Metal-Metal-to-Ligand Charge Transfer in Pt(II) Dimers Bridged by Pyridyl and Quinoline Thiols.

The investigation of two distinct species of square planar dinuclear Pt(II) dimers based on anti-[Pt(C∧N)(µ-N∧S)]2, where C∧N is either 2-phenylpyridine (ppy) or benzo(h)quinoline (bzq) and N∧S is pyridine-2-thiol (pyt), 6-methylpyridine-2-thiol (Mpyt), or 2-quinolinethiol (2QT), is presented. Each molecule was thoroughly characterized with electronic structure calculations, static UV-vis and photoluminescence (PL) spectroscopy, and cyclic voltammetry, along with transient absorbance and time-gated PL experiments. These visible absorbing chromophores feature metal-metal-to-ligand charge-transfer (MMLCT) excited states that originate from intramolecular d8-d8 metal-metal σ-interactions and are manifested in the ground- and excited-state properties of these molecules. All five molecules reported (anti-[Pt(ppy)(µ-Mpyt)]2 could not be isolated), three of which are newly conceived here, possess electronic absorptions past 500 nm and high quantum yield PL emission with spectra extending into the far red (λem > 700 nm), originating from the charge-transfer state in each instance. Each chromophore displays excited-state decay kinetics adequately modeled by single exponentials as recorded using dynamic absorption and PL experiments; each technique yields similar decay kinetics. The combined data illustrate that pyridyl and quinoline-thiolates in conjunction with select cyclometalates represent classes of MMLCT chromophores that exhibit excited-state properties suitable for promoting light-energized chemical reactions and provide a molecular platform suitable for evaluating coherence phenomena in transient metal-metal bond-forming photochemistry.

Understanding the influence of geometric and electronic structure on the excited state dynamical and photoredox properties of perinone chromophores.

In this work, a series of eight similarly structured perinone chromophores were synthesized and photophysically characterized to elucidate the electronic and structural tunability of their excited state properties, including excited state redox potentials and fluorescence lifetimes/quantum yields. Despite their similar structure, these chromophores exhibited a broad range of visible absorption properties, quantum yields, and excited state lifetimes. In conjunction with static and time-resolved spectroscopies from the ultrafast to nanosecond time regimes, time-dependent computational modeling was used to correlate this behavior to the relationship between non-radiative decay and the energy-gap law. Additionally, the ground and excited state redox potentials were calculated and found to be tunable over a range of 1 V depending on the diamine or anhydride used in their synthesis (Ered* = 0.45-1.55 V; Eox* = -0.88 to -1.67 V), which is difficult to achieve with typical photoredox-active transition metal complexes. These diverse chromophores can be easily prepared, and with their range of photophysical tunability, will be valuable for future use in photofunctional applications.

Energy Migration Processes in Re(I) MLCT Complexes Featuring a Chromophoric Ancillary Ligand.

We present the synthesis, structural characterization, electronic structure calculations, and ultrafast and supra-nanosecond photophysical properties of a series of five Re(I) bichromophores exhibiting metal to ligand charge transfer (MLCT) excited states based on the general formula fac-[Re(N∧N)(CO)3(PNI-py)]PF6, where PNI-py is 4-piperidinyl-1,8-naphthalimidepyridine and N∧N is a diimine ligand (Re1-5), along with their corresponding model chromophores where 4-ethylpyridine was substituted for PNI-py (Mod1-5). The diimine ligands used include 1,10-phenanthroline (phen, 1), 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline (bcp, 2), 4,4'-di-tert-butyl-2,2'-bipyridine (dtbb, 3), 4,4'-diethyl ester-2,2'-bipyridine (deeb, 4), and 2,2'-biquinoline (biq, 5). In these metal-organic bichromophores, structural modification of the diimine ligand resulted in substantial changes to the observed energy transfer efficiencies between the two chromophores as a result of the variation in 3MLCT excited-state energies. The photophysical properties and energetic pathways of the model chromophores were investigated in parallel to accurately track the changes that arose from introduction of the organic chromophore pendant on the ancillary ligand. All relevant photophysical and energy transfer processes were probed and characterized using time-resolved photoluminescence spectroscopy, ultrafast and nanosecond transient absorption spectroscopy, and time-dependent density functional theory calculations. Of the five bichromophores in this study, four (Re1-4) exhibited a thermal equilibrium between the 3PNI-py and the 3MLCT excited state, drastically extending the lifetimes of the parent model chromophores.

