC) in this complex avoids significant excited state energy losses as encountered in the prevailing class of d6 TM complexes of e.g. FeII and RuII. Furthermore, we have also shown [FeII(btz)3]2+ low-spin complex [5], a FeII analogue to [FeIII(btz)3]3+. It exhibits strong metal-to-ligand charge transfer (MLCT) absorption bands throughout the visible spectrum, and excitation of these bands gives rise to a 3MLCT state with a 528 ps excited state lifetime in CH3CN solution that is more than one order of magnitude longer compared to the MLCT lifetime of any previously reported FeII complex. Together, these results show that the FeII and FeIII oxidation states of the same Fe(btz)3 complex feature long lived MLCT and LMCT states, respectively. This demonstrates the versatility of iron N-heterocyclic carbene complexes as promising light-harvesters for a broad range of photophysical and photochemical applications, and on-going efforts for further improvements of the excited state dynamics of such systems will be outlined, including the very latest development of an iron compound [Fe(phtmeimb)2]+ with an excited state stable enough to emit luminescence with unprecedented 2 ns lifetime and with the ability to engage in bi-molecular electron transfer [6].

Systematic First-Principles Calculations of Charge Transfer Transitions of Transition Metal Ions (Sc3+, Ti3+, V3+, Cr3+, Mn3+, Fe3+) in α-Al2O3 with Structural Optimization

In order to calculate the LMCT energies for various trivalent TM ions in α-Al2O3 using realistic model clusters, we performed structural optimization using the CASTEP code and constructed the optimized TMO6 9- and TMAl13O6 30+ clusters (TM = Sc3+, Ti3+, V3+, Cr3+, Mn3+, Fe3+). We obtained the LMCT energies of TM ions by first-principles many-electron calculations using the DVME method. It was found that the differences between the LMCT energies of the optimized clusters and those of the relaxed clusters based on the crystal radii originate from the difference in the multiplet splitting of the LMCT states. The results also indicated that the LMCT energies of transition metal ions in α-Al2O3 can be calculated using small clusters composed of the central TM ion and the first-neighbor oxygen ions.

How minor structural changes generate major consequences in photophysical properties of RE coordination compounds; resonance effect, LMCT state

Lanthanide coordination compounds of the formula Na[Ln(L)4] (1Ln), where Ln = La3+, Eu3+, Gd3+, Tb3+, L = [L]− and HL = dimethyl(4-methylphenylsulfonyl)amidophosphate, were synthesized. Their structural and spectroscopic properties were discussed in detail based on X-ray diffraction measurements, IR spectroscopy, absorption and emission spectroscopy at 293 and 77 K and theoretical calculations of the intramolecular energy transfer (IET) rates. DFT calculations were used to investigate the 1Ln electronic properties required to calculate the transition rates. 30 and 22 pathways of intramolecular nonradiative energy transfer were examined in the case of 1Eu and 1Tb, respectively. It is shown that the main pathway for sensitization of the lanthanide emission is either the triplet (1Eu) or singlet (1Tb) transfer, occurring mainly through the exchange mechanism. The energy rates for energy transfer from S1 and T1 equal WS=1.53×105s−1 (1Eu), WT=5.14×106s−1 (1Eu) and WS=4.09×107s−1 (1Tb), WT=6.88×105s−1 (1Tb). The crucial role of the 7F5 level in the energy transfer process of 1Tb and the participation of the LMCT state in the depopulation of the ligand singlet state of 1Eu were demonstrated. The influence of the resonance effect on the splitting of the 7F1 level in 1Eu was analyzed. By comparing the properties of 1Ln with the properties of 2Ln coordination compounds, sharing the same ligand and crystallizing in the same crystallographic system (monoclinic), but with a different space group, it is demonstrated how slight structural changes can affect the photophysical properties of Ln compounds.

Ultrafast dynamics in LMCT and intraconfigurational excited states in hexahaloiridates(iv), models for heavy transition metal complexes and building blocks of quantum correlated materials.

The population and structural dynamics of IrCl62- is studied in acetonitrile and aqueous solutions in comparison to isoelectronic IrBr62- using ultrafast broadband, dispersed transient absorption, with both octahedra excited with 85 fs pulses at four different wavelengths, encompassing the first seven t2g-based electronic states. Ligand-to-metal charge transfer (LMCT) 420 or 490 nm excitation of IrCl62- into Uu'(2T2u) + Eu''(2T2u) states, superimposed due to Ham effect, or Uu'(2T1u), respectively, leads to symmetry lowering due to Jahn-Teller effect in these excited states with the subsequent 100 fs decay into Ug'(2T1g). This first LMCT state is formed vibrationally coherent in the 104 cm-1 t2g (scissor) or 243 cm-1 eg (out-of-phase-stretch) Jahn-Teller modes for the respective excitation wavelength. Direct excitation into Ug'(2T1g) at 600 nm and the intraconfigurational lowest excited Ug'(2T2g) state at 1900 nm helped to establish that Ug'(2T1g) decays via back electron transfer into Ug'(2T2g) (time constants, 3.55 ps in acetonitrile and 0.9 ps in water), and the decay of Ug'(2T2g) into the ground state is the rate-limiting relaxation step. The relaxation cascade of IrBr62- is similar with short-lived (≤100 fs) higher LMCT states, but the vibrational coherence is only observed in the Jahn-Teller t2g mode. Faster back electron transfer for IrBr62- is explained by the energy gap law. The intraconfigurational Ug'(2T2g) states, which are ∼5100 cm-1 above the ground state for both complexes, have a sub-nanosecond lifetime largely independent of the ligand nature (∼350 ps, acetonitrile).

