Lifetime Of Triplet Excited State Research Articles

Efficient Photooxidation of Sulfides with Amidated Alloxazines as Heavy-atom-free Photosensitizers.

Photooxidation utilizing visible light, especially with naturally abundant O2 as the oxygen source, has been well-accepted as a sustainable and efficient procedure in organic synthesis. To ensure the intersystem crossing and triplet quantum yield for efficient photosensitization, we prepared amidated alloxazines (AAs) and investigated their photophysical properties and performance as heavy-atom-free triplet photosensitizers and compared with those of flavin (FL) and riboflavin tetraacetate (RFTA). Because of the difference in the framework structure of AAs and FL and the introduction of carbonyl moiety, the absorption of FL at ∼450 nm is blue-shifted to ∼380 nm and weakened (ε = 8.7 × 103 for FL to ∼6.8 × 103 M–1 cm–1), but the absorption at ∼340 nm is red-shifted to ∼350 nm and enhanced by ∼50% (from ε = 6.4 × 103 for FL to ∼9.9 × 103 M–1 cm–1) in AAs. The intersystem crossing rates from the S1 to T1 are also enhanced in these AAs derivatives, while the fluorescence quantum yield decreases from ∼30 to ∼7% for FL and AAs, respectively, making the triplet excited state lifetime and the singlet oxygen quantum yield of AAs at least comparable to those of FL and RFTA. We examined the performance of these heave-atom-free chromophores in the photooxidation of sulfides to afford sulfoxides. In accordance with the prolonged triplet excited state lifetime and enhanced triplet quantum yield, 2–5-fold performance enhancements were observed for AAs in the photooxidation of sulfides with respect to FL. We proposed that the key reactive oxygen species of AA-sensitized photooxidation are singlet oxygen and superoxide radical anion based on mechanistic investigations. The research highlights the superior performance of AAs in photocatalysis and would be helpful to rationalize the design of efficient heavy-atom-free organic photocatalysts.

Unimolecular Photodynamic O2-Economizer To Overcome Hypoxia Resistance in Phototherapeutics.

Tumor hypoxia has proven to be the major bottleneck of photodynamic therapy (PDT) to clinical transformation. Different from traditional O2 delivery approaches, here we describe an innovative binary photodynamic O2-economizer (PDOE) tactic to reverse hypoxia-driven resistance by designing a superoxide radical (O2•-) generator targeting mitochondria respiration, termed SORgenTAM. This PDOE system is able to block intracellular O2 consumption and down-regulate HIF-1α expression, which successfully rescues cancer cells from becoming hypoxic and relieves the intrinsic hypoxia burden of tumors in vivo, thereby sparing sufficient endogenous O2 for the PDT process. Photosensitization mechanism studies demonstrate that SORgenTAM has an ideal intersystem crossing rate and triplet excited state lifetime for generating O2•- through type-I photochemistry, and the generated O2•- can further trigger a biocascade to reduce the PDT's demand for O2 in an O2-recycble manner. Furthermore, SORgenTAM also serves to activate the AMPK metabolism signaling pathway to inhibit cell repair and promote cell death. Consequently, using this two-step O2-economical strategy, under relatively low light dose irradiation, excellent therapeutic responses toward hypoxic tumors are achieved. This study offers a conceptual while practical paradigm for overcoming the pitfalls of phototherapeutics.

Trifluoromethylated Phenanthroline Ligands Reduce Excited-State Distortion in Homoleptic Copper(I) Complexes.

We report the synthesis and excited-state dynamics for a series of homoleptic copper(I) trifluoromethylated phenanthroline complexes with two, three, and four trifluoromethyl functional groups. Our analysis of the steady-state absorbance and emission, transient-absorption spectroscopy, and electronic-structure-theory calculations results enable in-depth analysis of the pseudo-Jahn-Teller distortion inhibition from increased steric hindrance of the trifluoromethyl functional group relative to the prototypical dimethyl phenanthroline complex. Surprisingly, our results demonstrate that the greatest degree of pseudo-Jahn-Teller distortion inhibition is achieved with trifluoromethylation of only the 2 and 9 positions by an unusual combination of steric hindrance and stabilization of a nondistorted 1MLCT manifold observed by transient kinetic lifetimes and optimized excited-state structures. The intersystem-crossing (ISC) lifetime for the 2,9-bis(trifluoromethyl)-1,10-phenanthroline Cu(I) complex is 69 ps, while the triplet excited-state lifetime and emission quantum yield are 106 ns and 4 × 10-3, respectively. Further trifluoromethylation of the phenanthroline yields a greater σ bond inductive withdrawing force on the phenanthroline nitrogens, ultimately resulting in weaker coordination to the copper. Last, the surprising success of the 2,9-bis(trifluoromethyl)-1,10-phenanthroline Cu(I) complex by adjusting both ligand sterics and electronic properties outlines a new strategy for developing long-lived Cu(I) charge-transfer complexes.

