Douglas M. Krein

Photophysical properties of a series of electron-donating and -withdrawing platinum acetylide two-photon chromophores.

To explore spectroscopic structure-property relationships in platinum acetylides, we synthesized a series of complexes having the molecular formula trans-bis(tributylphosphine)-bis(4-((9,9-diethyl-7-ethynyl-9H-fluoren-2-yl)ethynyl)-R)-platinum. The substituent, R = NH(2), OCH(3), N(phenyl)(2), t-butyl, CH(3), H, F, benzothiazole, CF(3), CN, and NO(2), was chosen for a systematic variation in electron-donating and -withdrawing properties as described by the Hammett parameter σ(p). UV/vis, fluorescence, and phosphorescence spectra, transient absorption spectra on the fs-ps time scale, and longer time scale flash photolysis on the ns time scale were collected. DFT and TDDFT calculations of the T(1) and S(1) energies were performed. The E(S) and E(T) values measured from linear spectra correlate well with the calculated results, giving evidence for the delocalized MLCT character of the S(1) state and confinement of the T(1) exciton on one ligand. The calculated T(1) state dipole moment ranges from 0.5 to 14 D, showing the polar, charge-transfer character of the T(1) state. The ultrafast absorption spectra have broad absorption bands from 575 to 675 nm and long wavelength contribution, which is shown from flash photolysis measurements to be from the T(1) state. The T(1) energy obtained from phosphorescence, the T(1)-T(n) transition energy obtained from flash photolysis measurements, and the triplet-state radiative rate constant are functions of the calculated spin density distribution on the ligand. The calculations show that the triplet exciton of chromophores with electron-withdrawing substituents is localized away from the central platinum atom, red-shifting the spectra and increasing the triplet-state lifetime. Electron-donating substituents have the opposite effect on the location of the triplet exciton, the spectra, and the triplet-state lifetime. The relation between the intersystem crossing rate constant and the S(1)-T(1) energy gap shows a Marcus relationship with a reorganization energy of 0.83 eV. The calculations show that intersystem crossing occurs by conversion from a nonpolar, delocalized S(1) state to a polar, charge-transfer T(1) state confined to one ligand, accompanied by conformation changes and charge transfer, supporting the experimental evidence for Marcus behavior.

Symmetry Breaking in Platinum Acetylide Chromophores Studied by Femtosecond Two-Photon Absorption Spectroscopy

We study instantaneous two-photon absorption (2PA) in a series of nominally quasi-centrosymmetric trans-bis(tributylphosphine)-bis-(4-((9,9-diethyl-7-ethynyl-9H-fluoren-2-yl) ethynyl)-R)-platinum complexes, where 11 different substituents, R = N(phenyl)2(NPh2), NH2, OCH3, t-butyl, CH3, H, F, CF3, CN, benzothiazole, and NO2, represent a range of electron-donating (ED) and electron-withdrawing (EW) strengths, while the Pt core acts as a weak ED group. We measure the 2PA cross section in the 540-810 nm excitation wavelength range by complementary femtosecond two-photon excited fluorescence (2PEF) and nonlinear transmission (NLT) methods and compare the obtained values to those of the Pt-core chromophore and the corresponding noncentrosymmetric side group (ligand) chromophores. Peak 2PA cross sections of neutral and ED-substituted Pt complexes occur at S0 → Sn transitions to higher energy states, above the lowest-energy S0 → S1 transition, and the corresponding values increase systematically with increasing ED strength, reaching maximum value, σ2 ∼ 300 GM (1 GM = 10-50 cm4 s), for R = NPh2. At transition energies overlapping with the lowest-energy S0 → S1 transition in the one-photon absorption (1PA) spectrum, the same neutral and ED-substituted Pt complexes show weak 2PA, σ2 < 30-100 GM, which is in agreement with the nearly quadrupolar structure of these systems. Surprisingly, EW-substituted Pt complexes display a very different behavior, where the peak 2PA of the S0 → S1 transition gradually increases with increasing EW strength, reaching values σ2 = 700 GM for R = NO2, while in the S0 → Sn transition region the peak 2PEF cross section decreases. We explained this effect by breaking of inversion symmetry due to conformational distortions associated with low energy barrier for ground-state rotation of the ligands. Our findings are corroborated by theoretical calculations that show large increase of the permanent electric dipole moment change in the S0 → S1 transition when ligands with strong EW substituents are twisted by 90° relative to the planar chromophore. Our NLT results in the S0 → S1 transition region are quantitatively similar to those obtained from the 2PEF measurement. However, at higher transition energy corresponding to S0 → Sn transition region, the NLT method yields effective multiphoton absorption stronger than the 2PEF measurement in the same systems. Such enhancement is observed in all Pt complexes as well as in all ligand chromophores studied, and we tentatively attribute this effect to nearly saturated excited-state absorption (ESA), which may occur if 2PA from the ground state is immediately followed by strongly allowed 1PA to higher excited states.

Spectroscopic structure-property relationships of a series of polyaromatic platinum acetylides.

