& BODIPY Chromophores

Tuning the Photophysical Properties of BODIPY Molecules byp-Conjugation with Fischer Carbene Complexes**

Gong Ming Chu,[a] Andr�s Guerrero-Mart�nez,[b] Israel Fern�ndez,*[a] andMiguel �ngel Sierra*[a]

Abstract: The synthesis, structure, and photophysical prop-erties of novel BODIPY–Fischer alkoxy-, thio-, and aminocar-bene dyads are reported. The BODIPY chromophore is di-rectly attached to the carbene ligand by an ethylenic spacer,thus forming donor–bridge–acceptor p-extended systems.The extension of the p-conjugation is decisive in the equilib-rium geometries of the dyads and is clearly reflected in thecorresponding absorption and emission spectra. Whereasthe BODIPY fragment is mainly isolated in aminocarbenecomplexes, it is fully conjugated in alkoxycarbene deriva-

tives. The former thus exhibit the characteristic photophysi-cal properties of BODIPY units, whereas complete suppres-sion of the BODIPY fluorescence emission is observed in thelatter, as a direct consequence of the strong electron-accept-ing character of the (CO)5M=C moiety. As the p-acceptorcharacter of the metal–carbene group can be modified, theelectronic properties of the conjugated BODIPY can betuned. Density functional calculations have been carried outto gain insight into the photophysical properties.

Introduction

Since the synthesis of the first 4,4-difluoro-4-borata-3a-azonia-4a-aza-s-indacene in 1968 by Treibs and Kreuzer,[1] boradiazain-dacenes (BODIPYs) have become highly useful molecular enti-ties in fields such as materials science or medicinal chemistry.[2]

This is mainly due to the fascinating optical properties associ-ated with this family of compounds. Thus, their excellent ther-mal and photochemical stabilities, high fluorescence quantumyields, negligible triplet-state formation, and intense absorp-tion profiles have transformed BODIPY species into quite at-tractive materials with applications in, for instance, biomole-cules labeling, ion sensing and signaling, energy transfer cas-settes, or light harvesting systems.[2] For these reasons, it is notsurprising that numerous synthetic methods have been devel-oped to produce new BODIPY derivatives with interesting andtunable photophysical properties.[2, 3] In this sense, the attach-

ment of BODIPY units to a transition-metal center to preparea dyad is highly attractive. The great number of possible modi-fications (that is, variation of the transition metal and/or its co-ordination sphere) that can be effected in these systems may,in principle, allow the properties of the BODIPY–transition-metal dyad to be tuned. However, in most of the reporteddyads the metal centers were attached onto the peripheralmoieties of the BODIPY fragment[4–6] rather than to its p-conju-gated core.[7, 8] As a result, the impact of the transition-metalfragment on the properties associated with the BODIPY chro-mophore is not that remarkable.

At this point, we turned our attraction to Group 6 Fischercarbene complexes because of their exceptional properties aselectron-withdrawing groups.[9] The p-acceptor ability of thesespecies has resulted not only in their wide use in organic syn-thesis (that is, Fischer carbene complexes behaving similarly toa Lewis acid complexed carbonyl function),[10] but also in theiremerging role as building blocks in the field of new materi-als.[11]

As the modification of the electronic properties of the car-bene ligand has proved to be relatively easy,[9] we hypothe-sized that the direct attachment by p-conjugation ofa Group 6 Fischer carbene complex to the p-core of BODIPYcould have a significant impact on the photophysical proper-ties of the BODIPY moiety. Herein we report the successful ac-complishment of such hypothesis, which allows the prepara-tion of new organometallic compound–BODIPY dyads possess-ing tunable photophysical properties.

[a] G. M. Chu, Dr. I. Fern�ndez, Prof. Dr. M. �. SierraDepartamento de Qu�mica Org�nicaFacultad de Qu�mica, Universidad Complutense de Madrid28040 Madrid (Spain)Fax: (+ 34) 913944310E-mail : israel@quim.ucm.es

sierraor@quim.ucm.es

[b] Dr. A. Guerrero-Mart�nezDepartamento de Qu�mica F�sica IFacultad de Ciencias Qu�micas, Universidad Complutense de Madrid28040 Madrid (Spain)

[**] BODIPY=boradiazaindacene

Supporting information for this article (including NMR spectra of isolatedcompounds, excitation spectra, Cartesian coordinates, and total energies ofall of the stationary points discussed in the text) is available on the WWWunder http://dx.doi.org/10.1002/chem.201303952.

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Full PaperDOI: 10.1002/chem.201303952

Results and Discussion

Synthesis and structure of the new BODIPY derivatives

Alkoxy-BODIPY–Fischer carbene complexes 3 were preparedfrom the pentacarbonyl[(ethoxy)(methyl)]carbenechromium(0)or tungsten(0) complexes 1 a or 1 b and the correspondingformyl-BODIPY 2 a-c in the presence of Et3N and TMSCl follow-ing the reaction conditions reported by Aumann andHeinen.[12] Thus, Et3N (4 equiv), trimethylsilyl chloride (TMSCl,3 equiv), and complex 1 (1.5 equiv) were added to a solutionof the aldehyde (1 equiv) in anhydrous Et2O at room tempera-ture and the mixture was stirred for 24 h. Removal of the sol-vent and purification of the residue by column chromatogra-phy allowed the isolation of the new BODIPY complexes 3 inmoderate to good yields (Scheme 1). Furthermore, aminocar-benes 4 were prepared from alkoxycarbene complexes 3 by anaminolysis reaction using dimethylamine at �100 8C in anhy-drous tetrahydrofuran in excellent yields.

