Triplet Excited-state Quantum Research Articles

DFT based predictive model for BODIPY triplet quantum yields

Triplet photosensitizers have been developed for use in a variety of applications such as photodynamic therapy, photocatalysis, and organic LEDs. All of these require a high triplet excited-state quantum yield, which is often measured as the singlet oxygen quantum yield (ФΔ). A common method to increase ФΔ is through the heavy-atom-effect achieved by the addition of heavy-atoms to the chromophore – such as the heavy halogens bromine and iodine. Previously, our group developed an empirical model using DFT to predict the ФΔ of heavy-atom containing chromophores. The model correlates a natural atomic orbital composition calculation of the heavy-atoms to the ФΔ of a chromophore. Herein, we modify the original model to apply specifically to halogenated boron dipyrromethene (BODIPY) dyes. In addition to developing a method to predict how changes in the structure of BODIPY dyes affects the ФΔ, this model provides insight into why different structural changes have differing impacts to the ФΔ. The BODIPY core has several unique substitution positions that can be halogenated; however, the model indicates that the 2- and 6-positions on BODIPY (IUPAC numbering) have the greatest impact on the ФΔ with yields changing from 0.00 to 0.90 when replacing the two protons with iodides. Although the original model made using xanthene type dyes provided reasonably good agreement with BODIPY dyes, the new reparametrized model allows for more accurate prediction of the ФΔ of BODIPY type chromophores and provides insight in the importance of substituent location to guide future chromophore design.

Synthesis, Photophysics, and Reverse Saturable Absorption of trans-Bis-cyclometalated Iridium(III) Complexes (C^N^C)Ir(R-tpy)+ (tpy = 2,2′:6′,2″-Terpyridine) with Broadband Excited-State Absorption

Extending the bandwidth of triplet excited-state absorption in transition-metal complexes is appealing for developing broadband reverse saturable absorbers. Targeting this goal, five bis-terdentate iridium(III) complexes (Ir1-Ir5) bearing trans-bis-cyclometalating (C^N^C) and 4'-R-2,2':6',2″-terpyridine (4'-R-tpy) ligands were synthesized. The effects of the structural variation in cyclometalating ligands and substituents at the tpy ligand on the photophysics of these complexes have been systematically explored using spectroscopic methods (i.e., UV-vis absorption, emission, and transient absorption spectroscopy) and time-dependent density functional theory (TDDFT) calculations. All complexes exhibited intensely structured 1π,π* absorption bands at <400 nm and broad charge transfer (1CT)/1π,π* transitions at 400-600 nm. Ligand structural variations exerted a very small effect on the energies of the 1CT/1π,π* transitions; however, they had a significant effect on the molar extinction coefficients of these absorption bands. All complexes emitted featureless deep red phosphorescence in solutions at room temperature and gave broad-band and strong triplet excited-state absorption ranging from the visible to the near-infrared (NIR) spectral regions, with both originating from the 3π,π*/3CT states. Although alteration of the ligand structures influenced the emission energies slightly, these changes significantly affected the emission lifetimes and quantum yields, transient absorption spectral features, and the triplet excited-state quantum yields of the complexes. Except for Ir3, the other four complexes all manifested reverse saturable absorption (RSA) upon nanosecond laser pulse excitation at 532 nm, with the decreasing trend of RSA following Ir2 ≈ Ir4 > Ir1 > Ir5 > Ir3. The RSA trend corresponded well with the strength of the excited-state and ground-state absorption differences (ΔOD) at 532 nm for these complexes.

Synthesis and photophysics of reverse saturable absorbing heteroleptic iridium(iii) complexes bearing 2-(7-R-fluoren-2′-yl)pyridine ligands

Three new heteroleptic cationic iridium(III) complexes (1-3) bearing 5,5'-bis(7-benzothiazolylfluoren-2'-yl)-2,2'-bipyridine (N^N) and 2-(7-R-fluoren-2'-yl)pyridine (R = H, naphthalimide, NPh2) (C^N) ligands were synthesized and characterized. Their photophysical properties were investigated systematically by UV-vis absorption, emission, and transient absorption spectroscopy, and the UV-vis absorption spectra were simulated by time-dependent density functional theory (TDDFT). All complexes exhibit ligand-centered (1)π,π* transitions with minor contributions from (1)ILCT (intraligand charge transfer, π(benzothiazolylfluorene) → π* (bpy)) and (1)MLCT (metal-to-ligand charge transfer, dπ(Ir) → π* (bpy)) transitions below 475 nm, and very weak (1,3)MLCT and (1,3)LLCT (ligand-to-ligand charge transfer, π(C^N) → π* (bpy)) transitions above 475 nm. Complexes 1-3 are weakly emissive in fluid solution at room temperature and in glassy matrix at 77 K. The emission of 2 in toluene solution is possibly dominated by the substituted bipyridine ligand-localized (3)π,π* emission, while 1 and 3 likely exhibit (3)MLCT/(3)LLCT dominated emission in toluene solution. All complexes possess relatively strong triplet transient absorption (TA) from visible to NIR region, where reverse saturable absorption (RSA) could occur. Nonlinear transmission experiments at 532 nm using ns laser pulses demonstrate that all three complexes exhibit strong RSA, with the RSA strength following this trend: 1 >3 > 2. The different substituents on the fluorenyl groups of the C^N ligands alter the energy and lifetime of the lowest singlet and triplet excited states. They also affect the ground-state and excited-state absorption characteristics. The differences in the ratio of the excited-state absorption to that of the ground state and the triplet excited-state quantum yield result in the different degrees of RSA. Complexes 1-3 are strong reverse saturable absorbers at 532 nm and could potentially be used as broadband nonlinear absorbing materials.

