Nonplanar Butterfly-Shaped π-Expanded Pyrrolopyrroles

Area of Science:

• Organic Chemistry
• Materials Science
• Photochemistry

Background:

• Curved polycyclic aromatic hydrocarbons (PAHs) with double-helicene structures are promising for molecular photonics.
• Aza-analogues offer unique electronic and structural properties.

Purpose of the Study:

• To synthesize and characterize large aza-analogues of curved PAHs with a double-helicene structure.
• To investigate their conformational behavior, electronic properties, and photophysical characteristics, particularly solvatochromism.

Main Methods:

• Multi-step synthesis using readily available starting materials.
• X-ray crystallography for structural determination.
• Nuclear Magnetic Resonance (NMR) spectroscopy for conformational analysis.
• Density Functional Theory (DFT) calculations for energy barrier assessment.
• UV-Vis absorption and fluorescence spectroscopy.
• Time-resolved spectroscopy to probe excited-state dynamics.

Main Results:

• Successful synthesis and characterization of three novel heterocyclic nanographenes.
• Identification of three conformational isomers (meso, P,P, M,M) with low interconversion energy barriers.
• Experimental and DFT results confirm the low energy differences between conformers.
• Optical absorption is insensitive to solvent polarity.
• Fluorescence exhibits pronounced solvatochromism, attributed to distinct locally excited and charge-transfer states.

Conclusions:

• The synthesized aza-analogues are promising building blocks for molecular photonics.
• The observed solvatofluorochromism in centrosymmetric molecules without strong substituents is a significant finding.
• The distinct behavior of locally excited and charge-transfer states explains the observed photophysical properties.