Origin Of Excited States Research Articles

Mechanochromic and Solvent-Induced Luminescence of a Supramolecular Pt(II)-Bipyridine Complex with Di(4-pyridylmethyl)aminedithiocarbamate.

[Pt(bpy)(DPMACS2)]2Cl2•3H2O (1•3H2O) (bpy = 2,2'-bipyridine, DPMACS2 = di(4-pyridylmethyl)aminedithiocarbamate) was synthesized and characterized by X-ray diffraction studies, and its crystal structure displayed intermolecular Pt(II)···Pt(II) contacts of 3.471 and 5.065 Å. Upon excitation, 1•3H2O showed broad luminescence at 538 nm, which was red-shifted and enhanced to 560 nm while cooling to 77 K. To this end, the B3LYP/LanL2DZ calculation results were performed to clearly explain their excited-state origin. Moreover, complex 1•3H2O displayed a dramatic mechanochromic shift from 538 to 608 nm while grinding, and the above red-shift was also observed while exposed to air within 1 day, suggestive of the simultaneous mechanochromic and solvent-induced luminescence. It is noted that the luminescence almost reverted to the original luminescence at 535-542 nm upon immersion in various solvents for the ground samples of complex 1•3H2O. In addition, the luminescence for the acetone-immersed ground samples returned to 608 nm in 1 min. The possible interactions between halogenated solvents and the free pyridyl groups in DPMACS2, which were not expected for acetone, have been proposed to be responsible for such a dramatic difference in this study.

Substituent-Mediated Transformation of Polynuclear Gold(I)-Sulfido Complexes—From Pentanuclear to Octadecanuclear Cluster-to-Cluster Transformation

Substituent-Mediated Transformation of Polynuclear Gold(I)-Sulfido Complexes—From Pentanuclear to Octadecanuclear Cluster-to-Cluster Transformation

Uncovering the Origin of the Emitting States in Bi3+-Activated CaMO3 (M = Zr, Sn, Ti) Perovskites: Metal-To-Metal Charge Transfer Versus s–p Transitions

After more than a century of studies on the optical properties of Bi3+ ions, the assignment of the nature of the emissions and the bands of the absorption spectra remain ambiguous. Here, we report an insight into the spectroscopy of Bi3+-activated CaMO3 perovskites (M = Zr, Sn, and Ti), discussing the factors driving the metal-to-metal charge transfer and sp -> s2 transitions. With the aim to figure out the whole scenario, a combined experimental and theoretical approach is employed. The comparison between the temperature dependence of the photoluminescence emissions with the temperature dependence of the exciton energy of the systems has led to an unprecedented evidence of the chargetransfer character of the emitting states in Bi3+-activated phosphors. Low-temperature vacuum ultraviolet spectroscopy together with the design of the vacuum-referred binding energy diagram of the luminescent center is exploited to shed light on the origin of the absorption bands. In addition, the X-ray absorption near the edge structure unambiguously confirmed the stabilization of Bi3+ in the Ca2+ site in both CaSnO3 and CaZrO3 perovskites. This breakthrough into the understanding of the excited-state origin of Bi3+ could pave the way toward the design of a new generation of effective Bi3+-activated phosphors.

Third-order nonlinear optical properties of a π-conjugated phenoxazinium compound: Mechanism and dynamic response

The third-order nonlinear optical response of a novel phenoxazinium salt dissolved in acetonitrile was investigated using a modified time-resolved pump–probe measurement with phase object at 532 nm with pico-second laser pulse. The experimental results clearly indicate that the observed nonlinear response should be of excited-state origin. The compound exhibits strong reverse saturable absorption and self-focusing effects plus long excited-state lifetimes. The nonlinear absorptive and refractive parameters of the phenoxazinium salt were evaluated by combination of picosecond pump–probe measurements and nanosecond Z-Scan experiments. The experiment results demonstrate this compound is a promising nonlinear optical material.

