Fe(III) complexes with prolonged luminescence lifetimes and symmetry-breaking charge separation

doi:10.21203/rs.3.rs-4002627/v1

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Fe(III) complexes with prolonged luminescence lifetimes and symmetry-breaking charge separation

https://doi.org/10.21203/rs.3.rs-4002627/v1

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Iron(III) complexes with long-lived luminescence in fluid solution at room temperature (up to 6.5 ns, significantly longer than benchmark compounds) have been prepared, by taking advantage of the excited-state equilibration approach, for the first time applied to earth-abundant, iron(III) compounds. Pyrene subunits have been covalently-linked by methylene linkage to [Fe(phtmeimb)₂]⁺ (1; phtmeimb is {phenyl[tris(3-methylimidazol-1-ylidene)]borate}⁻), so obtaining complex 3, and have been used to synthesize [Fe(pytmeimb)₂]⁺ (2; pytmeimb is {2-pyrene[tris(3-methylimidazol-1-ylidene)]borate}⁻). In 2 and 3, the ligand-to-metal charge transfer (²LMCT) state decays by energy transfer to closely-lying p-p* pyrene triplets. Equilibrated states are so formed, resulting in ²LMCT emission from the Fe(III) subunit, in which pyrene triplets serve as long-lived excited-state reservoirs. In 2, symmetry-breaking charge separation in dilute fluid solution at room temperature also occurs.

Physical sciences/Chemistry/Physical chemistry/Excited states

Physical sciences/Chemistry/Physical chemistry/Energy transfer

Transition metal complexes featuring photoactive properties are at the center of a large interest for application in various fields, ranging from solar energy conversion ^1-3 to light emitting technologies⁴, bio-imaging^5,6 and phototherapy⁷. However, such photoactive transition metal complexes are mostly made of precious and relatively less abundant metals, with ruthenium playing the major role⁸, and this represents a serious limitation for their possible use on a large scale. In recent years, many efforts have been devoted to preparing photoactive, luminescent metal complexes made of earth-abundant metals^9-23. Within this framework, iron compounds look quite interesting, since iron is one the most abundant metals on Earth. Elusive for years, in recent times iron compounds exhibiting relatively intense luminescence (quantum yields around 2%) have finally been synthesized^{10,11,16-18,21}. Despite many efforts, however, the room temperature luminescence lifetimes of iron complexes in solution remain relatively short: in fact, an excited-state lifetime of 2 ns is reported for [Fe(phtmeimb)₂]⁺ (1; phtmeimb = {phenyl[tris(3-methylimidazol-1-ylidene)]borate} anion), the iron(III) compound exhibiting the unquestioned longest-lived luminescence so far^16,24-26. Longer-lived luminescence is highly desirable, particularly for solar energy conversion processes involving inter-molecular photoinduced energy and/or electron transfer. Here, we introduce the excited-state equilibration approach as a method to obtain iron compounds with prolonged luminescence lifetimes and demonstrate that it opens the way to longer-lived luminescent iron complexes.

The excited-state equilibration approach is based on thermal equilibration between the lowest-lying excited state of the metal-based luminophore and the closely-lying triplet state of suitable organic chromophores with long-lived excited states. When the metal-based and the organic chromophores are covalently-linked (so allowing for non-negligible electronic coupling), energy transfer can take place from the metal-based chromophore to the organic chromophore triplet excited state. Due to the large difference between intrinsic decays of the individual chromophores, Boltzmann distribution leads to excited-state equilibration by fast reversible energy transfer (when the two involved excited states are close enough in energy), with the result of obtaining metal-based luminescence output powered by the organic triplet state, with this latter playing the role of an energy reservoir^27-35. In such a way, luminescence lifetime of the metal-based chromophore can be significantly prolonged, whereas luminescence quantum yield is unaffected or slowly decreased. The excited-state equilibration approach is exemplified in Fig. 1 ³⁶. This method has been successfully employed for prolonging luminescence lifetimes of several metal complexes, allowing to reach luminescence lifetimes even much longer than 50 microseconds for Ru(bpy)₃²⁺-type (bpy=2,2’-bipyridine) compounds^29,31,35, but it has never been applied to earth-abundant metal complexes (with the rare exception of semiprecious copper complexes³⁷), to the best of our knowledge.

To obtain Fe(III) compounds exhibiting excited-state equilibration, we synthesized two derivatives of 1 containing pyrene subunits, i.e. compounds 2 and 3 (see structural formulas in Fig. 1). The selection of pyrene chromophores was based on the close energy between the luminescent, doublet ligand-to-metal charge transfer (²LMCT) state of 1, assumed to lie at 582 nm (16), and the lower-lying triplet state of pyrene (³p-p*), known to be around 598 nm^34,38. Although the different spin multiplicity of the two states could slow down the intercomponent energy transfer process, here we demonstrate that, possibly owing to the presence of the (heavy) iron metal, energy transfer occurs fast enough to promote excited-state equilibration.

The ²LMCT emissions of both 2 and 3 exhibit biphasic decays, with shorter-lived components corresponding to excited-state equilibration and longer-lived components corresponding to the lifetime of the equilibrated state. The lifetime of 1 is so increased in 2 and 3, reaching 3.1 and 6.5 ns in 2 and 3, respectively, in dichloromethane fluid solution at room temperature; these luminescence lifetime values are the longest lifetimes ever reported for iron(III) complexes in fluid solution at room temperature.

