Triplet Excited-state Absorption Research Articles

Investigation of Optical Nonlinearities and Transient Dynamics in a Stilbenzene Derivative

The temporal properties of nonlinear absorption of a stilbenzene derivative have been investigated by time-resolved pump-probe techniques with picosecond laser pulses at the wavelength of 532 nm. With the help of an additional nanosecond and picosecond open-aperture Z-scan technique, numerical simulations based on a new five-energy level model are used to interpret the experimental results, and the nonlinear optical parameters of this new compound are determined. The five-energy level model is described by two-photon absorption (TPA) excited from ground-state-induced excited-state absorption (ESA) including singlet ESA and triplet ESA. Under the nanosecond pulse excitation, a TPA excited from the triplet excited state contributes to the nonlinear absorption, and the value of the TPA is found to be 1 order of magnitude larger than those usually found for TPA excited from the ground state.

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.

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.

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.

Excited State Absorption Study in Hematoporphyrin IX

We present the study of the excited state absorption of Hematoporphyrin IX dissolved in dimethyl sulfoxide. All measurements were carried out using open aperture Z-scan and white-light continuum pump-probe with picosecond pulses to avoid triplet excited state absorption. Without the latter contribution, the results obtained with both techniques show a transition to a high singlet excited state. The vibronic progression of the Q-band is observed due to photobleaching of the ground state. In addition, we show that the excited state presents reverse saturable absorption for most of the spectral range studied. A long relaxation component of the first singlet excited state was evidenced with the pump-probe experiment. This result is in agreement with fluorescence lifetime and fluorescence quantum yield measurements. In order to elucidate the origin of the nonlinear effects, we used a three-level energy diagram to describe the principal singlet-singlet transitions.

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.

Photophysical Properties of C60 Colloids Suspended in Water with Triton X-100 Surfactant: Excited-State Properties with Femtosecond Resolution

We examine the photophysics of a colloidal suspension of C(60) particles in a micellar solution of Triton X-100 and water, prepared via a new synthesis which allows high-concentration suspensions. The particle sizes are characterized by transmission electron microscopy and dynamic light scattering and found to be somewhat polydisperse in the range of 10-100 nm. The suspension is characterized optically by UV-vis spectroscopy, femtosecond transient absorption spectroscopy, laser flash photolysis, and z-scan. The ground-state absorbance spectrum shows a broad absorbance feature centered near 450 nm which is indicative of colloidal C(60). The transient absorption dynamics, presented for the first time with femtosecond resolution, are very similar to that of thin films of C(60) and indicate a strong quenching of the singlet excited state on short time scales and evidence of little intersystem crossing to a triplet excited state. Laser flash photolysis reveals that a triplet excited-state absorption spectrum, which is essentially identical in shape to that of molecular C(60) solutions, does indeed arise, but with much lower magnitude and somewhat shorter lifetime. Z-scan analysis confirms that the optical response of this material is dominated by nonlinear scattering.

Synthesis and characterization of the multi-photon absorption and excited-state properties of a neat liquid 4-propyl 4′-butyl diphenyl acetylene

The synthesis, characterization, and quantitative electronic structure modeling of multi-photon absorption properties of a neat liquid L34 (4-propyl 4′-butyl diphenyl acetylene) are reported. The liquid is (linearly) transparent in the visible spectrum, but possesses large two-photon absorption (2PA) and 2PA-induced singlet and triplet excited-state absorption as measured by the Z-scan technique and non-linear transmission measurements using both picosecond and nanosecond pulses. The most dominant contributions to the intensity-dependent non-linear absorption come from the 2PA-induced triplet excited states in the nanosecond time regime. We also present transient absorption spectra of the liquid obtained by nanosecond laser-flash photolysis and compare these results with electronic structure calculations. The energy of the absorption bands, both singlet and triplet are in reasonable agreement with calculations performed with Gaussian 03. The experimentally measured spectra and theoretical electronic structure modeling provide information about the energy levels of the excited states of this liquid, including 2PA and 2PA-induced process that is responsible for its non-linear optical properties.

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.

Tetra-2,3-pyrazinoporphyrazines with Externally Appended Pyridine Rings. 6. Chemical and Redox Properties and Highly Effective Photosensitizing Activity for Singlet Oxygen Production of Penta- and Monopalladated Complexes in Dimethylformamide Solution