To develop a structure-spectroscopic property relationship in platinum acetylides having poly(aromatic hydrocarbon) ligands, we synthesized a series of chromophores with systematic variation in the number of fused aromatic rings (nFAR) and ligand topology (polyacene (L), polyphenanthrene (Z), or compact(C)). We measured ground-state absorption, fluorescence, and phosphorescence spectra. We also performed nanosecond and femtosecond transient absorption experiments. To extend the range of compounds in the structure-property relationship, we did DFT calculations on an expanded series of chromophores. Both the DFT results and experiments show that the S(1) and T(1) state energies are a function of both nFAR and the ligand topology. In the L chromophores, the S(1) and T(1) state energies decrease linearly with nFAR. In contrast, the S(1) and T(1) state energies of the Z chromophores oscillate around a fixed value with increasing nFAR. The C chromophores have behavior intermediate between the L and Z chromophores. A parallel series of calculations on the ligands shows the same behavior. The S(1)-S(n) energy obtained from ultrafast time-resolved spectra has a linear variation in nFAR. The rate constant for nonradiative decay, k(nr), was calculated from the S(1) state lifetime and decreases with an increasing number of π electrons in the aromatic ring. The result is consistent with the spin-orbit coupling caused by the central platinum heavy atom decreasing with larger nFAR. The present work shows that the framework developed for the analysis of poly(aromatic hydrocarbon) properties is useful for the understanding of the corresponding platinum acetylide complexes.

Two-Photon Spectroscopy of a Series of Platinum Acetylides: Conformation-Induced Ground-State Symmetry Breaking

With the goal of elucidating electronic and conformational effects on structure-spectroscopic property relationships in platinum acetylides, we synthesized a series of nominally centrosymmetric chromophores trans-Pt(PBu3)2(C≡C-Phenyl-X)2, where X = diphenylamino (DPA), NH2, OCH3, t-Bu, CH3, H, F, benzothiazole (BTH), CF3, CN, and NO2. We collected one- and two-photon absorption spectra and also performed density functional theory (DFT) and time-dependent (TD) DFT calculations on the ground- and excited-state properties of these compounds. The DFT calculations revealed facile rotation between the two ligands, suggesting that the compounds exhibit nonplanar ground-state conformations in solution. TDDFT calculation of the S1 state energy and transition dipole moment for a nonplanar conformation gave good agreement with experiment. Two-photon absorption spectra obtained from these compounds allowed estimation of the change of permanent electric dipole moment upon vertical excitation from ground state to S1 state. The values are small Δμ < 1.0 D for neutral substituents such as CH3, H, and F but increase sharply to Δμ ≈ 11 D for electron-accepting NO2. When in a nonplanar conformation, the corresponding calculated Δμ values showed good agreement with the experimental data indicating that the two-photon spectra result from nonplanar ground-state conformations. Previously studied related chromophores having extended conjugation ( Rebane, A.; Drobizhev, M.; Makarov, N. S.; Wicks, G.; Wnuk, P.; Stepanenko, Y.; Haley, J. E.; Krein, D. M.; Fore, J. L.; Burke, A. R.; Slagle, J. E.; McLean, D. G.; Cooper, T. M. J. Phys. Chem. A 2014 , 118 , 3749 - 3759 ) show similar dependence of Δμ on the substituents, which allows us to conclude that the excited-state properties of these floppy chromophores are a function of the electronic properties of the substituents, ligand size, and nonplanar molecular conformation.

Hot isostatic pressing of transition metal ions into chalcogenide laser host crystals

This paper describes a technique using a hot isostatic pressing (HIP) for the diffusion of transition metal ions into chalcogenide laser host crystals. Thin layers of chromium metal are sputtered onto the surface of zinc selenide and zinc sulfide crystals before treatment in a HIP chamber. The transmissivities, excited state lifetimes, and diffusion rates are measured for various dopant concentrations. Efficiency, spectral output, and tuning data are also measured for a Cr:ZnSe laser. The diffusion rate of 5.48 × 10−8 cm2/s is two orders of magnitude faster than other techniques reported in the literature, and the sub 140 pm measured linewidth is more than 350 times smaller than what is typical of commercially available crystals. Preliminary results for Fe:ZnSe, Co:ZnSe, and Ni:ZnSe are presented as well.

Measurement of refractive indices of CdSiP 2 at temperatures from 90 to 450 K

Ordinary and extraordinary refractive indices of CdSiP2 were measured and a Sellmeier equation was obtained for the first time to our knowledge over the temperature range 90 to 450 K. The index values were used to calculate the crystal temperature and phase-matching angle dependence of the generated wavelengths in the nonlinear frequency conversion of a range of pump wavelengths. A good match was obtained between the calculated values of the wavelengths and some experimental measurements.

Femtosecond laser writing of electro-optic crystalline structures in glass

Over the last decade it has been demonstrated that nonlinear optical (NLO) crystals can be grown by laser precipitation in customized glasses and used for electro-optic applications. It has been further demonstrated that this novel crystal growth technique is capable of fabricating nonlinear waveguide structures, where the polar axis of the crystal is aligned along the growth direction. Femtosecond precipitation of NLO crystals in glass has the potential to be a low-cost method of creating functional optical elements. In order to realize this goal, the orientation of the NLO crystals must be carefully controlled. In the present study, a widely used electro-optical crystal, Lithium Niobate, was precipitated in 33LiO2-33Nb2O5-34SiO2 (mol%) (LNS) glass, forming NLO crystalline structures in an amorphous matrix. Glass fabrication techniques for making high quality glass, and the crystallization parameter space were explored to determine the optimal conditions for smooth and continuous crystal growth. The crystalline orientation of the precipitated lithium niobate was determined for a variety of writing conditions, and the growth technique was extended to multi-dimensional structures.