Triplet Excited-state Absorption Cross Section Research Articles

The Leap from Organic Light-Emitting Diodes to Organic Semiconductor Laser Diodes

In recent years, organic light-emitting device technology has expanded from organic light-emitting diodes (OLEDs) to organic semiconductor laser diodes (OSLDs) with the progress of sophisticated mo...

Effects of Extended π-Conjugation in Phenanthroline (N∧N) and Phenylpyridine (C∧N) Ligands on the Photophysics and Reverse Saturable Absorption of Cationic Heteroleptic Iridium(III) Complexes

Five new iridium(III) complexes (3,8-R-Phen)Ir(2-(3-R′-phenyl)pyridine)2 (1, R = fluoren-2-yl, R′ = H; 2, R = 7-benzothiazolylfluoren-2-yl, R′ = H; 3, R = H, R′ = fluoren-2-yl; 4, R = H, R′ = 7-benzothiazolylfluoren-2-yl; 5, R = R′ = 7-benzothiazolylfluoren-2-yl) with fluoren-2-yl or 7-benzothiazolylfluoren-2-yl substituent on the 2-phenylpyridine (C∧N) and/or phenanthroline (N∧N) ligands were synthesized and characterized. Their photophysical properties were investigated systematically via UV–vis absorption, emission, and transient difference absorption spectroscopy. Time-dependent density functional theory (TDDFT) calculations were performed to complement the experimental data and aid in our understanding of the characters of optical transitions. In addition, reverse saturable absorption was demonstrated at 532 nm for all complexes using nanosecond laser pulses. All complexes possess a weak low-energy tail that is attributed to 1,3MLCT (metal-to-ligand charge transfer)/1,3LLCT (ligand-to-ligand charge transfer) transitions. The major absorption bands below 475 nm for 1–5 arise from the 1π,π* and intraligand (1ILCT) transitions within the N∧N or C∧N ligands. Attachment of fluoren-2-yl or 7-benzothiazolylfluoren-2-yl substituents on N∧N ligand results in stronger red-shifts of the main absorption band to 400 and 408 nm for 1 and 2, respectively, as compared to 321 and 361 nm in complexes 3 and 4 with the same substituents on the C∧N ligands. In contrast, the 1,3MLCT/1,3LLCT transitions in 1 and 2 are just slightly red-shifted as compared to those in 3 and 4. The emission of complexes 1 and 2 is attributed to the N∧N ligand-centered 3π,π* state with some admixture of 3MLCT/3ILCT/3LLCT characters for 1. In contrast, the emission of 3 and 4 emanates exclusively from the 3MLCT/3LLCT states. For complex 5, which contains 7-benzothiazolylfluoren-2-yl substituents on both the C∧N and the N∧N ligands, the emission predominantly arises from the 3MLCT/3LLCT states with a small portion of N∧N ligand-localized 3π,π* character. All complexes exhibit broadband triplet excited-state absorption in the visible to the near-IR region, with the major absorption bands bathochromically shifted in 1 and 2 as compared to those in 3 and 4. The stronger excited-state absorption leads to dramatic reverse saturable absorption (RSA) at 532 nm for nanosecond laser pulses. The RSA strength decreases as 5 ≈ 2 > 4 ≈ 1 > 3, which is primarily determined by the ratio of the triplet excited-state absorption cross section relative to that of the ground state. Extended π-conjugation in the N∧N ligand obviously increases the RSA of complexes 1, 2, and 5 in comparison to those with π-conjugated substituents only on the C∧N ligands (3 and 4).

Platinum Chloride Complexes Containing 6-[9,9-Di(2-ethylhexyl)-7-R-9H-fluoren-2-yl]-2,2′-bipyridine Ligand (R = NO2, CHO, Benzothiazol-2-yl, n-Bu, Carbazol-9-yl, NPh2): Tunable Photophysics and Reverse Saturable Absorption