Sterically hindered Re- and Mn-CO2 reduction catalysts for solar energy conversion.

Novel molecular Re and Mn tricarbonyl complexes bearing a bipyridyl ligand functionalised with sterically hindering substituents in the 6,6'-position, [M(HPEAB)(CO)3(X)] (M/X = Re/Cl, Mn/Br; HPEAB = 6,6'-{N-(4-hexylphenyl)-N(ethyl)-amido}-2,2'-bipyridine) have been synthesised, fully characterised including by single crystal X-ray crystallography, and their propensity to act as catalysts for the electrochemical and photochemical reduction of CO2 has been established. Controlled potential electrolysis showed that the catalysts are effective for electrochemical CO2-reduction, yielding CO as the product (in MeCN for the Re-complex, in 95 : 5 (v/v) MeCN : H2O mixture for the Mn-complex). The recyclability of the catalysts was demonstrated through replenishment of CO2 within solution. The novel catalysts had similar reduction potentials to previously reported complexes of similar structure, and results of the foot-of-the-wave analysis showed comparable maximum turnover rates, too. The tentative mechanisms for activation of the pre-catalysts were proposed on the basis of IR-spectroelectrochemical data aided by DFT calculations. It is shown that the typical dimerisation of the Mn-catalyst was prevented by incorporation of sterically hindering groups, whilst the Re-catalyst undergoes the usual mechanism following chloride ion loss. No photochemical CO2 reduction was observed for the rhenium complex in the presence of a sacrificial donor (triethylamine), which was attributed to the short triplet excited state lifetime (3.6 ns), insufficient for diffusion-controlled electron transfer. Importantly, [Mn(HPEAB)(CO)3Br] can act as a CO2 reduction catalyst when photosensitised by a zinc porphyrin under red light irradiation (λ > 600 nm) in MeCN : H2O (95 : 5); there has been only one reported example of photoactivating Mn-catalysts with porphyrins in this manner. Thus, this work demonstrates the wide utility of sterically protected Re- and Mn-diimine carbonyl catalysts, where the rate and yield of CO-production can be adjusted based on the metal centre and catalytic conditions, with the advantage of suppressing unwanted side-reactions through steric protection of the vacant coordination site.

The effects of extended π-conjugation in bipyridyl ligands on the tunable photophysics, triplet excited state and optical limiting properties of Pt(ii) naphthalimidyl acetylide complexes.

Pt(ii) complexes that exhibit long-lived triplet excited state lifetimes are promising for optical power limiting materials. The introduction of large π-conjugated substituents can switch the triplet excited state to a long-lived 3π,π* state. Herein, we report four Pt(ii) diimine complexes with high π-conjugation via inserting an aryl group on the diimine ligand. Their photophysical properties were investigated using spectroscopic techniques. All the complexes exhibit strong ground absorption bands in their UV-Vis absorption spectra (maximum peaks ranging from 370 to 530 nm) and long-lived emission and triplet excited states. The insertion of π-conjugated substituents induces a pronounced red-shift in the ground state absorption and longer emission lifetime. Broadband transient absorption spectra in the visible-NIR region and Z-scan properties under 532 nm were carried out on the Pt(ii) diimine complexes, resulting in a remarkably strong reverse saturable absorption at 532 nm for nanosecond laser pulses. Otherwise, the high π-conjugation in the bipyridyl ligand increases the reverse saturable absorption. Therefore, these Pt(ii) diimine complexes with high π-conjugation are excellent candidates for devices that require strong reverse saturable absorption.