It is here also shown that luminescence quantum yield is significantly decreased in 2 in comparison with 1 and 3, by the occurring of charge separation upon symmetry breaking. Charge separation has been reported for 1, when the excited state of 1 is quenched by electron transfer by another molecule of 1^39-41, and this has been claimed as a potential route towards photochemical applications. Actually, symmetry-breaking charge separation is extensively studied because of its impact on photovoltaics and artificial photosynthesis⁴². However, for 1 charge separation has been reported at high concentration³⁹, cryogenic temperature in frozen solutions⁴⁰, or in thin films⁴¹, conditions favoring close distance between chromophores. Here, we show that charge separation in 2 can already occur in dilute (even at concentration as low as 10^-5 M) fluid solution at room temperature, probably thanks to the presence of pyrene subunits which enhances aggregation.

Detailed synthesis and characterization of 2 and 3 are reported in the Supplementary Material (see Figs S1-S16). The absorption and emission spectra of 2 and 3 (and of 1, for comparison purposes) in acetonitrile at room temperature (RT) are shown in Fig. 2. The absorption spectra in the visible are dominated by spin-allowed LMCT transitions; for 2 and 3, intense structured absorption features in the UV region are due to spin-allowed transitions centered in the pyrene subunits³⁸. At any excitation wavelength, only a single emission is present for 1–3, which is identical in shape for all the compounds. Emission is slightly blue shifted on passing from CH₃CN to CH₂Cl₂ solution for all the compounds, as usually expected for CT emissions (see Supplementary Material, Figure S17). The luminescence decays of 2 and 3 in CH₂Cl₂ at RT are biphasic (Fig. 3), whereas in CH₃CN only 2 exhibits a biexponential decay. In EtOH/MEOH 4:1 (v/v) rigid matrix at 77 K, 2 and 3 exhibit a somewhat structured emission, still very similar in shape one another (Fig. S18). Luminescence quantum yields of 3 in both CH₃CN and CH₂Cl₂ at RT are independent of excitation wavelength within the range 380–520 nm and are about 0.02 whereas for 2 are two orders of magnitude smaller, in both solvents. Table 1 collects all the relevant spectroscopic and photophysical data of 2 and 3, as well as some relevant data of 1. Redox data of 1–3 in acetonitrile are reported in Table S1 and cyclic and differential pulse voltammetry experiments of 2 and 3 are shown in Fig. S19, in which also attribution of the various processes to specific subunits is reported. In particular, the reduction process around − 0.77 V vs SCE is assigned to the Fe(III)/Fe(II) process, the oxidation process at + 0.64 V vs SCE is assigned to the Fe(IV)/Fe(III) oxidation, the oxidation processes occurring in the range + 1.2–1.6 V vs SCE are attributed to pyrene-based oxidation and the process around + 2.28 V is assigned to oxidation of the methylimidazole subunits.

The absorption spectra and redox behavior of 2 and 3 clearly indicate that only weak interaction occurs between the pyrene and the Fe(III) complex subunits, with each component exhibiting localized excited and redox states. The ²LMCT excited state level of 2 and 3 in acetonitrile at RT is estimated to be similar to that of 1, reported at 582 nm (2.13 eV¹⁶), whereas the energy of the ³p-p* pyrene state in 2 and 3 is approximated by the highest-energy features of the 77 K emission of A and B, that is 591 (2.10 eV) and 601 nm (2.06 eV), respectively (Fig. S18). Excitation of the LMCT state of 2 and 3 can therefore lead to population of the almost isoenergetic triplet state of pyrene by energy transfer⁴³. The intrinsic decay rate constant of the pyrene triplet state in solution is quite slow, even when accelerated by the proximity of a heavy atom (of the order of 10⁵ s^-1 or slower^44,45, so back energy transfer can compete with decay to the ground state for pyrene triplet states, for small energy difference between the involved excited states, as it is the present case. According to these considerations, the faster component of the luminescence decay in CH₃CN of 2, having a lifetime of 0.3 ns (Table 1), can be assigned to the emission from the initially populated ²LMCT state. Comparison with the 2 ns lifetime of 1 in the same experimental conditions¹⁶ suggests that the ²LMCT state in 2 is partially quenched by reversible energy transfer to the closely-lying triplet state of pyrene in 2, lying at 591 nm, with a rate constant of 2.8 x 10⁹ s^-1 (note that in the present case the quenching rate constant would be the sum of forward and backward energy transfer processes, so corresponding to the equilibration rate constant; details on calculation is reported in Supplementary Material). The longer-lived component of the emission of 2 (2.8 ns, Table 1) is attributed to the lifetime of the equilibrated state⁴⁶. In these experimental conditions, the luminescence of 3 is monoexponential (Table 1): the larger distance between the pyrene subunits and the metal complex in 3 with respect to 2 (see structural formulas in Fig. 1) makes probably energy transfer to pyrene triplet state too slow to compete with direct decay of the ²LMCT state of 3 to the ground state (5 x 10⁸ s^-1 in acetonitrile, assumed identical to the decay of 1), although driving force in 3 could be slightly more favorable. The situation is different in CH₂Cl₂, where the luminescence decay of ²LMCT state is biexponential for both 2 and 3 (see Table 1 and Fig. 3), most likely also because of the solvent dependence of the RT emission of 2 and 3 (Fig. S17), which favors energy transfer to pyrene triplet state (this latter is unsensitive to solvent polarity). The more interesting results are exhibited by 3: the shorter component, that is the prompt emission from the initially populated ²LMCT state, has a lifetime of 1.5 ns, indicating that the equilibration rate constant is 2 x 10⁸ s^-1 (see Supplementary Material), competitive with the intrinsic decay of 3 in this solvent (4.2 x 10⁸ s^-1, from the luminescence lifetime of 1, that is 2.4 ns). Luminescence lifetime of the equilibrated state is 6.5 ns, so showing that the luminescence of 3 in dichloromethane is about 2.7 times prolonged with respect to 1, by taking advantage of the excited-state equilibration approach; therefore, this approach can efficiently be used to prolong Fe(III) emission, without modifying the coordination environment of the metal chromophore or significantly losing emission quantum yield (F in 1 and 3 are 0.021 and 0.019, respectively, Table 1), in spite of the different spin multiplicity of the involved excited states.

