Triplet Transient Difference Absorption Research Articles

Synthesis, photophysics, and reverse saturable absorption of platinum complexes bearing extended π-conjugated C^N^N ligands

The synthesis of ligands 1-L-6-L that feature the 6-[9,9-di(2-ethylhexyl)-7-R-9H-fluoren-2-yl]-2,2'-bipyridine (C^N^N) core (R = 4-R'-phenylethynyl with R' = NO2, benzothiazol-2-yl (BTZ), H and OCH3 or R = 4'-BTZ-phenyl or BTZ) and their platinum complexes 1-6 were reported in this paper. The photophysical properties of these ligands and the Pt(II) complexes, including the UV-vis absorption spectra, emission characteristics at room temperature and at 77 K, and the triplet transient difference absorption spectra, were systematically investigated in order to understand the effects of the substituent at the 4-position of the 1-ethynylphenyl component and the extension of π-conjugation between the C^N^N core and the BTZ substituent. Reverse saturable absorption (RSA) of complexes 1-6 was demonstrated at 532 nm using 4.1 ns laser pulses. The UV-vis absorption spectra of 1-L-6-L are featured by strong (1)π,π* transitions in the blue spectral region, and the absorption bands are effectively red-shifted by substitution at the 4-position of the ethynylphenyl motif and by the extended π-conjugation of the linkage. A similar effect was observed for the fluorescence spectra of these ligands in CH2Cl2 at room temperature, but the nature of the fluorescence varies from (1)π,π* fluorescence in 3-L and 6-L, to intraligand charge transfer ((1)ILCT) fluorescence in 1-L, 2-L and 5-L; while 4-L possesses mixed (1)π,π*/(1)ILCT characters. All ligands exhibit moderate triplet transient absorption (TA) in the visible spectral region, with substitution at the 4-position of the ethynylphenyl component broadening of the TA bands, while extended π-conjugation of the linkage inducing red-shifts of the TA bands. For Pt(II) complexes 1-6, their UV-vis absorption spectra constitute red-shifted (1)π,π* transitions and low-energy metal-to-ligand charge transfer ((1)MLCT/(1)ILCT) tails. The emission of these complexes at room temperature in CH2Cl2 predominantly originates from the C^N^N core localized (3)π,π* state, probably mixed with minor (3)MLCT character. 4-Position substitution and extended π-conjugation on the ligands exert a negligible effect on the shape and energy of the emission spectra. Similar to their respective ligands, 1-6 all exhibit broader and red-shifted TA spectra with respect to their ligands and both the 4-position substitution and extended π-conjugation bathochromically shift the TA band maxima. The nonlinear transmission experiments carried out for 1-6 at 532 nm reveal that all complexes exhibit strong reverse saturable absorption (RSA), and the degree of RSA follows this trend: 6 <4 <5 <2 ≤3 <1. The RSA performance is efficiently improved by electron-withdrawing substituents (NO2 and BTZ) and by extending the π-conjugation; while electron-donating substituent (OCH3 in 4) decreases the RSA at 532 nm.

Near-Infrared Emission of Lanthanide(III) Texaphyrin Complexes

Lanthanide texaphyrin derivatives containing Er3+, Nd3+, and Yb3+ ions exhibit near-IR emission from the f–f transition of the lanthanide ions. Both the triplet transient difference absorption spectral measurement and singlet oxygen generation measurement suggest energy transfer from the macrocyclic ligand to the lanthanide ions. Electron-transfer mechanism could also play a role in the NIR emission of Yb–Tex complex.

Synthesis and photophysical studies of back-to-back dinuclear platinum terpyridine complexes with different substituents on the bridging ligand

Four back-to-back dinuclear platinum terpyridine complexes with different substituents on the bridging ligand (1a–1d) were synthesized and characterized. Their electronic absorption, photoluminescence and triplet transient difference absorption were systematically investigated. All complexes exhibit strong 1MLCT/1ILCT absorption bands in the visible region, which significantly red-shifts when electron-donating substituents are introduced on the conjugated bridge and blue-shifts when electron-withdrawing substituents are present. Excitation of 1a and 1d in solution at their respective low-energy absorption band at room temperature results in an orange and red luminescence, respectively, which can be tentatively attributed to the 1MLCT/1ILCT excited state. These complexes exhibit intense broad triplet transient difference absorption in the visible to the near-IR region, which likely arises from the ligand-localized states (3π,π∗ or 3ILCT). Electron-donating substituent causes a pronounced red-shift, while electron-withdrawing substituents induce a blue-shift of the triplet transient absorption bands.

