Zdenek Havlas
@cas.cz
Theoretical Chemistry, Institute of Ogranic Chemistry and Biochemistry
Czech Academy of Sciences
RESEARCH INTERESTS
theoretical chemistry
computing
organic chermistry
solar cell
Scopus Publications
Scopus Publications
Joseph L. Ryerson, Joel N. Schrauben, Andrew J. Ferguson, Subash Chandra Sahoo, Panče Naumov, Zdeněk Havlas, Josef Michl, Arthur J. Nozik, and Justin C. Johnson
American Chemical Society (ACS)
Rithwik Tom, Siyu Gao, Yi Yang, Kaiji Zhao, Imanuel Bier, Eric A. Buchanan, Alexandr Zaykov, Zdeněk Havlas, Josef Michl, and Noa Marom
American Chemical Society (ACS)
Zdeněk Havlas and Jiří Kaleta
American Chemical Society (ACS)
Jin Wen, Michał Turowski, Paul I. Dron, Jakub Chalupský, Robin Grotjahn, Toni M. Maier, Steven M. Fatur, Zdeněk Havlas, Justin C. Johnson, Martin Kaupp,et al.
American Chemical Society (ACS)
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 ...
Joseph L. Ryerson, Alexandr Zaykov, Luis E. Aguilar Suarez, Remco W. A. Havenith, Brian R. Stepp, Paul I. Dron, Jiří Kaleta, Akin Akdag, Simon J. Teat, Thomas F. Magnera,et al.
AIP Publishing
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.
Alexandr Zaykov, Petr Felkel, Eric A. Buchanan, Milena Jovanovic, Remco W. A. Havenith, R. K. Kathir, Ria Broer, Zdeněk Havlas, and Josef Michl
American Chemical Society (ACS)
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.
Eric A. Buchanan, Jiří Kaleta, Jin Wen, Saul H. Lapidus, Ivana Císařová, Zdeněk Havlas, Justin C. Johnson, and Josef Michl
American Chemical Society (ACS)
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.
Eric A. Buchanan, Zdeněk Havlas, and Josef Michl
The Chemical Society of Japan
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...
Jin Wen, Bowen Han, Zdeněk Havlas, and Josef Michl
American Chemical Society (ACS)
The previously reported ( Duman et al., J. Org. Chem . 2012 , 77 , 4516 ) calculated state energies of monomeric difluoroborondipyrromethene (BODIPY) and its axial dimer would suggest that these dyes are promising candidates for singlet fission, and the dimer was computed to have an unusual low-lying doubly excited state. We find that these results were affected by the use of an imbalanced active space in multireference calculations and are not correct. Multistate complete-active-space second-order perturbation theory (MS-CASPT2/cc-pVDZ) calculations using an [8,8] (8 electrons in 8 orbitals) active space for the monomer and a [16,16] active space for the dimer reproduce quite well the observed excitation energies of the S1 states of both, and yield T1 excitation energies well in excess of half of the S1 excitation energies. We conclude that neither BODIPY monomer nor its axial dimer would permit exothermic singlet fission and are not worthy of investigation as potentially useful candidates, and that the unusual low-energy doubly excited states of the dimer were artifacts.
Jan Plutnar, Magdaléna Hromadová, Nicolangelo Fanelli, Šárka Ramešová, Zdeněk Havlas, and Lubomír Pospíšil
American Chemical Society (ACS)
Reduction and oxidation of 4,7-dimethyl-1,3-dimethoxybenzimidazolium cation (1) and the related dihydroxy analogue (2) were investigated by electrochemical and in situ spectroelectrochemical methods. Quantum chemical methods were applied to UV–vis spectra in native, reduced, and oxidized forms. Compounds were searched for possible formation of a cation radical and a dication radical suitable for the singlet fission effect. Indeed 1 yields by oxidation the target quinoidal structure. However, the reduction step for both compounds is coupled with very fast dimerization and prevents obtaining a stable target form. The complex mechanism of the reduction process yields electrochemical current oscillation. Estimation of the maximum Ljapunov exponent proves characteristics of the deterministic chaos.