Compound 2 also exhibits a biexponential luminescence decay in CH₂Cl₂, with an equilibration rate constant – connected to the lifetime of the shorter component of the decay - of 8.0 x 10⁸ s^-1. However, the emission lifetime of the longer-lived component, assigned to the equilibrated state, is only increased to 3.1 ns.

A significative difference between 2 and 3 (and 1) is the emission quantum yield of 2, reduced by two orders of magnitude both in diluted (concentration about 3 x 10^− 5 M) CH₃CN or CH₂Cl₂ solution (e.g., F is 0.0004 in CH₃CN at RT, see Table 1) with respect to those of 3 and 1. Pump-probe spectroscopy shines light on this behavior. Figures 4a and S20 show the transient absorption spectra (TAS) of 2 in deoxygenated acetonitrile (laser pulse at 400 nm): the initial transient absorption spectrum is characterized by a bleaching in the 480–560 nm region, an apparent bleaching - due to stimulated emission - in the 650–750 nm region, and transient absorptions at about 450 nm and in the 560–650 nm region (TAS in the full wavelength scale is shown in Fig. S21). Such initial spectrum is assigned to the LMCT excited state, also in close similitude with that of 1 in acetonitrile at RT^16,39,40. The initial spectrum evolves with a time constant of 60 ps, with formation of a structured absorption in the 550–800 nm region and an intense, increased absorption around 450 nm (Figs. 4a, S20). On literature basis^16,21,39,40, the structured absorption in the 550–800 nm region is attributed to formation of Fe(IV), whereas the increased, intense absorption around 450 nm is assigned to Fe(II) formation. Such attributions are further confirmed by spectroelectrochemistry experiments (Fig. S22). These results indicate that in CH₃CN, inter-molecular interaction takes place, and the ²LMCT state deactivates via symmetry-breaking charge separation in the aggregate system, with formation of an Fe(II)-Fe(IV) dimer, which then decays to the ground state by charge recombination with a time constant of 250 ps (Fig. S20). It should be noted that the concentration of 2 used for TAS (about 1.5 x 10^− 3 M) is two orders of magnitude larger than that used for the luminescence experiments, so justifying why no clear evidence of formation of pyrene triplet state is shown by the TAS experiments. In fact, the symmetry-breaking charge separation process in the aggregate system (k_cs = 1.67 x 10¹⁰ s^-1) is much faster than the equilibration rate constant (2.8 x 10⁹ s^-1, see above). Comparison between emission quantum yields of 2 and 1 in dilute solutions suggests that more than 90% of 2 is present in the aggregate form already in dilute condition, and the percentage is expected to be larger in the higher concentration used for the TAS experiments, so decay of the (eventual) isolated 2 is largely obscured in TAS experiments. On the contrary, in luminescence experiments the recorded emission is due to isolated species: the aggregation process, leading to emission quenching by symmetry-breaking charge separation, is therefore the reason for the strongly reduced emission quantum yield of 2 in dilute solution.