The structures of carbene complexes 3 and 4 were estab-lished by NMR experiments and by comparison with the datareported for related chromium(0) and tungsten(0) a,b-unsatu-rated Fischer carbene complexes.[12–14] Thus, the carbenecarbon atom directly attached to chromium(0) in alkoxycar-benes 3 appears at d�325 ppm in the corresponding 13C NMRspectra, while a value of d�270 ppm was found in aminocar-benes 4 as a result of the well-known higher p-donation of theNMe2 group to the pz atomic orbital of the carbene carbonatom compared to the OEt group. Moreover, the olefinic hy-drogen atoms of the tether joining the BODIPY and the metal–

carbene fragments exhibit a coupling constant of J = 15–17 Hz,which is attributable to an E stereochemistry of the doublebond. With these characteristics, two possible isomers of thecarbene complexes 3 and 4, which maintain the E-stereochem-istry and the s-trans arrangement but with a different relativeorientation of the BODIPY and metal–carbene fragments, canbe envisaged (Scheme 2).

Density functional theory (DFT) calculations carried out atthe B3LYP/def2-SVP level (see the computational details in theExperimental Section) on complexes 3 aa (where the OEtgroup was replaced by a methoxy group) and 4 aa reveal thatboth isomers are nearly isoenergetic in the gas phase(DE(3 aaI–3 aaII) = 0.4 kcal mol�1 and DE(4 aaI–4 aaII) = 0.1 kcalmol�1). Interestingly, a X-ray diffraction analysis of suitable crys-tals of carbene complex 3 ba (grown in pentane/ethyl acetatesolution at �20 8C) indicates that in the solid state, isomer I ispreferred (Figure 1).[15] This suggests that the crystal packinghas a strong influence on the conformation adopted by thesespecies. Moreover, and as expected, the ethoxy group directlyattached to the carbene carbon atom adopts the so-called anticonformation (that is, the CH2 group is oriented towards theCO wall of the metal fragment), which is the preferred confor-mation for most of the alkoxy Fischer carbene complexes notonly in the gas phase but also in the solid state.[16]

Another important geometrical feature revealed by this ex-perimental–computational study is the high planarity exhibitedby alkoxycarbenes 3 (computed Cr-C-C-C dihedral angle of 0.08and 0.48 in 3 aa and 3 ac, respectively)[17] which reflects wellthe extent of p-conjugation between the BODIPY and metal–carbene fragments through the ethylene bridge. Differently,these fragments are not coplanar in aminocarbenes 4 (comput-ed Cr-C-C-C dihedral angle of 55.88 in 4 aa), thus indicating thelack of p-conjugation between them (Figure 2). This markeddifference in the structures of complexes 3 and 4 can be as-cribed to the stronger p-donating ability of the nitrogen atomto the to the pz atomic orbital of the carbene carbon atomcompared to the more electronegative oxygen atom. As a con-sequence, the p-acceptor nature of the pentacarbonylmetal–carbene fragment is clearly diminished in aminocarbenes 4,which cannot accept additional electronic density from theBODIPY fragment, which in turn behaves as a p-donor groupin alkoxycarbene complexes 3 (see below). An important con-clusion can be drawn from this simple structural analysis : theethylene-BODIPY fragment is mainly isolated in aminocarbenecomplexes 4, whereas it is fully conjugated in alkoxycarbeneScheme 1. Synthesis of carbene complexes 3 and 4.

Scheme 2. Possible isomers of complexes 3 (X = OEt) and 4 (X = NMe2).

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complexes 3. Therefore, itshould be expected that the eth-ylene-BODIPY moiety in com-plexes 4 would behave similarly(in terms of photophysical prop-erties) to BODIPY units possess-ing an ethylene group in posi-tion 2, while markedly differentproperties should be expectedfor alkoxy substituted complexes3 (see below).

The enhanced p-conjugationin complexes 3 should be in ad-dition reflected in measurablechanges in the equilibrium ge-ometry of these species. Asa consequence of the strong p-acceptor ability of the {(CO)5M =

C(OR)} fragment, the contribu-tion of the resonance structure IIdepicted in Scheme 3 becomesmore important in the descrip-tion of the equilibrium geometryof complexes 3. In contrast, reso-nance form III describes betterthe equilibrium geometry ofcomplexes 4. The population ofform II thus involves the centralC2=C3 double bond becomingweaker whereas the C1�C2bond becomes stronger. This be-comes clear when comparingthe corresponding Wiberg BondIndices (WBIs) computed for 3 aaand 4 aa : WBI(C1�C2): 1.21versus 1.12 and WBI(C2=C3):1.61 versus 1.71, respectively.Moreover, the strong p-donor

ability of the NMe2 compared to the OEt group is clearly re-flected in the Cr�Ccarbene bond strength, which is significantlyweaker in 4 aa (WBI = 0.74) than in 3 aa (WBI = 0.88).

