Porphyrin–Bodipy combination: synthesis, characterization and antenna effect

Abstract

This paper described the synthesis of multichromophoric and π-conjugated macromolecules consisting of two equal Bodipy units and one porphyrin unit (free base). Then, a zinc(II) complex of this macromolecule based on Bodipy and porphyrin was prepared by a classical complex reaction. The formation of nitroporphyrin, aminoporphyrin, Bodipy–porphyrin (PB) and Bodipy–metalloporphyrin (Zn–PB) was confirmed by ¹H-NMR, ¹³C-NMR, elemental analysis, UV-vis/fluorescence spectra, melting point and ESI-TOF-MS techniques. Moreover, the photophysical properties of chromophoric groups such as absorption, emission and quantum yields were investigated for the photoinduced energy transfer in three different organic solvents. The result indicated that the emission of Bodipy in the Bodipy–porphyrin combination was significantly quenched with the formation of the free base and zinc(II) complex of porphyrin. Efficient energy transfer from the Bodipy unit to the porphyrins (free base or Zn–chelate) was observed between 88% and 96% due to high π-conjugation. The energy transfer also occurred from Bodipy to porphyrin though the high polarity of acetonitrile.

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1. Introduction

The design of light-harvesting devices such as porphyrins,^1–4 phthalocyanines^5–8 and Bodipys^9–11 has attracted a great deal of interest due to their unique electronic, optical and structural properties, narrow emission bands with broad absorption wavelength and photostability. Their potential applications have been intensively investigated in many science and technology areas such as chemosensors, liquid crystals, solar energy cells, electrochromic agents, photodynamic therapy.^12–16 The presence of a conjugated system in a macromolecule (porphyrin, Bodipy, phthalocyanine etc.) provides a variety of advantages for electron transfer mechanisms. While the poor absorption and emission characteristic of porphyrins (450–500 nm),^17,18 Bodipy fluorophores were widely employed as light-harvesting antenna group that its derivatives had excellent photochemical properties with strong absorption bands in the visible to near-infrared region.^19–21 The molecules including both porphyrin and Bodipy had a more attractive profile by optoelectronic researchers while a lot of methods were developed to enhance the use of these compounds in optoelectronic devices.^22–24 Some strategies were developed for improving limited photophysical profile of porphyrin derivatives in the blue-green region of the spectrum. So, porphyrins were modified with Bodipy as antenna chromophore for efficient intramolecular energy transfer.^23,25 Lazarides and co-workers described a combination of Bodipy–porphyrin. The synthesis of multichromophore arrays consisting of two Bodipy units axially bound to porphyrin complexes that they investigated to electron or energy transfer mechanism among chromophores. Among chromophore groups, a more effective light harvesting was achieved by using of a phenolate linker.¹⁸

Herein a new multichromophoric agent was designed based on Bodipy–porphyrin (Scheme 1) for the investigation of energy transfer mechanism. For this research work, one aminoporphyrin unit and one dual-Bodipy unit were prepared and these compounds were combined with an appropriate reaction and then obtained to Zn(II) complex based on porphyrin–Bodipy. The idea here was the light collected by antenna groups (Bodipy's) was transferred to Zn(II) porphyrin units which in its excited state donates electrons to generate a charge separated state and the light harvesting. The energy transfer in the multichromophoric compounds was occurred predominantly via a linker-mediated through-bond rather than a through-space. The synthesis and photophysical studies were augmented by theoretical calculations and the excited-state properties of the chromophore groups.

Scheme 1 The synthesis route of chromophore groups.

2. Experimental

2.1. Materials

The Bodipy and porphyrins were prepared under an atmosphere of nitrogen. Dry dichloromethane (DCM) was obtained from its refluxing with CaH₂. Thin layer chromatography (TLC) was used on silica gel plates (Merck, Kieselgel 60, 0.25 mm thickness) with long-wavelength (365 nm). Purifications were performed on column silica gel (Acros, silica gel 60, 32–70 mesh). The deuterated solvents (CDCl₃, DMSO) for NMR spectroscopy and the other chemicals were purchased from Sigma-Aldrich; pyrrole, 4-nitrobenzaldehyde, 3,4,5-trimethoxybenzaldehyde, 1,3,5-benzenetricarbonyl chloride, 2,4-dimethyl-3-ethylpyrrole, triethylamine, boron trifluoride diethyl ether complex (BF₃·OEt₂); solvents; petroleum ether (40–60%), ethyl acetate, 36% HCl, dichloromethane, chloroform were provided from Merck.

