The fundamental influence of the structure and substitution of radical photoinitiators was investigated via a trifold combination of pulsed-laser polymerization with subsequent electrospray-ionization mass spectrometry (PLP-ESI-MS), femtosecond transient absorption (fs-TA) spectroscopy, and quantum chemistry. For the first time, a library of benzoin-derived photoinitiators with varied substitution patterns was synthesized. In the PLP-ESI-MS study, different photoinitiators were compared pairwise in so-called cocktail experiments, enabling the direct comparison of their initiation efficiency. In the fs-TA experiments, the transient response was observed after UV excitation in the visible spectral region, allowing for a description of excited state dynamics, which was analyzed with the aid of TD-DFT calculations. Ab initio calculations were undertaken to determine the reactivity of the radical fragments generated from these photoinitiators and to quantify the influence of various substituents on the rate of addition to monomer. In summary, the influence of the substituent on the initiation efficiency, intersystem crossing (ISC) behavior, excited state dynamics, and the extinction coefficients were analyzed. Hence, relaxation pathways and reaction mechanisms were optimized to explain disparate initiation efficiencies of a wide range of newly designed photoinitiators with varying substitution patterns. The strongly divergent absorptivities of the different photoinitiators and their corresponding initiation efficiencies underline that the absorptivity of a molecule is by no means an unequivocal measure for its reactivity when excited at a specific wavelength. In fact, the most efficient initiators are governed by one nπ* singlet state with a very low extinction coefficient at the excitation wavelength and one or two triplet states with nπ* character.

Five para-substituted monoacyltrimethylgermane derivatives, i.e., p-fluorobenzoyl­trimethylgermane (pFBG, λmax = 405 nm), p-methoxy­benzoyltrimethyl­germane (pMBG, λmax = 397 nm), benzoyltrimethyl­germane (pHBG, λmax = 409 nm), p-cyanobenzoyl­trimethylgermane (pCBG, λmax = 425 nm), and p-nitrobenzoyl­trimethylgermane (pNBG, λmax = 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 para-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 p-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.

We report the photophysical properties of diarylethene‐based photoswitches (DAE) which were incorporated into the backbones of macrocyclic peptides of variable sizes (6, 10 and 14 amino acids: cDAE‐6, cDAE‐10, and cDAE‐14). The insertion leads to a consistent bathochromic shift in UV/Vis absorption bands, a considerable peptide‐size‐dependent fluorescence intensity increase (up to a factor of 1.7), and a drastically reduced photoisomerization efficiency leading to formation of the ring‐closed DAE fragment. The compounds were studied by time‐resolved photoinduced broadband absorption spectroscopy, revealing a ring closure reaction for the unconstrained DAE reference system and, to a lesser extent, for cyclic cDAE‐14. The smallest cyclic peptide cDAE‐6 does not undergo any photoisomerization process at all. Furthermore, the DAE fragment ring closure reaction in the reference system is concurrent with intersystem crossing that is complete within a few picoseconds. Further analysis showed dominant triplet absorption throughout the observed spectral window (350–700 nm) in all macrocyclic systems. As a consequence, ring closure is favored in the unconstrained system in contrast to the cyclic peptides. Our studies demonstrate the importance of understanding the population dynamics of DAE rotamers when designing efficiently photocontrollable diarylethene‐based peptidomimetics.

Early-time dynamics of nitroaromatics and its coressponding bases can give valuable insights into photo-induced reactions relevant to atmospheric and environmental processes. In this work, femtosecond broadband absorption spectroscopy between 350 and 700 nm has been applied to explore the ultrafast dynamics of o-, p- and m-nitrophenol anions (NP-) in basic organic and aqueous solution. Excitation at 400 nm promotes these compounds into the first bright electronic singlet state, which is a charge-transfer state. A surprising finding for all nitrophenolates was a characteristic, spectrally broad stimulated emission (SE) from the electronically excited state into the ground state. The corresponding lifetime was on the order of a few hundred femtoseconds for o- and p-NP- while it was roughly ten times larger for m-NP-. In line with earlier observations, the SE is governed by an out-of-plane torsional motion of the nitro group, leading to a close energetic approach of the relevant electronically excited singlet and ground states. Subsequent dynamics can be assigned to excited state absorption and ground state relaxation due to energy dissipation of the vibrational modes to the solvent that occur for up to several tens of picoseconds. No longer-lasting transient absorption (TA) was found; instead, a complete recovery of the ground state bleaching was observed indicating that triplet state relaxation is either not significantly involved in this spectral part or shifted to other regions. In the aqueous system, time constants for all processes are much smaller than in organic solution, a fact that can be explained by the larger dipole moment of the solvent and the correspondingly stronger intermolecular coupling between NP- and the aqueous solvent.

Bis(mesitoyl)phosphinic acid and its sodium salt display a unique photo-induced reactivity: both derivatives stepwise release two mesitoyl radicals and, remarkably, metaphosphorous acid (previously postulated as transient species in the gas phase), providing a new phosphorus-based reagent.

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 BH. 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.

N,N‐dimethylaminobenzophenone (DMABP) represents an intramolecular donor–π–acceptor system resulting in luminescent properties that are highly sensitive to the local surrounding due to twisted intramolecular charge transfer (TICT). In this study, DMABP was covalently linked to a single (ss) or double‐stranded (ds) DNA and studied by a combination of UV/Vis absorption and fluorescence spectroscopy as well as femtosecond pump–probe measurements. As a result of embedding DMABP into a DNA environment, a drastic increase of the fluorescence quantum yield (QY) by a factor of more than 100 was detected, as further evidenced by an increase of the lifetime of the relevant excited states from less than 10 ps to more than 100 ps. A direct comparison between the ss‐ and ds‐DNA systems further demonstrates the high sensitivity to the surrounding area by means of a two‐fold difference in QY and fluorescence lifetimes. As a consequence, the respective DNA moiety (stacking and folding knot) affects significantly the competition between radiationless relaxation processes and fluorescence by interacting with DMABP.

The C-nucleoside based on the hydroxyquinoline ligand (Hq) is complementary to itself and forms stable Hq-Hq pairs in double-stranded DNA. These artificial Hq-Hq pairs may serve as artificial electron carriers for long-range photoinduced electron transfer in DNA, as elucidated by a combination of gel electrophoretic analysis of irradiated samples and time-resolved transient absorption spectroscopy. For this study, the Hq-Hq pair was combined with a DNA-based donor-acceptor system consisting of 6-N,N-dimethylaminopyrene conjugated to 2'-deoxyuridine as photoinducible electron donor, and methyl viologen attached to the 2'-position of uridine as electron acceptor. The Hq radical anion was identified in the time-resolved measurements and strand cleavage products support its role as an intermediate charge carrier. Hence, the Hq-Hq pair significantly enhances the electron hopping capability of DNA compared to natural DNA bases over long distances while keeping the self-assembly properties as the most attractive feature of DNA as a supramolecular architecture.