Absorption and emission properties

Figure 3 a, b shows the UV/Vis spectra of all of the dyads 3 and4, respectively, recorded at 298 K in hexane with a concentra-tion of 10�5 mol L�1 (see also Table 1). The spectra of alkoxycar-bene complexes 3 exhibit two main absorption bands at thevisible region (Figure 3 a). The less intense peak at about425 nm can be tentatively assigned to a ligand-field (LF) transi-tion involving the carbene ligand. Additionally, the intense andbroad band at around 570 nm can be assigned to a metal-to-ligand charge transfer (MLCT) absorption band. Moreover,a shoulder is found at about 520 nm in the MLCT band, whichis more noticeable in 3 aa and 3 ab complexes, that can be rea-sonably assigned to a transition involving the BODIPY frag-

Figure 1. ORTEP of compound 3 ba. Ellipsoids are set at 30 % probability.

Figure 2. Optimized geometries (B3LYP/def2-SVP level) of complexes 3 aaand 4 aa.

Scheme 3. Resonance forms for complexes 3 (X=OEt) and 4 (X=NMe2).

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ment by comparison with the spectra of the parent formyl-BODIPY species 2 (see inset in Figure 3 a). This assignment is ingood agreement with the corresponding maxima (lmax

�527 nm) of the excitation spectra of alkoxycarbene com-plexes 3 (see Table 1 and the Supporting Information). Differ-ent results are obtained in aminocarbene complexes 4 (Fig-ure 3 b), which exhibit more intense MLCT absorption bands atabout 530 nm. Noticeably, no significant differences have beenobserved in the UV/Vis spectra of the tungsten(0) carbenecomplexes 3 ba and 4 ba.

Compared to related styryl ethoxycarbene complexes,[14] thepresence of the BODIPY substituent in complexes 3 inducesa significant bathochromic shift on the MLCT absorption band(from ca. 475 nm in styryl ethoxycarbene complexes to ca.570 nm in complexes 3). This is a quite remarkable result as it

was demonstrated that the position of the MLCT band in thistype of complexes remains mostly unaltered regardless theelectronic nature of the substituent attached to the styrylgroup linked to the metal–carbene fragment.[14] A comparablered-shift has only been observed in biscarbene complexes,where the cooperative effect of both {(CO)5M = C(OR)} frag-ments greatly enhances the p-conjugation between the tetherand the metal–carbene moieties.[18] Therefore, dyads 3 can beconsidered as new push–pull systems with extended p-conju-gation, where the BODIPY fragment behaves as a p-donor sub-stituent forced by the strong electron-withdrawing effect ofthe pentacarbonylmetal–carbene unit through the ethylenicspacer.[19]

Differently, the bathochromic shift induced in the MLCT ab-sorption band of aminocarbene complexes 4 is not that signifi-cant (labs�530 nm), reflecting the lower extension of the p-conjugation between the BODIPY and the metal–carbene frag-ments in these complexes. Moreover, and as expected fromthe lack of coplanarity between the indacene and the C8-arylfragment owing to the steric hindrance of the indacenemethyl groups (Figures 1 and 2), no dependence of the posi-tion of the MLCT band with the substitution in the C8-arylfragment was observed in complexes 3 or 4.

Time-dependent DFT (TD-DFT) calculations were carried outon complexes 3 aa and 3 ac to determine the nature of the ver-tical transitions associated with the observed UV/Vis absorp-tions. A very good agreement between the vertical excitationenergies (computed in hexane as solvent) and the wavelengthsof the absorption maxima in the experimental UV/Vis spectrawas found, which allows the accurate assignment of the exper-imentally observed bands. Thus, our calculations indicate thatthe band at about 570 nm is the result of the one-electronpromotion from the HOMO to the LUMO. As shown inFigure 4, the HOMO is a p-molecular orbital that is mainly lo-cated at the metal fragment with a small contribution at theBODIPY moiety. In contrast, the LUMO is a p-molecular orbitalthat is fully delocalized within the indacene moiety. Therefore,the observed band can indeed be considered as a MLCT bandwith a remarkable p–p* character. As expected, no contribu-tion of the aryl fragment attached to the C8 position of the in-dacene into the HOMO or the LUMO is observed, thus confirm-ing the spectator role of this substituent on the electronicproperties of these BODIPY–metal carbene dyads.

According to our TD-DFT calculations, the band appearing atabout 425 nm is mainly the result of the promotion of oneelectron from the HOMO�2 to the LUMO. As shown inFigure 4, the HOMO�2 is again a p-molecular orbital that ismainly localized in the indacene fragment with only a smallcontribution coming from the metal fragment. In agreementwith our initial tentative assignment, this band can be consid-ered as a p–p* LF transition involving the BODIPY moiety.

To compare the energy transfer process after excitation ofthe energy-push component in dyads 3 and 4, we have toconsider that no selective excitation on the BODIPY absorptionband is possible owing to the full overlap with the MLCT ab-sorption band. However, the spectra of complexes 3 and 4show isoabsorptive points at about 460 nm with the parent

Figure 3. a) UV/Vis spectra of the alkoxycarbene complexes 3 aa, 3 ab, 3 ac,and 3 ba. The inset shows the UV/Vis spectra of the parent formyl-BODIPY’s2 a, 2 b and 2 c. b) UV/Vis spectra of the aminocarbene complexes 4 aa, 4 ab,and 4 ba. All of the measurements were carried out in a diluted solution inhexane (concentration 10�5 mol L�1, optical density <0.1) at 298 K.