UV-visible measurements were recorded with a Perkin Elmer Lambda 25 UV-vis spectrophotometer using quartz cells of 1.0 cm path length. Fluorescence measurements were carried out in a PerkinElmer LS 55 spectrofluorimeter at room temperature. Elemental analyses were carried out using TruSpec elemental analyzer. ¹H and ¹³CNMR spectra were recorded on a Varian 400 MHz spectrometer. The mass measurements were recorded by a Bruker Compass Data Analysis 4.0 (ESI-TOF-MS) (the mass spectrometers were used a micrOTOFQ and a maXis quadrupole time-of-flight mass spectrometer). In order to investigate intramolecular charge transfer nature of derivatives, pump probe experimental setup (Spectra Physics, Helios) with white light continuum was used. Experiments were conducted with 656 nm pump wavelength and the source for the pump and probe pulses were derived from the fundamental output of Integra-C (780 nm, 2 mJ per pulse, fwhm = 130 fs). Typically, 2500 excitation pulses were averaged for 5 s to obtain the transient spectrum at a set delay time. All measurements were conducted at 298 K.

2.2. Synthesis of compounds

2.2.1. The synthesis of 5-(4-nitrophenyl)-10,15,20-tris(3,4,5-trimethoxylphenyl)porphyrin (nitroporphyrin). 3,4,5-Trimethoxybenzaldehyde (1.96 g, 10 mmol) and 4-nitrobenzaldehyde (0.38 g, 2.5 mmol) were refluxed in presence of 45 mL propionic acid at 100 °C. Pyrrole (3.45 mL, 50 mmol) was then added dropwise and the mixture stirred for another 30 min.²⁶ Upon cooling to room temperature, the mixture acid was evaporated in vacuo and a dark purple solid was obtained. The crude product was purified column chromatography (dichloromethane–ethylacetate = 4 : 1).

¹H-NMR [400 MHz, CDCl₃]: 8.66 (8H, m, Py-H), 8.35 (d, 4H, ArH), 8.04 (d, 2H, ArH), 7.78 (d, 8H, ArH), 7.05 (d, 2H, ArH), 3.95 (s, 27H, OCH₃), −2.77 (bs, 2H, NH). ¹³C-NMR [100 MHz, CDCl₃]: 20.6, 21.4, 101.0, 113.5, 114.5, 115.9, 119.9, 120.2, 121.4, 130.6, 131.8, 132.3, 136.4, 144.5, 144.6, 146.8, 160.3. ESI-TOF-MS [+H⁺]; 930.3.

2.2.2. The synthesis of 5-(4-aminophenyl)-10,15,20-tris(3,4,5-trimethoxylphenyl)porphyrin (aminoporphyrin). Under nitrogen atmosphere, was obtained through restoring nitroporphyrin (1.43 g, 1.6 mmol) in 36% HCl, and the solution was refluxed at for 2.5 h. The SnCl₂ was added as reductant, (0.35 g, 1.6 mmol).²⁶ After the reaction finished, ammonia solution was used to adjust the pH of solution to 7–8. Then, the crude product was extracted by dichloromethane and purified using column chromatography (chloroform–cyclohexane = 1 : 1 as an eluent). A purple solid was obtained (1.18 g, 85%). ¹H-NMR (CDCl₃, 400 MHz): 8.61 (m, 8H, Py-H), 8.31 (d, 4H, ArH), 8.03 (d, 2H, ArH), 7.75 (d, 8H, ArH), 7.05 (d, 2H, ArH), 4.03 (s, 2H, NH₂), 3.93 (s, 27H, –OCH₃), −2.78 (s, 2H, NH). ¹³C-NMR [100 MHz, CDCl₃]: 20.5, 101.4, 113.7, 114.1, 114.8, 115.6, 115.9, 119.9, 120.2, 121.8, 130.4, 131.2, 132.5, 136.1, 144.6, 160.3. ESI-TOF-MS [+H⁺]; 900.2.