Six new platinum(II) chloride complexes 1-6 containing a 6-[9,9-di(2-ethylhexyl)-7-R-9H-fluoren-2-yl]-2,2'-bipyridine (R = NO2, CHO, benzothiazol-2-yl (BTZ), n-Bu, carbazol-9-yl (CBZ), NPh2) ligand were synthesized and characterized. The influence of the electron-donating or electron-withdrawing substituent at the 7-position of the fluorenyl component on the photophysics of these complexes was systematically investigated by spectroscopic methods and simulated by time-dependent density functional theory (TDDFT). Electron-withdrawing or -donating substituents exert distinct effects on the photophysics of the complexes. All complexes feature a low-energy, broad (1)MLCT (metal-to-ligand charge transfer)/(1)ILCT (intraligand charge transfer)/(1)π,π* absorption band (tail) above ca. 430 nm and a major absorption band(s) between 320 and 430 nm, which admix (1)MLCT, (1)π,π*, (1)ILCT, and/or (1)LLCT (ligand-to-ligand charge transfer) characters. The contributions of different configurations to the major absorption band(s) vary depending on the nature of the substituent. Strong electron-donating or -withdrawing substituents (NPh2 and NO2) and the aromatic substituent BTZ cause a pronounced red-shift of the absorption spectra of 1, 3, and 6. All complexes are emissive at room temperature and at 77 K. The emitting excited state is dominated by (3)π,π* character in 1-3, with some contributions from (3)MLCT in 1 and 2, while the emission is predominantly from the (3)MLCT state for 4 and 5 but with some (3)π,π* character. For 6, the emitting state is (3)ILCT in nature. With the increased electron-donating ability of the substituent, the (3)π,π* character diminishes while charge transfer character increases. All complexes exhibit broad and strong triplet excited-state absorption (TA) from the near-UV to the near-IR spectral region. The TA band maxima are red-shifted for complexes 1-3 (which possess the electron-withdrawing substituents) compared to those of 4-6 (which contain electron-donating substituents). All complexes manifest strong reverse saturable absorption (RSA) for a nanosecond laser pulse at 532 nm, which originates from the much stronger triplet excited-state absorption than the ground-state absorption of 1-6 in the visible spectral region. The strength of RSA follows this trend: 4 ≈ 5 < 1 ≈ 3 < 2 < 6, which is primarily determined by the ratio of the triplet excited-state absorption cross section relative to that of the ground-state absorption (σex/σ0) at 532 nm. The σex/σ0 ratios (116-261) of 1-6 at 532 nm are much larger than those of most of the reverse saturable absorbers reported in the literature, with the ratio of 6 (σex/σ0 = 261) being among the largest values reported to date. This makes complexes 1-6, especially 6, very promising reverse saturable absorbers.

Synthesis, Structural Characterization, Photophysics, and Broadband Nonlinear Absorption of a Platinum(II) Complex with the 6-(7-Benzothiazol-2′-yl-9,9-diethyl-9 H-fluoren-2-yl)-2,2′-bipyridinyl Ligand

A platinum complex with the 6-(7-benzothiazol-2'-yl-9,9-diethyl-9H-fluoren-2-yl)-2,2'-bipyridinyl ligand (1) was synthesized and the crystal structure was determined. UV/Vis absorption, emission, and transient difference absorption of 1 were systematically investigated. DFT calculations were carried out on 1 to characterize the electronic ground state and aid in the understanding of the nature of low-lying excited electronic states. Complex 1 exhibits intense structured (1)π-π* absorption at λ(abs)<440 nm, and a broad, moderate (1)MLCT/(1)LLCT transition at 440-520 nm in CH(2)Cl(2) solution. A structured (3)π-π*/(3)MLCT emission at about 590 nm was observed at room temperature and at 77 K. Complex 1 exhibits both singlet and triplet excited-state absorption from 450 nm to 750 nm, which are tentatively attributed to the (1)π-π* and (3)π-π* excited states of the 6-(7-benzothiazol-2'-yl-9,9-diethyl-9H-fluoren-2-yl)-2,2'-bipyridine ligand, respectively. Z-scan experiments were conducted by using ns and ps pulses at 532 nm, and ps pulses at a variety of visible and near-IR wavelengths. The experimental data were fitted by a five-level model by using the excited-state parameters obtained from the photophysical study to deduce the effective singlet and triplet excited-state absorption cross sections in the visible spectral region and the effective two-photon absorption cross sections in the near-IR region. Our results demonstrate that 1 possesses large ratios of excited-state absorption cross sections relative to that of the ground-state in the visible spectral region; this results in a remarkable degree of reverse saturable absorption from 1 in CH(2)Cl(2) solution illuminated by ns laser pulses at 532 nm. The two-photon absorption cross sections in the near-IR region for 1 are among the largest values reported for platinum complexes. Therefore, 1 is an excellent, broadband, nonlinear absorbing material that exhibits strong reverse saturable absorption in the visible spectral region and large two-photon-assisted excited-state absorption in the near-IR region.

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.

Excited-state absorption of a bipyridyl platinum(II) complex with alkynyl-benzothiazolylfluorene units

The singlet excited-state lifetime of a bipyridyl platinum(II) complex containing two alkynyl-benzothiazolylfluorene units was determined to be 145+/-105 ps by fitting femtosecond transient difference absorption data, and the triplet quantum yield was measured to be 0.14. A ground-state absorption cross section of 6.1 x 10(-19) cm(2) at 532 nm was deduced from UV-visible absorption data. Excited-state absorption cross sections of (6.7+/-0.1) x 10(-17) cm(2) (singlet) and (4.6+/-0.1) x 10(-16) cm(2) (triplet) were obtained by using a five-level dynamic model to fit open-aperture Z scans at picosecond and nanosecond pulse widths and a variety of pulse energies. For this complex, the ratio of the triplet excited-state absorption cross section to the ground-state absorption cross section--long used as a figure of merit for reverse saturable absorbers--thus stands at 754, to our knowledge the largest ever reported at 532 nm wavelength.

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.