Strong Influence of the Ancillary Ligand over the Photodynamic Anticancer Properties of Neutral Biscyclometalated IrIII Complexes Bearing 2-Benzoazole-Phenolates.

In this paper, the synthesis, comprehensive characterization and biological and photocatalytic properties of two series of neutral IrIII biscyclometalated complexes of general formula [Ir(C^N)2 (N^O)], where the N^O ligands are 2-(benzimidazolyl)phenolate-N,O (L1, series a) and 2-(benzothiazolyl)phenolate-N,O (L2, series b), and the C^N ligands are 2-(phenyl)pyridinate or its derivatives, are described,. Complexes of types a and b exhibit dissimilar photophysical and biological properties. In vitro cytotoxicity tests conclusively prove that derivatives of series a are harmless in the dark against SW480 cancer cells (colon adenocarcinoma), but express enhanced cytotoxicity versus the same cells after stimulation with UV or blue light. In contrast, complexes of type b show a very high cytotoxic activity in the dark, but low photosensitizing ability. Thus, the ancillary N^O ligand is the main factor in terms of cytotoxic activity both in the dark and upon irradiation. However, the C^N ligands play a key role regarding cellular uptake. In particular, the complex of formula [Ir(dfppy)2 (L1)] (dfppy=2-(4,6-difluorophenyl)pyridinate) [3 a] has been identified as both an efficient photosensitizer for 1 O2 generation and a potential agent for photodynamic therapy. These capabilities are probably related to a combination of its notable cellular internalization, remarkable photostability, high photoluminescence quantum yield, and long triplet excited-state lifetime. Both types of complexes exhibit notable catalytic activity in the photooxidation of thioanisole and S-containing aminoacids with full selectivity.

Series of C^N^C Cyclometalated Pt(II) Complexes: Synthesis, Crystal Structures, and Nonlinear Optical Properties in the Near-Infrared Region.

It is currently challenged that nonlinear optical (NLO) properties in the near-infrared region (NIR) of metal complexes can be tunable with an assistant ligand. Herein, the linear and nonlinear photophysical properties of the novel C^N^C cyclometalated Pt(II) complexes with different substituents as auxiliary ligands are presented. The complexes displayed intense triplet metal/ligand-to-ligand charge-transfer (3MLCT/3LLCT) and intraligand 3π, π* emission at low-temperature. The excited-state characteristics are further confirmed over the TD-DFT calculations, transient absorption, and emission lifetimes. The Pt-3 possesses a relatively high quantum yield (9.1%), a moderate triplet excited-state lifetime (5.32 μs), and a broad excited-state absorption from the visible to the near-IR region. Interestingly, it was found that Pt-3 exhibited high 2PA cross section values (σ2 up to 367 GM at 820 nm), as well as good optical limiting properties over a tunable femtosecond laser. The relationships between the structures and properties were systematically investigated on the basis of crystal structural information. Hence, cyclometalated Pt(II) complexes would become candidates for the application of the NIR NLO materials.

Synthesis, luminescence and excited state absorption properties of conjugated D-π-A and D-π-D phenothiazine compounds

Four D-π-A or D-π-D phenothiazine compounds bearing different substituents on central π-bridge (R = -H (PTZ-1), -Br (PTZ-2), -CN (PTZ-3) and -OC12H25 (PTZ-4)) were synthesized and characterized. The influence of the substituents on the photophysics of these compounds is systematically investigated by spectroscopic methods and simulated by density functional theory (DFT) calculations. All compounds exhibit intense absorption bands in UV and visible region, which are influenced by the different substituents on the central π-bridge. Both electron-withdrawing and electron-donating substituents cause bathochromic shifts of the low energy 1π-π*/1ICT (intramolecular charge transfer) absorption bands. Meanwhile, all compounds are emissive in solution at room temperature (λem = 400–800 nm, Φem = 0.01–0.74). Their fluorescent quantum yields decrease when the electron-withdrawing substituents attached on the central π-bridge, but increase when electron-donating substituents were introduced. In addition, PTZ-1–PTZ-4 all exhibit strong triplet transient absorption (TA) in visible region. When the electron-donating substituents attached, the triplet excited-state molar extinction coefficient (190200 L mol−1 cm−1) and triplet excited-state lifetime (53.2 μs) of PTZ-4 were significantly enhanced. Their luminescent and excited state absorption properties could provide us a basis for elucidating the structure-property correlations of phenothiazine compounds and developing novel optical functional materials.