Photophysical Processes in Rhenium(I) Diiminetricarbonyl Arylisocyanides Featuring Three Interacting Triplet Excited States.

We present a series of four transition-metal complexes based on the rhenium(I) tricarbonyl 1,10-phenanthroline (phen) template, with a lone ancillary arylisocyanide (CNAr) ligand to yield metal-organic chromophores of the generic molecular formula [Re(phen)(CO)3(CNAr)]+ [CNAr = 2,6-diisopropylphenyl isocyanide (1), 4-phenyl-2,6-diisopropylphenyl isocyanide (2), 4-phenylethynyl-2,6-diisopropylphenyl isocyanide (3), and 4-biphenyl-2,6-diisopropylphenyl isocyanide (4)]. This particular series features varied degrees of π-conjugation length in the CNAr moiety, resulting in significant modulation in the resultant photophysical properties. All molecules possess long-lived [8-700 µs at room temperature (RT)], strongly blue-green photoluminescent and highly energetic excited states (λmax,em = 500-518 nm; Φ = 14-64%). Each of these chromophores has been photophysically investigated using static and dynamic spectroscopic techniques, the latter probed from ultrafast to supra-nanosecond time scales using transient absorption and photoluminescence (PL). Time-resolved PL intensity decays recorded as a function of the temperature were consistent with the presence of at least two emissive states lying closely spaced in energy with a third nonemissive state lying much higher in energy and likely ligand-field in character. The combined experimental evidence, along with the aid of electronic structure calculations (density functional theory and time-dependent density functional theory performed at the M06/Def2-SVP/SDD level), illustrates that the CNAr ligand is actively engaged in manipulating the excited-state decay in three of these molecules (2-4), wherein the triplet metal-to-ligand charge-transfer (3MLCT) state along with two distinct triplet ligand-centered (3LC) excited-state configurations (phen and CNAr) conspire to produce the resultant photophysical properties. Because the π conjugation within the CNAr ligand was extended, an interesting shift in the dominant photophysical processes was observed. When the CNAr conjugation length is short, as in 1, the phenanthroline 3LC state dominates, resulting in a configurationally mixed triplet excited state of both LC and MLCT character. With more extended π conjugation in the CNAr subunit (2-4), the initially generated 3LC(phen)/3MLCT excited state ultimately migrates to the CNAr 3LC state on the order of tens of picoseconds. Molecules 3 and 4 in this series also feature unique examples of inorganic excimer formation, as evidenced by dynamic self-quenching in the corresponding PL intensity decays accompanied by the observation of a short-lived low-energy emission feature.

Excited-State Processes of Cyclometalated Platinum(II) Charge-Transfer Dimers Bridged by Hydroxypyridines.

A series of four anti-disposed dinuclear platinum(II) complexes featuring metal-metal-to-ligand charge-transfer (MMLCT) excited states, bridged by either 2-hydroxy-6-methylpyridine or 2-hydroxy-6-phenylpyridine and cyclometalated with 7,8-benzoquinoline or 2-phenylpyridine, are presented. The 2-hydroxypyridine bridging ligands control intramolecular d8-d8 metal-metal σ interactions, affecting the frontier orbitals' electronic structure, resulting in marked changes to the ground- and excited-state properties of these complexes. Three of these molecules possess reversible one-electron oxidations in cyclic voltammetry experiments as a result of strong intramolecular metallophilic interactions. In this series of molecules, X-ray crystallography revealed Pt-Pt distances ranging between 2.815 and 2.878 Å; the former represents the shortest reported metal-metal distance for platinum(II) dimers possessing low-energy MMLCT transitions. All four molecules reported here display visible absorption bands beyond 500 nm and feature MMLCT-based red photoluminescence (PL) above 700 nm at room temperature with high PL quantum yields (up to 4%) and long excited-state lifetimes (up to 341 ns). The latter were recorded using both transient PL and transient absorption experiments that self-consistently yielded quantitatively identical excited-state lifetimes. The energy-gap law was successfully applied to this series of chromophores, documenting this behavior for the first time in molecules possessing MMLCT excited states. The combined data illustrate that entirely new classes of MMLCT chromophores can be envisioned using bridging pyridyl hydroxides in cooperation with various C^N cyclometalates to achieve photophysical properties suitable for excited-state electron- and energy-transfer chemistry.

Ultrafast photoinduced electron transfer in viologen-linked BODIPY dyes.

