Why does the orientation of azulene affect the two-photon activity of a porphyrinoid-azulene system?

Attaching a dipolar molecule in a symmetric system induces a major change in the electronic structure, which may be reflected as the enhancement of the optical and charge-transfer properties of the combined system as compared to the pristine ones. Furthermore, the orientation of the dipolar molecule may also affect the said properties. This idea is explored in this work by taking porphyrinoid molecules as the pristine systems. We attached azulene, a dipolar molecule, at various positions of five porphyrinoid cores and studied the effect on charge-transfer and one- and two-photon absorption properties using the state-of-the-art RICC2 method. The attachment of azulene produces two major effects - firstly it introduces asymmetry in the system and, secondly, being dipolar, it makes the resultant molecule dipolar/quadrupolar. Porphyrin, N-confused porphyrin, sub-porphyrin, sapphyrin, and hexaphyrin are used as core porphyrinoid systems. The change in charge-transfer has been studied using the orbital analysis and charge-transfer distance parameter for the first five singlet states of the systems. The effect of orientation of azulene on the said properties is also explored. The insights gained from our observations are explored further at the dipole and transition dipole moment levels using a three-state model.

A Curious Case of Two-Photon Absorption in n-Helicene and n-Phenylene, n=6-10: Why n=7 is Different?

n-Helicenes and n-Phenylenes are interesting examples of twisted molecules, where although the atoms are connected through conjugated π ${\pi }$ -bonds, the π ${\pi }$ -conjugation is largely hindered by the twisted nature of the bonds. Such structures provide a unique opportunity to study the effect of twisted π ${\pi }$ -system on non-linear optical properties. In this work, we studied the two-photon absorption in donor-acceptor substituted n-helicenes and n-phenylenes employing the state-of-the-art RI-CC2 method and reported a unique feature we observed in n=7 systems. We found that both 7-helicene and 7-phenylene systems exhibit largest two-photon absorption than other members in their respective classes. Furthermore, using generalized few-state model, we provided a detailed microscopic mechanism of this unique observation involving participation of different transition dipole moment vectors and their relative orientations.

Chromophore Planarity, -BH Bridge Effect, and Two-Photon Activity: Bi- and Ter-Phenyl Derivatives as a Case Study.

In this work, we have employed electronic structure theories to explore the effect of the planarity of the chromophore on the two-photon absorption properties of bi- and ter-phenyl systems. To that end, we have considered 11 bi- and 7 ter-phenyl-based chromophores presenting a donor-π-acceptor architecture. In some cases, the planarity has been enforced by bridging the rings at ortho-positions by -CH2 and/or -BH, -O, -S, and -NH moieties. The results presented herein demonstrate that in bi- and ter-phenyl systems, the planarity achieved via a -CH2 bridge increases the 2PA activity. However, the introduction of a bridge with the -BH moiety perturbs the electronic structure to a large extent, thus diminishing the two-photon transition strength to the lowest electronic excited state. As far as two-photon absorption activity is concerned, this work hints toward avoiding -BH bridge(s) to enforce planarity in bi- and ter-phenyl systems; however, one may use -CH2 bridge(s) to achieve the enhancement of the property in question. All of these conclusions have been supported by in-depth analyses based on generalized few-state models.

Designing a Propellane-based Nonlinear Optically Active System Absorbing in Three Different Wavelength Regions.

The aim of this work is to demonstrate the possibility of using propellane in designing a molecule that can absorb in three different wavelength regions and their nonlinear optical (NLO) activity can be fine-tuned by varying the three wings. We considered 22 tailor-made propellane derivatives consisting of phenyl, naphthyl, and biphenyl wings for this purpose. Using the state-of-the-art linear and quadratic response methods within TD-DFT and RI-CC2 theories and a suitable generalized few-state model that quantifies the effect of orientation of different transition moments on NLO properties, we discussed how and why the linear and nonlinear optical activity of propellane vary when the three wings are assembled successively to construct a full-propellane.

First-order hyperpolarizabilities of propellanes: elucidating structure-property relationships.

Following recent experimental work demonstrating strong nonlinear optical properties, namely second harmonic generation of light, in crystals composed of 16,20-dinitro-(3,4,8,9)-dibenzo-2,7-dioxa-5,10-diaza[4.4.4]propellane molecules [A. Miniewicz, S. Bartkiewicz, E. Wojaczynska, T. Galica, R. Zalesny and R. Jakubas, J. Mater. Chem. C, 2019, 7, 1255-1262] in this paper we aim to investigate "structure-property" relationships for a series of 16 propellanes presenting a wide palette of substituents with varying electron-accepting/donating capabilities. To that end, we use electronic- and vibrational-structure theories and a recently developed generalized few-state model combined with a range-separated CAM-B3LYP functional to analyze electronic and vibrational contributions to the first hyperpolarizability for the whole series of molecules. The variations in computed properties are large among the studied set of substituents and can reach an order of magnitude. It has been demonstrated that the maximum values of frequency-independent first hyperpolarizability are expected for strong electron-accepting NO2 substituents, but only at the preferred position with respect to the electronegative oxygen atom in the 1,4-oxazine moiety. This holds for electronic as well as vibrational counterparts.