Photophysical Properties and Two-Photon Polymerization Ability of a Nitroalkoxystilbene Derivative

The photophysical behavior of the trans -4-propoxy 4′-nitrostilbene ( PNS ) as well as its one- and two-photon polymerization properties are reported. A detailed solvatochromic analysis of the steady-state and time-resolved fluorescence indicates that the locally excited state of PNS (LE) undergoes multiple deactivation pathways which are sequentially triggered by the solvent polarity. In low polar solvent, an efficient intersystem crossing (ISC) identified as a S 1 →T n process leads to the population of the lowest triple state from which the trans−cis photoisomerization is mainly proceeding. From ns-laser flash photolysis measurements, it also appears that the T 1 state which exhibits a nπ* character can efficiently abstract a hydrogen from aliphatic amines and subsequently produces α-aminoalkyl radicals, highly reactive species for the photoinitiation of free-radical polymerization. As a consequence, PNS constitutes a good photoinitiating system. However, the further increase of the solvent polarity opens a new relaxation pathway which populates a highly polar TICT level to the detriment of ISC and trans−cis photoisomerization. A strong increase of the fluorescence quantum yield with a concomitant decrease of the trans−cis photoisomerization one is then observed. At the high end of the solvent-polarity scale, an adding non-radiative deactivation process associated with the twisting of the nitro group leads to a noticeable decrease of the fluorescence process. The photoinduced charge transfer process which largely impacts the photophysical feature of such a D−π−A system also promotes the two-photon absorption properties of PNS . The chromophore exhibits a sizable two-photon absorption spectrum in the 700−900 nm region with δ of about 180 ± 15 GM at 750 nm. In conjunction with its photoinitiating properties, the two-photon polymerization ability of PNS was finally demonstrated. A systematic comparison with a 2,7 diaminofluorene derivative used as reference for two-photon polymerization confirms the substantial improvement of the two-photon polymerization efficiency.