Anion‐Sensitive 2,4‐Dinitrophenylhydrazone‐Containing Terpyridine Derivative and Its Platinum Chloride Complex

AbstractTerpyridine derivative 1, bearing 2,4‐dinitrophenylhydrazone, and its platinum chloride complex 2 were synthesized and characterized. The UV/Vis, emission, and triplet transient difference absorption spectra of 1 and 2 were investigated. Both 1 and 2 exhibit broad and strong absorption in the visible spectral region (λmax = 404 nm for 1 and 433 nm for 2), which are dominated by the 1π–π* transition of the ligand. When excited at the respective absorption band maximum, 1 and 2 exhibit charge‐transfer fluorescence atapproximately 550 nm. Compounds 1 and 2 also exhibit a broad and strong 3π–π* excited‐state absorption band in the visible region (λmax = 520 nm with ϵT1–Tn = 4.74 × 104 L mol–1 cm–1for 1, and λmax = 590 nm with ϵT1–Tn = 3.45 × 104 L mol–1 cm–1 for 2). The triplet excited states of 1 and 2 are both long‐lived, with lifetimes longer than 2 ms. Compounds 1 and 2 are sensitive to basic anions such as F–, AcO–, and H2PO4–. Upon addition of these anions, the UV/Vis absorption spectra of these two compounds are significantly redshifted, which is reflected by the color change from yellow to purple for 1 and from yellow to blue for 2. These anions also quench the charge‐transfer emission band at 550 nm. Moreover, the triplet transient absorption of 1 and 2 in DMSO solution is quenched upon addition of these anions. These spectroscopic changes are attributed to the deprotonation of the hydrazone NH group, which causes intramolecular electron transfer. The deprotonation mechanism is supported by the upfield shifts of the 1H NMR signals of the 2,4‐dinitrophenylhydrazone group, the imine group, and the phenyl group, following the addition of excess F–.

Photophysics and Nonlinear Absorption of Cyclometalated 4,6-Diphenyl-2,2′-bipyridyl Platinum(II) Complexes with Different Acetylide Ligands

The photophysical properties of a series of 4,6-diphenyl-2,2'-bipyridyl platinum(II) complexes bearing different σ-alkynyl ancillary ligands (1a-1k) were systematically investigated. All complexes exhibit strong (1)π,π* absorption bands in the UV region; and broad, structureless charge-transfer band(s) in the visible region, which systematically red-shift(s) when the electron-donating ability of the para substituent on the phenylacetylide ligand increases. All complexes are emissive in solution at room temperature. When excited at the charge-transfer absorption band, the complexes exhibit long-lived orange emission (λ(max): 555-601 nm), which is attributed to a triplet metal-to-ligand charge transfer/intraligand charge transfer emission ((3)MLCT/(3)ILCT). Most of these complexes exhibit broad triplet transient difference absorption in the visible to the near-IR region, with a lifetime comparable to those measured from the decay of the (3)MLCT/(3)ILCT emission. The reverse saturable absorption (RSA) of these complexes were demonstrated at 532 nm using nanosecond laser pulses. The degree of RSA follows this trend: 1k ≈ 1a > 1c > 1f ≈ 1i > 1h ≈ 1b > 1e > 1d > 1g, which is mainly determined by the ratio of the triplet excited-state absorption cross section to that of the ground-state and the triplet excited-state quantum yield.

Synthesis and Photophysics of Platinum(II) 6-Phenyl-4-(9,9-dihexylfluoren-2-yl)-2,2′-bipyridine Complexes with Phenothiazinyl Acetylide Ligand

