List of Papers This thesis is based on the following papers, which are referred to in the text by their Roman numerals. I Zhu, J.; Dahlstrand, C.; Smith, J. R.; Villaume, S.; Ottosson, H. On the Importance of Clar Structures of Polybenzenoid Hydrocarbons as Revealed by the π-contribution to the Electron Localization Function. Symmetry 2010, 2, 1653- 1682. II Dahlstrand, C.; Yamazaki, K.; Zhu, J.; Villaume, S.; Kilså, K.; Ottosson, H. Substituent Effects on the Electron Affinities and Ionization Energies of Tria-, Penta-, and Heptafulve- nes: A Computational Study. J. Org. Chem. 2010, 75(23), 8060-8068. III Tong, H.; Dahlstrand, C.; Villaume, S.; Zhu, J.; Piqueras, M. C.; Crespo, R.; Ottosson, H. Fulvenes: Compounds for which the Singlet-Triplet Energy Gaps are Closely Linked to Aromaticity and Aromaticity Differences. Manuscript (2012) IV Dahlstrand, C.; Rosenberg, M.; Kilså, K.; Ottosson. H. Explo- ration of the π-Electronic Structure of Singlet, Triplet, and Quintet States of Fulvenes, and Fulvalenes Using the Elec- tron Localization Function. J. Phys. Chem. Accepted (2012) V Rosenberg, M.; Dahlstrand, C.; Ottosson, H.; Kilså, K. Ma- nipulation of Excited State Energies in Fulvenic Molecules. Preliminary manuscript (2012) VI Dahlstrand, C.; Jahn, B.; Grigoriev, A.; Villaume, S.; Ahuja, R., Ottosson, H. Tuning the Band Gap of Polyfulvenes by Use of “Handles”: On the Effects of Exocyclic Substitution, Ben- zannulation, and Ring Methylation. Manuscript (2012) Reprints were made with permission from the respective publishers. Works not included in this thesis: • Rosenberg, M.; Dahlstrand, C.; Kilså, K.; Ottosson, H. Excited State Aromaticity and Antiaromaticity: Opportunities for Photo- physical and Photochemical Rationalizations. Chem. Rev. Manu- script (2012) • Dahlstrand, C. Aromaticity Effects in Polybenzenoid Hydrocar- bons and in Substituted Fulvenes: A Computational Study. Thesis for the degree of Licentiate of technology (2010) Author Contribution The author wishes to clarify his contributions to the included papers. I Performed a large part of the calculations and contributed partly to manuscript writing. II Performed a large part of the calculations and contributed ex- tensively to project development, data analysis and manuscript writing. III Performed a majority of the calculations, and contributed exten- sively to data analysis and manuscript writing. IV Performed a majority of the calculations. Contributed exten- sively to project development, manuscript writing and data analysis. V Preformed all synthetic work and provided the initial computa- tional results. VI Performed all oligomer calculations and a few initial PBC cal- culations. Contributed extensively to project development, manuscript writing, and data analysis. Contents 1. Introduction...............................................................................................13 1.1. Single Molecule Electronics..............................................................13 1.2. Electronics based on Molecular Materials ........................................14 2. Aromaticity ...............................................................................................15 2.1. A Short Historical Background .........................................................15 2.2. Different Types of Aromaticity.........................................................16 2.3. Spin State Multiplicity ......................................................................17 2.4. Aromaticity in the Singlet Ground State ...........................................18 2.4.1. Aromaticity Criteria...................................................................18 2.5. Aromaticity in the Lowest Triplet and Quintet Excited States..........19 2.6. Aromaticity Indices...........................................................................19 2.6.1. Harmonic Oscillator Model of Aromaticity (HOMA)...............19 2.6.2. Nucleus Independent Chemical Shifts (NICS) ..........................20 2.6.3. NICS scan ..................................................................................20 2.6.4. Electron Localization Function (ELF).......................................21 3. Computational Quantum Chemistry .........................................................22 3.1. Elementary Quantum Mechanics ......................................................22 3.2. Hartree-Fock (HF) and post-Hartree-Fock Methods.........................23 3.3. Basis Sets...........................................................................................24 3.4. Density Functional Theory (DFT).....................................................24 3.5. Calculating Molecular Properties......................................................26 4. Aromaticity of Polybenzenoid Hydrocarbons...........................................27 4.1. Properties of Polybenzenoid Hydrocarbons ......................................27 4.1.2. Clar Resonance Structures.........................................................27 4.1.3. Properties of the Electron Localization Function ......................29 4.1.4. Describing PBHs through the π-Component of the Electron Localization Function ..........................................................................31 4.2. Influence on π-Electronic Structure due to the Fusion of Ethylene or Benzene Ring Fragments onto PBHs .......................................................33 4.2.1. Fusion of Ethylene onto a PBH .................................................33 4.2.2. Fusion of a Benzene ring onto a PBH........................................33 4.3. Influence on the π-Electronic Structure of Benzene due to Distortions of the σ-Framework .................................................................................35 5. Aromatic Chameleons...............................................................................37 5.1. Electron Affinity and Ionization Energy ...........................................37 5.1.1. Tetrathiafulvalene and tetracyanoquinodimethane....................38 5.1.2. Basis Set Dependence of the Outer Valence Greens’ Function Calculations .........................................................................................39 5.1.3. Orbital Symmetry Considerations .............................................40 5.2. Donor-Acceptor Dyads......................................................................43 5.3. Excited States of Pentafulvenes ........................................................45 5.3.1. Rotation about the Exocyclic Bond of Substituted Pentafulvene.46 5.3.2. The Gauge Including Atomic Orbitals Method for Nuclear Magnetic Shieldings ............................................................................47 5.3.3. The Connection Between Aromaticity and the Singlet-Triplet Energy Gap..........................................................................................47 5.3.4. NICS-scan on a Potential Ground State Triplet.........................49 5.3.5. Dependence of HOMA, NICS, and ΔEST on the C-C Bond Lengths ................................................................................................50 5.4. π-Electronic Structure of Other Fulvenoid Compounds ...................54 5.4.1. Fulvenes and Fulvalenes Investigated by the π-Component of the Electron Localization Function......................................................54 6. Excited State Properties of Substituted Fluorenones and Dibenzofulvenes 58 6.1. Synthetic Aspects ..............................................................................58 6.2. Excited State Properties of Substituted Fluorenones and Dibenzofulvenes.......................................................................................60 6.2.2. Time-Dependent Density Functional Theory ............................60 6.2.3. Polarizable Continuum Model (PCM).......................................61 6.2.4. Exchange of O for C(CN)2.........................................................61 6.2.5. Substitution at the 2- and 7- positions .......................................62 7. Fulvenes in Polymeric Systems ................................................................66 7.1. Conducting Polymers ........................................................................66 7.2. Oligomer Extrapolations ...................................................................67 7.3. Periodic Boundary Conditions (PBC) ...............................................69 7.3.1. Fulvenoid or Quinoid Polyfulvenes...........................................69 7.3.2. Effect of Benzannulation ...........................................................70 7.3.3. Benzannulation and Aromaticity ...............................................72 8. Concluding Remarks.................................................................................73 9. Summary in Swedish ................................................................................74 9.1. Aromaticitet.......................................................................................74 9.2. Polybensenoida kolväten...................................................................75 9.3. Aromatiska kameleonter....................................................................75 9.4. Ledande polymerer............................................................................77 9.5. Slutsats ..............................................................................................78 Acknowledgements.......................................................................................79 References.....................................................................................................81