Synthesis and Functionalization of Corroles. an Insight on Their Nonlinear Optical Absorption Properties

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Current Organic Synthesis, 2014, 11, 29-41 29 Synthesis and Functionalization of Corroles. An Insight on Their Nonlinear Optical Absorption Properties

Carla I.M. Santosa, Joana F.B. Barataa, Mário J.F. Calvetea, Luís S.H.P. Valea, Danilo Dinib*, Moreno Meneghettic, Maria G.P.M.S. Nevesa*, Maria A.F. Faustinoa, Augusto C. Toméa and José A.S. Cavaleiroa aDepartment of Chemistry & QOPNA, University of Aveiro, 3810-193 Aveiro, Portugal; bDepartment of Chemistry, University of Rome “La Sapienza”, 00185 Rome, Italy; cDepartment of Chemical Sciences, University of Padua, 35131 Padua, Italy

Abstract: The development of efficient synthetic methodologies and post-functionalization procedures gave access to a wide variety of new corrole derivatives with potential applications in different areas. This paper reviews the preparation of meso-triaarylcorroles and their functionalization, namely via cycloaddition transformations, and describes the nonlinear optical (NLO) properties of a series of metal-free and gallium corroles with different peripheral substituents. The corroles studied were the free base 5,10,15-tris- (pentafluorophenyl)corrole, the corresponding gallium(III) complex and adducts obtained from 1,3-dipolar cycloaddition reaction. Corre- lations between the structure and the NLO effect of optical power limiting were attempted in our comparative analysis. The present contribution is one of the first reports on the effect of reverse saturable absorption in the visible spectrum generated by corrole-based complexes.

Keywords: Corroles, cycloaddition reactions, functionalization, nanosecond pulse, optical limiting, reverse saturable absorption.

1. INTRODUCTION In analogy with porphyrins and phthalocyanines, corroles [1] are considered now an independent class of molecules within the N N C F C6F5 largest family of tetrapyrrolic compounds. This upgrade is mainly + C6F5CHO 6 5 M due to the high number of corrole derivatives that have been syn- N thesized in the last decade [2], and the knowledge of their own dis- H N N tinctive properties, derived from the structural characteristics con- ferred by the low symmetry [3], ring tension due to the direct - pyrrole connection [4], trivalent coordination core [5] and C6F5 tautomeric isomerism [6]. In terms of electronic structure, corroles are aromatic macrocycles with a 18 -electron conjugated system 1, M=3H similar to porphyrins, [7] which are able to stabilize high-valence 1a, M=Ga(py) metal ions through ring coordination much more effectively than porphyrins. In the review here presented we will focus on the syn- Scheme 1. thesis of meso-substituted corroles, particularly the 5,10,15- tris(pentafluorophenyl)corrole, as well as its functionalization, be particularly suitable for the synthesis of A3-type corroles from along with an insight on the nonlinear optical properties of such aldehydes with strong electron-withdrawing substituents. For in- materials. stance, the 5,10,15-tris(pentafluorophenyl)corrole 1 was obtained in 11% yield. The route developed by Paolesse and co-workers [22] was based on the acidic conditions developed by Adler for the syn- 2. CORROLE SYNTHESIS thesis of meso-tetraarylporphyrins [23]. In this case, corrole 1 was Although corroles are not found in biological systems, the obtained in low yield (4%) [24]. Based on these synthetic ap- number of articles dedicated to the synthesis and applications of proaches, new progresses were made by other research groups. In these compounds has increased signiﬁcantly over the past decade, particular, Gryko´s group was able to refine the experimental condi- and currently the popularity of corroles is, in some cases, approach- tions giving access to meso-substituted A3- and trans-A2B-corroles ing that of the related porphyrins [8-20]. [25-29]. In fact, three different sets of experimental conditions were explored, based on the (reactivity of the aldehyde, catalyst, solvent, This huge progress started in 1999, with the development of ef- concentration, time etc), in order to open access to a series of meso- ficient synthetic methodologies leading to meso-substituted cor- A -corroles. The condensation of aldehyde with pyrrole, in a water- roles, such as 5,10,15-tris(pentafluorophenyl)corrole 1 (Scheme 1) 3 methanol mixture in the presence of HCl was also reported [30]. from simple procedures developed by Gross [21] and Paolesse [22]. Collman and Decréau also considered the use of microwave ir- The procedure proposed by Gross and co-workers [21] involves radiation in the synthesis of tri-arylcorroles [31]. Compared with the condensation of pyrrole in the absence of solvent and proved to the conventional heating methodology, the microwave irradiation affords noticeably cleaner reaction mixtures and higher yields of the corrole. Chauhan and Kumari [32] reported the synthesis in the *Address correspondence to these authors at the Department of Chemistry, University presence of Amberlyst 15 catalyst and under solvent-free condi- of Rome “La Sapienza”, 00185 Rome, Italy; Tel: +39-06-49913335; Fax: +39-06-490324; E-mail: [email protected], and Department of Chemistry, University of Aveiro tions. In these conditions 5,10,15-tris(pentafluorophenyl)corrole 1 3810-193 Aveiro, Portugal; Tel: +351234370710; E-mail: [email protected] was isolated in 30% yield. Recently Nocera et al. [33] carried out