Solid–Solid Phase Transitions in [trans-Pt(PMe3)2(C≡CC6H4R)2]-Containing Materials (R = O(CH2)nH; n = 6, 9, 12, and 15)

The title complexes were prepared in Hagihara conditions and were investigated by single crystal X-ray crystallography (n = 6, [Pt]C6; n = 12, [Pt]C12), X-ray powder diffraction (powder XRD), differential scanning calorimetry (DSC), thermal gravimetric analysis (TGA, [Pt]C12), and steady-state and time-resolved solid state UV–vis and emission spectroscopy at 298 and 77 K. [Pt]C6 complex exhibits no phase change with the temperature. Concurrently, [Pt]C9 (n = 9) and [Pt]C12 complexes exhibit an irreversible Tendo values of 104 and 119 °C, respectively, associated with a thermal annealing. Finally, [Pt]C15 (n = 15) complex exhibit a reversible thermal processes with a large hysteresis (Tendo = 126 °C, Texo = 140 °C) followed by a glass transition (Tendo = 146 °C, Texo = 69 °C) as depicted by DSC. These phase changes are accompanied by a decrease in triplet excited state lifetimes upon cycling the sample temperature over and under the transition temperatures. These various thermal processes induce a significant decrease in emission lifetimes, strongly suggesting the presence of a reorganization of the complexes in the solid state favoring more chain-chromophore contacts, thus promoting nonradiative “knocking” processes.

Large refractive index variations induced by accumulating triplet excitons under photoexcitation at low power

Herein, we present a method for the modification of the refractive index (n), based on employing an organic molecule with a long triplet excited-state lifetime. A host-guest material composed of a cyclic aromatic as the guest and an amorphous steroidal compound as the host was used to modulate n. The guest material exhibited a triplet lifetime longer than 1 s, and a high-density triplet excited-state population was obtained upon excitation with blue-violet light. The refractive index could be changed by 0.002, even when using a relatively low excitation power level of 100 mW cm−2.

Spin-orbit coupling dependent energy transfer in luminescent nonanuclear Yb-Gd / Yb-Lu clusters

In luminescent lanthanide (Ln(III)) complexes, the yield and the lifetime of triplet excited state of organic ligands are crucial factors that affect the ligands-to-Ln(III) energy transfer efficiency. Such factors are dependent on spin-orbit coupling induced by the Ln(III) ions that mixes different multiplicity states through heavy atom and paramagnetic effects. We investigated the role of these effects on the energy transfer efficiency in synthesized nonanuclear Yb-Gd / Yb-Lu clusters ([Ln9(µ-OH)10(butyl salicylate)16]NO3, Ln9 = YbnGd9-n or YbnLu9-n, n = 0, 1, 3, 7, and 9). Based on the intensity of the fluorescence and phosphorescence of the ligands, the spin-orbit coupling strength was in the order of Yb(III) > Gd(III) > Lu(III). Various photophysical processes affecting the energy transfer efficiency in YbnGd9-n and YbnLu9-n clusters are discussed from the perspective of spin-orbit coupling and give insight in how to optimize energy transfer efficiencies.

Tunable Rh2(II,II) Light Absorbers as Excited-State Electron Donors and Acceptors Accessible with Red/Near-Infrared Irradiation.

A series of dirhodium(II,II) paddlewheeel complexes of the type cis-[Rh2(μ-DTolF)2(μ-L)2][BF4]2, where DTolF = N,N'-di( p-tolyl)formamidinate and L = 1,8-naphthyridine (np), 2-(pyridin-2-yl)-1,8-naphthyridine (pynp), 2-(quinolin-2-yl)-1,8-naphthyridine (qnnp), and 2-(1,8-naphthyridin-2-yl)quinoxaline (qxnp), were synthesized and characterized. These molecules feature new tridentate ligands that concomitantly bridge the dirhodium core and cap the axial positions. The complexes absorb light strongly throughout the ultraviolet/visible range and into the near-infrared region and exhibit relatively long-lived triplet excited-state lifetimes. Both the singlet and triplet excited states exhibit metal/ligand-to-ligand charge transfer (ML-LCT) in nature as determined by transient absorption spectroscopy and spectroelectrochemistry measurements. When irradiated with low-energy light, these black dyes are capable of undergoing reversible bimolecular electron transfer both to the electron acceptor methyl viologen and from the electron donor p-phenylenediamine. Photoinduced charge transfer in the latter was inaccessible with previous Rh2(II,II) complexes. These results underscore the fact that the excited state of this class of molecules can be readily tuned for electron-transfer reactions upon simple synthetic modification and highlight their potential as excellent candidates for p- and n-type semiconductor applications and for improved harvesting of low-energy light to drive useful photochemical reactions.

