Ultrafast Dynamics of the First Excited-State of Quasi Monodispersed Single-Walled (9,7) Carbon Nanotubes

Time-resolved two color pump/probe spectroscopy was used to unravel the dynamics of ultrafast decay occurring upon population of the first optical bright excitonic level (E₁₁) in quasi-monodispersed, polymer-wrapped, single-walled (9,7)-carbon nanotubes (SWNTs) in toluene at room temperature. After resonant E₁₁ excitation, transfer of population to at least one optically dark level near E₁₁ was observed to take place within the first picosecond. In addition, phonon-assisted E₁₁-excitation led to transients similar to those observed upon resonant E₁₁-excitation indicating ultrafast vibrational relaxation convoluted with the temporal resolution of 60 fs

Elucidating the Early Steps in Photoinitiated Radical Polymerization via Femtosecond Pump–Probe Experiments and DFT Calculations

The excited states and dynamics of the three triplet radical photoinitiators benzoin (2-hydroxy-1,2-diphenylethanone, Bz), 2,4,6-trimethylbenzoin (2-hydroxy-1-mesityl-2-phenylethanone, TMB), and mesitil (1,2-bis­(2,4,6-trimethylphenyl)-1,2-ethanedione, Me)employed in our previous studies for quantifying net initiation efficiencies in pulsed laser polymerization with methacrylate monomers [Voll, D.; Junkers, T.; Barner-Kowollik, C. <i>Macromolecules</i> <b>2011</b>, <i>44</i>, 2542–2551]are investigated via both femtosecond transient absorption (TA) spectroscopy and density functional theory (DFT) methods to elucidate the underlying mechanisms causing different initiation efficiencies when excited at 351 nm. Bz and TMB are found to have very similar properties in the calculated excited states as well as in the experimentally observed dynamics. After excitation into the first excited singlet state (S₁) Bz and TMB undergo rapid intersystem crossing (ISC). The ISC can compete with ultrafast internal conversion (IC) processes because an excited triplet state (T_<i>n</i>) of nearly the same energy is present in both cases. ISC is therefore the dominating depopulation channel of S₁, and subsequent α-cleavage to produce radicals takes place on the picosecond time scale. In contrast, Me is excited into the second excited singlet state (S₂). In this case no isoenergetic triplet state is available, which inhibits ISC from competing with ultrafast deactivation processes. ISC is therefore assigned to be a minor deactivation channel in Me. Employing these findings, quantitative photoinitiation efficiency relations of Bz, TMB, and Me obtained by pulsed laser polymerization can be directly correlated with the relative TA intensities found in the femtosecond experiments. The ISC efficiency is thus a critical parameter for evaluating the overall photoinitiation efficiency and demonstrates that the employment of the herein presented method represents a powerful tool for attaining a quantitative picture on the suitability of a photoinitiator

An In-Depth Mechanistic Investigation of the Radical Initiation Behavior of Monoacylgermanes

Five <i>para</i>-substituted monoacyltrimethylgermane derivatives, i.e., <i>p</i>-fluorobenzoyl­trimethylgermane (pFBG, λ_max = 405 nm), <i>p</i>-methoxy­benzoyltrimethyl­germane (pMBG, λ_max = 397 nm), benzoyltrimethyl­germane (pHBG, λ_max = 409 nm), <i>p</i>-cyanobenzoyl­trimethylgermane (pCBG, λ_max = 425 nm), and <i>p</i>-nitrobenzoyl­trimethylgermane (pNBG, λ_max = 429 nm) are investigated via a combination of pulsed laser polymerization with subsequent electrospray ionization and mass spectrometry (PLP-ESI-MS) as well as femtosecond transient absorption spectroscopy. The relative initiation efficiencies of the initiating benzoyl radical fragments of pFBG, pMBG, and pHBG are determined using PLP-ESI-MS. The <i>para</i>-substituted derivatives with the electron-donating groups, pFBG and pMBG, display a factor 1.5 and 1.3, respectively, superior overall initiation efficiency compared to the unsubstituted pHBG. In contrast, the derivatives pCBG and pNBG carrying electron-withdrawing groups display only weak initiation behavior at a factor 4 higher total energy of ∼112 J (∼28 J for typical PLP experiments with pMBG, pFBG, and pHBG at ∼320 J and 90 000 pulses). The differences in the initiation efficiencies are representative for two classes of monoacyltrimethyl­germane initiators, i.e., efficient initiators and weak initiators, each distinct in their specific radical cleavage mechanism. The efficient initiators pMBG, pFBG, and pHBG show an ultrafast intersystem crossing within 2–4 ps after pulse irradiation and subsequent formation of benzoyl and trimethylgermyl radical fragments. In contrast, the weak initiators pCBG and pNBG relax to the ground state after photoexcitation via a dominating ultrafast internal conversion (IC) within 13 and 2 ps, respectively, disallowing effective initiation under typical PLP conditions (∼320 J/pulse with 90 000 pulses resulting in ∼28 J total energy per sample). pCBG features weak initiation behavior additionally forming methyl and <i>p</i>-cyanobenzoyl­dimethylgermyl radicals at a factor 4 higher total energy of ∼112 J. Consistent with a considerably faster IC relaxation, pNBG features a factor 10 weaker monomer conversion than pCBG