Table 1. Photophysical properties of complexes 3–5 and parent formyl-BODIPY compounds 2.[a]

Compound labs [nm][b] eabs[d] lem [nm][f] lmax [nm][g]

2 a 499 14.3 506 4982 b 498 10.8 506 4982 c 498 10.0 506 4983 aa 568 (549)[c] 3.2 (0.64)[e] 537 5273 ab 568 3.3 540 5273 ac 567 (545)[b] 3.1 (0.67)[e] 542 5283 ba 568 3.2 541 5274 aa 524 4.9 542 5274 ab 527 4.3 550 5274 ba 535 4.2 534 5275 ca 532 4.9 545 528

[a] All of the samples were recorded at 298 K in hexane (concentration of10�5 mol L�1, optical density <0.1). [b] Maximum of the UV/Vis spectrum.[c] Calculated TD-DFT excitation energy (in nm). [d] Molar extinction co-efficient e (L mol�1 cm�1 � 104). [e] Calculated TD-DFT oscillator strength.[f] Maximum of the emission spectrum using lexc = 460 nm. [g] Maximumof the excitation spectrum using lobs = 600 nm.

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formyl-BODIPY compounds 2, in which the BODIPY moiety isexpected to absorb a similar fraction of the incident light, thusallowing a qualitative understanding of the emissive proper-ties.

Figure 5 shows the emission spectra of complexes 3 and 4.Both type of dyads display broad emission bands centred atabout 545 nm, which are considerably red-shifted (ca. 40 nm)relative to the parent formyl-BODIPY compounds 2, owing tothe presence of conjugated systems in position 2 of the fluoro-phore that lowers its LUMO level and consequently, the energyof the emission transition.[2b] The emission redshifts are ingood agreement with those observed at the excitation spectra(see Table 1 and the Supporting Information), keeping theStokes shift of dyads 3 and 4 approximately constant with re-spect to the parent formyl-BODIPY species 2. From Figure 5, itis noteworthy that the alkoxycarbene complexes 3 show verylow emission intensities. Thepractically total suppression ofthe BODIPY-based emission canbe attributed to a very efficientenergy transfer occurring fromthe higher-lying BODIPY excitedstate to the lower-lying carbenestate owing to p-conjugationwith the metal–carbene frag-ment (see above). In contrast,aminocarbene complexes 4

show a small quenching of the fluorescence (about threetimes lower) when compared with the emission of parentformyl-BODIPY species 2 (see Figure 5). This result reveals thatthe p-conjugation between the ethylene-BODIPY and themetal–carbene fragments is strongly hindered by the introduc-tion of the NMe2 group. Taking into account the similar distan-ces between the BODIPY and metal–carbene moieties withindyads 3 and 4 (B···Ccarbene distance of 7.5 �), a Dexter-typeenergy transfer through-bond mechanism seems to be opera-tive in the observed suppression of the emission properties ofcarbene complexes 3. Therefore, our results indicate that theemission of BODIPY can be easily switched on/off by control-ling the extent of conjugation with the pentacarbonylmetal–carbene moiety.

To further confirm the effect of the p-conjugation on theproperties of these dyads, we finally synthesized complex 5 ca,which possesses a sulfur atom directly attached to the carbenecarbon atom. The synthesis of this complex was conductedsimilarly to complexes 3. Thus, from thiocarbene complex 1 cand formyl-BODIPY derivate 2 a in the presence of Et3N andTMSCl, dyad 5 ca was obtained in 30 % reaction yield(Scheme 4).

As the p-donor ability of the SEt group is lower than that ofNMe2 and the electronegativity of the sulfur atom is lowerthan the electronegativity of oxygen, an intermediate equilibri-

Figure 4. Computed molecular orbitals of complexes 3 aa and 3 ac (isosur-face value of 0.035 au).

Figure 5. Emission spectra of the alkoxycarbene complexes (c) 3 aa, 3 ab,3 ac, and 3 ba, and emission spectra of the aminocarbene complexes (g)4 aa, 4 ab, and 4 ba. Inset: emission spectra of the parent formyl-BODIPYcompounds 2 a, 2 b, and 2 c. All of the measurements were carried out ina diluted solution in hexane (concentration ca. 10�5 mol L�1, optical density<0.1) at 298 K (lexc = 460 nm).

Scheme 4. Synthesis of complex 5 ca.