Tetrakis-2,3-[5,6-di-(2-pyridyl)pyrazino]porphyrazinatopalladium(II) [Py 8TPyzPzPd] ( 1) and the corresponding pentapalladated species [(PdCl 2) 4Py 8TPyzPzPd] ( 2), dissolved (c approximately 10 (-5)-10 (-6) M) in preacidified dimethylformamide ([HCl] approximately 10 (-4) M), behave as potent photosensitizing agents for the production of singlet oxygen, (1)O 2, with Phi Delta values of 0.89 +/- 0.04 and 0.78 +/- 0.05, respectively. The related octacation [(2-Mepy) 8TPyzPzPd] (8+) ( 3), examined under similar experimental conditions, exhibits lower Phi Delta values, that is, 0.29 +/- 0.02 (as an iodide salt) and 0.32 +/- 0.02 (as a chloride salt). In view of the very high values of Phi Delta, the photophysics of complexes 1 and 2 has been studied by means of pump and probe experiments using ns laser pulses at 532 nm as excitation source. Both complexes behave like reverse saturable absorbers at 440 nm because of triplet excited-state absorption. The lifetimes of the triplet excited states are 65 and 96 ns for the penta- and mononuclear species, respectively. Fluorescence quantum yields (Phi f) are approximately 0.1% for both 1 and 2. Such low Phi f values for the two complexes are consistent with the high efficiency of triplet excited-state formation and the measured high yields of (1)O 2. Time-dependent density-functional theory (TDDFT) calculations of the lowest singlet and triplet excited states of the mono- and pentapalladated species help to rationalize the photophysical behavior and the relevant activity of the complexes as photosensitizers for the (1)O 2 ( (1)Delta g) generation.

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.

Nonlinear Optical Response of Chloroaluminiumphthalocyanine Encapsulated by Silica Core−Shell Particles

We report for the first time on the synthesis of core-shell particles containing chloroaluminiumphthalocyanine (ClAlPc) prepared using a sol-gel technique. These particles have the dye molecules at the core, encapsulated by silica shell. The mean size of the particle is determined from HRTEM studies and is found to be approximately 0.08 microm. The surface and bulk compositions of the core-shell particles are studied by XPS and EDAX measurements, respectively. Time-resolved fluorescent measurements indicate a decrease in fluorescence lifetime for the core-shell particles as compared to that of bare dye dissolved in ethanol. This is analyzed on the basis of available theoretical models. Third-order nonlinear optical effects are investigated by the Z-scan technique using 8 ns pulses at a wavelength of 532 nm from a frequency-doubled Nd:YAG laser. The analysis indicates that both singlet and triplet excited-state absorption contribute to nonlinear absorption.

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.

Synthesis, Crystal Structure, and Nonlinear Optical Behavior of β-Unsubstituted meso−meso E-Vinylene-Linked Porphyrin Dimers

[structure: see text] A vinylene-linked porphyrin dimer, with no substituents at the beta-positions, has been synthesized by CuI/CsF promoted Stille coupling. In the crystal structure of this dimer, the C(2)H(2) bridge is twisted by 45 degrees relative to the plane of the porphyrins. The absorption, emission spectra, and electrochemistry reveal substantial porphyrin-porphyrin pi-conjugation. The triplet excited-state absorption spectrum of this dimer makes it suitable for reverse saturable absorption at 710-900 nm.

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.

Understanding the One-Photon Photophysical Properties of a Two-Photon Absorbing Chromophore

A comprehensive one-photon photophysical study has been carried out on AF455, a known two-photon absorbing dye. AF455 is composed of an electron-accepting center with three arms that consist of a π-conjugated group with an electron-donating group at the terminal end of each arm. The objective of this work is to understand the one-photon excitation photophysical properties so that this knowledge will be carried into understanding the two-photon absorption properties. This was done by utilizing steady-state absorption, steady-state and time-resolved emission, femtosecond pump−probe, and nanosecond laser flash photolysis techniques. Through this study it was determined that AF455 undergoes an excited-state intramolecular charge transfer (ICT) upon absorption of a single photon. The extent of ICT stabilization is dependent on the solvent polarity, with increasing ICT stabilization in more polar solvents. The formation of the triplet excited state is small (<8%) and is effected only slightly by a change in the solvent polarity. With an increase in polarity the fluorescence quantum yield decreased and internal conversion was found to become more competitive. On the basis of a two-photon assisted excited-state absorption model we tied in the measured one-photon photophysical properties to understand why the nanosecond effective two-photon absorption cross section is much larger than the femtosecond intrinsic two-photon absorption cross section. We found that AF455 exhibits triplet excited-state absorption in the region of 800 nm that leads to enhancement of the nanosecond effective two-photon absorption cross section.

Structure Property Characterization of Nonlinear Optical Materials

ABSTRACTThis research is comprised of understanding the linear photophysical properties of various dyes to better understand the more complicated nonlinear optical properties. Determining structure property relationships of a series of structurally closely related chromophores is the key in understanding the drivers for the various photophysical properties. In this paper we survey the effect of physically changing the Pt poly-yne structure on the S0-S1 and T1-Tn absorption properties for each of the chromophores. A series of structurally modified platinum poly-ynes have been studied using experimental methods including UV/Vis absorption and nanosecond laser flash photolysis. We found that with extension of the ligand length both the ground and triplet excited state absorption shift to lower energies. Comparing the absorption properties of the ligands and butadiynes with the platinum containing versions reveal that the S1 and Tn exciton is localized on one portion of the ligand with extension and not conjugated through the whole molecule. Changing the phosphine R group results in little effect to the absorption properties except when the R group is conjugated in the case of phenyl. However, changing the R group results in varied materials properties.