Denisa Geffertová, Syed Tahir Ali, Veronika Šolínová, Marcela Krečmerová, Antonín Holý, Zdeněk Havlas, and Václav Kašička
Elsevier BV
Capillary electrophoresis (CE) and quantum mechanical density functional theory (DFT) were applied to the investigation of the acid-base and electromigration properties of important compounds: newly synthesized derivatives of 5-azacytosine - analogs of efficient antiviral drug cidofovir. These compounds exhibit a strong antiviral activity and they are considered as potential new antiviral agents. For their characterization and application, it is necessary to know their acid-base properties, particularly the acidity constants (pKa) of their ionogenic groups (the basic N3 atom of the triazine ring and the acidic phosphonic acid group in the alkyl chain). First, the mixed acidity constants (pKamix) of these ionogenic groups and the ionic mobilities of these compounds were determined by nonlinear regression analysis of the pH dependence of their effective electrophoretic mobilities. Effective mobilities were measured by CE in a series of background electrolytes in a wide pH range (2.0-10.5), at constant ionic strength (25mM) and constant temperature (25°C). Subsequently, the pKamix values were recalculated to thermodynamic pKa values using the Debye-Hückel theory. The thermodynamic pKa value of the NH+ moiety at the N3 atom of the triazine ring was found to be in the range 2.82-3.30, whereas the pKa of the hydrogenphosphonate group reached values from 7.19 to 7.47, depending on the structure of the analyzed compounds. These experimentally determined pKa values were in good agreement with those calculated by quantum mechanical DFT. In addition, DFT calculations revealed that from the four nitrogen atoms in the 5-azacytosine moiety, the N3 atom of the triazine ring is preferentially protonated. Effective charges of analyzed compounds ranged from zero or close-to-zero values at pH 2 to -2 elementary charges at pH≥9. Ionic mobilities were in the range (-16.7 to -19.1)×10-9m2V-1s-1 for univalent anions and in the interval (-26.9 to -30.3)×10-9m2V-1s-1 for divalent anions.
Eric A. Buchanan, Zdeněk Havlas, and Josef Michl
Elsevier
Abstract After a brief review of electronic aspects of singlet fission, we describe a systematic simplification of the frontier orbital (HOMO/LUMO) model of singlet fission and Davydov splitting in a pair of rigid molecules. In both instances, the model includes electron configurations representing local singlet excitation on either chromophore, charge transfer in either direction, and triplet excitation in both chromophores (biexciton). The resulting equations are simple enough to permit complete searches for local extrema of the square of the electronic matrix element and to evaluate the effect of intermolecular interactions on the exoergicity of singlet fission and on the biexciton binding energy in the six-dimensional space of rigid dimer geometries. The procedure is illustrated on results for the six best geometries for dimers of ethylene and of an indigoid heterocycle with 24 carbon, nitrogen, and oxygen atoms.
Juraj Galeta, Michal Šála, Martin Dračínský, Milan Vrábel, Zdeněk Havlas, and Radim Nencka
American Chemical Society (ACS)
A straightforward synthesis of pyrimido[4,5-d]pyridazines from pyrimidines and tetrazines under basic conditions is reported. Deprotonated, substituted 5-halopyrimidines readily react with variously substituted tetrazines in a highly regioselective manner via a complex reaction pathway, which was supported by DFT calculations. This mechanism leads to the empirically observed regioisomers without going through the conceivable hetaryne intermediate. These results on 5-halopyrimidines led to development of the methodology for preparation of opposite regioisomers based on 6-halopyrimidines.
Joel N. Schrauben, Akin Akdag, Jin Wen, Zdenek Havlas, Joseph L. Ryerson, Millie B. Smith, Josef Michl, and Justin C. Johnson
American Chemical Society (ACS)
Two isomers of both the lowest excited singlet (S1) and triplet (T1) states of the directly para, para'-connected covalent dimer of the singlet-fission chromophore 1,3-diphenylisobenzofuran have been observed. In one isomer, excitation is delocalized over both halves of the dimer, and in the other, it is localized on one or the other half. For a covalent dimer in solution, such "excitation isomerism" is extremely rare. The vibrationally relaxed isomers do not interconvert, and their photophysical properties, including singlet fission, differ significantly.