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um geometry for thiocarbene 5 ca should be expected be-tween the extreme situations represented by the ethoxycar-bene 3 aa and the aminocarbene 4 aa. Indeed, the optimizedgeometry of 5 ca at the B3LYP/def2-SVP level (Figure 6) showsa computed Cr-C1-C2-C3 dihedral angle of 31.08 (which is in-termediate between 0.08 and 55.88 computed for 3 aa and4 aa, respectively). Furthermore, the corresponding WBIs of1.62 and 0.84 for the C2�C3 and Cr�C1 bonds, respectively,confirms well that the extent of p-conjugation in thiocarbeneis also intermediate between that in alkoxy- and aminocarbenecomplexes. As a consequence, the corresponding MLCT bandmaximum appears at about 532 nm (intermediate betweenamino- and alkoxycarbenes; see Table 1) and again, corre-sponds to the HOMO!LUMO vertical excitation according toTD-DFT calculations. Not surprisingly, the fluorescence ofBODIPY in thiocarbene 5 ca is only partially quenched(Figure 7). Thus, an emission band at 545 nm appears in thecorresponding emission spectrum, although its relative intensi-ty respect to the parent formyl-BODIPY compounds 2 is clearlymuch lower (about six times lower). Although the emission ofthis complex is therefore lower than that observed for its ami-nocarbene analogue 4 aa, it is not suppressed as it occurs inalkoxycarbenes. This result supports well the idea that thephotophysical properties of the BODIPY fragment can betuned by subtle modifications of the electronic nature of the

metal–carbene fragment, which controls the extent of p-conju-gation of the dyad.

Conclusion

From the joint experimental–computational study reportedherein, it becomes obvious that the photophysical propertiesof the BODIPY chromophore can be modified, in principle atwill, by means of direct p-conjugation with Fischer carbenecomplexes. The extent of p-conjugation is decisive in alteringthe equilibrium geometry, and consequently, the photophysicalproperties of the BODIPY chromophore. Thus, when an alkoxy-carbene complex is present in the BODIPY–metal carbenedyad, the p-conjugation is maximized. This is reflected in a sig-nificant bathochromic shift of the corresponding MLCT band,which is the result of the p-donor behavior of the BODIPYmoiety. In contrast, p-conjugation is strongly diminished whenan aminocarbene complex is present in the dyad. As a conse-quence, the BODIPY moiety is now mainly isolated in thesecompounds which exhibit a similar fluorescence than formyl-BODIPY derivatives. In contrast, as the electronic structure ofthe BODIPY fragment is strongly altered by p-conjugation withthe alkoxycarbene moiety, complexes 3 exhibit a completesuppression of the fluorescence associated with the BODIPYchromophore. An intermediate situation is found in BODIPY–thiocarbene complex dyads owing to lower electronegativity(compared to oxygen) and p-donor ability (compared to nitro-gen) of the sulfur atom, which confirms the importance of p-conjugation in controlling the properties of the BODIPY units.

It is hoped that the present study will be of future use inmaterials chemistry owing to the ease of tuning the photo-physical properties of BODIPY units by subtle modifications onthe metal–carbene moiety.

Experimental Section

Computational details

Geometry optimizations without symmetry constraints were carriedout using the Gaussian 09 suite of programs[20] using the B3LYP[21]

functional in combination with the double-z plus polarizationdef2-SVP basis sets[22] for all atoms. This level is denoted B3LYP/def2-SVP. Stationary points were characterized as minima by calcu-lating the Hessian matrix analytically at this level. Calculations ofabsorption spectra were accomplished by using the time-depen-dent density functional theory (TD-DFT)[23] method at the samelevel. TD-DFT calculations were performed in n-hexane as solventusing the polarizable continuum model (PCM) method.[24] The as-signment of the excitation energies to the experimental bands wasperformed on the basis of the energy values and oscillatorstrengths. The B3LYP Hamiltonian was chosen because it wasshown to provide reasonable UV/Vis spectra for a variety of chro-mophores,[25] including Fischer carbene complexes[14, 18] and otherorganometallic species.[26] Wiberg bond indices were computed byusing the natural bond orbital (NBO) method.[27]

Figure 6. Optimized geometry (B3LYP/def2-SVP level) of complex 5 ca.

Figure 7. Comparison between emission spectra of the dyads 3 aa, 4 aa, and5 ca (lexc = 460 nm). All of the measurements were carried out in a dilutedsolution in hexane (concentration 10�5 mol L�1, optical density <0.1) at298 K.

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General experimental procedures

Reactions were carried out under an argon atmosphere. The sol-vents used in this work were purified by distillation and were fresh-ly distilled immediately before use. Triethylamine (Et3N) was dis-tilled from calcium hydride, whereas tetrahydrofuran (THF) and di-ethylether (Et2O) were distilled from sodium/benzophenone.Flame-dried glassware was used for moisture-sensitive reactions.Silica gel (Merck: 230–400 mesh) was used as stationary phase forpurification of crude reaction mixtures by flash column chromatog-raphy. Identification of products was made by thin-layer chroma-tography (Kieselgel 60F-254). UV light (l= 254 nm) and 5 % phos-phomolybdic acid solution in 95 % EtOH were used to develop theplates. NMR spectra were recorded at 25 8C in CDCl3, on a BrukerAvance 300 (300 MHz for 1H and 75 MHz for 13C). Chemical shiftsare given in ppm relative to CDCl3 (1H, 7.27 ppm and 13C,77.0 ppm). IR spectra were taken on a MIR (8000–400 cm�1) spec-trometer as solid films by slow evaporation of the solvent usingthe ATR (attenuated total reflectance) technique. MS spectra(HRMS) were acquired on a QTOF 6520: HP-1200 (Agilent Technolo-gies) mass spectrometer or a FTMS Bruker APEX Q IV.

