Organic molecule boosts phosphorescence 10x; can improve medical diagnosis

Scientists have discovered a new organic molecule that is more efficient in phosphorescence.

According to the scientists, the thienyl diketone molecule achieves phosphorescence that is over 10 times faster than traditional materials.

The team that made the discovery is from Osaka University in Japan.

Room-temperature phosphorescence (RTP) from metal-free organic molecules has been an area of research for years.

Challenges in enabling metal-free phosphorescence

Although classical RTP materials have been used for diverse applications, including organic light-emitting diodes (OLEDs) and bioimaging, they are mainly precious-metal complexes of Iridium or Platinum.

The paper states that organic RTP needed to overcome several challenges to match the output derived from traditional materials.

One of the most significant challenges is the small phosphorescence transition probability ( i.e. , rate constant k _p ), causing poor RTP quantum yields for heavy-metal complexes and even less for conventional organic compounds.

The addition of heavy atoms such as bromine, iodine, selenium, and tellurium along with carbonyl functionalities is a common approach utilized for enhancing rate constant. But this poses another problem as they are associated with high chemical reactivity and instability.

However, with the new metal-free organic molecule, efficient narrowband RTP was observed in solutions, amorphous polymer matrices, and crystalline solids.

The paper also states that a substantial phosphorescence rate was also experimentally confirmed, which “was the key to outstanding RTP quantum yields in solution (38% under Argon) and in polymer films (up to 54% in air).”

Uses of the organic molecule discovery

Phosphorescence is used in applications such as OLEDs and several types of medical diagnostics, including cancer.

It occurs when light is emitted at low temperatures caused by the absorption of radiation (such as X-rays or ultraviolet light) and continues for a noticeable time after this radiation has stopped.

The research can provide the basis for designing new organic phosphorescent materials that are not dependent on heavy or rare metals. With proper research, they can also outdo the existing materials which have been in use for years.

The findings will also come in handy and be a gamechanger in the fields of medical diagnostics, OLEDs, lighting, and more.

Besides, the method can pave the way for the development of organic phosphors whose development is not damaging to the environment and offers cheaper alternatives to precious metal – based phosphors.

The research was published in the journal Chemical Science in June.

Abstract

We report metal-free organic 1,2-diketones that exhibit fast and highly efficient room-temperature phosphorescence (RTP) with high colour purity under various conditions, including solutions. RTP quantum yields reached 38.2% in solution under Ar, 54% in a polymer matrix in air, and 50% in crystalline solids in air. Moreover, the narrowband RTP consistently dominated the steady-state emission, regardless of the molecular environment. Detailed mechanistic studies using ultrafast spectroscopy, single-crystal X-ray structure analysis, and theoretical calculations revealed picosecond intersystem crossing (ISC) followed by RTP from a planar conformation. Notably, the phosphorescence rate constant k _p was unambiguously established as ∼5000 s ⁻¹ , which is comparable to that of platinum porphyrins (representative heavy-metal phosphor). This inherently large k _p enabled the high-efficiency RTP across diverse molecular environments, thus complementing the streamlined persistent RTP approach. The mechanism behind the photofunction has been elucidated as follows: (1) the large k _p is due to efficient intensity borrowing of the T ₁ state from the bright S ₃ state, (2) the rapid ISC occurs from the S ₁ to the T ₃ state because these states are nearly isoenergetic and have a considerable spin–orbit coupling, and (3) the narrowband emission results from the minimal geometry change between the T ₁ and S ₀ states. Such mechanistic understanding based on molecular orbitals, as well as the structure-RTP property relationship study, highlighted design principles embodied by the diketone planar conformer. The fast RTP strategy enables development of organic phosphors with emissions independent of environmental conditions, thereby offering alternatives to precious-metal based phosphors.