Ab initio molecular dynamics of thiophene: the interplay of internal conversion and intersystem crossing.

The fast and slow components of the relaxation of photoexcited thiophene have been investigated by means of SHARC (surface hopping including arbitrary couplings) molecular dynamics based on multiconfiguration electronic structure calculations. Triplet states are included to ascertain their role in the relaxation process. After thiophene is excited to the S1 state, ultrafast dynamics (τfast = 96 fs) initiates a ring opening due to cleavage of a carbon sulfur bond and simultaneous ring puckering. This time constant is in agreement with previous experimental and theoretical studies. The subsequent dynamics of the open-ring structures is characterized by the interplay of internal conversion and intersystem crossing. For the open-ring structures, the S0, S1, T1 and T2 states are nearly degenerate and the spin-orbit couplings are large. The underlying potential energy surface is flat and long-lived open-ring structures in the singlet as well as in the triplet states are formed. Both the participation of triplet states and the shape of the energy surface explain the experimentally observed slow ring closure in the ground state.

Unravelling Photochemical Relationships Among Natural Products from Aplysia dactylomela.

Aplydactone (1) is a brominated ladderane sesquiterpenoid that was isolated from the sea hare Aplysia dactylomela together with the chamigranes dactylone (2) and 10-epi-dactylone (3). Given the habitat of A. dactylomela, it seems likely that 1 is formed from 2 through a photochemical [2 + 2] cycloaddition. Here, we disclose a concise synthesis of 1, 2, and 3 that was guided by excited state theory and relied on several highly stereoselective transformations. Our experiments and calculations confirm the photochemical origin of 1 and explain why it is formed as the sole isomer. Irradiation of 3 with long wavelength UV light resulted in a [2 + 2] cycloaddition that proceeded with opposite regioselectivity. On the basis of this finding, it seems likely that the resulting regioisomer, termed "8-epi-isoaplydactone", could also be found in A. dactylomela.

Deactivation pathways of thiophene and oligothiophenes: internal conversion versus intersystem crossing.

Oligothiophenes and polythiophenes are building blocks of organic-based energy conversion materials. Therefore the lifetime of the excited states plays a central role. As a first step to understand the factors influencing the performance, we investigated the deactivation processes from the first excited state S1 of thiophene and small oligothiophenes containing up to four rings using quantum chemical calculations. For thiophene a low-lying S1/S0 conical intersection seam is easily accessible and drives the fast internal conversion. In oligothiophenes barriers inhibit this passage while deactivation pathways via intersystem crossing channels open. The first one is responsible for the high triplet quantum yields and takes place shortly after the Franck-Condon region. The second one occurs in the vicinity of a local S1 minimum. The calculated spin-orbit coupling strength together with the singlet-triplet energy gaps can explain the decreasing triplet and increasing fluorescence quantum yields for growing chain length. From the triplets the ground state is reachable by inter-ring torsions and T1/S0 intersections. The present results allow a deeper understanding of the deactivation pathways of thiophene and small oligothiophenes and are of potential interest for the photophysics of longer oligothiophenes and polythiophenes used in optical devices.

Hole-transfer induced energy transfer in perylene diimide dyads with a donor-spacer-acceptor motif.

We investigate the photoinduced dynamics of perylene diimide dyads based on a donor-spacer-acceptor motif with polyyne spacers of varying length by pump-probe spectroscopy, time resolved fluorescence, chemical variation and quantum chemistry. While the dyads with pyridine based polyyne spacers undergo energy transfer with near-unity quantum efficiency, in the dyads with phenyl based polyyne spacers the energy transfer efficiency drops below 50%. This suggests the presence of a competing electron transfer process from the spacer to the energy donor as the excitation sink. Transient absorption spectra, however, reveal that the spacer actually mediates the energy transfer dynamics. The ground state bleach features of the polyyne spacers appear due to the electron transfer decay with the same time constant present in the rise of the ground state bleach and stimulated emission of the perylene energy acceptor. Although the electron transfer process initially quenches the fluorescence of the donor it does not inhibit energy transfer to the perylene energy acceptor. The transient signatures reveal that electron and energy transfer processes are sequential and indicate that the donor-spacer electron transfer state itself is responsible for the energy transfer. Through the introduction of a Dexter blocker unit into the spacer we can clearly exclude any through bond Dexter-type energy transfer. Ab initio calculations on the donor-spacer and the donor-spacer-acceptor systems reveal the existence of a bright charge transfer state that is close in energy to the locally excited state of the acceptor. Multipole-multipole interactions between the bright charge transfer state and the acceptor state enable the energy transfer. We term this mechanism coupled hole-transfer FRET. These dyads represent a first example that shows how electron transfer can be connected to energy transfer for use in novel photovoltaic and optoelectronic devices.

Photochemical formation of intricarene.

Sunlight is the ultimate driver of biosynthesis but photochemical steps late in biosynthetic pathways are very rare. They appear to play a role in the formation of certain furanocembranoids isolated from Caribbean corals. One of these compounds, intricarene, has been suspected to arise from an intramolecular 1,3-dipolar cycloaddition involving an oxidopyrylium. Here we show, by a combination of experiments and theory, that the oxidopyrylium forms under photochemical conditions and that its cycloaddition occurs via a triplet state. The formation of a complex by-product can be rationalized by another photochemical step that involves a conical intersection. Our work raises the question whether intricarene is biosynthesized in the natural habitat of the corals or is an artefact formed during workup. It also demonstrates that the determination of exact irradiation spectra, in combination with quantum chemical calculations, enables the rationalization of complex reaction pathways that involve multiple excited states.

Norbornenes in inverse electron-demand Diels-Alder reactions.

Significant differences in the reactivity of norbornene derivatives in the inverse electron-demand Diels-Alder reaction with tetrazines were revealed by kinetic studies. Substantial rate enhancement for the exo norbornene isomers was observed. Quantum-chemical calculations were used to rationalize and support the observed experimental data.

Beyond the van der Lugt/Oosterhoff model: when the conical intersection seam and the S1 minimum energy path do not cross.

The photoinduced ring-opening reaction of cyclohexadiene (CHD) is a textbook example for electrocyclic reactions. In this paper we report the complete "minimum energy path" of the low-lying region of the conical intersection space reaching from the closed to the open ring side. The general role of conical intersections (CoIns) is to provide the locus for ultrafast transfer between electronic states, in the present case, to the closed or open form in the ground state after photoexcitation. The seam was calculated with use of an analytic approach in which the intersection space in the vicinity of a CoIn is described to second order. The topography of the seam was investigated, revealing minimum energy and transition state structures. In addition the energy profile of the seam was rationalized with valence bond (VB) theory. The geometrical changes along the seam have been related to the motions along the excited state minimum energy path (S(1)-IRC-MEP) in a conceptual model highlighting the quasiparallel orientation of seam and IRC-MEP. Our model shows that even though the van der Lugt and Oosterhoff concept predicts the formation of an avoided crossing along the S(1)-IRC-MEP, it provides an incomplete description of the decay process to the ground state. The latter requires, in addition, vibrational motions orthogonal to the MEP, directed toward the CoIn seam.