Pump-probe spectroscopy has also been used to further characterize the excited state of 3 in deoxygenated 1,2-dichloroethane (DCE), since prolonged luminescence lifetime is recorded in CH₂Cl₂ (pump-probe spectroscopy in CH₂Cl₂ is complicated by technical problems, so we used DCE as the solvent for pump-probe experiments). Like for 2, the concentrations used for pump-probe experiments of 3 are much higher than those employed for luminescence experiments, so direct comparison between luminescence and pump-probe experiments could not be straightforward. Figures 4b and S23 show the TAS of 3 in DCE. The first spectrum recorded within the laser pulse duration (about 250 fs; excitation wavelength 400 nm) has less to do with the TAS of the ²LMCT state: rather, it recalls the TAS of pyrene excimers, characterized by broad transient absorption features peaking at about 515 nm⁴⁷. Pyrene radical cation or anion formation is excluded, on analyzing the radical anion and cation spectra of the free ligand B (Fig. S24), which do not show any significative absorption over 500 nm. This indicates that an excimer is rapidly formed (time constant in the fs timescale) upon 400 nm light excitation of 3, suggesting that 3, like 2, is largely present in an aggregate form at the concentration used for pump-probe spectroscopy (1.5 mM). The pyrene excimer spectrum evolves – with a time constant of 1.6 ps - into a spectrum which shows an intense absorption band at about 460 nm and other bands at about 500 and 560 nm (Figs. 4b and S23). These features can receive contributions from both the triplet excited state of pyrene and the ²LMCT state. Indeed, the transient absorption spectrum of pyrene triplet of B (Fig. S25) exhibits an intense band around 415 nm and moderate absorption at about 490 and 515 nm, in good agreement with literature data of alkyl-pyrene species⁴⁸, and that of the ²LMCT state has absorption peaks around 450 nm and a broad absorption in the 560–650 nm range (see above). Therefore, in the transient absorption spectrum of 3 recorded after 3 ps from pulse (Figs. 4b and S23), the band at about 500 nm can be due to triplet-triplet pyrene-based transitions, the band at about 560 nm can receive main contribution from the ²LMCT state, and the intense band at about 460 nm can receive contributions from transitions involving both pyrene triplet and LMCT states. This tends to suggest that the pyrene excimer state deactivates to the lower-lying pyrene-based triplet and LMCT states. A successive decay process, with a time constant of 16 ps, is also present (Fig. 4b and S23): during such a process the transient absorption band at about 460 nm remains roughly constant, whereas the band at about 500 nm decreases more significantly. The overall decay process of the excimer is tentatively assigned to the formation of the equilibrated ³p-p*/²LMCT state. The equilibrated state finally decays to the ground state on a time scale too long to be safely measured with our pump-probe apparatus (time limit, 3.3 ns), but that anyway appears to be of the order of few ns, in fair agreement with the longer-lived luminescence lifetime of 3 in CH₂Cl₂ (6.5 ns, see above)⁴⁹.

Finally, the different decay of excited 2 and 3 in concentrated solutions studied by pump-probe experiments, that is symmetry-breaking charge separation for 2 and ultrafast excimer formation leading to excited-state equilibration for 3, warrants some further comments. Although aggregation in concentrated solution of 2 and 3 is mainly attributed to pyrene-pyrene interactions in both cases, steric hindrance at pyrene subunits is much larger for 2 and this could hinder efficient pyrene-pyrene coupling needed for excimer formation in this latter species. Moreover, in aggregated species, the metal-metal distance in 3 would be much larger than in 2 (see Fig. 1), and this could reduce electronic coupling between the metal centers, making symmetry-breaking a non-competitive decay process in 3.

Summarizing, the photophysical properties of the new species 2 and 3 indicate the potential of the excited-state equilibration approach to achieve long-lived luminescence involving iron complexes, therefore opening a new strategy towards earth-abundant metal complexes with long-lived excited states. Noteworthy, prolonging the luminescence lifetime from 2 ns to 7 ns has a significative effect on the efficiency of bimolecular reactions involving excited states. For example, considering a simple Stern-Volmer behavior and assuming a diffusion rate constant of 10¹⁰ s^-1 and a quencher concentration of 0.05 M, for a diffusion-controlled quenching process (as most electron transfer processes are), a reaction involving a luminophore with 2 ns lifetime could be 50% efficient at the best, whereas the reaction efficiency would increase to approximate 80% when the luminophore lifetime is 7 ns. Moreover, the excited-state equilibration occurs despite the different spin multiplicity of the involved excited states, so representing an unprecedented case, to the best of our knowledge, for energy transfer between different subunits of a multichromophoric species. A careful design of coupled earth-abundant metal complexes and suitable organic chromophores – for example, increasing the energy of the metal-based CT excited state and lowering that of the organic-based triplet states, therefore increasing the percentage of the long-lived organic chromophore-based triplet state in the equilibrated state – could lead to a further increase in luminescence lifetimes so boosting the potential of such sustainable metal complexes for photochemical applications.

Moreover, a somewhat unexpected result is the ultrafast symmetry-breaking charge separation exhibited by 2 in dilute solution, which indicates that such process does not necessary require highly-concentrated solutions or low temperature to occur for this type of iron complexes.

Absorption, emission and lifetimes measurements

UV/Vis absorption spectra were taken on a Jasco V-560 spectrophotometer. For steady-state luminescence measurements, a HORIBA Jobin Yvon Fluorolog-3 spectrofluorimeter was used, equipped with a Hamamatsu R928P photomultiplier tube. The spectra were corrected for photomultiplier response using a program purchased with the fluorimeter. For the luminescence lifetimes, the same instrument was used equipped with a HORIBA Delta-Hub module and HORIBA NanoLED solid-state pulsed diodes (<1.4 ns pulse width at 456 nm) were employed. Emission quantum yields for deaerated solutions were determined using the optically diluted method⁵⁰. As luminescence quantum yield standards, we used [Ru(bpy)₃]²⁺ (bpy = 2,2'-bipyridine)⁵¹.

Electrochemical measurements

Cyclic voltammetry (CV) and differential pulse voltammetry (DPV) were made using an Autolab PGSTAT 12 potentiostat/galvanostat instrument controlled with GPES software and connected to an electrochemical cell with a three electrodes setup. The measurement apparatus is composed of a glassy carbon working electrode, a platinum wire counter electrode and an Ag/Ag⁺electrode as reference electrode, using the Fc/Fc+ couple (+0.395 V vs. SCE) as internal reference. Measurements were performed in degassed (Ar) acetonitrile (purchased from Merk, spectroscopic grade) solutions of the desired species (0.5 mM) using 0.05 M tetrabutylammonium hexafluorophosphate (TBAPF₆, Sigma Aldrich, electrochemical grade, ≥99.0 %) as the supporting electrolyte.

