Photon upconversion: from two-photon absorption (TPA) to triplet–triplet annihilation (TTA)

Abstract

Organic upconversion is a unique process in which low-energy light (usually NIR light) is converted to high-energy light through either the two-photon absorption (TPA) mechanism or the triplet-triplet annihilation (TTA) mechanism. Both TPA upconversion (TPA-UC) and TTA upconversion (TTA-UC) have been actively investigated in recent years due to their intriguing applications in optics, biophotonics, and solar energy utilization. Although they show some similarity (i.e., belonging to the nonlinear two-quantum process and needing focused excitation light), TPA-UC and TTA-UC are very different, such as in mechanism, characteristics involved, molecular design and potential applications. Here, we collectively reviewed these two kinds of upconversion processes and compared their respective characteristics and merits. We also present recent advances made in the areas of TPA- and TTA-UC, the remaining challenges and opportunities, with particular emphasis on molecular engineering of these two active upconversion materials.

New Styryl Phenanthroline Derivatives as Model D−π−A−π−D Materials for Non‐Linear Optics

“…The results listed in Table reveal that there is no solvent effect on the TPA efficiency, in spite of the considerable difference in polarity between the two media considered. The order of magnitude of the TPA cross‐sections (hundreds of GM) is highly promising and in line with that of other push‐pull dipolar systems with good NLO responses ,,…”

“…The notable fluorosolvatochromism of the present compounds, producing large Stokes shifts that increase with solvent polarity, allowed the hyperpolarizability coefficient to be estimated. Both the hyperpolarizability coefficients (β) and the TPA cross‐section (δ), evaluated by means of Two photon excited fluorescence, resulted rather large and comparable to those of other organic compounds considered NLO materials ,,. They exhibited the same trend that well matches that of the push‐pull character granted by the electron‐donor capability of the different substituents: X3 < X5 < X4 , with the latter showing the highest value.…”

“…Nowadays the development of active and controllable materials for Non‐Linear Optics (NLO) is the subject of intensive investigations from both the theoretical and practical points of view, for their potential applications in the domain of optoelectronics and photonics . Although different classes of materials have been studied, the attention has recently shifted to organic materials as the molecular design can tailor the NLO responses for targeted applications …”

Sandwich‐Type Porphyrin Complexes as Efficient Sensitizers for Triplet‐Triplet Annihilation Upconversion

“…It is thought that the TTET of sandwichlike porphyrin complexes occurs mainly between their dark (nonemissive) triplet excited sates and the triplet acceptor DPA, i. e. the weakly phosphorescent or non‐phosphorescent transition metal complex can be used for TTA upconversion . The higher upconversion quantum yields of sandwichlike porphyrin complexes may be caused by their longer triplet lifetimes, because the TTET process is actually a two‐molecular quenching procedure, long‐lived triplet excited state will increase the diffusion distance and make the encounter of the sensitizer and the acceptor more likely . The efficiency of TTET can be estimated by the triplet‐triplet quenching constant of DPA (k q ).…”

Sensitizer-Free Photon Upconversion in Single-Component Brominated Aromatic Crystals

“…The radiative S 1 ‐to‐S 0 decay results in emission with an anti‐Stokes shift (Figure S1A in the Supporting Information). While TPA reveals UC raging from near‐infrared (NIR) to visible region, it usually requires high‐power pulsed laser (> 10 6 W cm −2 ) as an excitation source, which restricts its applications …”