Light-Induced Spin Crossover in an Intermediate-Spin Penta-Coordinated Iron(III) Complex.

(PMe3)2FeCl3 is an Fe(III) complex that exists in the intermediate-spin ground state in a distorted trigonal bipyramidal geometry. An electronic state with high-spin configuration lies in close vicinity to the ground state, making it a potential spin crossover candidate. A mechanistic account of the spin crossover from the lowest quartet state (Q0) to the lowest sextet state (S1) of this complex is provided by exploring both thermal and light-induced pathways. The presence of a large barrier between the two spin states suggests a possible thermal spin crossover at a rather high temperature. The light-induced spin crossover is investigated by employing complete active space self-consistent field calculations together with dynamic correlation and spin-orbit coupling for the lowest seven quartet and lowest five sextet states. The system in the Q0 state upon light absorption is excited to the optically bright Q4 LMCT state. By following minimum energy pathways along the electronic states, two light-induced pathways for spin crossover are identified. From the Q4 state, the system can photo-regenerate the ground intermediate-spin state (Q0) through an internal conversion of Q4/Q3 followed by Q3/S1 and S1/Q0 intersystem crossings. In an alternate route, through Q4/S2 intersystem crossing followed by S2/S1 internal conversion, the system can complete the spin crossover from the Q0 to S1 state.

Well-tuned white-light-emitting behaviours in multicenter-Ln polyoxometalate derivatives: A photoluminescence property and energy transfer pathway study.

White light-emitting diodes (WLEDs) are of scientific significance in terms of their wide applications, and few uminescent materials based on white-light-emitting polyoxometalate (POM) derivatives have been reported till now. Herein, a series of organic chromophores modified POM derivatives [N(CH3)4]3K2Ln(C7H5O2)(H2O)2(α-PW11O39)]·11H2O (Ln3+ = Eu3+ (1), Tb3+ (2), Tm3+ (3), Lu3+ (4)) and multicenter-Ln analogues [N(CH3)4]3K2EuxTbyTm1-x-y(C7H5O2)(H2O)2(α-PW11O39)·11H2O (5-11) were synthesized successfully and were characterized by various physico-chemical analysis. The investigations indicate the white-light-emitting behavior can be well tuned by adjusting the molar ratio of Eu3+/Tb3+/Tm3+ = 0.06:0.10:0.84 in 9. The energy transfer process from organic benzoic and POM ligands to Eu3+, Tb3+ and Tm3+ emitting centers were detected through time-resolved emission spectroscopy (TRES) and the comparison of excitation of single-, double-, treble-Ln3+ mixed, indicating the energy can transfer from the photoexcitation O → M LMCT state of POM components and π → π* transition of organic ligand to sensitize the emissions of Ln3+ ions via intramolecular energy transition mechanism. The energy transfer between Eu3+ and Tb3+, Tm3+ and Eu3+, Tm3+ and Tb3+ ions also have been recorded and carefully studied by TRES and variations of Tm3+ luminescence lifetime in this context, and the results show a low-effectively process of energy transfer between Tm3+/Eu3+, Tm3+/Tb3+ ions and a relatively good energy transfer efficiency between Eu3+/Tb3+ ions.

FeII Hexa N-Heterocyclic Carbene Complex with a 528 ps Metal-to-Ligand Charge-Transfer Excited-State Lifetime.

The iron carbene complex [FeII(btz)3](PF6)2 (where btz = 3,3'-dimethyl-1,1'-bis(p-tolyl)-4,4'-bis(1,2,3-triazol-5-ylidene)) has been synthesized, isolated, and characterized as a low-spin ferrous complex. It exhibits strong metal-to-ligand charge transfer (MLCT) absorption bands throughout the visible spectrum, and excitation of these bands gives rise to a 3MLCT state with a 528 ps excited-state lifetime in CH3CN solution that is more than one order of magnitude longer compared with the MLCT lifetime of any previously reported FeII complex. The low potential of the [Fe(btz)3]3+/[Fe(btz)3]2+ redox couple makes the 3MLCT state of [FeII(btz)3]2+ a potent photoreductant that can be generated by light absorption throughout the visible spectrum. Taken together with our recent results on the [FeIII(btz)3]3+ form of this complex, these results show that the FeII and FeIII oxidation states of the same Fe(btz)3 complex feature long-lived MLCT and LMCT states, respectively, demonstrating the versatility of iron N-heterocyclic carbene complexes as promising light-harvesters for a broad range of oxidizing and reducing conditions.