HO O

C6F5 SO N OH 2 H OH HO N N M C6F5 HO O N N SO2 N OH H OH C6F5 3

O O O O C6F5 N SO2 O O O O

N N C F M O 6 5 O N O O N N SO2 O O O O C6F5 4 O

SO2Cl

ClO2S C6F5

N N NH O SO C F M C6F5 2 6 5 N N O N N C6F5 M N N

C F 6 5 2 SO2 NH O C6F5 5

C6F5 SO N COOH 2 H

N N M C6F5 N N SO2 COOH N H 6 C6F5

C6F5

SO2 NH N N C6F5 M N N N H O H N C H SO NH N C6F5 2 7 S O N H H Scheme 2. Synthesis and Functionalization of Corroles Current Organic Synthesis, 2014, Vol. 11, No. 1 31

NH HN

C6F5 C6F5

N HN

C6F5

pentacene, 1,2,4-trichlorobenzene , 200 °C

NH HN NH HN NH HN C F C F 6 5 6 5 C F C F + C6F5 C6F5 + 6 5 6 5 N HN N HN N HN

C6F5 C6F5 C6F5 8 9 10 Scheme 3. the synthesis of 5,10,15-tris(pentafluorophenyl)corrole in large The introduction of nitro and amino groups at the -pyrrolic po- scale. sitions of corroles allowed further modifications of these compounds [55-61]. The ionic liquid [Bmim][BF4] was also reported as a suitable reaction medium in the preparation of meso-substituted trans-A2B- Approaches involving reagents or catalysts based on transition corroles [34]. metal compounds are also being explored to modify the corrole core. Most of these strategies are based on transformations cata- 3. FUNCTIONALIZATION OF CORROLES lysed by palladium(0) using meso-4,6-dichloropyrimidin-5-yl- substituted corroles and meso-arylcorroles bearing bromine sub- The progress in synthetic methodologies leading to meso- stituents in -pyrrolic positions or in the phenyl groups [62-72]. triarylcorroles was accompanied by a considerable effort to develop The versatility of cycloaddition reactions was explored by different strategies for the functionalization of meso-triarylcorroles. For ex- groups, including ours, as an efficient methodology to functionalize ample, the nucleophilic aromatic substitution reaction of the para-F corrole core. For instance, the reaction of corrole 1 with pentacene atoms in 5,10,15-trispentafluorophenylcorrole 1, which is fre- afforded the expected oxidized Diels-Alder adducts 8 and 9 along quently used for the modification of porphyrins [35], is often used with the [4+4] cycloadduct 10 (Scheme 3) [73]. for its functionalization. Osuka and co-workers explored this approach to introduce several amines into corrole 1 [36], while Following interest in Diels-Alder reactions, the reactivity of Cavaleiro and co-workers used it to link galactose residues [37] or corrole 11 (Scheme 4) bearing a vinyl group as 2 and 4 compo- to functionalize it with silica particles [16]. Maes and co-workers nents was also studied [17,18]. This derivatives was obtained considered the nucleophilic aromatic substitution of chloro atoms in through a Wittig reaction, from the corresponding formyl derivative the 4,6-dichloropyrimidin-5-yl units of meso-pyrimidinyl- obtained by Vilsmeier-Haack reaction [56, 74]. substituted corroles as a post-functionalization of this type of plat- Considering the vinyl corrole 11 as diene, its reactivity was form [38, 39]. tested with 1,4-benzoquinone, 1,4-naphthoquinone and dimethyl Several procedures were established for the insertion of halogen acetylenedicarboxylate affording the expected oxidized compounds atoms (Br, I, Cl) into the -pyrrolic positions of the corrole macro- 12, 13 and 15, respectively (Scheme 4). The unexpected formation cycle [38, 40-46]. Conventional procedures, such as sulfonation, of compound 14 with dimethyl acetylenedicarboxylate was justified chlorosulfonation afforded the expected derivatives in excellent by a cyclotrimerization process [75]. yields. In particular, the 2,17-dichlorosulfonated corrole 2 has been As dienophile, the Diels-Alder cycloadducts 16 were obtained extensively used as a versatile precursor of corroles with sulfona- in the presence of o-quinone methides generated in situ from mide groups to be used for advanced applications and materials Knoevenagel reaction of 4-hydroxycoumarin or 4-hydroxy-6- [47-54]. The water-soluble carbohydrate corroles 3, pegylated cor- methylcoumarin with paraformaldehyde (Scheme 4) [17]. roles 4, donor-acceptor dyads 5, amino acid corroles 6 and corrole- biotin conjugates 7 are examples of such compounds (Scheme 2). Consequent to interest in 1,3-dipolar cycloaddition reactions in- Some of these derivatives showed promising cytotoxic and antitu- volving tetrapyrrolic macrocycles [76, 77], 5,10,15-tris(pentafluoro- mor activities. phenyl)corrole-3-carbaldehyde 17 was used as a precursor of the 32 Current Organic Synthesis, 2014, Vol. 11, No. 1 Santos et al.