Two platinum 6-phenyl-4-(9,9-dihexylfluoren-2-yl)-2,2'-bipyridine complexes (4 and 5) with phenothiazinyl acetylide ligand were synthesized and characterized. Their UV-vis absorption and emission characteristics in solution and in Langmuir-Blodgett (LB) film were systematically investigated. The triplet transient difference absorption and nonlinear absorption properties were also studied for these complexes. Both complexes exhibit a broad metal-to-ligand charge transfer/intraligand charge transfer/ligand-to-ligand charge transfer ((1)MLCT/(1)ILCT/(1)LLCT) absorption band between 400 and 500 nm and a (3)MLCT/(3)ILCT/(3)pi,pi* emission band at approximately 594 nm at room temperature, which blue shifts at 77 K. Both UV-vis absorption and emission spectra show negative solvatochromic effect. The triplet excited-state lifetime at room temperature for complex 4 is approximately 1.2 micros, which is longer than that for complex 5 (approximately 600 ns). The emission quantum yield of complex 4 in toluene is 0.18 and 0.053 for complex 5. Both of the complexes also exhibit broad and moderately strong triplet transient absorption from the near-UV to the near-IR spectral region. However, 5 exhibits stronger reverse-saturable absorption than complex 4 does at 532 nm for nanosecond laser pulses. This is attributed to the weaker ground-state absorption but stronger triplet excited-state absorption at 532 nm for 5 than for 4, which leads to a larger ratio of excited-state absorption cross section to ground-state absorption for 5 than 4. In addition, LB films of 4 and 5 were prepared and characterized by AFM technique. The UV-vis absorption and emission spectra of the LB films of 4 and 5 were also investigated and compared with those obtained in solution.

Cyclometalated Platinum(II) 6-Phenyl-4-(9,9-dihexylfluoren-2-yl)-2,2′-bipyridine Complexes: Synthesis, Photophysics, and Nonlinear Absorption

A series of mononuclear and dinuclear cyclometalated platinum(II) 6-phenyl-4-(9,9-dihexylfluoren-2-yl)-2,2'-bipyridine complexes (F-1-F-5) were synthesized and their photophysical properties were systematically investigated. All complexes exhibit strong (1)pi,pi* absorption bands in the UV region, and a broad, structureless charge transfer band in the visible region. The charge-transfer band is broadened and red-shifted for F-3-F-5 compared to those for F-1 and F-2 because of the electron-donating acetylide ligand and the involvement of the ligand-to-ligand charge transfer character. The molar extinction coefficients for the dinuclear complex F-5 are much higher than those for the mononuclear complexes F-1-F-4, indicating the electronic coupling through the bridge ligand. All complexes are emissive in solution at room temperature and in glassy matrix at 77 K. When excited at the charge transfer absorption band, the complexes exhibit a long-lived red/orange emission around 600 nm, which is attributed to a triplet metal-to-ligand charge transfer/intraligand charge transfer emission ((3)MLCT/(3)ILCT). For emission at 77 K, the emitting state is tentatively assigned as (3)MLCT for F-2-F-4, and (3)MLCT/(3)pi,pi* for F-1 and F-5 taking into account the emission energy, the shape of the spectrum, the lifetime, and the thermally induced Stokes shift. F-1-F-4 exhibit broad triplet transient difference absorption in the visible to the near-IR region, with a lifetime comparable to those measured from the decay of the (3)MLCT/(3)ILCT emission. Therefore, F-1-F-4 give rise to a strong reverse saturable absorption for ns laser pulses at 532 nm. Z-scan experiments were carried out at 532 nm using both ns and ps laser pulses, and the experimental data was fitted by a five-band model to extract the singlet and triplet excited-state absorption cross sections. The degree of reverse saturable absorption follows this trend: F-1 = F-2 > F-3 > F-4 > F-5, which is mainly determined by the ratio of the triplet excited-state absorption cross-section to that of the ground-state and the triplet excited-state quantum yield. Comparison of the photophysics of F-1, F-2, and F-3 to those of their corresponding Pt complexes without the fluorenyl substituent discovers that F-1-F-3 exhibit larger molar extinction coefficients for their low-energy charge transfer absorption band, longer triplet excited-state lifetimes, higher emission quantum yields, and increased ratios of the excited-state absorption cross-section to that of the ground-state.

Back-to-Back Dinuclear Platinum Terpyridyl Complexes: Synthesis and Photophysical Studies

