Max Gmelch

Wavelength‐discriminating systems typically consist of heavy benchtop‐based instruments, comprising diffractive optics, moving parts, and adjacent detectors. For simple wavelength measurements, such as lab‐on‐chip light source calibration or laser wavelength tracking, which do not require polychromatic analysis and cannot handle bulky spectroscopy instruments, lightweight, easy‐to‐process, and flexible single‐pixel devices are attracting increasing attention. Here, a device is proposed for monotonously transforming wavelength information into the time domain with room‐temperature phosphorescence at the heart of its functionality, which demonstrates a resolution down to 1 nm and below. It is solution‐processed from a single host–guest system comprising organic room‐temperature phosphors and colloidal quantum dots. The share of excited triplet states within the photoluminescent layer is dependent on the excitation wavelength and determines the afterglow intensity of the film, which is tracked by a simple photodetector. Finally, an all‐organic thin‐film wavelength sensor and two applications are demonstrated where this novel measurement concept successfully replaces a full spectrometer.

Biluminescent organic systems displaying fluorescence and phosphorescence at room temperature under aerated atmosphere still remain uncommon to date. Especially the utilization of the room temperature phosphorescence (RTP) is limited, as it is effectively quenched by oxygen, rendering it incompatible with many applications for use in ambient conditions. So far, only encapsulation to prevent oxygen penetration into the films or the use of special steroid hosts are known concepts to allow conventional biluminescent emitters to show RTP. Here, the design and synthesis of a novel water‐soluble biluminescent emitter is reported. After dispersion in a host matrix, and using host/guest hydrogen‐bonding interactions, easy‐processable highly phosphorescent transparent thin‐films are realized. Their luminescent properties can be activated at will under aerated atmosphere for several weeks without degradation of the system. The polymer (host) mixture can even be readily used for 3D printing, leading to a large range of accessible phosphorescent objects.

In recent years, there has been a growing interest in purely organic materials showing ultralong room‐temperature phosphorescence with lifetimes in the range of seconds. Still, the longest known phosphorescence lifetimes are only achieved with crystalline systems so far. Here, a rational design of a completely new family of halogen‐free organic luminescent derivatives in amorphous matrices, displaying both conventional fluorescence and phosphorescence is reported. Hydrogen bonding between the newly developed emitters and an ethylene‐vinyl alcohol copolymer (Exceval) matrix, which efficiently suppresses vibrational dissipation, enables bright long‐lived phosphorescence with lifetimes up to 2.6 s at around 480 nm. The importance of the chosen matrix is shown as well as the implementation in an organic programmable luminescent tag.

Amorphous purely organic thin-films are able to show efficient phosphorescence under ambient conditions at room temperature (RTP). This opens the perspective to a wide range of new applications, which have attracted lots of interest in the field of material science recently. Therefore, an increasing number of different molecules displaying RTP have already been reported. Whereas the efficiency, the lifetime, or the oxygen sensitivity is frequently discussed, the origin of RTP mainly remains vague. Often, material design rules tend to the development of increasingly complex structures. Here, the well-known Tetra-N-phenylbenzidine (TPD), an archetypical material showing highly efficient fluorescence and RTP, is broken down to its fragments. As the complexity of the system decreases with the molecule’s size, spectroscopic investigation of this molecular family enables a deeper understanding of the appearance of RTP. With spectral and time-resolved measurements, RTP can be detected for all compounds containing a biphenyl core, with lifetimes up to 0.9 s under inert gas conditions. These findings form the basis of a deeper understanding of the appearance of RTP in organic molecules and therefore allow for a more focused investigation of new materials.

The development of organic materials displaying ultralong room‐temperature phosphorescence (URTP) is a material design‐rich research field with growing interest recently, as the luminescence characteristics have started to become interesting for applications. However, the development of systems performing under aerated conditions remains a formidable challenge. Furthermore, in the vast majority of molecular examples, the respective absorption bands of the compounds are in the near ultraviolet (UV) range, which makes UV excitation sources necessary. Herein, the synthesis and detailed analysis of new luminescent organic metal‐free materials displaying, in addition to conventional fluorescence, phosphorescence with lifetimes up to 700 ms and tailored redshifted absorption bands, allowing for deep blue excitation, are reported. For the most promising targets, their application is demonstrated in the form of organic programmable tags that have been recently developed. These tags make use of reversible activation and deactivation of the URTP by toggling between the presence and absence of molecular oxygen. In this case, the activation can be achieved with visible light excitation, which greatly increases the use case scenarios by making UV sources obsolete.

Simple and cheap organic thin films turn into high-quality programmable tags through writing and erasing with light only.

For almost 70 years, Förster resonance energy transfer (FRET) has been investigated, implemented into nowadays experimental nanoscience techniques, and considered in a manifold of optics, photonics, and optoelectronics applications. Here, we demonstrate for the first time simultaneous and efficient energy transfer from both donating singlet and triplet states of a single photoluminescent molecular species. Using a biluminescent donor that can emit with high yield from both excited states at room temperature allows application of the FRET framework to such a bimodal system. It serves as an exclusive model system where the spatial origin of energy transfer is exactly the same for both donating spin states involved. Of paramount significance are the facts that both transfers can easily be observed by eye and that Förster theory is successfully applied to state lifetimes spanning over 8 orders of magnitude.

The aggregation of conjugated polymers and electronic coupling of chromophores play a central role in the fundamental understanding of light and charge generation processes. Here we report that the predominant coupling in isolated aggregates of conjugated polymers can be switched reversibly between H-type and J-type coupling by partially swelling and drying the aggregates. Aggregation is identified by shifts in photoluminescence energy, changes in vibronic peak ratio, and photoluminescence lifetime. This experiment unravels the internal electronic structure of the aggregate and highlights the importance of the drying process in the final spectroscopic properties. The electronic coupling after drying is tuned between H-type and J-type by changing the side chains of the conjugated polymer, but can also be entirely suppressed. The types of electronic coupling correlate with chain morphology, which is quantified by excitation polarization spectroscopy and the efficiency of interchromophoric energy transfer that is revealed by the degree of single-photon emission.