N Py N

C6F5 Ga C6F5 N N

C6F5 12 O

O O

O N Py N N Py N O C6F5 Ga C6F5 C6F5 Ga C6F5 N Py N N N C6F5 Ga C6F5 N N R1 N N C6F5 11 C6F5 COOMe COOMe 13 C6F5 16 MeOOC MeOOC COOMe

COOMe N Py N N Py N C F C F C6F5 Ga C6F5 + 6 5 Ga 6 5 N N N N

C6F5 C6F5 15 14 Scheme 4.

CHO azomethine ylide 18 (Scheme 5), generated in situ, in the presence of N-methylglycine. The ylide was trapped by several dipolaro- N Py N philes like quinones (1,4-benzoquinone, 1,4-naphthoquinone and 1,4-anthraquinone) [78], dimethyl fumarate and dimethyl acetyle- C6F5 Ga C6F5 nedicarboxylate [74] affording corroles 19-27 (Fig. 1). N N In the particular case of 1,4-naphthoquinone and 1,4-anthraqui- N-methylglycine none, besides the expected dehydrogenated cycloadducts 22 and 25, toluene, reflux the quinone-fused corroles 23 and 26 were also obtained. Their for- C6F5 mation can be explained by an 1,5-electrocyclization of the azomethine ylide to a pyrrolo[3,4-b]corrole followed by a Diels-Alder reac- 17 tion with 1,4-naphthoquinone (or 1,4-anthraquinone) [78]. Cavaleiro N Me and co-workers found that the gallium(III) complex 1a reacts with the azomethine ylide formed in situ from the reaction of p-formalde- hyde and N-methylglycine, by affording the dimethylaminomethyl- N Py N corroles [79].

C6F5 Ga C6F5 A similar approach was followed by Gryko and co-workers to prepare free-base corrole-C60 dyads covalently linked in a meso- N N position through rigid and semi-rigid spacers [80, 81].

4. NONLINEAR TRANSMISSION PROPERTIES OF C6F5 CORROLES 18 The advent of nonlinear optics closely followed the invention of the laser [82], with the studies of Franken et al. [83] in the 1960s, Scheme 5. Synthesis and Functionalization of Corroles Current Organic Synthesis, 2014, Vol. 11, No. 1 33

CO Me CO2Me H 2 H CO2Me MeO2C MeO2C MeO2C

N N N H H N N CH3 N N CH3 + N N CH3 C F Ga C6F5 Ga 6 5 Ga C6F5 Py Py Py 19 20 21 O

O O O N O O N N CH3 C F N Ga 6 5 + N N CH3 N N Py 24 C F Ga C6F5 Ga 6 5 Py Py 23 22

O O

O O N N + N N N CH3 N N CH3 N C F C F Ga C6F5 Ga 6 5 Ga 6 5 Py Py Py

25 26 27 Fig. (1). Structures of corrole adducts obtained via 1,3-dipolar cycloaddition reaction. in which coherent, high-intensity laser light having a centre wave- ties to the eyesight of military personnel, optical sensors and both length of 694 nm, irradiated on a sample of crystalline quartz pro- military and commercial aircraft pilots [85]. This has promoted duced radiation at twice the frequency (or half the wavelength, i.e. major developments for optical limiters in military applications in 347 nm) of the incident radiation. Since then, the definition of non- the quest of fulfilling the necessary requirements [86]. An NLO linear optical (NLO) effect evolved and nowadays we mean by this material acting as an optical limiter attenuates powerfully intense the reversible alteration of the optical properties of materials fol- optical beams thus displaying low transmittance under high- lowing the interaction between high-intensity optical fields and the intensity illumination. When lower light intensities interact with the materials themselves. The development of modern optical technol- OL material, the optical beam passes through the limiter material ogy demanded the ability to control the intensity of light in a prede- almost unabsorbed (Fig. 2). fined way. For the manipulation of optical beams in the passive method, NLO materials whose transmittance decreases significantly with increasing light fluence have received much attention since many years ago [84]. Materials with NLO transmission based on the above-mentioned optical response have an important application in the manipulation of optical signals in optical communication and other optical signal processes. These materials and related devices have been named optical limiters or optical power limiters. There is increasing interest in the development of organic optical limiting materials for applications ranging from the protection of optical sensors to laser irradiation and all-optical switching. Hasty progress in laser technology has resulted in new superior laser systems that are efficient, compact and operating at a range of wavelengths. As major consequence, several common applications are omnipresent in everyday life, such as CD players, scanners and lecture presenta- tion pointers, to give a few examples. Military research has also Fig. (2). Variation of the light intensity Iout transmitted by an ideal optical kept the pace: a larger presence of laser functions in ‘friendly roles’ limiter vs the incoming light intensity Iin. In the abscissa axis the threshold and in potential enemy weaponry evidence noteworthy vulnerabili- intensity Ilim at which Iout saturates is indicated. 34 Current Organic Synthesis, 2014, Vol. 11, No. 1 Santos et al.