New boron-dipyrromethene (BODIPY) dyes linked to viologen are prepared and their photophysical and electrochemical properties are investigated. Both synthesized molecules have similar electronic absorption spectra with the absorption maximum localized at 517 and 501 nm for dye 1 and dye 2, respectively. They exhibit well-defined redox behavior, highlighting the presence of BODIPY and viologen subunits, with little perturbation of the redox potential of both subunits with respect to the parent compounds. Both dyes are heavily quenched by photoinduced electron transfer from the BODIPY to the viologen subunit. The transient absorption technique demonstrates that dye 2 forms the viologen radical within a timeframe of 7.1 ps, and that the charge-separated species has a lifetime of 59 ps. Sustained irradiation of dye 2 in the presence of a tertiary amine allows for the accumulation of BODIPY-methyl-4,4'-bipyridinium (BODIPY-MV(+)), as observed by its characteristic absorption at 396 and 603 nm. However, dye 2 does not generate catalytic amounts of hydrogen under standard conditions.

Excited state processes of dinuclear Pt(II) complexes bridged by 8-hydroxyquinoline.

Dinuclear d8 Pt(II) complexes, where two mononuclear square planar Pt(II) units are bridged in an "A-frame" geometry, possess photophysical properties characterised by either metal-to-ligand-(MLCT) or metal-metal-ligand-to-ligand charge transfer (MMLCT) transitions determined by the distance between the two Pt(II) centres. When using 8-hydroxyquinoline (8HQH) as the bridging ligand to construct novel dinuclear complexes with general formula [C^NPt(µ-8HQ)]2, where C^N is either 2-phenylpyridine (1) or 7,8-benzoquinoline (2), triplet ligand-centered (3LC) photophysics results echoing that in a mononuclear model chromophore, [Pt(8HQ)2] (3). The lengthened Pt-Pt distances of 3.255 Å (1) and 3.243 Å (2) results in a lowest energy absorption centred around 480 nm assigned as having mixed LC/MLCT character by TD-DFT, mirroring the visible absorption spectrum of 3. Additionally, 1 and 2 exhibit 3LC photoluminescence with limited quantum yields (0.008) from broad transitions centred near 680 nm. Photoexcitation of 1-3 leads to an initially prepared excited state that relaxes within 15 ps to a 3LC excited state centred on the 8HQ bridge, which then persists for several microseconds. All the experimental results correspond well with DFT electronic structure calculations.

Bidirectional "ping-pong" energy transfer and 3000-fold lifetime enhancement in a Re(I) charge transfer complex.

The synthesis and photophysics of a new Re(I)-carbonyl diimine complex, Re(PNI-phen)(CO)(3)Cl, where the PNI-phen is N-(1,10-phenanthroline)-4-(1-piperidinyl)naphthalene-1,8-dicarboximide is reported. The metal-to-ligand charge transfer (MLCT) emission lifetime was increased approximately 3000-fold at room temperature with respect to that of the model complex [Re(phen)(CO)(3)Cl] as a result of thermal equilibrium between the emissive (3)MLCT state and a long-lived triplet ligand-centered ((3)LC) state on the PNI chromophore. This represents the longest excited state lifetime (τ = 651 µs) that has ever been observed for a Re(I)-based CT photoluminescence at room temperature. The energy transfer processes and the associated rate constants leading to the establishment of the excited state equilibrium were elucidated by a powerful combination of three techniques (transient visible and infrared (IR) absorption and photoluminescence), each applied from ultrafast to the micro/milliseconds time scale. The MLCT excited state was monitored by transient IR using CO vibrations through time intervals where the corresponding signals obtained in conventional visible transient absorption were completely obscured by overlap with strong transients originating from the pendant PNI chromophore. Following initial excitation of the (1)LC state on the PNI chromophore, energy is transferred to form the MLCT state with a time constant of 45 ps, a value confirmed in all three measurement domains within experimental error. Although transient spectroscopy confirms the production of the (3)MLCT state on ultrafast time scales, Förster resonance energy transfer calculations using the spectral properties of the two chromophores support initial singlet transfer from (1)PNI* to produce the (1)MLCT state by the agreement with the experimentally observed energy transfer time constant and efficiency. Intersystem crossing from the (1)MLCT to the (3)MLCT excited state is believed to be extremely fast and was not resolved with the current experiments. Finally, triplet energy was transferred from the (3)MLCT to the PNI-centered (3)LC state in less than 15 ns, ultimately achieving equilibrium between the two excited states. Subsequent relaxation to the ground state occurred via emission resulting from thermal population of the (3)MLCT state with a resultant lifetime of 651 µs. The title chromophore represents an interesting example of "ping-pong" energy transfer wherein photon excitation first migrates away from the initially prepared (1)PNI* excited state and then ultimately returns to this moiety as a long-lived excited triplet which disposes of its energy by equilibrating with the photoluminescent Re(I) MLCT excited state.