Two back-to-back terpyridine ligands using fluorenyl as bridging group (1-L and 2-L) and their corresponding dinuclear platinum(II) complexes (1 and 2) were synthesized and characterized. Their electronic absorption, photoluminescence, and the triplet transient difference absorption were systematically investigated. Both ligands possess intense (1)pi,pi* absorption in the UV region, and they exhibit structured (1)pi,pi* fluorescence around 400 nm. With addition of p-toluenesulfonic acid to the ligands, both the absorption band and the emission band are red-shifted because of the increased electron-withdrawing ability of the protonated terpyridines and possible mixture of some intraligand charge transfer (ILCT) character. For complexes 1 and 2, they both exhibit broad and strong absorption between 400 and 500 nm, which is assigned as the (1)pi,pi*/(1)ILCT/(1)MLCT (metal-to-ligand charge transfer) transition. The involvement of (1)ILCT in the lowest excited state is evident by the acid titration experiment of the ligands. At room temperature, the complexes exhibit dual emission that admixes fluorescence and phosphorescence from the (1,3)pi,pi*/(1,3)ILCT/(1,3)MLCT states. The assignment of the emitting states is based on the distinct emission lifetimes, different sensitivity to oxygen quenching, and different temperature dependency. Both complexes exhibit emission at 77 K, which is assigned as the mixture of (3)pi,pi*/(3)MLCT. 1 and 2 also exhibit two triplet excited-state absorption bands in the visible to the NIR region, which are tentatively attributed to the (3)pi,pi* and (3)MLCT/(3)ILCT state. In addition, the connection pattern between the fluorenyl component and the terpyridyl components influences the excited-state characteristics of both the ligands and the complexes. Ligand 1-L and its corresponding platinum complex 1 that have the triplet bond connection between the fluorenyl and terpyridyl components exhibit a red-shifted low-energy absorption band, an emission band, and a transient absorption band compared to ligand 2-L and complex 2 that have the fluorenyl directly attached to terpyridyl components. These differences could be rationalized by the enhanced conjugation between the fluorenyl and terpyridyl components in 1-L and 1 because of the better coplanarity induced by the triple bond.

3,4-Ethylenedioxythiophene (EDOT)-based π-conjugated oligomers: Facile synthesis and excited-state properties

A facile synthetic method to produce soluble 3,4-ethylenedioxythiophene (EDOT) oligomers was developed by using iron (III) nitrate nonahydrate as the oxidizing reagent. Two EDOT oligomers, i.e. octomers and octodecamers were obtained as the major products. Their UV–vis absorption, fluorescence at room temperature and 77K, and triplet transient difference absorption have been measured in CH2Cl2 solutions or glassy solutions. With increased conjugation length from EDOT octomers (A) to EDOT octodecamers (B), the UV–vis absorption, fluorescence and the triplet transient difference absorption band shift to longer wavelength. Both the oligomers also exhibit reasonably high efficiency to generate singlet oxygen.

Photophysics of Pt(II) 4,6-diphenyl-2,2′-bipyridyl complexes in solution and in LB film

Two amphiphilic platinum(II) 4,6-diphenyl-2,2′-bipyridyl complexes ( 1 and 2) were synthesized and characterized. LB films of these two complexes were prepared and characterized by AFM technique. Electronic absorption and emission characteristics of these complexes in solutions at room temperature, in glassy solutions at 77 K, and in LB films were studied. The emission energies of the complexes in LB films are similar to those in acetonitrile solutions at room temperature. However, the lifetimes of 1 and 2 in LB films are 4–7 times as long as those in CH 3CN solutions. The triplet transient difference absorption spectra of these complexes exhibit broad absorption in the visible region to the near-IR region. Introducing a hydroxyl substituent on the 4,6-diphenyl-2,2′-bipyridyl ligand favors the formation of intermolecular hydrogen bonding and thus helps the deposition of uniform and stable LB films and increases the self-quenching rate constant for emission of 2 in CH 3CN solution.

Influence of Alkoxyl Substituent on 4,6-Diphenyl-2,2′-bipyridine Ligand on Photophysics of Cyclometalated Platinum(II) Complexes: Admixing Intraligand Charge Transfer Character in Low-Lying Excited States

A series of platinum 4,6-diphenyl-2,2'-bipyridine complexes (6-10) with alkoxyl substituent on the 6-phenyl ring have been synthesized and characterized. The influence of the alkoxyl substituent on the nature of the low-lying excited states, and thus the photophysical properties, have been systematically investigated spectroscopically and theoretically. Complexes 6-10 exhibit a broad low-energy charge-transfer absorption band from 400 to 500 nm, which shows weak negative solvatochromic effect. They all emit at about 590 nm in fluid solutions at room temperature, with the emission energy essentially independent of the nature of the monodentate ligand and the polarity of solvent. The excited-state lifetimes of 6 and 7 are much longer (approximately 460-570 ns) than those of their corresponding "alkoxyl free" analogues 12 and 13 (approximately 40-100 ns) in CH(3)CN. Additionally, the emission quantum yields of 7-9 in CH(2)Cl(2) are quite high (0.15-0.21). Spectroscopic studies and Time-Dependent Density Functional Theory (TDDFT) calculations indicate that these unique photophysical properties are induced by the electron-donating ability of the alkoxyl substituent, which causes a mixture of the intraligand charge transfer (ILCT) with the metal-to-ligand charge transfer (MLCT)/ligand-to-ligand charge transfer (LLCT) in their low-lying excited states. Complexes 6-10 exhibit broad triplet transient difference absorption in the near-UV to the near-IR region, where reverse saturable absorption (RSA) could occur. Nonlinear absorption experiments at 532 nm for nanosecond laser pulses demonstrate that 6-9 are strong reverse saturable absorbers, while 10 exhibits weak RSA because of its larger ground-state absorption cross-section and its low triplet excited-state quantum yield.