For many practical applications of the effect of nonlinear increase of the number of possible excited states involved in the transmission (NLT), it is desirable to have reverse saturable absorb- electronic transitions of the corrole [108]. The presence of pyridine ers that allow the complete transmission of light at low optical (py) as axial ligand in the gallium complexes 1a, 25-27 is expected fields over a large spectral window. Many tetrapyrrolic-based mate- to introduce an axial dipole moment that accelerates the processes rials have been demonstrated to fulfill these conditions in the mid- leading to excited state absorption [109]. visible spectral range, including metalloporphyrins [87], metal- Complex 26 is characterized by the presence of 9,10- lophthalocyanines [88-92], metallonaphthalocyanines [93-97] and anthraquinone group fused with the corrole ring, conferring a more hemiporphyrazines [98, 99]. Like the other tetrapyrrolic counter- extended electronic conjugation when compared to the other com- parts, corroles present the important characteristic of displaying plexes 25 and 27. In particular, the dyad-complex 27 has a donor- spectral responses that are strongly dependent on the chemical acceptor (D-A) system with charge-separation features in the ex- structure [100]. Consequently, it is expected that linear and nonlin- cited state the fullerene being the acceptor moiety [80]. It is antici- ear optical properties of corroles are tunable opportunely via suit- pated that all the derivatives 1, 1a, 25-27 display the effect of re- able modifications of the corrole structure through the functionali- verse saturable absorption (RSA) [110] at the wavelength of analy- zation of the meso or -pyrrolic positions [19, 24, 55, 56, 73, 101, sis. To our knowledge this is the first demonstration of a nonlinear 102]. Besides the modulation of the opto-electronic properties, optical effect in the visible spectrum generated by a corrole-based these processes of structural modification are also performed on systems. metallocorroles with the aim of protecting the ring cavity and pre- venting the fragmentation of the corrole macrocycle [103]. With 4.1. Nonlinear Optical Transmission Measurements and these premises we decided to consider the so far unexplored possi- Spectroscopic Determinations bility of using a metal free and a series of gallium corroles, i.e. prototypic metallocorroles like zinc tetraphenylporphyrins in the Nonlinear optical transmission measurements were carried out family of metalloporphyrins [104], in the role of photoactive mate- with 9-ns pulses at 532 nm of a doubled Nd:YAG laser (Quantel rials for optical limiting (OL) purposes [105,106] through the de- YG980E) [111]. The energy transmitted by the sample was meas- termination of the nonlinear transmission of ns pulses at 532 nm. At ured with a pyroelectric detector (Scientech model SPHD25) at 2 this regard, we decided to compare the nonlinear transmission pro- Hz in an open-aperture configuration. Nonlinear transmission val- perties of corrole 1 and of gallium complexes 1a and cycloadducts ues are obtained as an average of 10 experimental determinations. 25-27 obtained via 1,3-dipolar cycloaddition reactions (Fig. 3). The intensity of the incident pulses was controlled with a /2 wave plate and a polarizing cube beam-splitter. The area of the incident beam on the sample was 0.050 cm2. The measurements were carried out in 2-mm-thick glass cells. The concentration of the solutions of O free-base and Ga(py) corroles 1, 1a, 25-27 used for the experiments was in the range 5-15*10-5 M in toluene. For a meaningful com- O parison of the nonlinear transmission properties all the solutions of NH HN corroles 1, 1a, 25-27 had linear transmission of T0 = 0.75 at the wavelength of analysis (532 nm). The photochemical stability of C6F5 C6F5 N the solutions under the conditions of intense irradiation necessary NH N N N CH3 for the evaluation of the OL performance was checked by compar- Ga C6F5 ing the linear transmission spectra before and after the measurement of nonlinear transmission. The linear optical spectra of the solutions Py C F were recorded with a Varian Cary 5 UV-vis spectrophotometer. 6 5 25 Emission and excitation spectra of corroles 1, 1a, 25-27 were de- 1, M=3H termined in degassed toluene solution using a Perkin-Elmer LS-50B 1a, M=Ga(py) spectrometer. Emission spectra were recorded utilizing solutions of corroles with absorbance 0.15 at the wavelength of excitation in order to minimize phenomena of self-absorption of the emitted O radiation.