Pump-probe measurements

Time-resolved transient absorption experiments were performed using a pump-probe setup based on the Spectra-Physics MAI-TAI Ti:sapphire system as the laser source and the Ultrafast Systems Helios spectrometer as the detector. The output of laser beam was splitted to generate pump and probe beam pulses with a beam splitter (85 and 15%). The pump pulse (400 nm, 1-2 mJ) was generated with a Spectra-Physics 800 FP OPA and focused in the 2 mm quartz cuvette containing the sample. The probe beam was delayed with a computer-controlled motion controller and then focused into a 2-mm sapphire plate to generate a white light continuum (spectral range 400–800 nm). The white pulse is then overlapped with the pump beam in the cuvette containing the sample. The effective time resolution was about 200 fs, and the temporal chirp over the white-light 450–750 nm was ca. 150 fs; the temporal window of the optical delay stage was 0-3200 ps. To cancel out orientation effects on the dynamics, the polarization direction of the linearly polarized probe pulse was set at a magic angle of 54.7° with respect to that of the pump pulse. All the transient spectra shown in the present paper are chirp corrected. The time-resolved data were analyzed with the Ultrafast Systems Surface Explorer Pro software.

Nanosecond transient absorption experiments were performed with an EOS broadband pump-probe nanosecond Transient Absorption Spectrometer (Ultrafast Systems LLC). The pump pulses at λ=356 nm (7 mJ) is generated with an actively Q-switched Nd:YAG laser with an integrated harmonics generator. The probe pulse (1 kHz, 0.5 ns pulse width), which was generated in a photonic fiber, was synchronized with the femtosecond amplifier. The measurements were carried out in-situ using 2 mm quartz cuvettes.

Spectroelectrochemistry measurements

UV-vis spectroelectrochemical measurements were obtained with a SPECAC Omni Cell System: an optically transparent thin-layer electrode (OTTLE) cell with the working Pt-mesh, twinned Ag-wire reference and Pt-mesh auxiliary electrodes melt-sealed into a polyethylene spacer – CaF2 windows and 0.25 mm path length. The UV-vis spectra were acquired with a JASCO V570 spectrophotometer concurrently applying a potential by using an Autolab multipurpose equipment interfaced to a PC. Acetonitrile (spectroscopic grade) was purchased from Merk while tetrabutylammonium hexafluorophosphate (electrochemical grade, ≥99.0 %) was purchased from Sigma Aldrich.

Experimental uncertainties are as follows: absorption maxima, 2 nm; molar absorption, 15%; luminescence maxima, 4 nm; luminescence lifetimes, 10%; luminescence quantum yields, 15%; transient absorption decay and rise rates, 10%; redox potentials, 15 mV.

Acknowledgments

We thank the Italian Ministry of Foreign Affairs and International Cooperation, PGR project on Artificial Photosynthesis, collaboration Italy-Japan, Grant JP21GR09 (SC, FP, SG) and the European Union (NextGenerationEU), through the MUR-PNRR project SAMOTHRACE, grant ECS00000022 (SC, FP, AA) for funding.

Author contributions:

Conceptualization: SC, FP

Methodology: SC, FP, SG, AMC

Investigation: FP, SG, AMP, AA

Visualization: FP, SG

Funding acquisition: SC

Writing – original draft: SC, FP

Writing – review & editing: SC, FP, SG, AMC, AA

Competing interests:

Authors declare that they have no competing interests.

Data and materials availability:

All data are available in the main text or the supplementary information.

