Paulo J. Coelho

Synthesis and spectrokinetic studies of spiro[thioxanthene-naphthopyrans]

Abstract Several new spiro[thioxanthene-naphthopyrans] were prepared from substituted naphthols using a one step reaction. The photochromic properties of the new compounds were studied under flash photolysis and continuous irradiation. Compared to reference compounds general significant bathochromic shifts in the absorption spectra of the open forms, faster thermal bleaching kinetics and interesting fatigue resistances were observed.

Photochromic fused-naphthopyrans without residual color.

A series of new photochromic fused-naphthopyrans with an alkyl bridge between the pyran ring and the naphthalenic core was synthesized in several steps from 4-(bromomethyl)benzocoumarin. The presence of the alkyl bridge in these new fused-naphthopyrans prevents the formation of one long-lived photoisomer and therefore has a dramatic effect on their photochromic properties: UV irradiation of common naphthopyrans gives rise to two isomeric colored photoisomers, one of which fades very slowly and is responsible for a persistent residual color. UV excitation of these new uncolored fused-naphthopyrans leads to the formation of only one colored photoisomer that fades completely to the uncolored state in few seconds/minutes following a monoexponential decay law, thus avoiding the problem of the residual coloration typically observed with naphthopyrans.

Synthesis and photochromic behaviour of novel 2H-chromenes derived from fluorenone

Abstract The synthesis of 2 H -chromenes based on a fluorenone nucleus is described. Whereas 2,2-diphenyl-2 H -pyranofluorenones 4 – 7 and dicyanomethylidene-2 H -pyranofluorenes 8 – 10 do not exhibit photochromism, 2,2-diphenyl-2 H -pyranofluorenols exhibit photochromic behaviour. The spectrokinetic properties of these compounds in solution are reported.

Synthesis of photochromic dyes based on annulated coumarin systems

Novel 2H-chromenes derived from hydroxycoumarins were synthesized, and their photochromic behaviour was studied under flash-photolysis conditions, showing a wide absorption range in the visible region. All the compounds exhibit low fluorescence, which apparently has no negative effect on their photochromic properties.

Preventing the formation of the long-lived colored transoid-trans photoisomer in photochromic benzopyrans.

A new photochromic fused benzopyran presenting a bridge between the pyran double bond and the benzenic ring was prepared. While the UV irradiation of usual benzopyrans leads to the formation of two colored photoisomers with very different thermal stabilities, studies by laser flash photolysis showed that the presence of this particular bridge prevents the formation of the undesirable long-lived colored TT isomer and therefore after laser irradiation the colored solution fades following a fast monoexponential decay.

Synthesis and Photochromic Behavior of Novel Annelated 2H‐Chromenes Derived from Hydroxy‐9H‐xanthen‐9‐ones

The synthesis of four novel pyrano-xanthenones derived from hydroxy-9H-xanthen-9-ones is described, and their photochromic properties in solution are reported. All compounds synthesized exhibit a good colorability, making them good potential dyes. The presence of the chromone system fused in the 7,8-position seems to lead to more stabilized colored forms.

Fast Color Change with Photochromic Fused Naphthopyrans

Photochromic molecules can reversibly develop color upon irradiation with UV light. These smart molecules, mainly in the naphthopyran family, have been applied with success to ophthalmic lenses that darken quickly under sunlight and revert to the uncolored state after several minutes in the dark. This slow adaptation to the absence of light is one of the limitations and is due to the formation of an unwanted photoisomer. We have designed a new naphthopyran with a bridged structure which prohibits the formation of the undesirable, persistent photoisomer and thus shows a very fast switching between the uncolored and colored states. UV irradiation of a hybrid siloxane matrix doped with the new fused naphthopyran leads to the formation of a pink coloration bleaching in a few milliseconds, in the absence of light, at room temperature. This new fused naphthopyran is easily prepared in three steps from readily accessible precursors and is amenable to structural modifications to tailor color and lifetime of the colored photoisomer.

Fast and fully reversible photochromic performance of hybrid sol–gel films doped with a fused-naphthopyran

A new type of photochromic naphthopyran with a fused structure was embedded in an organic–inorganic amine–alcohol–silicate hybrid gel producing colourless, transparent and flexible photochromic films. Under UV-vis light these materials developed, in less than 30 s, intense yellow-orange colours that faded completely in the dark in 3 min. This behaviour contrasts with the usual performance of common naphthopyrans that always generate two coloured species which fade with different kinetics, leading to the formation of a residual colour that persists for several minutes/hours. The structural design of this new naphthopyran prevents the formation of the long-lived coloured species and therefore fully reversible photo-switchable materials can be easily obtained.

First report of a permanent open form of a naphthopyran

The synthesis of a permanent open form of a naphthopyran is described for the first time. The open form of the 10-hydroxy-2H-naphtho[1,2-b]pyran, obtained by reaction of 1,8-dihydroxy-naphthalene with 1,1-diphenylprop-2-yn-1-ol under acid catalysis, is stable due to the establishment of an intramolecular hydrogen bond.

Photochromic and photophysical properties of new benzo- and naphtho[1,3]oxazine switches

A new set of photochromic hindered benzo- and naphtho[1,3]oxazines with fast switching speed between the closed and open form were synthesised. Laser excitation of these uncoloured molecules (with ∼10 ns FWHM pulses) leads to the cleavage of the C-O bond and opening of the oxazine ring in less than 20 ns, generating a thermally unstable coloured zwitterionic species that reverts in few nanoseconds to the initial oxazine closed form. For these derivatives a detailed spectroscopic and photophysical study was undertaken involving measurements of absorption and phosphorescence spectra, phosphorescence quantum yields (ϕ(Ph)), phosphorescence lifetimes (τ(Ph)), triplet energies (E(T)), singlet oxygen (ϕ(Δ)) and ring opening photoreaction (ϕ) quantum yields, reisomerization kinetics and the zwitterionic form lifetime. A remarkable solvent effect on the lifetime of the photogenerated species was observed.