An experimental and theoretical investigation was carried out on a series of platinum-acetylide oligomers of the general structure Ph-CC-[PtL2-CC-(1,4-Ph)-CC-]n-PtL2-CC-Ph (where n = 1, 2, 3, 4, 6; Ph = phenyl, 1,4-Ph = 1,4-phenylene; L = P(n-Bu)3, and the geometry at Pt = trans). The objective of this work is to understand the geometry and electronic structure of the ground and triplet excited states of Pt-acetylide oligomers. The experiments carried out include temperature-dependent absorption and photoluminescence spectroscopy (80-298 K range) and ambient temperature transient absorption spectroscopy. Density functional theory (DFT) and time-dependent density functional theory (TD-DFT) calculations were carried out on several of the oligomers using the hybrid Becke's three-parameter functional with the B3LYP correlation functional and the SDD basis set. The combined experimental and theoretical results provide very clear evidence that the triplet excited state is localized on a chromophore consisting approximately of a single -[PtL2-CC-(1,4-Ph)-CC-PtL2]- repeat unit. DFT calculations indicate that in the ground state conformers that differ in the (rotational) orientation of the 1,4-phenylenes with respect to the plane defined by the PtL2(C)2 units (twisted = t and planar = p) are very close in energy (difference of <1 kcal.mol-1). By contrast, in the triplet excited state, the p conformer is 3 kcal.mol-1 lower in energy than the t conformer. The torsional geometry change in the triplet state is reflected in the low-temperature phosphorescence spectra of the short oligomers, where separate emission bands are seen from the t and p conformers.

Pi-conjugated dendrimers are an important class of materials for optoelectronic devices, especially for light-harvesting systems. We report here a theoretical investigation of the optical response and of the excited-state properties of three-arm and four-arm phenyl-cored dendrimers for photovoltaic applications. A variety of theoretical methods are used and evaluated against each other to calculate vertical transition energies, absorption and excitation spectra with vibronic structure, charge transport, and excitonic behavior upon photoexcitation and photoemission processes. Photophysical phenomena in these dendrimers are, in general, better explained with ab initio methods rather than with semiempirical techniques. Calculated reorganization energies were found to correlate well with the device photocurrent data where available. The excitons formed during photoexcitation are calculated to be more delocalized than the ones formed after vibrational relaxation in the excited states for fluorescence emission. The localization of excitons in emission processes is a result of geometrical changes in the excited state coupled with vibronic modes. Correlated electron-hole pair diagrams illustrate breaking of pi-conjugation in three-arm dendrimers due to meta linkage of arms with the core, whereas four-arm dendrimers are not affected by such breaking due to presence of ortho and para branching. Yet, ortho branching causes large twist angles between the core and the arms that are detrimental to pi-electron system delocalization over the structure.

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A new luminescent oxygen and temperature sensor has been developed that utilizes two luminescent dyes, 5,10,15,20-tetrakis(pentafluorophenyl)porphyrin platinum(II) (PtTFPP, the oxygen sensor) and tris(1,10-phenanthroline)ruthenium(II) dichloride (Ruphen, the temperature sensor). The two dyes are dispersed in an oxygen-permeable polymer binder consisting of a copolymer of 4-tert-butylstyrene (tBS) and 2,2,2-trifluoroethyl methacrylate (p-tBS-co-TFEM). To alleviate energy transfer and other quenching interactions between the two luminescent dyes in the p-tBS-co-TFEM binder, the Ruphen temperature sensor is encapsulated in polyacrylonitrile (PAN) polymer nanospheres that are prepared by coprecipitation of PAN and Ruphen from N,N-dimethylformamide solution. The temperature and air-pressure response of the emission from the sensor film is fully characterized by using emission spectroscopy. The emission from the two luminescent dyes is spectrally well-separated. The intensity of the Ruphen emission varies strongly with temperature (approximately 1.4% degrees C(-1)), whereas the intensity of the PtTFPP emission varies with temperature and air pressure. The two-dye luminescent coating is useful as a pressure-sensitive paint (PSP), where the emission from the Ruphen temperature sensor is used to correct for the temperature dependence of the pressure response of the PtTFPP sensor. To demonstrate the PSP application, a coupon coated with the sensor was imaged using a CCD camera, and the CCD images were analyzed by intensity ratio methods. Spectroscopic studies were also carried out on a sensor that contains three dyes in order to demonstrate the feasibility of including an intensity reference dye along with the temperature and pressure dyes into the sensor.

