The main goal of the project was to develop a methodology for modifying organic fluorophores in such a way as to significantly increase the values of two photophysical parameters important in the context of potential applications: photostability and fluorescence brightness. This goal was achieved in several ways. The first one involved chemical modification of the molecule. Several new porphyrin derivatives with substituents containing cyclooctatetraene (COT), known as an effective triplet state quencher, were obtained. This modification allowed for a huge (three orders of magnitude) increase in photostability in anaerobic conditions. However, in solutions containing oxygen, photostability increased for zinc porphyrins, while it decreased in the case of "free bases". Similar photodegradation quantum yield values were obtained for both groups, which indicates a similar mechanism of photodestruction in both types of porphyrins. The above hypothesis was confirmed by studies on the photooxidation products of tetraphenylporphyrins.

 
The synthesis of COT-containing derivatives turned out to be very important from the point of view of single-molecule fluorescence, in which the triplet lifetime was dramatically shortened to several dozen nanoseconds compared to unsubstituted porphyrins (hundreds of microseconds). This enabled the recording of emission spectra of individual fluorophores.

 
Other types of substituents - electron-donating or electron-withdrawing - were used to functionalize porphycene. Several dozen derivatives containing amino, nitro, fluoro-, chloro- and bromoporphycene groups, as well as derivatives with alkyl and/or aryl substituents were obtained. A very important result was the demonstration that the photostability of nitro derivatives is many (about a thousand) times greater than that of aminoporphycenes. A rather unexpected result of the aminoporphycene research was the correction of previous work that suggested the existence of double emission coming from two tautomeric forms. One of these emissions was shown to come from a photo-oxidation product.

 
Detailed photophysical studies of the newly obtained compounds resulted in the detection of an interesting correlation between the fluorescence quantum yield (in the range of as much as four orders of magnitude) and the strength of intramolecular hydrogen bonding. To explain this, a model was proposed according to which the above relationship (the stronger the bond, the faster the non-radiative deactivation) is caused by a quantum effect: delocalization of internal protons in a cavity composed of four nitrogen atoms).

 

The second way to increase photostability was to create intermolecular hydrogen bonds between the fluorophore and the alcohol, i.e. the proton donor/acceptor. In this type of complexes, additional non-radiative depopulation processes of the S₁ state are activated (proton transfer in the excited state and internal conversion), which reduces the efficiency of occupying the triplet state (which is the precursor of the photoproduct). This resulted in an up to 200-fold increase in photostability. It should be noted that reducing the lifetime of the singlet state results in a lower fluorescence quantum yield, which reduces the brightness. However, for fluorescence from single molecules, we observe a strong increase in the emission signal, because in this case the brightness is determined by the efficiency of occupying the triplet state and its decay time.

  
The third type of methodology for increasing photostability was used for a single fluorophore (a phthalocyanine derivative) placed in an optical microcavity. The Purcell effect (increase in the radiative constant) leads to a shortening of the lifetime of the S₁ state, and as a consequence, the probability of transition to the triplet state is reduced, as a result of which the molecule becomes more photostable.