Optimization of Triplet Excited State and Singlet Oxygen Quantum Yields of Picolylamine–Porphyrin Conjugates through Zinc Insertion

We synthesized a new class of picolylamine-porphyrin conjugates 1-3 and have investigated the effect of heavy atom insertion on their intersystem crossing efficiency through spin-orbit perturbations. By incorporating zinc ions in the core as well as periphery positions of the porphyrin ring, we have successfully optimized their triplet excited state quantum yields and their efficiency to generate singlet oxygen. Uniquely, the picolylamine-porphyrin conjugate 3 having five zinc ions exhibited a triplet excited state quantum yield of ca. 0.97 and a sensitized singlet oxygen generation yield of ca. 0.92. In contrast, the free base porphyrin derivative 1 exhibited ca. 0.64 and 0.5 of the triplet excited state and singlet oxygen quantum yields, respectively. Our results demonstrate that the insertion of zinc metal ions in the picolylamine-porphyrin conjugates not only quantitatively enhances the triplet excited state and singlet oxygen yields but also imparts hydrophilicity, thereby their potential use as sensitizers in photodynamic therapy and green photooxygenation reactions.

Platinum(II) Complexes Bearing 2‐(9,9‐Dihexadecyl‐7‐R‐fluoren‐2‐yl)‐1,10‐phenanthroline Ligands: Synthesis, Photophysics and Reverse Saturable Absorption

AbstractSix platinum(II) complexes bearing 2‐(9,9‐dihexadecyl‐7‐R‐fluoren‐2‐yl)‐1,10‐phenanthroline ligands (R = NO2, CN, I, H, benzothiazole, thiophene for ligands 1–6, respectively) were synthesized and their photophysics and reverse saturable absorption (RSA) were systematically investigated. All complexes exhibit intense absorption bands between 350 and 450 nm, which are attributed to 1π,π*/1ILCT (intraligand charge transfer) transitions, and broad 1MLCT (metal to ligand charge transfer) transitions between 475 and 550 nm. All complexes are emissive at room temperature in a variety of solvents and at 77 K in butyronitrile glassy matrix, with the long‐lived, structured emission appearing between 600 and 750 nm. The emitting states are assigned to predominantly the 3π,π* state with an admixture of 3MLCT characters. Upon ns laser excitation at 355 nm, all complexes display broad, moderately strong transient absorption from the visible to the near‐IR region. As a result, RSA is observed from all complexes at 532 nm for ns laser pulses. The strength of the RSA for these complexes follows the trend: Pt‐2 &gt; Pt‐3 &gt; Pt‐4 &gt; Pt‐1 &gt;&gt; Pt‐5 &gt; Pt‐6, which correlates well with the trend of the estimated ΦTσex/σ0 values (where ΦT is the triplet excited state quantum yield, and σex and σ0 are the excited‐state and ground‐state absorption cross‐section, respectively) at 532 nm. It is revealed that substitution at the 7‐position of the fluorenyl component exerts a distinct effect on the photophysics and RSA of the complexes. Aromatic substituents such as benzothiazol‐2‐yl and 2‐thionyl cause pronounced red‐shifts of the absorption bands, the emission bands, and the transient difference absorption bands, but a decrease of the RSA at 532 nm.