2.2.3. The synthesis of 3,5-{bis[4,4-difluoro, 8-(2,6-diethyl, 1,3,5,7-tetramethyl-4-bora-3a,4a-diaza-s-indacene)]}benzoylchloride (Bodipy). Bodipy complex was prepared according to the known procedure¹⁶ and purified by some purification techniques. To a solution of 2,4-dimethyl-3-ethylpyrrole (1.8 mL, 13.3 mmol) in dry dichloromethane (100 mL), 1,3,5-benzenetricarbonyl chloride (0.883 g, 3.33 mmol) was added dropwise at room temperature and under N₂. The solution was heated and stirred to 60 °C for 5 h. After cooling of the solution, triethylamine (TEA) (10 equiv.) was added to the residual solid, the mixture was stirred at room temperature for 30 min under N₂, and boron trifluoride diethyl etherate (15 equiv.) was added. The reaction monitored by TLC and three different points were appeared. The solution was stirred at 60 °C for 2 h and the final residue was purified by column chromatography (petroleum ether–EtOAc; in 4 : 1 = 3 : 1 ≥ 2 : 1 ratio) and obtained to a red-orange solid. M.p.: 319–321 °C. ¹H-NMR (400 MHz, CDCl₃): δ (ppm) 7.82 (d, 2H, ArH), 7.53 (dd, 1H, ArH), 2.54 (s, 12H, CH₃), 2.26 (q, 8H, CH₂), 1.51 (s, 12H, CH₃), 0.97 (t, 12H, CH₃). ¹³C-NMR (100 MHz, CDCl₃): δ (ppm) 168.1, 148.1, 140.3, 133.1, 131.4, 130.2, 127.2, 126.5, 119.7, 17.2, 14.8, 13.2, 12.7. Anal. calc. C₄₁H₄₇B₂ClF₄N₄O; C, 66.11; H, 6.36; N, 7.52; found: C, 66.25; H, 6.77; N, 7.43. ESI-TOF-MS [+H⁺]; m/z: 744.3.

2.2.4. The synthesis of {{3,5-bis[4,4-difluoro, 8-(2,6-diethyl, 1,3,5,7-tetramethyl-4-bora-3a,4a-diaza-s-indacene)]}}phenyl-{4-{5-[10,15,20-tris(3,4,5-trimethoxylphenyl)porphyrin]}benzamide (PB). To a solution of aminoporphyrin (0.1 g, 0.11 mmol), TEA (0.35 mL) and 4-dimethylaminopyridine (cat.) in DCM (10 mL) at 0 °C, was added Bodipy (0.083 g, 0.11 mmol), and the resulting solution stirred was allowed to warm to r.t. and stirred for 18 h, then heated at reflux for further 18 h. The mixture was extracted with DCM–water (3 times). The organic layers were combined and dried (NaSO₄), filtered and evaporated in vacuo. The crude mixture was purified using column chromatography (DCM–EtOAc; 1 : 1) and obtained a purple-red solid. 0.14 g, 79%. M.p. > 350. ¹H NMR (400 MHz, CDCl₃): δ (ppm) 8.72–8.62 (m, 8H, Py-H), 8.29 (d, 4H, J = 8.0 Hz, ArH), 8.03 (d, 2H, J = 8.0 Hz, ArH), 7.87 (d, 2H, J = 7.8 Hz, ArH), 7.73 (d, 8H, J = 8.0 Hz, ArH), 7.55 (dd, 1H, ArH), 7.10 (bs, 1H, NH), 7.04 (d, 2H, J = 8.0 Hz, ArH), 3.99 (s, 27H, –OCH₃), 2.51 (s, 12H, CH₃), 2.23 (q, 8H, CH₂), 1.55 (s, 12H, CH₃), 0.99 (t, 12H, CH₃), −2.55 (bs, 2H, NH). ¹³C NMR (100 MHz, CDCl₃): δ (ppm) 168.1, 160.3, 148.2, 144.3, 140.6, 136.6, 133.2, 132.7, 131.7, 131.3, 130.9, 130.6, 127.3, 126.4, 120.9, 120.3, 119.5, 119.3, 115.6, 115.4, 114.9, 114.5, 113.6, 101.4, 20.9, 17.6, 14.9, 13.8, 12.9. Anal. calc. C₉₄H₉₅B₂F₄N₉O₁₀; C, 70.19; H, 5.95; N, 7.84; found: C, 69.89; H, 6.15; N, 7.71. ESI-TOF-MS [+H⁺]; m/z: 1608.1.