Excited-state absorption of mono-, di- and tri-nuclear cyclometalated platinum 4,6-diphenyl-2,2′-bipyridyl complexes

The ground-state absorption cross-sections ( σ g), triplet excited-state absorption cross-section ( σ T) at 532 nm, singlet excited-state absorption cross-sections ( σ s) at various visible wavelengths, singlet and triplet excited-state lifetimes, and triplet quantum yields of three cyclometalated platinum(II) 4,6-diphenyl-2,2′-bipyridyl complexes, are reported. The presence of metal–metal and π–π interactions in the dinuclear and trinuclear complexes results in a significant increase in their respective σ g’s in the visible spectral region. As a result, the ratio of σ s/ σ g and σ T/ σ g at each wavelength is significantly greater for the mononuclear complex than for the dinuclear and the trinuclear complexes.

Determination of the triplet excited-state absorption cross section in a polyfluorene by energy transfer from a phosphorescent metal complex

The triplet excited-state $({T}_{1}\ensuremath{\rightarrow}{T}_{n})$ absorption cross section of poly(9,9-dioctylfluorene-alt-benzothiadiazole) (F8BT) was calculated from quasi-steady-state photoinduced absorption measurements of films of the polymer blended with iridium (III) [(bis 4,6-difluorophenyl)-pyridinato-$N,\text{ }\mathrm{C}{2}^{\ensuremath{'}}$] picolinate (FIrpic). Steady-state photoluminescence spectra and photoinduced absorption results showed that quenching of the FIrpic phosphorescence originated from triplet energy transfer from FIrpic to F8BT. By comparing the strength of the photoinduced absorption with the known rate of triplet generation in the F8BT, the triplet excited-state absorption cross section of F8BT was found to be $3.1\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}16}\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{2}\ifmmode\pm\else\textpm\fi{}20%$. This value is relatively small compared with that for poly($p$-phenylenevinylene)s, but is consistent with previous quantum-chemical calculations. Accurate knowledge of the triplet excited-state absorption cross section is valuable for quantifying the populations of nonemissive triplet excitons in polymer-based devices.

Higher-order triplet interaction in energy-level modeling of excited-state absorption for an expanded porphyrin cadmium complex

Recent measurements of transmission versus fluence for a methanol-solvated asymmetric pentaazadentate porphyrin-like (APPC) cadmium complex, [(C6H4-APPC)Cd]Cl, showed the limitations of current energy-level models in predicting the transmission behavior of organic reverse saturable absorbers at fluences greater than 1 J/cm². A new model has been developed that incorporates higher-order triplet processes and accurately fits both nanosecond and picosecond transmission-versus-fluence data. This model has provided the first known determination of a higher triplet excited-state absorption cross section and lifetime for an APPC complex and also described a previously unreported feature in the transmission-versus-fluence data. The intersystem crossing rate and the previously neglected higher triplet excited-state absorption cross section are shown to govern the excited-state population dynamics of methanol-solvated [(C6H4-APPC)Cd]Cl most strongly at more-practical device energies.

Nonlinear light absorption in meso-substituted tetrabenzoporphyrin and tetraarylporphyrin solutions

The characterization of fluence-dependent nonlinear absorption and the excited state absorption cross-sections of the zinc complexes of tetrabenzoporphyrin with meso-substituents of phenyl and thienyl (Ia and Ib), and the zinc and lead complexes of porphyrin with meso-substituent of 3,4,5-trimethoxyphenyl (IIa and IIb) are determined with the Z-scan method. The results from Ia and Ib are comparable with the literature report. The lead porphyrin of IIb gives the largest triplet excited state absorption cross-section (σ ex T =3.3×10 −16 cm 2) among these compounds, and shows very strong optical limiting power as a result. Whereas we obtain the smallest σ ex T=2.5×10 −17 cm 2 for the zinc porphyrin bearing four electron withdrawing groups of benzoquinone (compound III), which exhibits unique linear absorption property at 532 nm. This result could be due to the rapid decay of its lowest triplet state to yield both long-lived triplet biradicals and S 0 states.

Substituted tetra-2,3-pyrazinoporphyrazinino copper(II) complexes: synthesis and nonlinear optical refractive and absorptive properties

Tetra-substituted tetra-2,3-[5,6-(9,10-phenanthro)pyrazino]porphyrazinino copper(II) complexes (AzaPhc, 5a–d) have been prepared via complexation of mono-substituted 5,6- (9,10-phenanthro)-2,3-dicyanopyrazines with copper(I) chloride. Their nonlinear optical properties have been studied by the Z-scan measurements at 532 nm. We found that their effective nonlinear refractive indices n 2 eff (ranging from −4.6×10 −10 to −10.0×10 −10 e.s.u.) are about two to four times larger than the octa-substituted phthalocyaninine complex, and their triplet excited-state absorption cross sections σ ex τ (ranging from 8.2×10 −19 to 9.8×10 −19 cm 2) are about an order less than the phthalocyanine compound. The one-photon excitation and the heating effect have been applied to interpret our experimental results.