4′-(5′′′-R-Pyrimidyl)-2,2′:6′,2′′-terpyridyl (R = H, OEt, Ph, Cl, CN) Platinum(II) Phenylacetylide Complexes: Synthesis and Photophysics

A series of new square-planar 4'-(5'''-R-pyrimidyl)-2,2':6',2''-terpyridyl platinum(II) phenylacetylide complexes ( 1a- 5a) bearing different substituents (R = H, OEt, Ph, Cl, CN) on the pyrimidyl ring have been synthesized and characterized. The electronic absorption, photoluminescence, and triplet transient difference absorption spectra were investigated. All of the complexes exhibit broad, moderately strong absorption between 400 and 500 nm that can be tentatively assigned to the metal-to-ligand charge transfer ( (1)MLCT) transition, possibly mixed with some ligand-to-ligand charge transfer ( (1)LLCT) character. Photoluminescence arising from the (3)MLCT state was observed both in fluid solutions at room temperature and in a rigid matrix at 77 K. The (1)MLCT/ (1)LLCT absorption bands and the (3)MLCT emission bands for 1a- 5a red-shift in comparison to those of the corresponding 4'-toly-2,2':6',2''-terpyridyl platinum(II) phenylacetylide complex. In addition, the energies of the (1)MLCT/ (1)LLCT absorption and the (3)MLCT emission bands exhibit a linear correlation with the Hammett constant (sigma p) of the 5'''-substituent on the pyrimidyl ring. The lifetime of the (3)MLCT emission at room temperature is governed by the energy gap law. The triplet transient difference absorption spectra of 1a- 5a exhibit a broad absorption band from 500 to 800 nm, and a bleaching band between 420 and 500 nm. Complex 5a, which contains the -CN substituent, exhibits a lower-energy triplet absorption band at 785 nm and a shorter lifetime (130 ns) in CH 3CN than 2a, which has the -OEt substituent, does (lambda T1-Tn (max) = 720 nm, tau T = 660 ns). The triplet excited-state absorption coefficients at the band maxima for 1a- 5a vary from 36 600 L.mol (-1).cm (-1) to 115 090 L.mol (-1).cm (-1), and the quantum yields of the triplet excited-state formation range from 0.19 to 0.66. All complexes exhibit a moderate nonlinear transmission for nanosecond laser pulses at 532 nm. Moreover, these complexes can generate singlet oxygen efficiently in air-saturated CH 3CN solutions, with the singlet oxygen generation quantum yield (Phi Delta) varying from 0.24 to 0.46.

Photophysics and Nonlinear Absorption of Peripheral-Substituted Zinc Phthalocyanines

The photophysical properties, such as the UV-vis absorption spectra, triplet transient difference absorption spectra, triplet excited-state extinction coefficients, quantum yields of the triplet excited state, and lifetimes of the triplet excited state, of 10 novel zinc phthalocyanine derivatives with mono- or tetraperipheral substituents have been systematically investigated in DMSO solution. All these complexes exhibit a wide optical window in the visible spectral range and display long triplet excited-state lifetimes (140-240 mus). It has been found that the complexes with tetrasubstituents at the alpha-positions exhibit a bathochromic shift in their UV-vis absorption spectra, fluorescence spectra, and triplet transient difference absorption spectra and have larger triplet excited-state absorption coefficients. The nonlinear absorption of these complexes has been investigated using the Z-scan technique. It is revealed that all complexes exhibit a strong reverse saturable absorption at 532 nm for nanosecond and picosecond laser pulses. The excited-state absorption cross sections were determined through a theoretical fitting of the experimental data using a five-band model. The complexes with tetrasubstituents at the alpha-positions exhibit larger ratios of triplet excited-state absorption to ground-state absorption cross sections (sigma T/sigma g) than the other complexes. In addition, the wavelength-dependent nonlinear absorption of these complexes was studied in the range of 470-550 nm with picosecond laser pulses. All complexes exhibit reverse saturable absorption in a broad visible spectral range for picosecond laser pulses. Finally, the nonlinear transmission behavior of these complexes for nanosecond laser pulses was demonstrated at 532 nm. All complexes, and especially the four alpha-tetrasubstituted complexes, exhibit stronger reverse saturable absorption than unsubstituted zinc phthalocyanines due to the larger ratio of their excited-state absorption cross sections to their respective ground-state absorption cross sections.