2.2.5. The synthesis of {{3,5-bis[4,4-difluoro, 8-(2,6-diethyl, 1,3,5,7-tetramethyl-4-bora-3a,4a-diaza-s-indacene)]}}phenyl-{4-{5-[10,15,20-tris(3,4,5-trimethoxylphenyl)porphyrin]}benzamide zinc(II) (Zn–PB). To a solution of PB (80.0 mg, 50 mmol) in 10 mL of DCM was added a methanolic solution of Zn(OAc)₂·2H₂O (3 equiv.) and sodium acetate (2 equiv.), and the mixture was refluxed for 2 h. The reaction was monitored by TLC. After cooling to room temperature, the reaction mixture was extracted three times with water (10 mL), dried over sodium sulfate, and filtered. Zn–PB was obtained in almost quantitative yield as a claret red solid. M.p. > 350. ¹H NMR (400 MHz, DMSO): δ (ppm) 8.75–8.69 (m, 8H, Py-H), 8.32 (d, 4H, J = 4.8 Hz, ArH), 8.11 (d, 2H, J = 6.0 Hz, ArH), 7.93 (d, 2H, J = 7.8 Hz, ArH), 7.71 (d, 8H, J = 8.0 Hz, ArH), 7.55 (dd, 1H, ArH), 7.12 (bs, 1H, NH), 7.00 (d, 2H, J = 4.0 Hz, ArH), 3.97 (s, 27H, –OCH₃), 2.52 (s, 12H, CH₃), 2.25 (q, 8H, CH₂), 1.52 (s, 12H, CH₃), 1.04 (t, 12H, CH₃). ¹³C NMR (100 MHz, CDCl₃): δ (ppm) 169.2, 160.7, 148.4, 144.5, 140.9, 136.1, 133.3, 132.9, 132.1, 131.4, 130.2, 130.4, 127.3, 126.1, 121.2, 120.5, 119.1, 118.9, 115.7, 115.5, 115.3, 114.1, 113.4, 101.1, 20.8, 17.5, 14.8, 14.0, 12.8. Anal. calc. C₉₄H₉₃B₂F₄N₉O₁₀Zn; C, 67.53; H, 5.61; N, 7.54; found: C, 67.89; H, 5.75; N, 7.31. ESI-TOF-MS [+H⁺]; m/z: 1671.7.

3. Result and discussions

The porphyrins (nitro and amino) and Bodipy were synthesized according to the literature procedure.^16,26 Nitroporphyrin was prepared by the condensation of aldehydes (3,4,5-trimethoxybenzaldehyde and 4-nitrobenzaldehyde) with pyrrole in the presence of propionic acid. Aminoporphyrin was obtained from the reduction of nitroporphyrin by SnCl₂. Bodipy was synthesized by the reaction of triple-acyl chloride of kryptopyrrole in the presence BF₃·OEt₂ and triethylamine. Target compound, PB, was easily prepared by the reaction of acyl-chloride with aryl-amino. The final complex, Zn–PB was obtained from the complex reaction of PB and Zn(OAc)₂. All compounds were characterized by ¹H-NMR, ¹³C-NMR, elemental analysis or ESI-TOF-MS. The single crystal could be not obtained from PB and Zn–PB for X-ray studies. The perfect match between experimental and simulated ionic patterns undoubtedly confirmed the structure of all compounds. In ¹H-NMR of PB, the methyl and ethyl protons of Bodipy unit appeared around 2.51–0 ppm (singlet, triplet or quartet) while nine methoxy protons of porphyrin was observed at 3.99 as a singlet peak. The broad NH₂ peak of porphyrin shifted from 4.03 to 7.10 ppm by the binding of Bodipy group. Moreover, other aromatic peaks shifted slightly toward down-area or up-area. ¹H-NMR spectrum of Zn–PB given similarly the peaks except for the pyrrolic protons of porphyrin unit disappeared due to the complexation effect.

The absorption spectra of compounds (nitro-porphyrin, amino-porphyrin, Bodipy and PB) were shown in Fig. 1. The absorption of nitroporphyrin and aminoporphyrin given similarly a curve with four hills (421, 516, 553, 592, 651; 425, 517, 554, 595, 650 nm, respectively) while Bodipy given one characteristic peak at 538 nm. The absorption features of nitroporphyrin are slightly red shifted by 1–3 nm compared to the corresponding transitions of the compounds. On the other hand, Q-bands of Bodipy (535 nm) were broader with higher absorbance at the same concentration. However, in the absorption spectrum of PB was observed two broad with shoulder at 424 and 538 nm and a little peak at 656 nm. The Soret band of aminoporphyrin bathochromically shifted to 424 nm (3 nm) in the absorption spectrum of PB due to the allowed transition from S₀/S₂. Q bands of porphyrin and Bodipy in same area overlapped and the peak at 538 nm broadened (due to the transition from S₀/S₁). The red-shifts in absorbance maxima and broadening of Q-band in PB and Zn–PB in comparison to Bodipy or aminoporphyrin suggest to the presence of strong intermolecular interactions among chromophores. The red-shifts in absorbance maxima and broadening of Q-band in PB and Zn–PB in comparison to Bodipy or aminoporphyrin suggest to the presence of strong intermolecular interactions among chromophores.