Revisiting fluorenone photophysics via dipolar fluorenone derivatives.

The nonradiative decay of four dipolar fluorenone derivatives (FODs) was systematically investigated using steady state and time-resolved UV-vis absorption and fluorescence measurements combined with cyclic voltammetry. Analysis of the frontier orbital localization of the global minimum geometry and the vertical transitions was carried out from DFT calculations. The first singlet excited state was found to be π-π* in all derivatives regardless of the polarity of the solvent. Charge separation/recombination dominates the singlet excited state deactivation for carbazole-containing FODs. Intersystem crossing (ISC) operates exclusively in the 3,6-disubstituted variants as evidenced by phosphorescence experiments. In the case of CPAFO36, ISC competes disadvantageously with CT deactivation.

Accessing the triplet manifold of naphthalene benzimidazole-phenanthroline in rhenium(I) bichromophores.

The steady-state and ultrafast to supra-nanosecond excited state dynamics of fac-[Re(NBI-phen)(CO)3(L)](PF6) (NBI-phen = 16H-benzo[4',5']isoquinolino[2',1':1,2]imidazo[4,5-f][1,10]phenanthrolin-16-one) as well as their respective models of the general molecular formula [Re(phen)(CO)3(L)](PF6) (L = PPh3 and CH3CN) has been investigated using transient absorption and time-gated photoluminescence spectroscopy. The NBI-phen containing molecules exhibited enhanced visible light absorption with respect to their models and a rapid formation (<6 ns) of the triplet ligand-centred (LC) excited state of the organic ligand, NBI-phen. These triplet states exhibit an extended excited state lifetime that enable the energized molecules to readily engage in triplet-triplet annihilation photochemistry.

Ligand-triplet migration in iridium(iii) cyclometalates featuring π-conjugated isocyanide ligands.

The manipulation of the triplet excited state manifold leads to large differences in the photophysical properties within a given class of metal-organic chromophores. By the appropriate choice of ancillary ligand, large changes can be made both to the order and nature of the lowest excited states and therefore to the resulting photophysical properties. Herein, a series of four bis-2-phenylpyridine (ppy) cyclometalated Ir(iii) compounds bearing two arylisocyanide ligands were synthesized and photophysically characterized to understand the effects of using ancillary ligands featuring systematic changes in π-conjugation. By varying the arylisocyanide ligands, the photoluminescence quantum yield ranged from 5% to 49% and the excited state lifetime ranged between 24 µs and 2 ms. These variations in photophysical response are consistent with lowering the triplet ligand-centered (3LC) state of the arylisocyanide ligand as the π system was extended, confirmed by 77 K photoluminescence emission spectra and ultrafast transient absorption experiments. The latter analysis gleaned detailed insight into the importance of the interplay of the 3LC state of the phenylpyridine and arylisocyanide ligands in these polychromophic Ir(iii) molecules.

Visible-Light-Driven Triplet Sensitization of Polycyclic Aromatic Hydrocarbons Using Thionated Perinones.

Metal-free chromophores that efficiently generate triplet excited states represent promising alternatives with respect to transition metal-containing photosensitizers, such as those featuring metal-to-ligand charge transfer excited states. However, such molecular constructs have remained underexplored due to the unclear relationship(s) between molecular structure and efficient/rapid intersystem crossing. In this regard, we present a series of three thionated perinone chromophores serving as a newly conceived class of heavy metal-free triplet photosensitizers. We demonstrate that thionation of the lone C═O substituent in each highly fluorescent perinone imparts red-shifted absorbance bands that maintain intense extinction coefficients across the visible spectrum, as well as unusually efficient triplet excited state formation as inferred from the measured singlet O2 quantum yields at 1270 nm (ΦΔ = 0.78-1.0). Electronic structure calculations revealed the emergence of a low energy S1 (n → π*) excited state in the proximity of a slightly higher energy S2 (π → π*) excited state. The distinct character in each of the two lowest-lying singlet state manifolds resulted in the energetic inversion of the corresponding triplet excited states due to differences in electron exchange interactions. Rapid S1 → T1 intersystem crossing was thereby facilitated in this manner through spin-orbit coupling as predicted by the El Sayed rules. The lifetimes of the resultant triplet excited states persisted into the microsecond time regime, as measured by transient absorbance spectroscopy, enabling effective bimolecular triplet sensitization of some common polycyclic aromatic hydrocarbons. The synthetically facile interchange of a single O atom to an S atom in the investigated perinones resulted in marked changes to their photophysical properties, namely, conversion of dominant singlet state fluorescence in the former to long-lived triplet excited states in the latter. The combined results suggest a general strategy for accessing long-lived triplet excited states in organic chromophores featuring a lone C═O moiety residing within its structure, valuable for the design of metal-free triplet photosensitizers.

