1-, 3-, 6-, and 8-Tetrasubstituted Asymmetric Pyrene Derivatives with Electron Donors and Acceptors: High Photostability and Regioisomer-Specific Photophysical Properties.

The systematic synthesis of five 1-, 3-, 6-, and 8-tetrasubstituted asymmetric pyrenes with electron donor and acceptor moieties is presented, together with an examination of their photophysical properties. Pyrene derivative PA1, containing one formyl and three piperidyl groups, showed bright solvatochromic fluorescence from green (λem = 557 nm, ΦFL = 0.94 in hexane) to red (λem = 648 nm, ΦFL = 0.50 in methanol), suggesting potential applications for PA1 as an environmentally responsive probe. Although the synthesis of simple 1- and 3-disubstituted pyrene derivatives is considered difficult, PA13, with two formyl groups at the 1- and 3-positions and two piperidyl groups at the 6- and 8-positions, could be synthesized successfully. PA13 exhibited less pronounced solvatochromism, but displayed a narrow fluorescent band with high ΦFL in all solvents (ΦFL > 0.75). Moreover, its absorption band displayed an exceptional bathochromic shift compared to the other derivatives (e.g., λabs = 480 and 522 nm in ethanol for PA1 and PA13, respectively), suggesting that such modifications of pyrene may be quite important for the modulation of its energy gap. Additionally, all compounds exhibited exceptionally high photostability, which highlights the advantage of these new dyes and provides new insights on the design of photostable fluorophores.

Suppressed Triplet Exciton Diffusion Due to Small Orbital Overlap as a Key Design Factor for Ultralong-Lived Room-Temperature Phosphorescence in Molecular Crystals.

Persistent room-temperature phosphorescence (RTP) under ambient conditions is attracting attention due to its strong potential for applications in bioimaging, sensing, or optical recording. Molecular packing leading to a rigid crystalline structure that minimizes nonradiative pathways from triplet state is often investigated for efficient RTP. However, for complex conjugated systems a key strategy to suppress the nonradiative deactivation is not found yet. Here, the origin of small rates of a nonradiative decay process from triplet states of conjugated molecular crystals showing RTP is reported. Optical microscopy analysis showed that, despite a favorable molecular stacking, an aromatic crystal with strong RTP is characterized by small diffusion length and small values of the diffusion coefficient of triplet excitons. Quantum chemical calculations reveal a large overlap between the lowest unoccupied molecular orbitals but very small overlap between the highest occupied molecular orbitals (HOMOs). Inefficient electron exchange caused by the small overlap of HOMOs prevents triplet excitons from diffusing over long distances and consequently from quenching at defect sites inside the crystal or at the crystal surface. These results will allow design of comprehensive molecular structures to obtain molecular solids with more efficient RTP.

Nanoscale triplet exciton diffusion via imaging of up-conversion emission from single hybrid nanoparticles in molecular crystals.

Up-conversion materials composed of donor and acceptor molecules which convert low energy photons into higher energy ones by triplet-triplet annihilation can improve the sensitivity of photocatalysts or the efficiency of solar cells. The use of crystalline materials can lead to a decrease in the up-conversion threshold intensity due to increased diffusion length LT of triplet excitons. Here, we demonstrate direct microscopic imaging of triplet exciton diffusion in polycrystalline films. The generation of high local density of triplet states is achieved by functionalizing alumina nanospheres with donor molecules and dispersing them in the acceptor films. Diffusion of the triplet excitons and up-converted emission are reflected in enlarged microscopic images of the nanoparticles. Analysis provides the average LT values of 491 nm for the acceptor of polycrystalline anthracene and 172 nm for polycrystalline 9,10-diphenylanthracene, and reveals large distributions of the diffusion characteristics. These average values are more than an order of magnitude larger than LT obtained by a conventional method, and highlight the need for accurate nanoscale characterization of the up-conversion materials.