Computational Studies of Photophysical and Aromatic Properties of Porphyrinoids and Acenes

Expanded porphyrins, porphyrinoids, porphycenes and N-doped nanographenes are multi- ring molecules whose aromaticity cannot be easily predicted based on NMR chemical shifts. The magnetically induced current-density susceptibility and the ring-current path- ways have been elucidated for these molecules at ab initio and density functional theory (DFT) levels using the gauge-including magnetically induced current (GIMIC) method. Calculations showed that the lowest electronic transition of the antiaromatic molecules are purely magnetic transitions which is also the main reason why these molecules sustain large net paratropic ring currents. The photophysical properties of expanded porphyrins, acenes and pyrene have also been studied using quantum mechanical methods. The absorption spectrum including vibrational bands of the acenes and pyrene were simulated and compared with those obtained in high-resolution measurements of the absorption spectra in the visible range. The vibra- tional contributions to the absorption spectra were obtained by using a time-generating function approach, which is computationally faster than alternative approaches. The 0-0 transition energies of acenes and pyrene are reported. Various vibrational modes contributing to the vibrational fine structure of these molecules have been identified. The quantum yield of luminescence was determined for the expanded porphyrins by calculating rate constants for radiative and non-radiative transitions between excited electronic states and the ground stateExpanderade porfyriner, porfyrinoider, porfycener och kvävedopade nanografener är molekyler som består av ett flertal ringar. De här molekylernas aromaticitet kan inte enkelt bestämmas utifrån NMR kemiska skift. Den magnetiskt inducerade strömsuskeptibiliteten och ringströmmarnas rutter har utforskats för de här molekylerna med ab initio-metoder och täthetsfunktionalteorinivå med hjälp av beräkningsmetoden GIMIC (Gauge-including magnetically induced currents). Beräkningarna visar att de lägst liggande elektroniska övergångarna är helt magnetiska i antiaromatiska molekyler och det här är orsaken till att sådana molekyler har starka paratropiska ringströmmar. I arbetet har de fotofysikaliska egenskaperna för expanderade porfyriner, acener och pyren har också studerats med kvantmekaniska metoder. Aceners och pyrens absorptionsspektrum och de vibrationella banden har simulerats och jämförts med spektrum som är uppmätta med hög resolution inom våglängdsområdet för synligt ljus. De vibrationella bidragen till absorptionsspektrumen bestämdes med en tidsgenereringsmetod som är snabbare än alternativa tillvägagångssätt. Energierna för 0-0 -övergångarna för acener och pyren har bestämts. Därtill har vibrationsmoder som bidrar till vibrationsfinstrukturen i spektrumen för de här molekylerna identifierats. Kvantutbytet för luminescensen för expanderade porfyrener bestämdes i arbetet genom att beräkna hastighetskonstanter för både radiativa och icke-radiativa övergångar mellan exciterade elektroniska tillstånd och grundtillstå

Aromatic Pathways in Porphycene Derivatives Based on Current-Density Calculations

Magnetically induced current densities are reported for porphycenes at the density functional theory level using gauge-including atomic orbitals, which ensure gauge-origin independence and a fast basis-set convergence of the current densities. We have analyzed the current densities by using the gauge-including magnetically induced current (GIMIC) method. The porphycenes are found to be aromatic. They sustain strong diatropic ring currents. The ring-current pathways have been obtained by integrating the strength of the current density passing selected bonds. The calculations show that the ring current of the porphycenes divides into an outer and inner pathways at the pyrrolic rings. Thus, the ring current involves all 26 pi electrons of the porphycenes, which is similar to the ring current of porphin. No significant local ring currents are sustained by the pyrrolic rings. Dihydroporphycene with four inner hydrogens is found to be antiaromatic with weakly aromatic pyrrolic rings. The six-membered rings in benzoporphycene sustain local paratropic ring currents. The global ring current of dibenzoporphycene divides into an outer and inner pathway at the benzoic rings. Interactions between the inner hydrogen and the neighbor nitrogen are found to be more significant for differences in the H-1 NMR shieldings than variations in global ring-current strengths. We found that the antiaromatic dihydroporphycene has a larger HOMO-LUMO gap but a smaller optical gap than the aromatic porphycene.Peer reviewe

Computational Studies of the Aromatic and Photophysical Properties of Carbaporphyrinoid