Influence of meta- and para-phosphonated diphenylanthracene on photon upconversion in self-assembled bilayers

It is well known that the rate and efficiency of photon upconversion via triplet–triplet annihilation (TTA-UC) are strongly dependent on the energetics of the sensitizer and acceptor molecules. In rigid scaffoldings, where the dyes are fixed in position, the structure and orientation of the molecules also presumably play an important role. We investigate how the variation in the position of the phosphonate surface binding group on 9,10-diphenylanthracene influences TTA-UC in self-assembled bilayers on ZrO2. Interestingly, meta- or para-substitution of the anthracene dye with phosphonate groups had minimal influence on their energetics, surface loadings, or sensitizer to acceptor triplet energy transfer efficiency. However, the TTA-UC efficiency is three times lower with the meta-substituted dye, which is attributed to its threefold decrease in its triplet excited state lifetime relative to the para-substituted dye.

Molecular Road Map to Tuning Ground State Absorption and Excited State Dynamics of Long-Wavelength Absorbers.

Realizing chromophores that simultaneously possess substantial near-infrared (NIR) absorptivity and long-lived, high-yield triplet excited states is vital for many optoelectronic applications, such as optical power limiting and triplet-triplet annihilation photon upconversion (TTA-UC). However, the energy gap law ensures such chromophores are rare, and molecular engineering of absorbers having such properties has proven challenging. Here, we present a versatile methodology to tackle this design issue by exploiting the ethyne-bridged (polypyridyl)metal(II) (M; M = Ru, Os)-(porphinato)metal(II) (PM'; M' = Zn, Pt, Pd) molecular architecture (M-(PM')n-M), wherein high-oscillator-strength NIR absorptivity up to 850 nm, near-unity intersystem crossing (ISC) quantum yields (ΦISC), and triplet excited-state (T1) lifetimes on the microseconds time scale are simultaneously realized. By varying the extent to which the atomic coefficients of heavy metal d orbitals contribute to the one-electron excitation configurations describing the initially prepared singlet and triplet excited-state wave functions, we (i) show that the relative magnitudes of fluorescence (k0F), S1 → S0 nonradiative decay (knr), S1 → T1 ISC (kISC), and T1 → S0 relaxation (kT1→S0) rate constants can be finely tuned in M-(PM')n-M compounds and (ii) demonstrate designs in which the kISC magnitude dominates singlet manifold relaxation dynamics but does not give rise to T1 → S0 conversion dynamics that short-circuit a microseconds time scale triplet lifetime. Notably, the NIR spectral domain absorptivities of M-(PM')n-M chromophores far exceed those of classic coordination complexes and organic materials possessing similarly high yields of triplet-state formation: in contrast to these benchmark materials, this work demonstrates that these M-(PM')n-M systems realize near unit ΦISC at extraordinarily modest S1-T1 energy gaps (∼0.25 eV). This study underscores the photophysical diversity of the M-(PM')n-M platform and presents a new library of long-wavelength absorbers that efficiently populate long-lived T1 states.