Balzani, V., Moggi, L., Manfrin, M. F., Bolletta, F. & Gleria, M. Solar energy conversion by water photodissociation: transition metal complexes can provide low-energy cyclic systems for catalytic photodissociation of water, Science189, 852-856 (1975). doi: 10.1126/science.189.4206.852.
Meyer, T. J., Sheridan, M. V. & Sherman, B. J. Mechanisms of molecular water oxidation in solution and on oxide surfaces, Chem. Soc. Rev.46, 6148-6169 (2017), and refs. therein. doi:10.1039/C7CS00465F.
O’Regan, B. & Graetzel, M. A low-cost, high-efficiency solar cell based on dye-sensitized colloidal TiO2 films, Nature353, 737-740 (1991). doi:10.1038/353737a0.
Idris, M., Kapper, S. C., Tadle, A. C., Batagoda, T., Ravinson, D. S. M., Abimbola, O., Djurovich, P. I., Kim, J., Forrest, S. R. & Thompson, M. E. Blue emissive fac/mer-Iridium (III) NHC carbene complexes and their application in OLEDs, Adv. Opt. Mater. 9, 2001994 (2021), and refs. therein. doi:10.1002/adom.202001994.
Lo, K.K.-W. Molecular design of bioorthogonal probes and imaging reagents derived from photofunctional transition metal complexes, Acc. Chem. Res.53, 32-44 (2020). Doi:10.1021/acs.accounts.9b00416.
Caporale, C. & Massi, M. Cyclometalated Ir(III) complexes for life science, Coord. Chem. Rev.363, 71-91 (2018). Doi: 10.1016/j.ccr.2018.02.006.
Sancho-Albero, M., Facchetti, G., Panini, N., Meroni, M., Bello, E., Rimoldi, I., Zucchetti, M., Frapolli, R. & De Cola, L. Enhancing Pt (IV) complexes' anticancer activity upon encapsulation in stimuli‐responsive nanocages, Adv. Healthcare Mater.12, 2202932 (2023) and refs. therein. Doi: 0.1002/adhm.202202932.
Campagna, S., Puntoriero, F., Nastasi, F., Bergamini, G. & Balzani, V. Photochemistry and photophysics of coordination compounds: ruthenium, Top. Curr. Chem., 280, 117-214 (2007). DOI: 10.1007/128_2007_133.
McCusker, J. K. Electronic structure in the transition metal block and its implications for light harvesting, Science363, 484–488 (2019). Doi: 10.1126/science.aav9104.
Wenger, O. S. Is iron the new ruthenium?, Chem. Eur. J.25, 6043 –6052 (2019). Doi: 10.1002/chem.201806148.
Förster, C. &. Heinze, K. Photophysics and photochemistry with Earth-abundant metals-fundamentals and concepts, Chem. Soc. Rev.49, 1057-1070 (2020).
Wegeberg, C. & Wenger, O. S. Luminescent first-row transition metal complexes, JACS Au1,1860–1876 (2021). doi: 10.1021/jacsau.1c00353.
Zhang, Y., Lee, T. S., Favale, J. M., Leary, D. C., Petersen, J. L., Scholes, G. D., Castellano, F. N. & Milsmann, C. Delayed fluorescence from a zirconium(iv) photosensitizer with ligand-to-metal charge-transfer excited states, Nat. Chem.12, 345–352 (2020). Doi: 10.1038/s41557-020-0430-7.
Wegeberg, C., Häussinger, D. & Wenger, O. S. Pyrene-Decoration of a chromium(0) tris(diisocyanide) enhances excited state delocalization: a strategy to improve the photoluminescence of 3d₆ metal complexes, J. Am. Chem. Soc.143, 15800-15811 (2021).doi: 10.1021/jacs.1c07345.
Herr, P., Kerzig, C., Larsen, C. B., Häussinger, D. & Wenger, O. S. Manganese(i) complexes with metal-to-ligand charge transfer luminescence and photoreactivity, Nat. Chem. 13, 956–962 (2021).
Kjaer, K. S., Kaul, N., Prakash, O., Chabera, P., Rosemann, N. W., Honarfar, A., Gordivska, O., Fredin, L. A., Bergquist, K.-E., Häggström, L., Ericsson, T., Lindh, L., Yartsev, A., Styring, S., Huang, P., Uhlig, J., Bendix, J., Strand, D., Sundström, V., Persson, P., Lomoth, R. & Wärnmark, K. Luminescence and reactivity of a charge-transfer excited iron complex with nanosecond lifetime, Science363, 249–253 (2019). Doi: 10.1126/science.aau7160.
Aydoga, A., Bangle, R. E., Cadranel, A., Turlington, M. D., Conroy, D. T., Cauët, E., Singleton, M. L., Meyer, G. J., Sampaio, R. N., Elias, B. & Troian-Gautier, L. Accessing photoredox transformations with an iron(III) photosensitizer and green light, J. Am. Chem. Soc. 143, 15661-15673 (2021). Doi: 10.1021/jacs.1c06081.
Leis, W., Argüello Cordero, M.A., Lochbrunner, S., Schubert, H. & Berkefeld, A. A photoreactive iron(II) complex luminophore, J. Am. Chem. Soc.144, 1169-1173 (2022). Doi: 10.1021/jacs.1c13083.
Kaufhold, S., Rosemann, N. W., Chábera, P., Lindh, L., Losada, I. B., Uhlig, J., Pascher, T., Strand, D., Wärnmark, K., Yartsev, A. & P. Persson, P. Microsecond photoluminescence and photoreactivity of a metal-centered excited state in a hexacarbene−Co(III) complex, J. Am. Chem. Soc. 143, 1307−1312 (2021).
Pal, A. K., Li, C., Hanan, G. S. & Zysman-Colman, E. Blue-emissive cobalt(III) complexes and their use in the photocatalytic trifluoromethylation of polycyclic aromatic hydrocarbons, Angew. Chem. Int. Ed.57, 8027-8031 (2018). Doi: 10.1002/anie.201802532.