Porphyrins and porphyrin derivatives are naturally occurring molecules, whereas carbaporphyrinoids are synthesized porphyrin derivatives. They have received much attention in recent years by the scientific community due to their diverse potential applications in technological developments such as molecular electronic devices and conversion of solar energy. However, the full utilization of this class of compounds can not be realized without an in-depth understanding of their chemical and physical properties. Two of such properties are aromaticity and optical properties. In this thesis, the aromatic properties and the light absorption spectra in the ultraviolet and visible (UV/Vis) range have been studied computationally for some recently synthesized carbaporphyrins and carbachlorins using first-principle computational approaches. In the first part of the thesis, the background of carbaporphyrinoids and some examples of naturally occurring porphyrins and porphyrin derivatives are delineated. The second and third part review theoretical and computational methods that are employed in studies of the molecular aromaticity and electronic excitation spectra of molecules. The computational studies of magnetically induced current densities and electronic excitation energies are discussed in the fourth chapter. The obtained results are also presented in chapter four and the main conclusions are summarized in the last chapter. The study shows that all the carbaporphyrinoids studied sustain a magnetically induced ring current in the porphyrin macro ring. This indicates that they are aromatic according to the ring-current criterion. However, the calculated ring-current pathways differ from those predicted from the nucleus independent chemical shift (NICS) calculations and the current pathways deduced from H NMR spectroscopy studies. The vertical excitation energies which is akin to the ultraviolet-visible spectrum obtained experimentally for some of the selected carbaporphyrinoids also showed deviations from those of the experimental values. These deviations can be ascribed to solvent effects as in the calculation of the vertical excitation energies, solvent effects were not accounted for

Computational Studies of Aromatic and Photophysical Properties of Expanded Porphyrins

Magnetically induced current densities and ring-current pathways have been calculated at density functional theory (DFT) and second-order Moller-Plesset perturbation theory (MP2) levels of theory for a set of expanded porphyrins consisting of five or six pyrrolic rings. The studied molecules are sapphyrin, cyclo [6]pyrrole, rubyrin, orangarin, rosarin, and amethyrin. Different functionals have been employed to assess the functional dependence of the ring-current strength susceptibility. Vertical singlet and triplet excitation energies have been calculated at the second-order approximate coupled cluster (CC2), expanded multiconfigurational quasi-degenerate perturbation theory (XMC-DPT2), and time-dependent density functional theory levels. The lowest electronic transition of the antiaromatic molecules was found to be pure magnetic transitions providing an explanation for the large paratropic contribution to the total current density. Rate constants for different nonradiative deactivation channels of the lowest excited states have been calculated yielding lifetimes and quantum yields of the lowest excited singlet and triplet states. The calculations show that the spin-orbit interaction between the lowest singlet (S-0) and triplet (T-1) states of the antiaromatic molecules is strong, whereas for the aromatic molecule the spin-orbit coupling vanishes. The experimentally detected fluorescence from S-2 to S-0 of amethyrin has been explained. The study shows that there are correlations between the aromatic character and optical properties of the investigated expanded porphyrins.Peer reviewe

Calculation of vibrationally resolved absorption spectra of acenes and pyrene

The absorption spectra of naphthalene, anthracene, pentacene and pyrene in the ultraviolet-visible (UV-Vis) range have been simulated by using an efficient real-time generating function method that combines calculated adiabatic electronic excitation energies with vibrational energies of the excited states. The vertical electronic excitation energies have been calculated at the density functional theory level using the PBE0 functional and at the second-order approximate coupled-cluster level (CC2). The absorption spectra have been calculated at the PBE0 level for the studied molecules and at the CC2 level for naphthalene. The transition probabilities between vibrationally resolved states were calculated by using the real-time generating function method employing the full Duschinsky formalism. The absorption spectrum for naphthalene calculated at the PBE0 and CC2 levels agrees well with the experimental one after the simulated spectra have been blue-shifted by 0.48 eV and 0.12 eV at the PBE0 and CC2 level, respectively. The absorption spectra for anthracene, pentacene and pyrene simulated at the PBE0 level agree well with the experimental ones when they are shifted by 0.49 eV, 0.57 eV and 0.46 eV, respectively. The strongest transitions of the main vibrational bands have been assigned.Peer reviewe

New insights into aromatic pathways of carbachlorins and carbaporphyrins based on calculations of magnetically induced current densities

Magnetically induced current densities have been calculated and analyzed for a number of synthesized carbachlorins and carbaporphyrins using density functional theory and the gauge including magnetically induced current (GIMIC) method. Aromatic properties have been determined by using accurate numerical integration of the current flow yielding reliable current strengths and pathways that are related to the degree of aromaticity and the aromatic character of the studied molecules. All investigated compounds are found to be aromatic. However, the obtained aromatic pathways differ from those previously deduced from spectroscopic data and magnetic shielding calculations. For all studied compounds, the ring current divides into an outer and an inner branch at each pyrrolic subring, showing that all pi-electrons of the pyrrolic rings take part in the delocalization pathway. The calculations do not support the common notion that the main share of the current takes the inner route at the pyrrolic rings without an inner hydrogen and follows an 18 pi aromatic pathway. The aromatic pathways of the investigated carbaporphyrins and carbachlorins are very similar, since the current strength via the C-beta=C-beta' bond of the cyclopentadienyl ring of the carbaporphyrins is almost as weak as the current density passing the corresponding saturated C beta-C-beta' bond of the carbachlorins.Peer reviewe