Pt(II) diimine complexes bearing difluoro-boron-dipyrromethene acetylide ligands: Synthesis, photophysics, aggregation included emission and optical power limiting properties

A series of Pt(II) complexes bearing difluoro-boron-dipyrromethene (Bodipy) acetylide ligands and different alkyl/aryl substituted 2,2′-bipyridyl ligands (Pt-1 – Pt-3) were synthesized and characterized. Their photophysics, aggregation included emission (AIE) and optical power limiting properties were systematically investigated via UV−Vis absorption, emission, transient absorption, and nonlinear transmission spectroscopy/technique. These complexes exhibit 1π-π* transitions/metal-to-ligand charge transfer (1MLCT) absorption bands in UV-Vis spectral region and green fluorescence assigned to 1π-π* state mixed with 1MLCT character. All complexes exhibit obvious AIE in CH3CN-water solution system. When water fraction increased, nanoparticles of Pt(II) complexes were formed owing to the aggregation, and the emission intensity and triplet excited state lifetime both increased. In addition, complexes Pt-1 – Pt-3 all exhibit triplet transient absorption (TA) in visible region, where reverse saturable absorption (RSA) could occur. The strength of RSA at 532 nm follows the trend: Pt-2 > Pt-3 > Pt-1. The significant AIE, TA and OPL properties of these complexes would be useful for rational design of transition-metal complexes with high emission quantum yield, broadband excited-state absorption, long excited-state lifetime, and strong nonlinear absorption for multi-functional applications.

Photophysical Properties of Visible‐Light‐Harvesting PtII Bis(acetylide) Complexes

PtII complexes have attracted much attention due to their readily derivatizable molecular structures and efficient production of triplet excited states and have been extensively used in many areas. Conventional PtII complexes usually exhibit small molar absorption coefficients in the visible spectral region, and the triplet excited state lifetimes are short. However, for applications such as photocatalysis, triplet–triplet annihilation (TTA) upconversion, nonlinear optics, etc., visible‐light‐harvesting PtII bis(acetylide) complexes with long‐lived triplet excited state are cucial. We summarize strategies directed towards achieving these goals, through the attachment of appropriate strong visible‐light‐harvesting organic chromophores to the ligand to switch the S0→1MLCT (metal‐to‐ligand charge transfer) transition to the S0→1IL (intraligand) transition (π–π*) and to switch the lowest triplet excited state from the 3MLCT to the 3IL state. We also summarize the properties of PtII bis(acetylide) complexes that show broadband absorption and reverse saturate absorption for application in nonlinear optics. Singlet and triplet energy transfer and localization of the triplet excited state in multiple‐chromophore complexes are also discussed.

Long-Lived Triplet Excited States of Bent-Shaped Pentacene Dimers by Intramolecular Singlet Fission.

Intramolecular singlet fission (ISF) is a promising photophysical process to construct more efficient light energy conversion systems as one excited singlet state converts into two excited triplet states. Herein we synthesized and evaluated bent-shaped pentacene dimers as a prototype of ISF to reveal intrinsic characters of triplet states (e.g., lifetimes of triplet excited states). In this study, meta-phenylene-bridged TIPS-pentacene dimer (PcD-3Ph) and 2,2'-bipheynyl bridged TIPS-pentacene dimer (PcD-Biph) were newly synthesized as bent-shaped dimers. In the steady-state spectroscopy, absorption and emission bands of these dimers were fully characterized, suggesting the appropriate degree of electronic coupling between pentacene moieties in these dimers. In addition, the electrochemical measurements were also performed to check the electronic interaction between two pentacene moieties. Whereas the successive two oxidation peaks owing to the delocalization were observed in a directly linked-pentacene dimer (PcD) by a single bond, the cyclic voltammograms in PcD-Biph and PcD-3Ph implied the weaker interaction compared to that of p-phenylene-bridged TIPS-pentacene dimer (PcD-4Ph) and PcD. The femtosecond and nanosecond transient absorption spectra clearly revealed the slower ISF process in bent-shaped pentacene dimers (PcD-Biph and PcD-3Ph), more notably, the slower relaxation of the excited triplet states in PcD-Biph and PcD-3Ph. Namely, the quantum yields of triplet states (ΦT) by ISF approximately remain constant (ca. 180-200%) in all dimer systems, whereas the lifetimes of the triplet excited states became much longer (up to 360 ns) in PcD-Biph as compared to PcD-4Ph (15 ns). Additionally, the lifetimes of the corresponding triplet states in PcD-Biph and PcD-3Ph were sufficiently affected by solvent viscosity. In particular, the lifetimes of PcD-Biph triplet state in THF/paraffin (1.0 μs) increased up to approximately three times as compared to that in THF (360 ns), whereas those of PcD-4Ph were quite similar in both solvent.