Prakash, O., Lindh, L., Kaul, N., Rosemann, N. W., Losada, I. B., Johnson, C., Chabera, P., Ilic, A., Gupta, A. K., Uhlig, J., Ericsson, T., Häggström, L., Huang, P., Bendix, J., Strand, D., Yartsev, A., Lomoth, R., Persson, P. & Wärnmark, K. Photophysical integrity of the iron(III) scorpionate framework in iron(III)−NHC complexes with long-lived ²LMCT excited states, Inorg. Chem.61, 17515-17526 (2022). doi: 10.1021/acs.inorgchem.2c02410.
Sinha, N., Wegeberg, C., Häussinger, D., Prescimone, A. & Wenger, O. S. Photoredox-active Cr(0) luminophores featuring photophysical properties competitive with Ru(II) and Os(II) complexes, Nat. Chem.,15, 1730-1736 (2023). doi: 10.1038/s41557-023-01297-9.
Glacer, F., Aydogan, A., Elias, B. & Troian-Gautier, L. The great strides of iron photosensitizers for contemporary organic photoredox catalysis: On our way to the holy grail?, Coord. Chem. Rev., 500, 215522 (2024). doi: 10.1016/j.ccr.2023.215522.
A luminescence lifetime of about 5 ns has recently been reported for a Fe(III) compound²⁵. However, that luminescence has been questioned as due to an unidentified impurity ²⁶.
Steube, J., Kruse, A., Bokareva, O. S., Reuter, T., Demeshko, S., Schoch, R., Argüello Cordero, M. A., Krishna, A., Hohloch, S., Meyer, F., Heinze, K., Kühn, O., Lochbrunner, S. & Bauer, M. Janus-type emission from a cyclometalated iron(iii) complex, Nat. Chem.15, 468-474 (2023). Doi: 10.1038/s41557-023-01137w.
Johnson, C. E., Schwartz, J., Deegbey, M., Prakash, O., Sharma, K., Huang, P., Ericsson, T., Häggström, L., Bendix, J., Gupta, A. K., Jakubikova, E., Wärnmark, K. & Lomoth, R. Ferrous and ferric complexes with cyclometalating N-heterocyclic carbene ligands: a case of dual emission revisited, Chem. Sci., 14, 10129-10139 (2023). DOI: 10.1039/d3sc02806b.
Ford, W. E. & Rodgers, M. A. J. Reversible triplet-triplet energy transfer within a covalently linked bichromophoric molecule, J. Phys. Chem. 96, 2917-2920 (1992). Doi: 10.1021/j100186a026.
Tyson, D. S. & Castellano, F. N. Intramolecular singlet and triplet energy transfer in a ruthenium(II) diimine complex containing multiple pyrenyl chromophores,J. Phys. Chem. A103, 10955- 10960 (1999). Doi: 10.1021/jp992983w.
Simon, J. A., Curry, S. L., Schmehl, R. H., Schatz, T. R., Piotrowiak, P., Jin X. & Thummel, R. P. Intramolecular electronic energy transfer in ruthenium(II) diimine donor/pyrene acceptor complexes linked by a single C−C bond, J. Am. Chem. Soc., 119, 11012-11022 (1997). Doi: 10.1021/ja970069f.
Wilson, G. J., Laukikonis, A., Sasse, W. H. F. & Mau, A. W.-H. Excited-State processes in ruthenium(II) bipyridine complexes containing covalently bound arenes, J. Phys. Chem. A101, 4860-4866 (1997). Doi: 10.1021/jp970667g.
Tyson, D. S., Luman, C. R., Zhou, X. & Castellano, F. N. New Ru(II) chromophores with extended excited-state lifetimes, Inorg. Chem.40, 4063-4071 (2001). Doi: 10.1021/ic010287g.
Michalec, J. F., Bejune, S. A., Cuttell, D. G., Summerton, G. C., Gertenbach, J. A., Field, J. S., Haines, R. J. & McMillin, D. R. Long-lived emissions from 4’-Substituted Pt(trpy)Cl⁺ complexes bearing aryl groups. influence of orbital parentage, Inorg. Chem. 40, 2193-2200 (2001).doi: 10.1021/ic0013126.
Passalacqua, R., Loiseau, F., Campagna, S., Fang, Y.-Q. & Hanan, G. S. In search of ruthenium(II) complexes based on tridentate polypyridine ligands that feature long-lived room-temperature luminescence: the multichromophore approach., Angew. Chem. Int. Ed.42, 1608-1611 (2003). Doi: 10.1002/anie.200250613.
Maubert, B., McClenaghan, N. D., Indelli, M. T. & Campagna, S. Absorption spectra and photophysical properties of a series of polypyridine ligands containing appended pyrenyl and anthryl chromophores and their ruthenium(II) and osmium(II) complexes, J. Phys. Chem. A107, 447-455 (2003). Doi: 10.1021/jp0213497.
McClenaghan, N. D., Leydet, Y., Maubert, B., Indelli, M. T. & Campagna, S. Excited-state equilibration: a process leading to long-lived metal-to-ligand charge transfer luminescence in supramolecular complexes, Coord. Chem. Rev.249, 1336-1350 (2005) and refs. therein. Doi: 10.1016/j.ccr.2004.12.017.
The lifetime of the thermally-equilibrated excited state (1/k_c, referring to Fig.1) depends on the intrinsic decay rates k_a and k_b of the “isolated” subunits and on the percentage of each excited state in the equilibrated state, in its turn depending on the energy difference between the involved excited states, provided that the rate constants for forward and backward energy transfer are much faster than k_a and k_b, so that Boltzmann equilibration is effective. More details are reported in ref. 35. Lowering of the (longer-lived) triplet p-p* organic chromophore excited state with respect to the (shorter-lived) metal-based CT state (provided that backward energy transfer is still much faster than k_c) leads to a longer-lived equilibrated state, that is longer-lived CT emission.
Leydet, Y., Bassani, D., Jonosauskas, G. & McClenaghan, N. D. Equilibration between three different excited states in a bichromophoric copper(I) polypyridine complex, J. Am. Chem. Soc.129, 8688-8689 (2007). Doi: 10.1021/ja072335n.