Absorption and fluorescence spectroscopy

Experiments were carried out at 298 K, in hexane (concentration ofca. 10�6 mol L�1, optical density <0.1), using quartz cuvettes withoptical paths of 1 cm. UV absorption spectra were registered usinga UVICON XL spectrophotometer (Bio-Tex Instruments). Steady-state fluorescence spectra were recorded using an AMINCOBowman Series 2 spectrofluorometer, with 1.0 nm bandwidth foremission and excitation. The excitation wavelengths were fixed at460 nm and 490 nm in the measurement of the emission spectra.The emission wavelength was fixed at 600 nm for the determina-tion of all the excitation spectra.

All commercially available products were used without further pu-rification. The following BODIPY compounds were prepared ac-cording to literature: 2 a,[28] 2b,[29] 2 c.[28] The following metal–car-bene complexes were prepared according to previously describedmethods: Pentacarbonyl[(ethoxy)(methyl)carbene]chromium(0),[30]

pentacarbonyl[(ethoxy)(methyl)carbene]tungsten(0),[31] pentacar-bonyl[1-(ethylthio)(methyl)carbene]chromium(0).[32]

General procedure of Aumann’s reaction

Et3N (4 mmol) and TMSCl (3 mmol) were added to a solution of thecorresponding aldehyde (1 mmol) in anhydrous Et2O (5 mL). Imme-diately a solution of the corresponding Fischer carbene complex(1.5 mmol) in anhydrous Et2O (1 mL) were added to the solutionmixture at room temperature. The mixture was stirred until totaldisappearance of the starting material (checked by TLC). The sol-vent was removed under reduced pressure and the product puri-fied by flash chromatography on silica gel using argon under pres-sure.

Complex 3 aa

The carbene complex 1 a (227 mg, 0.86 mmol) in dry Et2O (0.6 mL)was added to a solution of 2 a (200 mg, 0.57 mmol), Et3N (0.7 mL,5.02 mmol), and TMSCl (220 mL, 1.71 mmol) in dry Et2O (3.0 mL).The reaction mixture was stirred at room temperature for 24 h. Thesolvent was removed under reduced pressure and purified by flashchromatography to yield compound 3 aa as a purple solid(271 mg, 78 %). 1H NMR (300 MHz): d= 7.60–7.5 (m, 8 H), 7.37 (m,4 H), 7.20 (d, J = 15.4 Hz, 1 H), 7.13 (d, J = 15.4 Hz, 1 H), 6.12 (s, 1 H),

6.11 (s, 1 H), 5.06–4.96 (m, 4 H), 2.75 (s, 6 H), 2.61 (s, 6 H), 1.65 (t, J =7.1 Hz, 6 H), 1.54 (s, 3 H), 1.53 (s, 3 H), 1.42 ppm (s, 6 H); 13C NMR(75 MHz) d= 326.4, 325.4, 224.5, 224.4, 217.3, 217.2, 159.8, 159.7,155.9, 155.8, 146.2, 146.1, 145.4, 142.4, 142.2, 140.8, 140.1, 138.9,138.2, 134.8, 134.4, 133.2, 131.0, 130.9, 129.5, 129.4, 129.3, 129.3,128.1, 127.9, 127.8, 126.1, 125.1, 124.6, 124.3, 123.4, 75.2, 75.1, 15.4,15.3, 15.1, 14.9, 14.8, 14.7, 14.4, 14.3, 13.1, 13.0 ppm; IR (CH2Cl2):n= 2052, 1921, 1539, 1511, 1192, 664 cm�1; HRMS (ESI): m/z calcdfor C29H26BCrF2N2O6 [M + H] 599.1258, found 599.1252.

Complex 3 ab

The carbene complex 1 a (503 mg, 1.90 mmol) in dry Et2O (1.3 mL)was added to a solution of 2 b (465 mg, 1.27 mmol), Et3N (1.5 mL,10.75 mmol), and TMSCl (500 mL, 3.93 mmol) in dry Et2O (6.8 mL).The reaction mixture was stirred at room temperature for 24 h. Thesolvent was removed under reduced pressure and purified by flashchromatography to yield compound 3 ab as a purple solid(298 mg, 38 %). 1H NMR (300 MHz): d= 7.57 (d, J = 15.3 Hz, 1 H),7.37 (d, J = 7.7 Hz, 2 H), 7.17 (d, J = 7.7 Hz, 2 H), 7.16 (d, J = 15.3 Hz,1 H), 6.11 (s, 1 H), 5.02 (q, J = 7.1 Hz, 2 H), 2.74 (s, 3 H), 2.61 (s, 3 H),2.48 (s, 3 H), 1.65 (t, J = 7.1 Hz, 3 H), 1.56 (s, 3 H), 1.45 ppm (s, 3 H).13C NMR (75 MHz): d= 326.2, 224.4, 217.2, 159.6, 155.8, 146.3, 142.9,140.8, 139.5, 138.8, 133.5, 131.4, 131.2, 130.0, 127.7, 125.5, 124.4,123.2, 75.2, 21.4, 15.1, 14.9, 13.9, 13.8, 13.2 ppm; IR (CH2Cl2): n=2053, 1929, 1540, 1516, 1190, 665 cm�1; HRMS (ESI): m/z forC30H28BCrF2N2O6 [M + H] 613.1415, found 613.1431.