Nonlinear Absorbing Cationic Iridium(III) Complexes Bearing Benzothiazolylfluorene Motif on the Bipyridine (N∧N) Ligand: Synthesis, Photophysics and Reverse Saturable Absorption

Four new heteroleptic cationic Ir(III) complexes bearing benzothiazolylfluorene motif on the bipyridine (N∧N) (1 and 2) and phenylpyridine (C∧N) (3 and 4) ligands are synthesized and characterized. The influence of the position of the substituent and the extent of π-conjugation on the photophysics of these complexes is systematically investigated by spectroscopic methods and simulated by time-dependent density functional theory (TDDFT). The complexes exhibit ligand-centered (1)π,π* transitions with admixtures of (1)ILCT (π(benzothiazolylfluorene) → π*(bpy)) and (1)MLCT (metal-to-ligand charge transfer) characters below 475 nm, and very weak (1,3)MLCT and (1,3)LLCT (ligand-to-ligand charge transfer) transitions above 475 nm. The emission of these complexes at room temperature in CH2Cl2 solutions is ascribed to be predominantly from the (3)MLCT/(3)LLCT states for 1 and from the (3)π,π* state for 2, while the emitting state of 3 and 4 are assigned to be an admixture of (3)MLCT, (3)LLCT, and (3)π,π* characters. The variations of the photophysical properties of 1-4 are attributed to different degrees of π-conjugation in the bipyridine and phenylpyridine ligands induced by different positions of the benzothiazolylfluorenyl substituents on the bipyridine ligand and different extents of π-conjugation in the phenylpyridine ligands, which alters the energy and lifetime of the lowest singlet and triplet excited states. 1-4 all possess broadband transient absorption (TA) upon nanosecond laser excitation, which extends from the visible to the NIR region. Therefore, 1-4 all exhibit strong reverse saturable absorption (RSA) at 532 nm for ns laser pulses. However, the TA of complexes 1, 2, and 3 are much stronger than that of 4. This feature, combined with the difference in ground-state absorption and triplet excited-state quantum yield, result in the difference in RSA strength, which follows this trend: 1 ≈ 2 ≈ 3 > 4. Therefore, complexes 1-3 are strong reverse saturable absorbers at 532 nm and could potentially be used as broadband nonlinear absorbing materials.

In Vitro Demonstration of Apoptosis Mediated Photodynamic Activity and NIR Nucleus Imaging through a Novel Porphyrin

We synthesized a novel water-soluble porphyrin THPP and its metalated derivative Zn-THPP having excellent triplet excited state quantum yields and singlet oxygen generation efficiency. When compared to U.S. Food and Drug Administration approved and clinically used sensitizer Photofrin, THPP showed ca. 2-3-fold higher in vitro photodynamic activity in different cell lines under identical conditions. The mechanism of the biological activity of these porphyrin systems has been evaluated through a variety of techniques: 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) assay, comet assay, poly(ADP-ribose)polymerase (PARP) cleavage, CM-H(2)DCFDA assay, DNA fragmentation, flow cytometric analysis, fluorescence, and confocal microscopy, which confirm the apoptotic cell death through predominantly reactive oxygen species (ROS). Moreover, THPP showed rapid cellular uptake and are localized in the nucleus of the cells as compared to Hoechst dye and Photofrin, thereby demonstrating its use as an efficient sensitizer in photodynamic therapy and live cell NIR nucleus imaging applications.

Photophysics and nonlinear absorption of 4,4′-diethynylazobenzene derivatives terminally capped with substituted aromatic rings

The photophysical properties, nonlinear absorption characteristics and the influence of the substituted aromatic rings on the nature of excited states of a series of 4,4′-diethynylazobenzene derivatives ( 1a – 1f ) were systematically investigated. All compounds exhibit reverse saturable absorption at 532 nm and could potentially be used as nonlinear absorbing materials. ► Photophysics of 4,4′-diethynylazobenzene derivatives was systematically investigated. ► Excited state properties can be tuned by alternation of the terminal substituents. ► Compounds exhibit varied reverse saturable absorption at 532 nm for ns laser pulses. The photophysical properties of a series of 4,4′-diethynylazobenzene derivatives terminally capped with substituted aromatic rings ( 1a : R = phenyl; 1b : R = 4-(diphenylamino)phenyl; 1c : R = 4-(9 H -carbazol-9-yl)phenyl; 1d : R = 9 H -fluoren-2-yl; 1e : R = biphenyl-4-yl; 1f : R = naphthalen-2-yl) were systematically investigated. All compounds exhibit strong 1 π , π * absorption bands in the UV region; and a broad, structureless charge-transfer band/shoulder in the visible region (except for 1a ), which systematically red-shifts when electron-donating substituents are introduced to the terminal phenyl rings, but blue-shifts when π -conjugation of the terminal aromatic ring increases. All compounds are emissive in solution at room temperature and at 77 K. When excited at the low-energy absorption band, the compounds emit fluorescence between 369 and 419 nm, which can be attributed to 1 π , π */ 1 ICT (intramolecular charge transfer) state. Density functional theory (DFT) calculations on 1a – 1f in gas phase were also performed to gain insight into the nature of the ground electronic state and the low-lying excited electronic states. 1d – 1f exhibit strong triplet transient absorption band(s) in the visible spectral region, which are mainly attributed to the 3 π , π * state. Reverse saturable absorption (RSA) of these compounds was demonstrated at 532 nm using ns laser pulses. The degree of RSA follows this trend: 1b > 1c ≈ 1a > 1e > 1f > 1d , 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.

