Spectral and kinetic parameters of transient species in the photolysis of naphthylmethylideneiminospironaphthopyran by excitation at different wavelengths: Nano- and femtosecond laser photolysis

Abstract

The synthesis, photochemistry, and photophysics of multifunctional photochromic systems are cur rently the subject of intensive research. Such systems can be used as photochemical probes and labels to monitor fine properties of complex chemical and bio logical objects [1]. These compounds are also promis ing for the design of materials for next generation molecular electronics [2,3].In [4,5] we reported procedures for the synthesis of photobifunctional compounds (PBCs) of two types. The PBCs contain two structural moieties with differ ent photochemical properties, the spirocyclic (indoli nonaphthopyran or naphthooxazine) and azomethine units, which are bound into the molecule in different modes. In the photolysis of these PBCs, the opening of the spirocycle to give the relatively long livied, colored merocyanine form absorbing light in the range of 550-650 nm occurs in the one moiety, and intramolecular proton transfer and cis-trans isomerization to give the relatively short lived isomer absorbing at 400-500 nm occurs in the other [6][7][8][9][10][11][12]. It has been shown that these photoprocesses can be induced selectively in some PBCs by varying the wavelength of excitation light [11,12]. This feature of PBCs can be useful from the standpoint of their application in devices requiring targeted photoinitiation of different processes by vary ing the excitation light wavelength. 1,3 Dihydro 5 (2 hydroxy 1 naphthylmethylide neimino) 1,3,3 trimethylspiro[2H indole 2,3 [3H] naphtho[2,1 b]pyran] is an example of PBC in which the ratio of the efficiencies of photoprocesses strongly depends on the excitation light wavelength. Recently [11,12], we studied the spectral and kinetic parame ters of relatively long lived photolysis products of this compound and found that two photochromic pro cesses can occur simultaneously: the formation of the merocyanine form in the spironaphthopyran moiety and cis-trans photoisomerization in the azomethine moiety.In this paper, we present results of a femtosecond laser photolysis study of the dynamics of ultrafast pho tophysical processes in this PBC excited at wave lengths of about 340 and 490 nm in methanol solution and compare the results with the data obtained by nanosecond laser photolysis. 1 (Phenyliminomethyl) 2 naphthol (PN) and 1,3 dihydro 1,3,3 trimethylspiro[2H indole 2,3' [3H] naphtho[2,1 b]pyran] (SNP) model the moi eties of the PBC molecule under study: PHOTOCHEMISTRYAbstract-Intramolecular processes occurring in a photobifunctional compound (PBC) comprising the spironaphthopyran and hydroxyazomethine moieties have been studied in methanol solutions by femtosec ond laser photolysis using light with wavelengths of 340 and 490 nm. At the excitation wavelength of 490 nm, the cis-trans photoisomerization in the azomethine moiety occurs in the S 1 state. In the case of PBC photol ysis with 340 nm light, the opening of the spiro bond of the spiropyran moiety (formation of the X form) also takes place during relaxation of the S n state to the S 1 state followed by isomerization to...

Kinetics of photochemical reactions of biphotochromic compounds based on spironaphthopyran and enamine — conjugation effect

“…of the K t formation process is in the subpicosecond time scale as it is for spiro-compounds. 15,21,22,[27][28][29][30][31]…”

“…Therefore, two photoprocesses in BPC (1)cis-trans isomerization of the azomethine fragment and merocyanine formation from spirocycleoccur simultaneously in the course of relaxation of singlet excited states. 15…”

“…18 The rise time of the precursor of trans isomers (corresponding nonrelaxed singlet excited states) was 0.2 ps. 15 Since the excitation with visible light corresponds to the electronic transitions localized in the hydroxynaphthalydeneimine moiety of (2), which is insufficiently coupled with the spiro fragment, then the spiro bond rupture is inefficient in the time interval of the localized singlet excited state relaxation. In the case of BPC (1), the more efficient corresponding coupling results in the formation of a small amount of form B even under visible light excitation.…”

“…18 The relatively poor coupling between spiro and hydroxynaphthalydeneimine parts of (2) ensures an opportunity of spiro bond break in the course of high singlet excited state relaxation (the rise time of the B precursor after UV excitation of (2) was 0.15 ps ref. 15) rather than it would be relaxed to the low-lying singlet excited states localized on the hydroxynaphthalydeneimine fragment. On the other hand, most parts of the high singlet excited states of (1) relax principally to the low-lying singlet excited states localized in the azomethine part of (1) due to the efficient coupling and the form B still remains the minor product even under UV excitation of (1).…”

“…, where 2A E is (2) in the closed form of the spiro-cycle linked to the cis-enol form of the hydroxyazomethine fragment) studied earlier by us. 14,15 The first structural difference between BPC (1) and ( 2) is that in (1) there is a conjugation between π-electronic systems of two photochromic fragmentsspirocycle (naphthopyran) and hydroxyazomethine and in (2) the conjugation is expected to be between π-electronic systems of the indole and the hydroxyazomethine moieties. The second difference is that in (2) the indole fragment acts as the "aniline" part of the azomethine moiety whereas in (1) the naphthopyran fragment is a "phenol" type substituent (Scheme 2).…”