4.2. UV-vis Absorption and Emission Spectra of Corroles 1, 1a, O 25-27 The electronic absorption spectra of metal-free corrole 1 and Ga(III)(py) corroles 1a, 25 and 27 in toluene present a doubly N N N peaked Soret type absorption band in the range 350-450 nm with 5 -1 -1 Ga C6F5 molar extinction coefficients in the order of 10 M cm (Figs 4-8). N N CH3 In addition to that, there is another group of doubly peaked ab- Py C F Ga 6 5 sorption bands at 500-650 nm with Q-character [3, 112], which is 26 Py characterized by molar extinction coefficients in the order of 1- 27 5*10-4 M-1 cm-1 for corroles 1, 1a, 25, 27. The general presence of doubly peaked features in the absorption spectra of corroles derives Fig. (3). Corrole macrocycles studied for NLO properties. mainly from their lower degree of symmetry with respect to por- The aim was to deduce possible correlations between nonlinear phyrin and phthalocyanine analogs [113, 114]. transmission properties and the structural features of the five com- We noticed that the absorption spectrum of the corrole-fullerene pounds chosen. The presence of C6F5 on all three meso positions of dyad 27 (Fig. 8) did not present features associable to the fullerene the selected compounds have a favorable effect of such type of moiety. In the absorption spectrum of the dioxonaphthopyrrolyl fluorinated substituent on the optical limiting performance and substituted corrole 25 (Fig. 6) there is a slight broadening of the nonlinear optical properties of tetrapyrrolic complexes [107]. Also groups of Soret and Q-bands which could be due to the occurrence the unsymmetrical pattern of -substitution in metallocorroles 25- of some intermolecular aggregation in the solvent used to perform 27, impart a push-pull effect on the ring plane that contributes to the these studies. In the spectra of the anthraquinone substituted corrole Synthesis and Functionalization of Corroles Current Organic Synthesis, 2014, Vol. 11, No. 1 35

1.0x105

1.0x105 -1 -1

4 cm -1

5.0x10 cm

-1 5.0x104 / M / M

0.0 0.0 400 500 600 700 800 400 500 600 700 800

/ nm / nm

/ a.u. / a.u. em I em I

400 500 600 700 800 400 500 600 700 800 / nm / nm Fig. (6). (Top) absorption spectrum of corrole 25 in toluene; (bottom) exci- Fig. (4). (Top) absorption spectrum of free-base corrole 1 in toluene; (bot- tation (black trace) and emission (grey trace) spectra of corrole 25 in tolu- tom) excitation (black trace) and emission (grey trace) spectra of corrole 1 ene. The emission spectrum has been obtained upon excitation at 427 nm, in toluene. The emission spectrum has been obtained upon excitation at 412 which corresponds to the wavelength of maximum absorption of 25. The nm, which corresponds to the wavelength of maximum absorption for 1. excitation spectrum has been obtained by collecting the emission at 604 nm, The excitation spectrum has been obtained by collecting the emission at 648 i.e. the wavelength of maximum emission of 25. nm, i.e. the wavelength of maximum emission of 1. 4x104

3x104 2x105 -1 -1 2x104 cm cm

-1 -1 1x105

/ M 4 / M 1x10

0 0 400 500 600 700 800 400 500 600 700 800 / nm / nm

/ a.u. / a.u. em em I I

400 500 600 700 800 400 500 600 700 800 / nm / nm

Fig. (5). (Top) absorption spectrum of Ga(py) corrole 1a in toluene; (bot- Fig. (7). (Top) absorption spectrum of corrole 26 in toluene; (bottom) exci- tom) excitation (black trace) and emission (grey trace) spectra of corrole 1a tation (black trace) and emission (grey trace) spectra of corrole 26 in tolu- in toluene. The emission spectrum has been obtained upon excitation at 425 ene. The emission spectrum has been obtained upon excitation at 426 nm, nm, which corresponds to the wavelength of maximum absorption for 1a. which corresponds to the wavelength of maximum absorption of 26. The The excitation spectrum has been obtained by collecting the emission at 620 excitation spectrum has been obtained by collecting the emission at 676 nm, nm, i.e. the wavelength of maximum emission of 1a. i.e. the wavelength of maximum emission of 26. 36 Current Organic Synthesis, 2014, Vol. 11, No. 1 Santos et al.

All the excitation spectra of corroles 1, 1a, 25-27 have been de- 1.0x105 termined at the wavelengths of maximum emission individuated in the fluorescence spectra. These present profiles that recall the absorption spectra (Figs. 4-8). The corrole-fullerene dyad 27 was -1 characterized by relatively weak emission when compared to the

cm other corroles. It is believed that such a behavior is due to the rapid -1 5.0x104 occurrence of charge-transfer phenomena between the fullerene

/ M (acceptor) and corrole (donor) moieties with consequent decrease of the lifetime of the emissive state and reduction of fluorescence intensity [80]. This observation is particularly striking since the coordinating group Ga(III)(py) is known to warrant high yields of 0.0 fluorescence in corrole-based complexes with respect to other metal 400 500 600 700 800 / nm centers [117].

