Ab Initio Calculation of UV-vis Absorption of Parent Mg, Fe, Co, Ni, Cu, and Zn Metalloporphyrins Gaurab Ganguly, Zdenek Havlas, Josef Michl Inorganic Chemistry, 2024 Relativistic restricted active space (RAS) second-order multireference perturbation theory (MRPT2) methods, incorporating spin-orbit (SO) coupling perturbatively via state interaction (SO-MRPT2/RASSCF), were used to reproduce the absorption spectra of parent metalloporphyrins containing the Mg2+, Zn2+, Co2+, Ni2+, Cu2+, or FeCl2+ ions in the 12,500-40,000 cm-1 region. Particular attention was paid to the interaction between the porphyrin ring and the metal 3d electrons in states of different multiplicities (we used metal 3d and double d-shell or 3d' orbitals). For this class of compounds, the N-electron valence state perturbation theory (NEVPT2) method is superior to the complete active space perturbation theory (CASPT2) and successfully reproduces the energies of all four characteristic transitions (Q, B, N, and L) of closed-shell metalloporphyrins. Inclusion of SO coupling was found to have very little effect on excitation energies and oscillator strengths. For FeCl2+ porphyrin, we treated ligand-to-metal charge-transfer (LMCT; π,d), metal ligand field (d,d), and metal-to-ligand charge-transfer (MLCT; d,π*) transitions within the same framework. The broad and intense spectral features associated with its B (Soret) band are attributed to multiconfigurational LMCT (d,π*) bands involving strong metal-ligand orbital mixing.
Electronic States of 2,3-Diamino-1,4-naphthoquinone and Its N-Alkylated Derivatives Jin Wen, Michał Turowski, Paul I. Dron, Jakub Chalupský, Robin Grotjahn, et al. Journal of Physical Chemistry C, 2020 Diaminoquinones with a captodatively stabilized biradicaloid structure are candidates for singlet fission, but few such compounds are known. We report the solution spectroscopy and photophysics of ...
Structure and photophysics of indigoids for singlet fission: Cibalackrot Joseph L. Ryerson, Alexandr Zaykov, Luis E. Aguilar Suarez, Remco W. A. Havenith, Brian R. Stepp, et al. Journal of Chemical Physics, 2019 We report an investigation of structure and photophysics of thin layers of cibalackrot, a sturdy dye derived from indigo by double annulation at the central double bond. Evaporated layers contain up to three phases, two crystalline and one amorphous. Relative amounts of all three have been determined by a combination of X-ray diffraction and FT-IR reflectance spectroscopy. Initially, excited singlet state rapidly produces a high yield of a transient intermediate whose spectral properties are compatible with charge-transfer nature. This intermediate more slowly converts to a significant yield of triplet, which, however, does not exceed 100% and may well be produced by intersystem crossing rather than singlet fission. The yields were determined by transient absorption spectroscopy and corrected for effects of partial sample alignment by a simple generally applicable procedure. Formation of excimers was also observed. In order to obtain guidance for improving molecular packing by a minor structural modification, calculations by a simplified frontier orbital method were used to find all local maxima of singlet fission rate as a function of geometry of a molecular pair. The method was tested at 48 maxima by comparison with the ab initio Frenkel-Davydov exciton model.
Singlet Fission Rate: Optimized Packing of a Molecular Pair. Ethylene as a Model Alexandr Zaykov, Petr Felkel, Eric A. Buchanan, Milena Jovanovic, Remco W. A. Havenith, et al. Journal of the American Chemical Society, 2019 A procedure is described for unbiased identification of all π-electron chromophore pair geometry choices that locally maximize the rate of conversion of a singlet exciton into singlet biexciton (triplet pair), using a simplified version of the diabatic frontier orbital model of singlet fission (SF). The resulting approximate optimal geometries provide insight and are expected to represent useful starting points for searches by more advanced methods. The general procedure is illustrated on a pair of ethylenes as the simplest model of a π-electron system, but it is applicable to pairs of much larger molecules, with dozens of non-hydrogen atoms, and not necessarily planar. We first examine the value of |TA|2, the square of the electronic matrix element for SF with initial excitation fully localized on partner A, on a grid of several billion geometries within the six-dimensional space of physically realizable possibilities. The optimized pair geometries are found to follow the qualitative guidance proposed earlier. In the neighborhood of each local maximum of |TA|2, consideration of mixing with charge-transfer configurations and of excitonic interaction between partners A and B determines the SF energy balance and yields squared matrix elements |T*|2 and |T**|2 for the lower and upper excitonic states S* and S**, respectively. Assuming Boltzmann populations of these states, the geometry is further optimized to maximize k, the sum of the SF rates obtained from Marcus theory, and this reorders the suitable geometries substantially. At 87 pair geometries, the |T*|2 and |T**|2 values are compared with those obtained from high-level ab initio non-orthogonal configuration interaction calculations and found to follow the same trend. Finally, the biexciton binding energy at the optimized geometries is calculated. Altogether 13 significant local maxima of SF rate for a pair of ethylenes are identified in the physically relevant part of space that avoids molecular interpenetration in the hard spheres approximation. The three best geometries are twist-stacked, slip-stacked, and L-shaped. The maxima occur at the (5-dimensional) surfaces of seven 6-dimensional "parent" regions of space centered at physically inaccessible geometries at which the calculated SF rate is very large but the two ethylenes interpenetrate. The results are displayed in interactive graphics. The computer code ("Simple") written for these calculations is flexible in that it permits a choice of performing the search for local maxima in six dimensions on |TA|2, |T*|2, or k. It is available as freeware at https://cloud.uochb.cas.cz/simple.
Molecular Packing and Singlet Fission: The Parent and Three Fluorinated 1,3-Diphenylisobenzofurans Eric A. Buchanan, Jiří Kaleta, Jin Wen, Saul H. Lapidus, Ivana Císařová, et al. Journal of Physical Chemistry Letters, 2019 Crystal structures, singlet fission (SF) rate constants, and other photophysical properties are reported for three fluorinated derivatives of 1,3-diphenylisobenzofuran and compared with those of the two crystal forms of the parent. The results place constraints on the notion that the effects of molecular packing on SF rates could be studied separately from effects of chromophore structural changes by examining groups of chromophores related by weakly perturbing substitution if their crystal structures are different. The results further provide experimental evidence that dimer-based models of SF are not sufficiently general and that trimer- and possibly even higher oligomer-based or many-body models need to be formulated.
Optimal arrangements of tetracene molecule pairs for fast singlet fission Eric A. Buchanan, Zdeněk Havlas, Josef Michl Bulletin of the Chemical Society of Japan, 2019 In search for a qualitative understanding of the effects of molecular packing on singlet fission (SF) rate, a simplified version of the frontier orbital model is described and illustrated on a pair...