Interplay of Aromaticity and Antiaromaticity in N-Doped Nanographenes

The aromaticity of three nonplanar, fully conjugated aza-nanographenes built around a pyrrolo[3,2-b]pyrrole core is assessed through the application of two different computational procedures—GIMIC and NICS. We examine the calculated magnetically induced current densities (GIMIC) and nucleus-independent chemical shifts (NICS). The structural differences between these three apparently similar molecules lead to significantly different aromatic properties. GIMIC analysis indicates that the peripheral diatropic ring current of 3.9 nA/T for the studied bowl-shaped diaza-nanographene is the strongest, followed by the double [6]helicene which lacks seven-membered rings, and is practically nonexistent for the double [5]helicene possessing seven-membered rings. The biggest difference however is that in the two not-fully-fused molecules, the central pyrrole rings possess a significant diatropic current of about 4.1 nA/T, whereas there is no such current in the diaza-nanographene. Moreover, the antiaromaticity of the seven-membered rings is increasing while moving from double [5]helicene to diaza-nanographene (from −2.4 to −6.0 nA/T). The induced currents derived from NICSπ,zz-XY-scan analysis for all of the studied systems are in qualitative agreement with the GIMIC results. Subtle differences may originate from σ-electron currents in GIMIC or inaccuracy of NICSπ,zz values due to the nonplanarity of the systems, but the general picture is similar.Peer reviewe

New insights into aromatic pathways of carbachlorins and carbaporphyrins based on calculations of magnetically induced current densities

Magnetically induced current densities have been calculated and analyzed for a number of synthesized carbachlorins and carbaporphyrins using density functional theory and the gauge including magnetically induced current (GIMIC) method. Aromatic properties have been determined by using accurate numerical integration of the current flow yielding reliable current strengths and pathways that are related to the degree of aromaticity and the aromatic character of the studied molecules. All investigated compounds are found to be aromatic. However, the obtained aromatic pathways differ from those previously deduced from spectroscopic data and magnetic shielding calculations. For all studied compounds, the ring current divides into an outer and an inner branch at each pyrrolic subring, showing that all π-electrons of the pyrrolic rings take part in the delocalization pathway. The calculations do not support the common notion that the main share of the current takes the inner route at the pyrrolic rings without an inner hydrogen and follows an 18π aromatic pathway. The aromatic pathways of the investigated carbaporphyrins and carbachlorins are very similar, since the current strength via the Cβ[double bond, length as m-dash]Cβ′ bond of the cyclopentadienyl ring of the carbaporphyrins is almost as weak as the current density passing the corresponding saturated Cβ–Cβ′ bond of the carbachlorins

Computational Studies of Aromatic and Photophysical Properties of Expanded Porphyrins

Magnetically induced current densities and ring-current pathways have been calculated at density functional theory (DFT) and second-order Møller–Plesset perturbation theory (MP2) levels of theory for a set of expanded porphyrins consisting of five or six pyrrolic rings. The studied molecules are sapphyrin, cyclo[6]­pyrrole, rubyrin, orangarin, rosarin, and amethyrin. Different functionals have been employed to assess the functional dependence of the ring-current strength susceptibility. Vertical singlet and triplet excitation energies have been calculated at the second-order approximate coupled cluster (CC2), expanded multiconfigurational quasi-degenerate perturbation theory (XMC-DPT2), and time-dependent density functional theory levels. The lowest electronic transition of the antiaromatic molecules was found to be pure magnetic transitions providing an explanation for the large paratropic contribution to the total current density. Rate constants for different nonradiative deactivation channels of the lowest excited states have been calculated yielding lifetimes and quantum yields of the lowest excited singlet and triplet states. The calculations show that the spin–orbit interaction between the lowest singlet (<i>S</i>₀) and triplet (<i>T</i>₁) states of the antiaromatic molecules is strong, whereas for the aromatic molecule the spin–orbit coupling vanishes. The experimentally detected fluorescence from <i>S</i>₂ to <i>S</i>₀ of amethyrin has been explained. The study shows that there are correlations between the aromatic character and optical properties of the investigated expanded porphyrins