Complex 3 ac

The carbene complex 1 a (131 mg, 0.50 mmol) in dry Et2O (0.3 mL)was added to a solution of 2 c (126 mg, 0.33 mmol), Et3N (0.4 mL,1.32 mmol), and TMSCl (120 mL, 0.99 mmol) in dry Et2O (1.8 mL).The reaction mixture was stirred at room temperature for 24 h. Thesolvent was removed under reduced pressure and purified by flashchromatography to yield compound 3 ac as a purple solid (90 mg,43 %). 1H NMR (300 MHz): d= 7.57 (d, J = 15.4 Hz, 1 H), 7.21–7.05(m, 5 H), 6.12 (s, 1 H), 5.02 (q, J = 7.1 Hz, 2 H), 3.91 (s, 3 H), 2.74 (s,3 H), 2.61 (s, 3 H), 1.65 (t, J = 7.1 Hz, 3 H), 1.59 (s, 3 H), 1.48 ppm (s,3 H); 13C NMR (75 MHz): d= 326.2, 224.4, 217.2, 160.5, 159.6, 155.8,146.2, 142.5, 140.8, 138.8, 133.7, 131.4, 129.2, 126.4, 125.2, 124.5,123.2, 114.8, 75.2, 55.3, 15.1, 15.0, 14.9, 13.9, 13.3 ppm; IR (CH2Cl2):n= 2052, 1922, 1538, 1514, 1192, 664 cm�1; HRMS (ESI): m/z calcdfor C30H28BCrF2N2O7 [M + H] 629.1364, found 629.1361.

Complex 3 ba

The carbene complex 1 b (285 mg, 1.01 mmol) in dry Et2O (0.6 mL)was added to a solution of 2 a (170 mg, 0.46 mmol), Et3N (0.6 mL,4.30 mmol), and TMSCl (200 mL, 1.56 mmol) in dry Et2O (2.5 mL).The reaction mixture was stirred at room temperature for 24 h. Thesolvent was removed under reduced pressure and purified by flashchromatography to yield compound 3 ba as a purple solid(138 mg, 40 %). 1H NMR (300 MHz): d= 7.56–7.49 (m, 4 H), 7.38 (d,J = 15.5 Hz, 1 H), 7.32–7.29 (m, 2 H), 6.13 (s, 1 H), 4.88 (q, J = 7.2 Hz,2 H), 2.75 (s, 3 H), 2.61 (s, 3 H), 1.63 (t, J = 7.2 Hz, 3 H), 1.52 (s, 3 H),1.42 ppm (s, 3 H); 13C NMR (75 MHz): d= 300.9, 203.8, 197.9, 159.9,155.7, 146.3, 142.7, 142.4, 140.7, 134.4, 133.3, 131.1, 129.6, 129.4,129.3, 127.8, 124.8, 123.4, 78.0, 14.9, 14.7, 14.0, 13.9, 13.0 ppm; IR(CH2Cl2): n= 2061, 1913, 1538, 1510, 1191, 724 cm�1; HRMS (ESI):m/z calcd for C29H26BF2N2O6W [M + H] 731.1365, found 731.1345.

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Complex 5 ca

The carbene complex 1 c (132 mg, 0.47 mmol) in dry Et2O (0.5 mL)was added to a solution of 2 a (110 mg, 0.31 mmol), Et3N (0.4 mL,2.90 mmol), and TMSCl (120 mL, 0.93 mmol) in dry Et2O (1.7 mL).The reaction mixture was stirred at room temperature for 24 h. Thesolvent was removed under reduced pressure and purified by flashchromatography to yield compound 5 ca as a purple solid (57 mg,30 %). 1H NMR (300 MHz): d= 7.84 (d, J = 15.0 Hz, 1 H), 7.77 (d, J =15.0 Hz, 1 H), 7.57–7.53 (m, 3 H), 7.34–7.31 (m, 2 H), 6.17 (s, 1 H),3.15 (q, J = 7.6 Hz, 2 H), 2.83 (s, 3 H), 2.64 (s, 3 H), 1.59 (s, 3 H), 1.44(s, 3 H), 1.36 ppm (t, J = 7.6 Hz, 3 H); 13C NMR (75 MHz): d= 322.3,227.1, 217.7, 161.2, 155.6, 147.1, 144.6, 142.5, 140.6, 136.1, 134.2,133.8, 131.1, 129.6, 129.5, 127.9, 126.1, 124.0, 35.5, 15.1, 14.9, 14.5,12.9, 12.2 ppm; IR (CH2Cl2): n= 2047, 1919, 1538, 1506, 1162,655 cm�1; HRMS (ESI): m/z calcd for C29H26BCrF2N2O5S [M + H]615.1030, found 615.1006.

General procedure for the preparation of aminocarbenecomplexes

A solution of dimethylamine (2 equiv, 2.0 m in THF) was addeddropwise to a solution of the corresponding ethoxycarbene com-plex (1 equiv) in dry THF at �100 8C. The reaction mixture wasstirred under argon at this temperature until the starting materialwas totally consumed. The solvent was then evaporated in vacuoand the resulting crude reactions purified by flash column chroma-tography with argon under pressure.