Excited-State Triplet Equilibria in a Series of Re(I)-Naphthalimide Bichromophores.

We present the synthesis, structural characterization, electronic structure calculations, and the ultrafast and supra-nanosecond photophysical properties of a series of five bichromophores of the general structural formula [Re(5-R-phen)(CO)3(dmap)](PF6), where R is a naphthalimide (NI), phen = 1,10-phenanthroline, and dmap is 4-dimethylaminopyridine. The NI chromophores were systematically modified at their 4-positions with -H (NI), -Br (BrNI), phenoxy (PONI), thiobenzene (PSNI), and piperidine (PNI), rendering a series of metal-organic bichromophores (Re1-Re5, respectively) featuring variability in the singlet and triplet energies in the pendant NI subunit. Five closely related organic chromophores as well as [Re(phen)(CO)3(dmap)](PF6) (Re6) were investigated in parallel to appropriately model the photophysical properties exhibited in the bichromophores. The excited state processes of all molecules in this study were elucidated using a combination of transient absorption spectroscopy and time-resolved photoluminescence (PL) spectroscopy, revealing the kinetics of the energy transfer processes occurring between the appended chromophores. The spectroscopic analysis was further supported by electronic structure calculations which identified the origin of many of the experimentally observed electronic transitions.

Long-lived triplet excited state in a platinum(ii) perylene monoimide complex.

We report the synthesis and solution based photophysical properties of a new Pt(ii)-terpyridine complex coupled to a perylene monoimide (PMI) chromophoric unit through an acetylene linkage. This structural arrangement resulted in quantitative quenching of the highly fluorescent PMI chromophore by introducing metal character into the lowest energy singlet state, thereby leading to the formation of a long-lived PMI-ligand localized triplet excited state (τ = 8.4 µs). Even though the phosphorescence from this triplet state was not observed, highly efficient quenching of this excited state by dissolved oxygen and the observation of singlet oxygen photoluminescence in the near-IR at 1270 nm initially pointed towards triplet excited state character. Additionally, the coincidence of the excited state absorbance difference spectra from the sensitized PMI ligand using a triplet donor and the Pt-PMI complex provided strong evidence for this triplet state assignment, which was further supported by TD-DFT calculations.

Photoinduced charge separation in platinum acetylide oligomers.

The series of three donor-spacer-acceptor complexes, DPAF-Ptn-NDI, has been synthesized and characterized using time-resolved absorption spectroscopy. In these complexes, the donor is a (diphenylamino)-2,7-fluorenylene (DPAF) unit, the acceptor is a naphthalene diimide (NDI), and the spacers are a series of platinum acetylides of varying lengths, [-Pt(PBu(3))(2)-C≡C-Ph-C≡C-](n) (where Bu = n-butyl, Ph = 1,4-phenylene and n = 1, 2, and 3). Electrochemistry indicates that the DPAF-Ptn-NDI system has a charge transfer state at ca. 1.5 eV above the ground state that is based on one electron transfer from the DPAF donor to the NDI acceptor. Transient absorption spectroscopy on time scales ranging from 0.2 ps to 1 µs reveals that excitation of all of the complexes leads to production of the charge transfer state with nearly unit quantum efficiency. The rates for charge separation and charge recombination are not strongly dependent upon the length of the platinum acetylide spacer, suggesting that the spacer is actively involved in the electron (hole) transport processes. Analysis of the experimental results leads to a model in which charge separation and charge recombination occur by hole-hopping via states localized on the [-Pt(PBu(3))(2)-C≡C-Ph-C≡C-](n) bridge.