Fig. 1 The absorption spectra of the chromophore groups.

The emission spectra of chromophore compounds were recorded in DCM at room temperature at λ_exc 475 nm (Fig. 2). The characteristic emission peaks of Bodipy appeared as a single-broad band at 560 nm depending on the properties of solvent while the porphyrins showed to two the small emission bands with two hills around 660 nm and 725 nm, respectively. While the emission band of Bodipy at 562 nm was quenched by the binding of aminoporphyrin; two emission peaks at 660 and 728 nm enlarged. Moreover, both emission bands at 660 and 728 nm shifted hypsochromically to blue and the maximum of peaks appeared at 647 and 698 nm, respectively. After the zinc(II) addition, the emission peak of PB at 560 nm more quenched depending of the complex reaction. The basis of affinity depends on the match between the valence of zinc(II) and the electron donating ability of the nitrogens on the porphyrin. Moreover, it can be considered that the expected trends between ligand and metal ion are observed as hard–soft acid–base effects.¹² This effectively disturbs π-conjugation of porphyrin, giving rise to significant energy band gap, leading to photoinduced electron/energy transfer from Bodipy to porphyrin. These changes in the emission spectra of PB and Zn–PB could be attributed to a strong interaction between the porphyrin and Bodipy chromophores in the ground state. The existence of this electron transfer pathway was responsible for rapid activation of the Bodipy singlet excited state which resulted in almost complete presence of Bodipy-based fluorescence at normal condition.

Fig. 2 Emission spectra of chromophore groups in DCM (λ_exc 475 nm).

The photodynamics and nonlinear optical properties of PB and Zn–PB were investigated by ultrafast wavelength-dependent pump probe spectroscopy. As shown in Fig. 3, the spectra of the femtosecond transient absorption for the free base and the complex forms show to similar characteristics. The femtosecond transient absorption spectra of PB and Zn–PB references given in introduction²⁵ revealed the instantaneous formation of the Zn–PB singlet-excited state features. Here, the transient absorption spectra exhibited the absorption bands with a maximum around 450 nm, which decayed slowly to populate the corresponding triplet manifold with a maximum absorption at 470 nm. The absorption spectrum of PB in DCM at 1 ps shows characteristic bands assigned to the singlet excited state of the porphyrin. The energy transfer product of revealed faster decay than that observed for the Bodipy control compound in DCM. Upon increasing electron donating property, relative lifetime shortened as observed in the literature.²⁵

Fig. 3 (a) Femtosecond transient absorption spectra at different time intervals for PB and Zn–PB. (b) The fast decay of transient bands corresponding to intramolecular charge transfer state.

The excitation changes were represented at fixed emission (656 nm) in Fig. 4 which shows similar differences for PB and Zn–PB. The excitation spectra exhibit a suitable match with the absorption. These two properties indicate clearly an efficient S₁ energy transfer from Bodipy terminals to porphyrin.

Fig. 4 Excitation spectra of PB and Zn–PB in DCM (λ_em 656 nm).

The quantum yields were summarized in Table 1. By using different solvent, each of the porphyrins gave a typical emission with a quantum yield (0.10–0.13) at λ_exc 475 nm.²⁷ As expected, Bodipy showed a high quantum yield, 0.701. However, PB and Zn–PB have a quantum yield as 0.062 and 0.039, respectively. The decreasing in quantum yields could be attributed to the energy transfer from Bodipy terminals to porphyrin terminal. The Bodipy terminal behaved as light-harvesting antenna group that the received energy transferred to porphyrin unit and the energy roamed on all atoms of the macromolecule. The all changes in the photophysical properties of multichromophoric compounds could be depended on the conformation of these molecules. Both Bodipy units were close to porphyrin unit and the extended π-ring played a dominant role in an efficiency of energy transfer. The structural integrity and resemblance of the macromolecules were arrived by the photophysical and the spectroscopic studies. Macromolecules to mimic the ‘antenna-reaction center’ functionality of photosynthetic reaction center had been successfully constructed by coordinating double Bodipy with an acid chloride terminal to the porphyrin.²³ The HOMO/LUMO wave functions extend over the entire complex. In Table 1, we report the calculated levels for isolated PB and Zn–PB w molecules. Similar results were obtained in different solvents. HOMO and LUMO levels of PB and are very close in energy (around −5.65 eV for the HOMOs and around −4.40 eV for the LUMOs), which results in a strong interaction between these orbitals while HOMO/LUMOs of Zn–PB are farther in energy levels. This complex interaction pushes the LUMO level of Zn–PB toward higher energy. There is a good accord with the experimental observation that the presence of the zinc(II) metal increases the intensity of the low energy absorption peaks while the high energy one is reduced.