Turro, N. J. Modern Molecular Photochemistry (University Science Books, 1991).
Kaul, N. & Lomoth, R. The carbene cannibal: photoinduced symmetry-breaking charge separation in an Fe(III) n‐heterocyclic carbene, J. Am. Chem. Soc.143, 10816-10821 (2021). Doi: 10.1021/jacs.1c03770.
Rosemann, N. W., Lindh, L., Losada, I. B., Kaufhold, S., Pakash, O., Ilic, A., Schwarz, J., Wärnmark, K., Chabera, P., Yartsev, A. & Persson, P. Competing dynamics of intramolecular deactivation and bimolecular charge transfer processes in luminescent Fe(III) N-heterocyclic carbene complexes, Chem. Sci.14, 3569-3579 (2023). Doi: 10.1039/d2sc05357h.
Zhang, M., Johnson, C. E., Ilic, A., Schwarz, J., Johansson, M. B. & Lomoth, R. High-efficiency photoinduced charge separation in Fe(III)carbene thin films, I. Am. Chem. Soc.145, 19171-19176 (2023). Doi: 10.1021/jacs.3c05404.
Sebastian, E. & Hariharan, M. Symmetry-breaking charge separation in molecular constructs for efficient light energy conversion, ACS Energy Lett. 7, 696-711 (2022) and refs. therein. Doi:10.1021/acsenergylett.1c02412.
To carefully determine the excited state energies of ²LMCT and pyrene p-p* triplets in 2 and 3 is not straightforward. On assuming the highest-energy feature of the 77 K emission band of 2 and 3 (Fig. S18) as a bonafide approximation of the ²LMCT excited state energy of these complexes, values of 2.06 and 2.04 eV could be obtained, respectively, which are slightly different from the 2.13 eV estimated in acetonitrile for 1¹⁶, despite the ²LMCT absorption and room temperature emission bands of 1-3 are practically identical. However, the 77 K emission spectra are here recorded in EtOH/MeOH 4:1 (v/v) solvent mixture, and solvent polarity can affect the energy of CT states. In any case, the relevant excited states appear to be close enough in energy to make equilibration an effective process. Indeed, excited state equilibration has been reported even when a (shorter-lived) triplet metal-to-ligand charge-transfer (MLCT) state was lower-lying than a (longer-lived) triplet organic chromophore³⁴.
Birks, J. B. Photophysics of Aromatic Molecules (Wiley Interscience, 2nd Edition, 1970).
Harriman, A., Hissler, M. & Ziessel, R. Photophysical properties of pyrene-(2,2′-bipyridine) dyads, Phys. Chem. Chem. Phys., 1, 4203-4211 (1999). Doi:10.1039/A904390J.
The ²LMCT state of 2 and 3 can also be quenched by reductive electron transfer by the pyrene subunits. Electron transfer from pyrene to the excited iron subunit is thermodynamically allowed by ca. 0.08 eV, assuming 2.13 eV as the energy of ²LMCT state and considering the redox data reported in Table S1. However, electron transfer quenching is rarely expected to lead to excited-state equilibration and therefore does not agree with the prolonged lifetimes reported for 2 and 3. Anyway, some contribution from this deactivation route cannot be totally ruled out, particularly for 2 in acetonitrile.
Foggi, P., Pettini, L., Santa, I., Righini, R. & Califano, S. Transient absorption and vibrational relaxation dynamics of the lowest excited singlet state of pyrene in solution, J. Phys. Chem.99, 7439-7445 (1995). Doi: 10.1021/j100019a029.
Masuhara, H., Tanaka, J. A., Mataga, N., De Schryver, F. C. & Collart, P. Absorption spectra and dynamics of triplet state of bis[1-(1-pyrenyl)ethyl]ethers, Polymer J., 15, 915-917 (1983). doi.org/10.1295/polymj.15.915.
It should be pointed out that the pump-probe experiments here reported are performed with laser pulse at 400 nm. Other excitation wavelengths were not available for technical reasons.
Demas, J. N. & Crosby, G. A. Measurement of photoluminescence quantum yields, J. Phys. Chem., 75,991-1024 (1971). Doi: 10.1021/j100678a001.
Nakamaru, N. Synthesis, luminescence quantum yields, and lifetimes of trischelated ruthenium(II) mixed-ligand complexes including 3,3′-Dimethyl-2,2′-bipyridyl, Bull. Chem. Soc. Jpn., 55, 2697-2705 (1982). doi.org/10.1246/bcsj.55.2697.

Table 1

Absorption and luminescence data. Absorption and luminescence data of 1–3 in fluid solution at room temperature. Air-equilibrated and nitrogen-saturated samples exhibit the same emission lifetimes and quantum yields. Emission data are independent of excitation wavelength in the excitation range 380–550 nm.
	absorption	Luminescence, CH₃CN			Luminescence, CH₂Cl
	l_max, nm (e, M^− 1cm^− 1)^a	l_max, nm	t, ns	F	l_max, nm	t, ns	F
2	501 (4000)	655	0.3; 2.8^b	0.0004	645	0.8; 3.1 ^b	0.0007
3	501 (3200)	655	2.0	0.022	645	1.5; 6.5 ^b	0.019
1	501 (2950)	655	2.0	0.020	645	2.4	0.021
(a) Only the lowest-energy maximum is reported. (b) Biexponential decay.

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Fe(III) complexes with prolonged luminescence lifetimes and symmetry-breaking charge separation

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