Study of Model Systems for Bilirubin and Bilin Chromophores: Determination and Modification of Thermal and Photochemical Properties

Bilin chromophores and bilirubin are involved in relevant biological functions such as light perception in plants and as protective agents against Alzheimer and other diseases. Despite their extensive use, a deep rationalization of the main factors controlling the thermal and photochemical properties has not been performed yet, which in turn hampers further applications of these versatile molecules. In an effort to understand those factors and allow control of the relevant properties, a combined experimental and computational study has been carried out for diverse model systems to understand the interconversion between <i>Z</i> and <i>E</i> isomers. In this study, we have demonstrated the crucial role of steric hindrance and hydrogen-bond interactions in thermal stability and the ability to control them by designing novel compounds. We also determined several photochemical properties and studied the photodynamics of two model systems in more detail, observing a fast relaxation of the excited state shorter than 2 ps in both cases. Finally, the computational study allowed us to rationalize the experimental evidence

A Priori Prediction of Mass Spectrometric Product Patterns of Photoinitiated Polymerizations

We introduce a method for the a priori prediction of mass spectra of complex poly­(methyl methacrylate)­s initiated by photoinitiators featuring multiple cleavage points. The method is based on permutation mathematics using multinomial coefficients to predict the probability of each poly­(methyl methacrylate) species’ isotopic pattern contribution to the overall mass spectrum. The method assumes a statistical behavior for the cleavage of the photoinitiator. The excellent agreement of the predicted mass spectrum based on multinomial coefficients with the experimental mass spectrum confirms a multipoint cleavage mechanism of the assessed photoinitiators. We exemplify our method for the prediction of mass spectra of poly­(methyl methacrylate)­s initiated by four tetraacylgermane derivates and one bisacylgermane, recorded after visible light pulsed-laser polymerization by high resolution Orbitrap electrospray ionization mass spectrometry (ESI-MS). The excellent agreement of our approach with experimental data suggests that a wide array of polymer mass spectra of polymers initiated by initiators capable of multiple cleavage events can be quantitatively predicted

Dimolybdenum Paddlewheel as Scaffold for Heteromultimetallic Complexes: Synthesis and Photophysical Properties

A diphenylphosphine functionalized benzoic acid was applied for the synthesis of a homoleptic dimolybdenum-based metalloligand, exhibiting four symmetrically placed phosphine donor sites. This allowed subsequent treatment with gold­(I), rhodium­(I), and iridium­(I) precursors to obtain early–late heterometallic complexes as well as Lewis acid–base adducts with BH₃. The compounds were in-depth investigated by spectroscopic techniques, single-crystal X-ray diffraction, and femtosecond laser spectroscopy. The coordination of different metal fragments to the dimolybdenum metalloligand leads to a fine-tuning of the system’s optical properties, which correlates well with fluorescence quantum yield measurements. Nevertheless, triplet dynamics still remain the dominating channel in these systems with an intersystem crossing time constant below 1 ps

Ultrafast Dynamics of <i>o</i>‑Nitrophenol: An Experimental and Theoretical Study

The photolysis of <i>o</i>-nitrophenol (<i>o</i>-NP), a typical push–pull molecule, is of current interest in atmospheric chemistry as a possible source of nitrous acid (HONO). To characterize the largely unknown photolysis mechanism, the dynamics of the lowest lying excited singlet state (S₁) of <i>o</i>-NP was investigated by means of femtosecond transient absorption spectroscopy in solution, time-resolved photoelectron spectroscopy (TRPES) in the gas phase and quantum chemical calculations. Evidence of the unstable <i>aci</i>-nitro isomer is provided both in the liquid and in the gas phase. Our results indicate that the S₁ state displays strong charge transfer character, which triggers excited state proton transfer from the OH to the NO₂ group as evidenced by a temporal shift of 20 fs of the onset of the photoelectron spectrum. The proton transfer itself is found to be coupled to an out-of-plane rotation of the newly formed HONO group, finally leading to a conical intersection between S₁ and the ground state S₀. In solution, return to S₀ within 0.2–0.3 ps was monitored by stimulated emission. As a competitive relaxation channel, ultrafast intersystem crossing to the upper triplet manifold on a subpicosecond time scale occurs both in solution and in the gas phase. Due to the ultrafast singlet dynamics, we conclude that the much discussed HONO split-off is likely to take place in the triplet manifold