Effects of extending the π-conjugation of the acetylide ligand on the photophysics and reverse saturable absorption of Pt(ii) bipyridine bisacetylide complexes.

The synthesis and photophysics of four platinum(ii) bipyridine (bpy) bisacetylide complexes with different degrees of π-conjugation and an electron-donating diphenylamino (NPh2) or electron-withdrawing benzothiazolyl (BTZ) terminal substituent on the acetylide ligands are reported. The UV-vis absorption spectra of these complexes are composed of intense ligand-localized 1π,π* transitions at 330-430 nm and broad, moderately strong ligand-to-ligand charge transfer/metal-to-ligand charge transfer (1LLCT/1MLCT) transitions at 430-530 nm. All complexes are phosphorescent in solutions at room temperature and exhibit very broad and moderately strong triplet excited-state absorption in the visible to the NIR spectral region (425-800 nm). It is found that extending the π-conjugation of the acetylide ligands via adding one or two more ethynylfluorenyl unit(s) to the acetylide ligand does not change the energies of the 1π,π* and 1LLCT/1MLCT transitions pronouncedly except for increasing the molar extinction coefficients of the 1π,π* transitions. The emitting triplet excited states of the four complexes are the 3MLCT/3LLCT states and have the same energy. However, the complex that contains the tris(ethynylfluorenyl) units and the terminal NPh2 substituent on the acetylide ligand exhibits longer triplet lifetimes than the corresponding complex that has the bis(ethynylfluorenyl) units. The transient absorption band maxima of the complexes with tris(ethynylfluorenyl) units are slightly red-shifted in comparison to those of their respective counterparts with bis(ethynylfluorenyl) units. The nature of the terminal substituent does not influence the parentage and energies of the lowest singlet and triplet excited states. However, the triplet excited-state lifetimes of the complexes with the NPh2 terminal substituent on the bis(ethynylfluorenyl) or tris(ethynylfluorenyl) ligands are much longer than that of their counterpart with monofluorenylacetylide ligands; while the triplet lifetimes of the complexes containing the BTZ terminal substituent are similar to their counterpart with monofluorenylacetylide ligands. All complexes exhibit strong reverse saturable absorption (RSA) at 532 nm for nanosecond laser pulses.

Strong triplet excited-state absorption in a phenanthrolinyl iridium(III) complex with benzothiazolylfluorenyl-substituted ligands

Femtosecond transient difference absorption (fs TA) measurements, together with a series of open-aperture Z scans at picosecond and nanosecond pulse widths and a variety of pulse energies, were performed on a 1,10-phenanthrolinyl iridium(III) complex bearing ligands containing a benzothiazolylfluorenyl motif. An analysis of decay data from the fs TA experiment yields a value of 1.24±0.26 ns for the singlet excited-state lifetime τ(S) of the complex. By fitting the Z scans to a five-level dynamic model incorporating the independently measured value of τ(S) and previously reported values of the complex's triplet quantum yield (0.13) and triplet excited-state lifetime (230 ns), we obtain values of 3.5×10(-17) cm(2) (singlet) and 5.0×10(-16) cm(2) (triplet) for the excited-state absorption cross-sections of the complex in toluene solution at 532 nm; the latter value represents one of the largest triplet excited-state absorption cross-sections ever reported at this wavelength. The ratio of the triplet excited-state cross-section to the ground-state absorption cross-section exceeds 3800.

Long-lived Triplet Excitons Allowed by Intermolecular Hydrogen Bonding in Amorphous Matrix

A purely organic guest material with a long triplet excited state lifetime (τ > 1 s) was doped into two different π-conjugated amorphous hosts with similar glass transition temperatures (Tg). The phosphorescence quenching behavior of the guest material was investigated under vacuum conditions. When doped into a host that did not exhibit intermolecular hydrogen bonding, the τ of the guest decreased significantly at temperatures below the Tg of the host. Conversely, when doped into a host that did exhibit intermolecular hydrogen bonding, the τ remained almost unaffected at temperatures below the Tg of the host. The diffusion coefficient of oxygen, which was largely removed from the host materials under vacuum, was similar for both host materials. These results indicated that weak thermal diffusional motion of a host material intrinsically quenches long-lived triplet excitons of purely organic guest materials.