Complex 4 aa

A solution of dimethylamine in THF (80 mL, 0.16 mmol) was addedto a solution of 3 aa (50 mg, 0.08 mmol) in dry THF (0.7 mL) at�100 8C. The reaction mixture was stirred at room temperature for10 min. The solvent was removed under reduced pressure and pu-rified by flash chromatography to yield compound 4 aa as a pinkdark solid (40 mg, 80 %). 1H NMR (300 MHz): d= 7.52–7.49 (m, 3 H),7.31–7.28 (m, 2 H), 6.70 (d, J = 17.0 Hz, 1 H), 6.03 (s, 1 H), 5.74 (d, J =17.0 Hz, 1 H), 3.90 (s, 3 H), 3.44 (s, 3 H), 2.67 (s, 3 H), 2.58 (s, 3 H),1.45 (s, 3 H), 1.38 ppm (s, 3 H); 13C NMR (75 MHz): d= 270.8, 223.3,217.5, 156.8, 153.8, 144.1, 141.8, 139.7, 138.5, 135.9, 134.9, 129.2,129.1, 128.0, 122.3, 121.9, 114.3, 51.2, 45.6, 14.7, 14.5, 14.1,12.6 ppm; IR (CH2Cl2): n= 2051, 1919, 1539, 1514, 1192, 724 cm�1;HRMS (ESI): m/z calcd for C29H27BCrF2N3O5 [M + H] 598.1418, found598.1429.

Complex 4 ab

A solution of dimethylamine in THF (410 mL, 0.84 mmol) was addedto a solution of 3 ab (250 mg, 0.41 mmol) in dry THF (3.2 mL) at�100 8C. The reaction mixture was stirred at room temperature for10 min. The solvent was removed under reduced pressure and pu-rified by flash chromatography to yield compound 4 ab as a pinkdark solid (206 mg, 83 %). 1H NMR (300 MHz): d= 7.31 (d, J = 7.8 Hz,2 H), 7.16 (d, J = 7.8 Hz, 2 H), 6.70 (d, J = 16.9 Hz, 1 H), 6.02 (s, 1 H),5.74 (d, J = 16.9 Hz, 1 H), 3.90 (s, 3 H), 3.44 (s, 3 H), 2.67 (s, 3 H), 2.57(s, 3 H), 2.45 (s, 3 H), 1.47 (s, 3 H), 1.41 ppm (s, 3 H); 13C NMR(75 MHz): d= 270.5, 223.3, 217.5, 157.4, 156.6, 144.2, 142.2, 139.6,139.0, 138.5, 135.8, 132.2, 131.8, 129.9, 127.8, 126.2, 121.8, 114.4,51.2, 45.6, 21.4, 14.6, 14.1, 13.7, 12.7 ppm; IR (CH2Cl2): n= 2051,1917, 1539, 1517, 1196, 668 cm�1; HRMS (ESI): m/z calcd forC30H27BCrF2N3O5 [M�H] 610.1429, found 610.1425.

Complex 4 ba

A solution of dimethylamine in THF (190 mL, 0.38 mmol) was addedto a solution of 3 ba (138 mg, 0.19 mmol) in dry THF (1.6 mL) at�100 8C. The reaction mixture was stirred at room temperature for10 min. The solvent was removed under reduced pressure and pu-rified by flash chromatography to yield compound 4 ba as a pinkdark solid (120 mg, 87 %). 1H NMR (300 MHz): d= 7.52–7.50 (m,3 H), 7.31–7.29 (m, 2 H), 6.62 (d, J = 16.7 Hz, 1 H), 6.04 (s, 1 H), 6.03(d, J = 16.7 Hz, 1 H), 3.84 (s, 3 H), 3.41 (s, 3 H), 2.69 (s, 3 H), 2.58 (s,3 H), 1.47 (s, 3 H), 1.39 ppm (s, 3 H); 13C NMR (75 MHz): d= 251.9,203.4, 198.6, 157.0, 153.7, 144.3, 141.9, 140.1, 138.6, 134.7, 132.0,130.7, 129.2, 129.1, 127.9, 126.2, 122.0, 119.0, 53.8, 44.0, 14.7, 14.5,13.8, 12.6 ppm; IR (CH2Cl2): n= 2059, 1900, 1539, 1515, 1198,724 cm�1; HRMS (ESI): m/z calcd for C29H26BF2N3O5W [M + H]730.1525, found 730.1526.

Acknowledgements

The authors are grateful for financial support from the SpanishMINECO (CTQ2010–20714-C02–01/BQU and Consolider-Ingenio2010, CSD2007–00006) and CAM (S2009/PPQ-1634). G.M.C. ac-knowledges the MEC for a FPI grant. A.G.-M. acknowledges re-ceipt of a Ramn y Cajal Fellowship and funding from theSpanish MINECO (CTQ2010–18564).

Keywords: BODIPY · density functional calculations · Fischercarbenes · fluorescence · pi-conjugation

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150.

Received: October 9, 2013

Published online on December 30, 2013

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