Enhancement of optical properties of new purine nucleobases containing electron-donating and -withdrawing peripheral groups.

Nowadays, a great deal of attention has been focused on synthesizing highly fluorescent unnatural base analogs. This has been motivated by the need to overcome the lack of fluorescence of nucleic acids' natural bases. Fluorescent unnatural base analogs, such as purines, may be used in several applications, such as DNA or RNA optical spectroscopy studies. Moreover, for purines base analogs, the optical properties, for example, emission, can be tunable through molecular engineering, improving their applications as fluorescent probes. Looking in this direction, the synthesis and optical spectroscopic studies of a new set of purines base analogs are of foremost relevance. Here, an increase in the fluorescence quantum yield was observed in molecules with NH-π-CN arrangement. The two-photon absorption (2PA) cross-sections also increased for the lower energy 2PA state. The enhancement of both properties results in a two-photon brightness of 5 and 10 times higher than in compounds lacking the NH-π-CN arrangement. For the higher energy 2PA state, an excited state absorption contribution to the 2PA cross-section values was observed, that was verified through ultrafast transient absorption measurements. The higher 2PA brightness makes the purines base analogs promising candidates as fluorescent probes in RNA and DNA spectroscopic studies.

Two-photon absorption properties of BODIPY-like compounds based on BF2-naphthyridine complexes.

Boron dipyrromethene type molecules (BODIPYs) are versatile molecules which have been used for applications ranging from photodynamic therapy to solar cells (DSSC). However, these molecules usually do not present high two-photon absorption cross-sections, limiting their use in nonlinear optical applications. Herein, we study a series of BF2-naphthyridine based boron-complexes with electron-donating and withdrawing groups to increase their two-photon absorption. We have found two-photon absorption cross-sections up to approximately 270 GM, which corresponds to an increase of approximately five times in comparison to the average cross-section value reported for molecules with similar conjugation length, indicating such compounds as potential materials for nonlinear applications in both the visible and infrared spectral regions.

Low threshold Rhodamine-doped whispering gallery mode microlasers fabricated by direct laser writing.

The combination of the outstanding properties of whispering gallery modes with both the flexibility and ease of processing of polymers is particularly attractive for photonics applications. However, the versatile fabrication of polymeric nano/microdevices with the desired photonic performance has proven challenging. Here, we report on lasing in Rhodamine B doped whispering gallery mode microcavities fabricated by direct laser writing via two-photon polymerization. Threshold pump energies as low as 12 nJ were achieved for free-space pulsed excitation at 532 nm. To the best of our knowledge, this is the lowest laser threshold attained for microcavities fabricated in a single step of femtosecond laser writing, a remarkable feat that stands out from other fabrication methods.