Peter R. Craig, Zdeněk Havlas, Marianela Trujillo, Pawel Rempala, James P. Kirby, John R. Miller, Bruce C. Noll, and Josef Michl
American Chemical Society (ACS)
Properties of the tetraphenylcyclobutadienecyclopentadienylnickel(II) cation 1 and its tetra-o-fluoro derivative 1a have been measured and calculated. The B3LYP/TZP optimized geometry of the free cation 1 agrees with a single-crystal X-ray diffraction structure except that in the crystal one of the phenyl substituents is strongly twisted to permit a close-packing interaction of two of its hydrogens with a nearby BF4(-) anion. The low-energy parts of the solution electronic absorption and magnetic circular dichroism (MCD) spectra of 1 and 1a have been interpreted by comparison with TD-DFT (B3LYP/TZP) results. Reduction or pulse radiolysis lead to a neutral 19-electron radical, whose visible absorption and MCD spectra have been recorded and interpreted as well. The reduction is facilitated by ∼0.1 V upon going from 1 to 1a. Unsuccessful attempts to prepare several other aryl substituted derivatives of 1 by the classical synthetic route are described in the Supporting Information .
Zdenek Havlas and Josef Michl
Wiley
The choice of chromophores and of their mutual geometrical arrangement for optimized singlet fission (SF) rates are considered. The electronic matrix element that enters the Fermi golden rule for the rate of SF is worked out algebraically for a simple model, but the density of states factor is not analyzed here. The model treats only the highest occupied and lowest unoccupied orbitals of the partners. It provides an approximate formula that requires only the knowledge of the expansion coefficients of these orbitals and of overlap integrals between atomic orbitals on the partners to obtain an estimate of the electronic matrix element. An illustrative application to a pair of ethylene molecules suggests that favored geometries will be those in which one of the AOs on the first ethylene overlaps with both AOs on the second ethylene, while the other AO on the first ethylene overlaps with at least one, and preferably both, AOs of the second ethylene as little as possible.
Zdeněk Havlas, Jin Wen, and Josef Michl
AIP Publishing LLC
Singlet fission (SF) offers an opportunity to improve solar cell efficiency, but its practical use is hindered by the limited number of known efficient materials, limited knowledge of SF mechanism, mainly the relation between the dimer structure and SF efficiency and diffusion of the triplet states allowing injection of electrons into the solar cell semiconductor band. Here we report on our attempt to design new classes of chromophores and to study the relation between the structure and SF efficiency.
Jin Wen, Zdenĕk Havlas, and Josef Michl
American Chemical Society (ACS)
Singlet fission offers an opportunity to improve solar cell efficiency, but its practical use is hindered by the limited number of known efficient materials. We look for chromophores that satisfy the desirable but rarely encountered adiabatic energy conditions, E(T2) - E(S0) > E(S1) - E(S0) ≈ 2[E(T1) - E(S0)], and are small enough to permit highly accurate calculations. We provide a rationale for the use of captodative biradicaloids, i.e., biradicals stabilized by direct interaction between their radical centers, which carry both an acceptor and a donor group. A computation of vertical excitation energies of 14 structures of this type by time-dependent density functional theory (TD-DFT) yielded 11 promising candidates. The vertical excitation energies from S0 and T1 were recalculated by complete-active-space second-order perturbation theory (CASPT2), and five of the compounds met the above energy criteria. Their adiabatic excitation energies from the S0 into the S1, S2, T1, and T2 excited states were subsequently calculated, and three of them look promising. For 2,3-diamino-1,4-benzoquinone, adiabatic E(T1) and E(S1) energies were close to optimal (1.12 and 2.23 eV above the S0 ground state, respectively), and for its more practical N-peralkylated derivative they were even lower (0.63 and 1.06 eV above S0, respectively). PCM/CASPT2 results suggested that the relative energies can be further tuned by varying the polarity of the environment.
Gregg S. Kottas, Thierry Brotin, Peter F. H. Schwab, Kamal Gala, Zdeněk Havlas, James P. Kirby, John R. Miller, and Josef Michl
American Chemical Society (ACS)
The known (tetraphenyl-η4-cyclobutadiene)-η5-cyclopentadienylcobalt (1) and a series of its new substituted derivatives have been prepared. The electronic states of a few representatives have been characterized by absorption and magnetic circular dichroism. Time-dependent density functional theory has been used to arrive at spectral assignments for several prominent low-energy bands. The absorption spectra of the radical ions of 1 have also been recorded.