Optical absorption and emission properties of rubrene: insight from a combined experimental and theoretical study

Tetracene (Tc) and rubrene (Rub) are two prototype fluorescent molecules. Both molecules exhibit the same ‘fluorescent backbone’, but due to the additional phenyl groups, the backbone of Rub is twisted, whereas it is planar for Tc. In agreement with earlier investigations, optical spectroscopy of the respective solutions reveals that the S0→S1 transition in Rub is red-shifted with respect to Tc by ∼2000 cm−1 and that Rub exhibits a considerably larger Stokes shift. In order to unravel the physical origin of these differences, we have performed a detailed normal coordinate analysis and frequency calculations using density functional theory (DFT) in conjunction with linear response time-dependent DFT (TD-DFT) energy scan calculations. The calculations yield dimensionless normal coordinate displacements of the excited-state origin that were employed for the calculation of the vibrational finestructure of the absorption and fluorescence spectra of Tc and Rub. The purely theoretical displacements were subsequently refined through fitting to the experimental spectra using the time-dependent theory of electronic spectroscopy. The analysis reveals that the ∼2000 cm−1 red shift of the 0–0 vibronic band of Rub relative to Tc is mainly caused by the inductive effect of the phenyl substituents that leads to destabilization of the donor molecular orbital (MO) (the highest occupied molecular orbital (HOMO)). The large Stokes shift of 820 cm−1 observed for Rub is found to originate mainly from unresolved vibrational progressions involving low-frequency modes that are characterized by appreciable displacements in the excited state. The analysis shows that the spectra of Rub are strongly subject to temperature induced broadening, whereas for Tc this is much less significant.

Ab Initio Molecular Dynamics of Excited-State Intramolecular Proton Transfer Using Multireference Perturbation Theory

We present the first calculations of excited-state dynamics using ab initio molecular dynamics with a multireference perturbation theory description of the electronic structure. The new AIMS-CASPT2 method is applied to a paradigmatic excited-state intramolecular proton-transfer reaction in methyl salicylate, and the results are compared with previous ultrafast spectroscopic experiments. Agreement of AIMS-CASPT2 and experimental results is quantitative. The results demonstrate that the lack of an observed isotope effect in the reaction is due to multidimensionality of the reaction coordinate, which largely involves heavy-atom bond alternation instead of proton transfer. Using the dynamics results as a guide, we also characterize relevant minima on the ground and first singlet excited state using CASPT2 electronic structure theory. We further locate an S1/S0 minimal energy conical intersection, whose presence explains experimental observations of a sharp decrease in fluorescence quantum yield at excitation energies more than 1,300 cm-1 above the excited-state origin.

A Class of Luminescent Cyclometalated Alkynylgold(III) Complexes: Synthesis, Characterization, and Electrochemical, Photophysical, and Computational Studies of [Au(C∧N∧C)(C⋮CR)] (C∧N∧C = κ3C,N,C Bis-cyclometalated 2,6-Diphenylpyridyl)

A new class of luminescent cyclometalated alkynylgold(III) complexes, [Au(RC=N(R')=CR)(CCR' ')], i.e., [Au(C=N=C)(C triple bond CR'')] (HC=N=CH = 2,6-diphenylpyridine) R' ' = C6H5 1, C6H4-Cl-p 2, C6H4-NO2-p 3, C6H4-OCH3-p 4, C6H4-NH2-p 5, C6H4-C6H13-p 6, C6H13 7, [Au(tBuC=N=CtBu)(C triple bond CC6H5)] 8 (HtBuC=N=CtBuH = 2,6-bis(4-tert-butylphenyl)pyridine), and [Au(C=NTol=C)(CCC6H4-C6H13-p)] 9 (HC=NTol=CH = 2,6-diphenyl-4-p-tolylpyridine), have been synthesized and characterized. The X-ray crystal structures of most of the complexes have also been determined. Electrochemical studies show that, in general, the first oxidation wave is an alkynyl ligand-centered oxidation, while the first reduction couple is ascribed to a ligand-centered reduction of the cyclometalated ligand with the exception of 3 in which the first reduction couple is assigned as an alkynyl ligand-centered reduction. Their electronic absorption and luminescence behaviors have also been investigated. In dichloromethane solution at room temperature, the low-energy absorption bands are assigned as the pi-pi* intraligand (IL) transition of the cyclometalated RC=N(R')=CR ligand with some mixing of a [pi(C triple bond CR'') --> pi*(RC=N(R')=CR)] ligand-to-ligand charge transfer (LLCT) character. The low-energy emission bands of all the complexes, with the exception of 5, are ascribed to origins mainly derived from the pi-pi* IL transition of the cyclometalated RC=N(R')=CR ligand. In the case of 5 that contains an electron-rich amino substituent on the alkynyl ligand, the low-energy emission band was found to show an obvious shift to the red. A change in the origin of emission is evident, and the emission of 5 is tentatively ascribed to a [pi(CCC6H4NH2) --> pi*(C=N=C)] LLCT excited-state origin. DFT and TDDFT computational studies have been performed to verify and elucidate the results of the electrochemical and photophysical studies.