4.3. Nonlinear Transmission at 532 nm of Corroles 1, 1a, 25-27 The profiles of the nonlinear transmission vs the incident fluence of 9 ns laser pulses at 532 nm are presented in Fig. 9 for corroles 1, 1a, 25-27. / a.u.

em 0.8 I

0.7

0.6 400 500 600 700 800 / nm 1 0.5 1a Fig. (8). (Top) absorption spectrum of the fullerene-corrole dyad 27 in tolu- 25 26

ene; (bottom) excitation (black trace) and emission (grey trace) spectra of Transmittance at 532 nm 27 dyad 27 in toluene. The emission spectrum has been obtained upon excita- 0.4 tion at 427 nm, which corresponds to the wavelength of maximum absorption of 27. The excitation spectrum has been obtained by collecting the 0.01 0.1 1 emission at 607 nm, i.e. the wavelength of maximum emission of dyad 27. F / J cm-2 in

26 (Fig. 7) it is observed a wide broadening of the bands within the Fig. (9). Nonlinear variation of the transmittance at 532 nm of the toluene

Soret range 350-450 nm accompanied by an additional band cen- solutions of corroles 1, 1a and 25-27 with the incident fluence (Fin). Radia- tered at 465 nm. A red shift of the absorption bands on Q band- tion was produced with ns laser pulses. The linear transmittance at the region is observed when compared with the ones observed for cor- wavelength of analysis was 0.75 for all corrole solutions. These had the roles 1a and 25 (663 nm versus 600 nm) together with an increase concentration values: 6.7*10-5 M (1); 5.2*10-5 M (1a); 9.7*10-5 M (25); -4 -5 in the intensity of the Q band relatively to that of the Soret band. 1.33*10 M (26); 9.6*10 M (27). These features can be due to some intermolecular aggregation and We observed the decrease of transmission with increasing inci- also as a result of the expansion of the -system and the lowering of dent fluence for all corroles under investigation. In the nonlinear corrole symmetry [115]. Another evidence that is in agreement with optical (NLO) regime generated by the incident fluence ranging in these spectroscopic features is the fact that molar extinction coeffi- -2 the interval 0.01 < Fin < 1.2 J cm with 9 ns laser pulses there is no cients values were of the same order of magnitude (comprised in observation of the saturation of transmittance [118] for all corrole 4 -1 -1 the range 1-4*10 M cm ) for the Soret and Q-bands (Fig. 7), with solutions. Moreover, in the same range of Fin, corrole solutions a pronounced decrease of the absorption properties of 26 with re- having linear transmittance of 0.75 do not experience the condition spect to corroles 1, 1a, 25 and 27 in correspondence of the Soret of limiting threshold, i.e. do not reach the fluence value at which bands. the nonlinear transmission is half the linear transmission [119]. The observed NLO behavior of this series of corroles is indicative of the The emission and excitation spectra of corroles 1, 1a, 25-27 are shown in Figs. 4-8. The emission spectra of the five corrole deriva- occurrence of reverse saturable absorption (RSA) [110], i.e. the tives under investigation have been obtained upon excitation of the phenomenon of the reversible formation of excited states with larger absorption coefficients with respect to the parent ground state wavelength of maximum absorption in the interval of Soret bands. Corroles 1, 1a, 25-27 display a characteristic Stokes shift in the upon increase of the incident light flux [120]. The mechanism at the emission spectra with respect to the group of absorption bands of basis of such nonlinear optical behavior is sequential two- or multiphoton absorption (Fig. 10), in analogy with the photophysics of the the Q-region. The maxima of emission of 1, 1a, 25-27 are located in the range 600-650 nm, and are accompanied by vibronic structures other tetrapyrrolic macrocyclic analogs [110]. at higher wavelengths. The emission intensity of all corroles here Since the excited state(s) of corroles 1, 1a, 25-27 absorbs light considered was not dependent on the presence of molecular oxygen pulses with 9 ns of duration, the lifetime of the strongly absorbing in solution. This finding is indicative of the fluorescent nature of excited state of these corroles must be longer than 9 ns. Moreover, the emission in this series of cycloadducts [116]. In the sole case of if the strongly absorbing excited state(s) is not directly formed by corrole 26, i.e. the gallium complex with condensed anthraquinone the ground state upon absorption of the first photon, then the inter- as substituent, the emission spectrum is not obtained as mirror im- nal conversion time tIC = 1 / kISC (see Fig. 10 for the explanation of age of the group of Q absorption bands (Fig. 7). Coherent to what the meaning of kISC), i.e. the time required to form the strongly ab- observed in the absorption spectrum, the emission spectrum of 26 sorbing excited state, must be necessarily shorter than pulse dura- 8 -1 also presents features ascribable to the existence of multimolecular tion, i.e. tIC < 9 ns, or kISC > 1.1*10 s [121]. In the case of corroles species having distinct spectral/emissive features. the internal conversion process is expected to be an intersystem Synthesis and Functionalization of Corroles Current Organic Synthesis, 2014, Vol. 11, No. 1 37