Joseph L. Ryerson, Joel N. Schrauben, Andrew J. Ferguson, Subash Chandra Sahoo, Panče Naumov, Zdenĕk Havlas, Josef Michl, Arthur J. Nozik, and Justin C. Johnson
American Chemical Society (ACS)
Polycrystalline thin films of 1,3-diphenylisobenzofuran (1) with a morphology referred to here as α exhibit highly efficient singlet fission, producing two triplet states for every absorbed photon at 77 K, and about 1.4 triplet states per absorbed photon at room temperature. However, the triplet yield depends strongly on the specific crystalline form of 1, and for the morphology referred to as β the triplet yields are roughly an order of magnitude smaller. In this study, α, β, and mixed α/β films of 1 are prepared by thermal evaporation and solution drop-casting, and the structural and photophysical differences that may account for the very different triplet quantum yields are explored. The crystallites of 1 in thin films have been identified with two bulk crystal polymorphs grown from solution and structurally characterized. Analysis of absorption spectra of the films reveals a 600 cm–1 blue shift in the onset and a unique spectral profile for the form α crystallites as compared to form β. Intermolecular...
Justin C. Johnson, Akin Akdag, Matibur Zamadar, Xudong Chen, Andrew F. Schwerin, Irina Paci, Millicent B. Smith, Zdeněk Havlas, John R. Miller, Mark A. Ratner,et al.
American Chemical Society (ACS)
In order to identify optimal conditions for singlet fission, we are examining the photophysics of 1,3-diphenylisobenzofuran (1) dimers covalently coupled in various ways. In the two dimers studied presently, the coupling is weak. The subunits are linked via the para position of one of the phenyl substituents, in one case (2) through a CH2 linker and in the other (3) directly, but with methyl substituents in ortho positions forcing a nearly perpendicular twist between the two joint phenyl rings. The measurements are accompanied with density functional theory (DFT) and time-dependent DFT (TD-DFT) calculations. Although in neat solid state, 1 undergoes singlet fission with a rate constant higher than 10(11) s(-1); in nonpolar solutions of 2 and 3, the triplet formation rate constant is less than 10(6) s(-1) and fluorescence is the only significant event following electronic excitation. In polar solvents, fluorescence is weaker because the initial excited singlet state S1 equilibrates by sub-nanosecond charge transfer with a nonemissive dipolar species in which a radical cation of 1 is attached to a radical anion of 1. Most of this charge transfer species decays to S0, and some is converted into triplet T1 with a rate constant near 10(8) s(-1). Experimental uncertainties prevent an accurate determination of the number of T1 excitations that result when a single S1 excitation changes into triplet excitation. It would be one if the charge-transfer species undergoes ordinary intersystem crossing and two if it undergoes the second step of two-step singlet fission. The triplet yield maximizes below room temperature to a value of roughly 9% for 3 and 4% for 2. Above ∼360 K, some of the S1 molecules of 3 are converted into an isomeric charge-transfer species with a shorter lifetime, possibly with a twisted intramolecular charge transfer (TICT) structure. This is not observed in 2.
Akin Akdag, Zdeněk Havlas, and Josef Michl
American Chemical Society (ACS)
Of the five small biradicaloid heterocycles whose S(1), S(2), T(1), and T(2) adiabatic excitation energies were examined by the CASPT2/ANO-L-VTZP method, two have been found to meet the state energy criterion for efficient singlet fission and are recommended to the attention of synthetic chemists and photophysicists.
Matthew G. Fete, Zdeněk Havlas, and Josef Michl
American Chemical Society (ACS)
Cs salts of four of the title anions were prepared by fluorination of salts of partly methylated (n = 11, 10) or partly methylated and partly iodinated (n = 6, 5) CB(11)H(12)(-) anions. The CH vertex is acidic, and in the unhindered anion with n = 6 it has been alkylated. Neat Cs(+)[1-H-CB(11)(CF(3))(11)](-) is as treacherously explosive as Cs(+)[CB(11)(CF(3))(12)](-), but no explosions occurred with the salts of the other three anions. BL3YP/6-31G* gas-phase electron detachment energies of the title anions are remarkably high, 5-8 eV. Treated with NiF(3)(+) in anhydrous liquid HF at -60 °C, anions with n = 11 or 10 resist oxidation, whereas anions with n = 6 or 5 are converted to colored EPR-active species, presumably the neutral radicals [HCB(11)(CF(3))(n)F(11-n)](•). These are stable for hours at -60 °C after extraction into cold perfluorohexane or perfluorotri-n-butylamine solutions. On warming to -20 °C in a Teflon or quartz tube, the color and EPR activity disappear, and the original anions are recovered nearly quantitatively, suggesting that the radicals oxidize the solvent.