Intraligand fluorescence of M(III)(oxinate)3 under ambient conditions with M = Fe, Ru, Os and oxinate = 8-quinolinolate

The electronic spectra of the complexes M(III)(oxinate)3 with M=Fe, Ru, Os and oxinate=8-quinolinolate are dominated by oxinate to M(III) LMCT transitions. Nevertheless, these complexes are also characterized by a fluorescence at higher energies, which originates from the oxinate ligands. This green luminescence appears in solution as well as in the solid state under ambient conditions. The electronic coupling between the IL and the LMCT states is apparently not strong enough to lead to a complete quenching of the oxinate IL fluorescence.

Illuminating Iron: Mesoionic Carbenes as Privileged Ligands in Photochemistry

The new iron age: The stabilization of an iron(III) complex with mesoionic carbenes has been shown to lead to record-breaking excited-state lifetimes and an unprecedented spin-allowed radiative decay from a 2 LMCT state. These results set the stage for a sustainable future in the new "iron age".

Molecular and electronic structures of copper-cuprizone and analogues

A paramagnetic dinuclear copper(II) complex, [CuII(L−)Cl]2 (1) while LH=(E)-1,2-diphenyl-2-(2-(pyridine-2-yl)hydrozono)ethanone, an analogue of mono-hydrazone cuprizone (LmcpzH2), was isolated and substantiated by spectra, single crystal X-ray structure determination, unrestricted density functional theory (DFT) calculations and inter alia. Magnetic susceptibility measurement and EPR spectra confirmed the triplet state of 1 above 30K because of two paramagnetic copper(II) centers. Unrestricted DFT calculations on copper cuprizone species with doublet spin state established similar electronic features and disclosed that bis-deprotonated NN-chelation, [Cu(Lmcpz)2]2−, (cis or trans-2NN), is ∼3×103kJ/mol higher in energy than the mono-deprotonated NO-chelation, Cu(LmcpzH)2, (cis or trans-2NO) precluding the existence of so far reported 2NN products. The calculations revealed that in all isomers, due to the mixing of dx2−y2 orbital with a ligand group orbital as in 1, 48–50% spin density is delocalized over the LmcpzH2 ligand. In fluid solution, 1 absorbs strongly at 527nm due to LMCT elucidated by the time dependent (TD) DFT calculations and the excited LMCT state is fluorescent (λex=375nm; λem=405, 426nm; Φ=0.014).

Spotlight on the ligand: luminescent cyclometalated Pt(iv) complexes containing a fluorenyl moiety.

The synthesis, electrochemistry and photophysical properties of a family of Pt(iv) complexes with cyclometalated 2-(9,9-dimethylfluoren-2-yl)pyridine (flpy) are reported. Homoleptic and heteroleptic tris-cyclometalated complexes with a meridional configuration, mer-[Pt(C^N)2(flpy)]OTf, with C^N = flpy or cyclometalated 2-phenylpyridine (ppy), were prepared by reacting the bis-cyclometalated precursors [Pt(C^N)2Cl2] with flpyH in the presence of two equivalents of AgOTf. The corresponding facial isomers were obtained by photoisomerization. The Pt(iv) complexes with flpy display intense absorptions in the near-visible region and yellow phosphorescence in fluid solutions at 298 K, with quantum yields in the range 0.06-0.28 and lifetimes of hundreds of microseconds. The emissions arise from essentially (3)LC(flpy) states in all cases, with little metal-orbital contribution. However, computational calculations and experimental data demonstrate that subtle variations in the contribution of metal orbitals to the emitting state have a profound impact on quantum yields, while nonradiative deactivation through the thermal population of deactivating LMCT states does not have a significant influence.

Broadband ultrafast transient absorption of iron (III) tetraphenylporphyrin chloride in the condensed phase

Ultrafast transient absorption of iron (III) tetraphenylporphyrin chloride [Fe(III)TPPCl] is used to probe the excited state relaxation in room temperature solution. The ground state was excited either at 400nm near the Soret resonance or at 520nm in the Q resonance. Relaxation from the lowest lying porphyrin singlet (π, π∗) manifold results in formation of a LMCT (π, dπ) state on a time scale of 0.4–0.6ps followed by a metal-to-ligand charge transfer transition from the iron to the porphyrin on a time scale of 1.8–2.3ps, producing an excited 4T intermediate spin state of Fe(III)TPPCl. The system relaxes back to the high spin 6A1 ground state on a time scale of 13–18ps. The relatively slow electronic relaxation of the more symmetric Fe(III)TPPCl compound is contrasted with the much faster subpicosecond ground state recovery observed for less symmetric biologically active heme systems at room temperature.