extended network of electronic conjugation. In the case of corrole 25 this extension is not strongly coupled with the corrole macrocycle as testified by the lack of any absorption maxima shift in its linear optical spectra with respect to -unsubstituted Ga(py) corrole 1a. A different situation is observed for corrole 25 where the expansion of the -system is demonstrated by its spectroscopic data (Figs. 5-8). It is expected that the presence of such large conjugated substituents either condensed or not with the corrole macrocycle, impart an increase of the electronic polarizability which manifests itself with the relatively low fluence onsets of the NLO regimes for -2 25 and 26 (at Fin > 0.09 J cm , Fig. 9) with respect to beta- -2 unsubstituted corroles 1 or 1a (at Fin > 0.15 J cm , Fig. 9). Fig. (10). Jablonski diagram reporting the mechanism of sequential two- photon absorption of ns light pulses by corroles 1, 1a, 25-27 in the visible The NLO absorption profile of the corrole-fullerene dyad 27 did not present any distinctive signature or ameliorative effect on spectral range. S0, S1 and T1(2) indicate the ground state, first excited singlet state and first (second) excited triplet state, respectively. kISC indicates the the optical nonlinearity of 27 associated with the fullerene moiety rate of intersystem crossing. 0, 1, S1, T1 and T2 are the ground-state ab- since the NLO response of 27 at 532 nm for 9 ns pulses was less sorption cross-section, excited triplet state absorption cross-section, lifetime pronounced than that of the other two substituted corroles 25 and of the first excited singlet state, lifetime of the first excited triplet state and 26. This finding proves that possible charge transfer phenomena, lifetime of the second excited triplet state, respectively. which are quite common in fullerene-tetrapyrrolic macrocycles dyads and alter considerably the linear and NLO properties of the Table 1. Ground-state ( 0) and excited state ( 1) absorption cross- dyad with respect to the separated moieties [80], occur in dyad 27 at sections at 532 nm for corroles 1, 1a, 25-27. The ground- a lower rate with respect to that of the corrole-based excited state state absorption cross-section is correlated with the linear transition under ns pulse irradiation. transmittance To by the relationship To = exp(- 0NoL)

where No is number of absorbing molecules per cubic cen- The photostability of corroles 1, 1a, 25-27 in the NLO regime timeter, and L the optical path (in cm) in the absorbing has been systematically checked by comparing the transmission medium. Minimum nonlinear transmittance (Tmin) at 532 spectra of the corrole solutions taken before and after the recording nm obtained from the nonlinear transmission data of Fig. of nonlinear transmission curves (Figs. 11-15). (9). Tmin is correlated with s1 by the relationship Tmin = exp(- 1NoL) assuming that all molecules are present in the same The corroles we examined present quasi identical linear trans- absorbing excited state at the maximum fluence experi- mission spectra in the wavelength range of analysis before and after enced by the system [123] the determination of the nonlinear absorption profiles (Fig. 9). 100

2 2 Compound 0/cm Tmin 1/cm 80 1 3.6*10-17 0.61 6.1*10-17

1a 4.6*10-17 0.53 10.1*10-17 60

25 2.5*10-17 0.38 8.3*10-17 40 26 1.8*10-17 0.40 5.7*10-17 before NLT 27 2.5*10-17 0.45 6.9*10-17 20 Transmittance / % after NLT crossing the rate of which is increased by the presence of heavy 0 metals in the complex. This is evident by comparing the nonlinear 300 400 500 600 700 800 900 transmission curves of metal free-base corrole 1 and metallocorrole / nm 1a (Fig. 9), which differ solely for the nature of the central coordinating atom. The comparison shows that the onset of the nonlinear Fig. (11). Comparison of the transmission spectra of free-base corrole 1 regime occurs at lower fluence for metallocorrole 1a with respect to taken before and after the recording of the nonlinear transmission curve metal-free corrole 1. Moreover, compound 1a reaches a lower value (Fig. 9). of minimum transmittance (Tmin, Table 1) with respect to 1 at the 100 maximum fluence of analysis. These findings are mainly correlated with an acceleration of the process of intersystem crossing in pass- 80 ing from 1 to 1a although this is not the only possible cause of such a difference. In fact, one also has to take into account that free-base corrole 1 is characterized by a lower value of ground-state absorp- 60 tion cross-section (0) with respect to metallocorrole 1a at 532 nm (Table 1), which implies that the rate of optical pumping defined as 40 0 Iin / h is lower for corrole 1 with respect to 1a at the wavelength of analysis. Finally, another possible factor at the basis of the dif- 20 ferent NLO behaviors of 1 and 1a at 532 nm can be the difference Transmittance / % before NLT of excited state absorption cross-section values in the two macrocy- after NLT cles at that specific wavelength. 0 300 400 500 600 700 800 900 The best performing corroles in terms of optical limiting effect / nm are the Ga(py) corroles 25 and 26 since they reach the lowest values Fig. (12). Comparison of the transmission spectra of Ga(py) corrole 1a of Tmin at about 0.40 (Fig. 9 and Table 1). The structures of these taken before and after the recording of the nonlinear transmission curve species are characterized by the presence of substituents having an (Fig. 9). 38 Current Organic Synthesis, 2014, Vol. 11, No. 1 Santos et al.

100 of intermolecular aggregation and/or -system expansion (Fig. 7), does not present any decrease of the intensity of the absorption 80 band at about 465 nm (Figs. 7 and 14) which is generated by exci- tonic coupling in a possible stable intermolecular aggregate [115] 60 after the NLO experiment. This indicates the absence of any disag- gregating effect of the ns pulsed radiation on corrole 26 and proba- bly the spectroscopic features are due to a -system exten- 40 sion/alteration.