This manuscript reports the synthesis and photophysical investigation of two hexa-peri-hexabenzocoronenes (HBCs) that are functionalized with platinum(II) acetylide units of the type trans-(Ar-CC-)2Pt(PBu3)2. In one complex, the platinum is directly linked to the HBC chromophore by an ethynyl spacer, whereas in the second, the platinum is separated from the HBC via a 1,4-phenylene ethynylene spacer. The Pt-acetylide units introduce strong spin-orbit coupling into the HBC chromophore, giving rise to high yields of the triplet excited state along with moderately intense phosphorescence at ambient temperature. On the basis of emission spectroscopy, the triplet state of the HBC chromophore is located at 2.14 eV and the S-T splitting is 0.6 eV. The triplet-triplet absorption and radical cation absorption spectra of the Pt-HBCs are determined by laser flash photolysis. Aggregation of the Pt-HBCs in a poor solvent such as hexane leads to quenching of the triplet state, but spectroscopy provides no evidence for the formation of a triplet excimer, even under conditions where the molecules are strongly aggregated.

Density functional theory (DFT) calculations were performed on oligomers of 3,4-(ethylenedioxy)thiophene (EDOT), 4-(dicyanomethylene)-4H-cyclopenta[2,1-b:3,4-b′]dithiophene (CDM), and co-oligomers (CDM/ EDOT). Oligomer data were extrapolated to polymer values. Theoretical band gaps reproduce λ max from UV spectroscopy for PEDOT and are about 1 eV larger than electrochemical band gaps. λ max of PCDM/EDOT is predicted to be 0.42 eV smaller than that of PEDOT and 0.15 eV smaller than that of PCDM. PCDM/EDOT has a wide valence and an extremely narrow conduction "band". It is probably better not to refer to these localized states as a band at all. This rationalizes the mobility ratio of 500 between p-type and n-type charge carriers and the low n-type conductivity of PCDM/EDOT. The lack of dispersion of the conduction band is due to the very different EAs of EDOT and CDM.

A series of thiophene coupled acceptors were systematically investigated at the density functional theory level to reveal structure-property relationships for building blocks of materials used in organic photovoltaic applications. All of the acceptor groups studied in this work retain their aromaticity when coupled to thiophene groups as estimated from their aromatic stabilization energies. However, pure chains of acceptors may adopt quinoidal geometry along the conjugated backbone depending on the structure of interest. Spearman rank order correlation has been used to assess the relationships between the computed variables such as highest occupied molecular orbital, lowest unoccupied molecular orbital, E(g), oscillator strength, exciton binding energy, aromatic stabilization energy, etc. The relative acceptor strengths were plotted and electrostatic potential maps were generated to examine the charge distribution over the chromophores. It has been found that there is no correlation between acceptor strength and electron withdrawing ability of the acceptor. Electron rich and highly electronegative atoms within acceptor groups mainly affect the charge distribution over the acceptor geometry. Exciton binding energy increases with the increasing aromatic character of the acceptor group. The acceptor strength is inversely correlated with the oscillator strength for the lowest excited state transition.

We have prepared two series of first-generation thiophene-bridge dendrimers, with either three (3G1) or four (4G1) arms attached to a phenyl core, to elucidate their structure−property relationships. Optical properties were investigated with a combination of steady-state and time-resolved spectroscopic techniques. Steady-state spectroscopic data for the 3-arm dendrimers suggests that the exciton is delocalized over the α-conjugated thiophene segment and the phenyl core, but that the meta-linking of the dendrons prevents their electronic communication. In contrast, conjugation through the core to dendrons in the ortho and para positions is permitted in the 4-arm dendrimers, although the data suggest that the conjugation length does not extend over the full length of the α-conjugated sections of two coupled dendrons. This observation is due to steric interactions between neighboring arms, which forces the arms to twist and bend out of the plane of the phenyl core, and is particularly prevalent in disrupting the conjugation through the ortho positions. As expected, our results show that an increase in the bridge length results in an increase in the conjugation length for both dendrimers, and a subsequent red-shift of the absorption and emission. In addition, an increase in the dendron length results in an increase in the photoluminescence quantum yield and lifetime, suggesting that the ground and excited-state geometries are very similar and that the electronic transition is coupled to fewer vibrational modes.