PH sensitive photophysical and photochemical properties of a pentaazadentate porphyrin-like gadolinium (III) complex

The electronic absorption spectra, photobleaching property, emission, and triplet excited state characteristics of a pentaazadentate porphyrin-like gadolinium(III) complex have been investigated at acidic and basic conditions. The electronic absorption spectrum of this complex exhibits a Soret band at ca. 449 nm and a Q-like band at ca. 880 nm when the pH value of the solution is lower than 6.6. At basic conditions, the Q-like band blue shifts to ca. 624 nm, while the Soret band only shows an approximate 20 nm hyposochromic shift. The acidic solution is relatively stable upon exposure to ambient light, but the basic solution photobleached to colorless in approximately 3 hours. Irradiation of the Soret band of basic solutions at 420 nm causes faster photobleaching than irradiation of the Q-like band (624 nm). The emission of this complex at pH = 6.5 appears at ca. 915 nm (max.) and 1016 nm, which is a mirror image of the Q-like band, indicating the nature of the emitting state being the lowest singlet excited state. At pH = 9.0, the emission band shifts to ca. 585 nm (max.) and 630 nm (shoulder). The triplet transient difference absorption spectrum of the solution at pH = 6.5 exhibits a bleaching band at ca. 460 nm, a narrow positive band at ca. 450 nm, and a broad, moderately intense absorption band from 480 nm extending to the near-IR region (700 nm). The triplet excited state lifetime deduced from the decay of the transient absorption is approximately 81 ns.

Synthesis, Photophysics, and Optical Limiting of Platinum(II) 4‘-Tolylterpyridyl Arylacetylide Complexes

A series of 4'-tolylterpyridyl platinum(II) complexes with different arylacetylide ligands, namely, phenylacetylide, 4-bromophenylacetylide, 4-nitrophenylacetylide, 4-methoxyphenylacetylide, 4-dimethylaminophenylacetylide, 1-naphthylacetylide, and 3-quinolinylacetylide, were synthesized. Their photophysical properties, such as electronic absorption spectra, emission characteristics at room temperature and 77 K, and transient difference absorption spectra, have been investigated. All of these complexes exhibit a metal-to-ligand charge-transfer (1MLCT) transition at ca. 420-430 nm in their electronic absorption spectra. For ttpy-Ph, ttpy-C6H4Br-4, ttpy-C6H4OCH3-4, ttpy-C6H4N(CH3)2-4, and ttpy-Np, an additional solvatochromic charge-transfer band appears at ca. 460-540 nm. This band is sensitive to the para substituents on the phenylacetylide ligand and is tentatively assigned to a metal- or/and acetylide-to-terpyridyl charge-transfer transition (i.e., a 1MLCT or/and 1LLCT transition). All of the complexes exhibit room-temperature phosphorescence. The emission can be attributed to a 3MLCT state except for ttpy-C6H4NO2-4, for which the emission likely originates from an intraligand 3pi,pi* state involving the nitrophenylacetylide ligand. For ttpy-C6H4OCH3-4, ttpy-C6H4N(CH3)2-4, and ttpy-Np, there probably is more than one low-energy state in close energy proximity, resulting in multiple exponential decays. In addition, the triplet transient absorption difference spectra of ttpy-Ph, ttpy-C6H4Br-4, ttpy-C6H4NO2-4, and ttpy-Quin exhibit moderately intense, broad absorption bands in the visible region and extending into the near-IR region, which likely originate from the same excited state that emits or from a state that is in equilibrium with the emitting state. It appears that the electron-rich arylacetylide ligands, especially 4-methoxyphenylacetylide and 4-dimethylaminophenylacetylide, cause a decrease of the emission efficiency and disappearance of the transient absorption. In contrast, the complexes that exhibit positive absorption bands in the visible spectral region of the triplet transient difference absorption spectra show substantial optical limiting for nanosecond laser pulses at 532 nm.