Transmittance / % Transmittance 20 before NLT CONCLUSION after NLT 0 The access to new corroles via functionalization of meso- 300 400 500 600 700 800 900 triarylcorroles reached a high success due to the contribution of / nm different research groups all over the world. New derivatives are now accessible for different applications. In the present contribution Fig. (13). Comparison of the transmission spectra of the peripherally substituted Ga(py) corrole 25 taken before and after the recording of the nonlinear the NLO properties of the free base corrole 1 and of the gallium(III) transmission curve (Fig. 9). complexes 1a, 25-27 were determined. In this corrole series only the quinone fused metallocorrole 26 showed spectral features typi- 100 cal of intermolecular aggregation and/or expansion of the conjugated system. 80 The nonlinear transmission of the selected derivatives has been determined at the wavelength of analysis 532 nm in order to evalu- 60 ate their optical limiting properties of ns laser pulses. All corroles examined displayed the effect of reverse saturable absorption 40 (RSA) in the visible spectrum upon irradiation with ns laser pulses. Some correlations between the chemical structure of the corroles 20 here examined and their NLO behaviour could be determined. In Transmittance / % Transmittance before NLT after NLT particular, when corroles 1 and 1a were confronted we found that the replacement of central H atoms with Ga(py) as central coordi- 0 300 400 500 600 700 800 900 nating group brings about the lowering of the onset of the NLO / nm regime. This was mainly ascribed to the acceleration of the process of formation of the highly absorbing excited state in passing from Fig. (14). Comparison of the transmission spectra of the naphthoquinone free-base 1 to gallium(III)(pyridine) complex 1a by virtue of the substituted Ga(py) corrole 26 taken before and after the recording of the heavy-atom effect induced by the gallium atom. The peripherally nonlinear transmission curve (Fig. 9). substituted corroles 25-27 displayed the largest optical nonlineari- 100 ties. No evidence of electronic coupling between the corrole macrocycle and the electronically conjugated network of the substituents 80 in corroles 25-27 could be evinced from the combined analysis of the optical spectra and the NLO transmission. 60 CONFLICT OF INTEREST

40 The authors confirm that this article content has no conflict of interest. 20 before NLT ACKNOWLEDGEMENTS Transmittance / % Transmittance after NLT 0 Thanks are due to Fundação para a Ciência e a Tecnologia 300 400 500 600 700 800 900 (FCT, Portugal), European Union, QREN, FEDER and COMPETE / nm for funding the QOPNA research unit (project PEst- Fig. (15). Comparison of the transmission spectra of the corrole-fullerene C/QUI/UI0062/2013; FCOMP-01-0124-FEDER-037296). The dyad 27 taken before and after the recording of the nonlinear transmission projects PPTDC/DG/QUI/82011/2006 and PTDC/QUI/74150/2006 curve (Fig. 9). and the Portuguese National NMR Network, also supported this research. Santos, C.I.M., Barata J.F.B. and Calvete M.J.F. thank Remarkably high photostability is found for the non peripher- FCT for the grants SFRH/BD/64155/2009, SFRH/BPD/63237/2009 ally substituted corroles 1 and 1a, i.e. the systems with the weaker and SFRH/BPD/26775/2006, respectively. Dini D. acknowledges optical limiting properties, whereas corroles 25, 26 and, in particu- the financial support from the Italian Ministry of Instruction, Uni- lar corrole 27, i.e. the most limiting systems, experience enough versity and Research through the project PRIN 2010 (Protocol No. optical stress under ns pulses irradiation to display a more pro- 20104XET32). nounced decrease of linear transmission in the near UV and NIR after NLO performance while corrole 26 decreases its transmittance REFERENCES uniformly also in the visible range (Figs. 13-15). Since the spectral [1] Johnson, A.W.; Kay, I.T. Corroles. Part I. Synthesis. J. Chem. Soc., 1965, changes of corroles 25-27 consist mainly of an unstructured de- 1620-1629. crease of optical transmission, it is believed that the NLO experi- [2] Nardis, S.; Monti, D.; Paolesse, R. Novel aspects of corrole chemistry. Mini ment induces an increase of the concentration of the photoactive Rev. Org. Chem., 2005, 2, 355-374. species due to the partial evaporation of the solvent in NLO regime [3] (a) Hush, N.S.; Dyke, J.M.; Williams, M.L.; Woolsey, I.S. Electronic spectra of metal corrole anions J. Chem. Soc. Dalton Trans. 1974, 395-399; (b) following the heating of the sample with no alteration of the corrole Ghosh, A.; Jynge, K. Molecular structures and energetics of corrole isomers: chemical structure. It is remarkable that the naphthoquinone substi- a comprehensive local density functional theoretical study. Chem. Eur. J., tuted corrole 26, i.e. the system which presented spectral evidences 1997, 3, 823-833. Synthesis and Functionalization of Corroles Current Organic Synthesis, 2014, Vol. 11, No. 1 39

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Received: September 27, 2013 Revised: October 09, 2013 Accepted: October 10, 2013