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.

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.

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.

Synthesis and photophysics of 4′-R-2,2′;6′,2″-terpyridyl (R = Cl, CN, N(CH3)2) platinum(II) phenylacetylide complexes

Platinum terpyridyl (tpy) phenylacetylide complexes with -Cl, -CN, and -NMe(2) substituents on the 4'-position of the tpy ligand were synthesized and characterized. Their photophysical properties were systematically investigated. In addition, theoretical electronic structure calculations using density functional theory (DFT) and time-dependent (TD-DFT) approaches were carried out for complexes and ; the results of these calculations provided additional information on the nature of the electronic structures of the low-lying electronic states of these complexes, including the electron density distribution and the composition of the frontier molecular orbitals. Complexes exhibit moderately intense charge-transfer bands in the visible region, which are assigned to the (1)MLCT/(1)LLCT transitions. In comparison to their corresponding chloride complexes , these charge-transfer bands become broadened and red-shifted. Complexes emit at room temperature in CH(3)CN and CH(2)Cl(2) solutions and at 77 K in butyronitrile glassy solutions. At room temperature, the emission is tentatively attributed to (3)MLCT for and , and to a mixture of (3)MLCT/(3)ILCT/(3)pi,pi* for . Due to the admixture of (3)ILCT/(3)pi,pi* characters in its emitting state, displays a much higher emission quantum yield and longer emission lifetime compared to and . Replacing the chloride co-ligand in by phenylacetylide co-ligand clearly enhances the emission and prolongs the lifetime. exhibit a broad and moderately intense triplet excited-state absorption in the visible to the NIR region, with large excited-state absorption coefficients and moderately high triplet excited state quantum yields. Therefore, complexes , especially , have potential applications in organic light emitting devices (OLED) and as reverse saturable absorbing materials.

Axial Halogen Ligand Effect on Photophysics and Optical Power Limiting of Some Indium Naphthalocyanines

Three axially substituted complexes, 2,3-octa(3,5-di-tert-butylphenoxy)-2,3-naphthalocyaninato indium chloride (1a), 2,3-octa(3,5-di-tert-butylphenoxy)-2,3-naphthalocyaninato indium bromide (1b), and 2,3-octa(3,5-di-tert-butylphenoxy)-2,3-naphthalocyaninato indium iodide (1c) have been synthesized and their photophysical properties have been investigated. Optical power limiting of nanosecond (ns) and picosecond (ps) laser pulses at 532 nm using these complexes has been demonstrated. All complexes display strong Q(0,0) absorption and measurable emission in the near-infrared region and exhibit strong excited-state absorption in the range of 470-700 nm upon ns laser excitation. The different axial ligands show negligible effect on the linear absorption, emission, and transient difference absorption spectra. However, the excited-state lifetime, triplet excited-state quantum yield, and efficiency to generate singlet oxygen are affected significantly by the heavier axial ligand. Brominated and iodinated complexes 1b and 1c show higher triplet excited-state quantum yield, while chlorinated complex 1a has longer excited-state lifetime and is more efficient in generating singlet oxygen. The iodinated complex 1c displayed the best optical limiting due to the higher ratio of excited-state absorption cross section to ground state absorption cross section (sigma(eff)/sigma(0)).

Synthesis, photophysical and photochemical properties of substituted zinc phthalocyanines

The synthesis, photophysical and photochemical properties of the 4-({3,4,5-tris-[2-(2-ethoxyethoxy)ethyloxy]benzyl}oxy) and 4-({3,4,5-tris-[2-(2-ethoxyethoxy)ethyloxy]benzyl}thio) zinc(ii) phthalocyanines are reported for the first time. The new compounds have been characterized by elemental analysis, IR, (1)H and (13)C NMR spectroscopy, electronic spectroscopy and mass spectra. General trends are described for photodegradation, singlet oxygen, fluorescence and triplet excited state quantum yields, and triplet state and fluorescence lifetimes of these compounds in dimethylsulfoxide (DMSO). The fluorescence of the complexes was quenched by benzoquinone (BQ). The effects of the substitution on the photophysical and photochemical parameters of the zinc(II) phthalocyanines (6, 7 and 8) are also reported. Photophysical and photochemical properties of phthalocyanine complexes are very useful for PDT applications. The substituted Zn(II) phthalocyanines showed high triplet and singlet oxygen quantum yields. High singlet oxygen quantum yields are very important for Type II mechanism. Thus, these complexes show potential as Type II photosensitizers.