Table 1 The photophysical properties of chromophore groups in different solvents

Compound		λabs (nm)		λem^a (nm)	ΦY^b	ΦENT (%)	HOMO/LUMO [eV]
		Soret band	Q band
a Exc: 475 nm, in 1 × 10^−6 M concentration.b Rhodamine B (in EtOH) with a quantum yield of 0.68, 298 K.c Total quantum yield (Bodipy and porphyrin).d Look at ref. 26.
Nitro-porphyrin	DCM	421	516, 553, 592, 651	660, 720	0.131	n.a.	n.a.^d
	Toluene	422	517, 553, 591, 652	661, 721	0.122	n.a.	n.a.^d
	MeCN	419	513, 552, 590, 649	651, 714	0.101	n.a.	n.a.^d
Amino-porphyrin	DCM	425	517, 554, 595, 650	660, 728	0.123	n.a.	n.a.^d
	Toluenee	425	517, 553, 596, 651	660, 722	0.111	n.a.	n.a.^d
	MeCN	422	513, 551, 594, 648	653, 715	0.108	n.a.	n.a.^d
Bodipy	DCM	—	535	562	0.701	n.a.	n.a.
	Toluene	—	534	562	0.830	n.a.	n.a.
	MeCN	—	527	549	0.410	n.a.	n.a.
PB	DCM	428	538, 660	564, 647, 698	0.062^c	88	−5.53/−4.38
	Toluene	428	539, 660	564, 648, 698	0.051^c	94	−5.51/−4.41
	MeCN	425	536, 658	564, 640, 697	0.038^c	90	−5.65/−4.45
Zn–PB	DCM	429	540, 665	562, 648, 698	0.039^c	94	−4.99/−2.51
	Toluene	429	540, 665	562, 648, 698	0.037^c	96	−4.83/−2.55
	MeCN	428	538, 660	558, 647, 695	0.019^c	95	−5.15/−2.42

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The efficiencies of energy transfer of PB and Zn–PB (Table 1) were obtained by the equation

Φ_ENT = 1 − ΦF_PB/ΦF_Bodipy

in which Φ_ENT is the energy transfer percentage, ΦF_PB or ΦF_Zn–PB and ΦF_Bodipy are the fluorescence quantum yields.²⁷ The Bodipy emission of PB and Zn–PB was quenched significantly. These results indicated that hydrogens, zinc(II), which coordinated to pyrrolic nitrogens of porphyrin unit quenches the singlet state of Bodipy unit by enhancing the spin-forbidden deactivation processes.²³ The absorption/emission wavelengths and quantum yield of compounds hypsochromically/bathochromically changed and the absorption, emission intensities increased/decreased depending on polarity of solvent (Table 1). Acetonitrile more affected generally to the photophysical properties of the chromophore groups due its high polarity. The quantum yield decreased by the increasing polarity of solvents.²⁸ The quantum yields of compounds were affected by a change of solvent polarity and solvents had a strong effect on the emission maxima with increasing solvent polarity. The fluorescence bands were similarly shifted to blue depending on increasing polarity of solvent. The change in the quantum yield was attributed to competing excited electron transfer quenching of fluorescence.^29,30

4. Conclusion

The present paper has revealed photophysical-spectroscopic properties of highly π-conjugated Bodipy–porphyrin. While the emission band of Bodipy unit was quenched by the binding of aminoporphyrin; both emission bands of Bodipy–porphyrin combination at longer wavelength enlarged and the bands shifted hypsochromically to blue and the maximum of peaks appeared with a shorter wavelength. These changes in the emission spectra of free base and chelating form could be attributed to a strong interaction (energy transfer) between the porphyrin and Bodipy chromophores. Energy transfer from the photo-excited Bodipy antenna group to the porphyrin in the Bodipy–porphyrin combination is a highly effective process. The results indicated that the excellent photophysical properties of the Bodipy will highly enhance to the poor absorption and emission characteristic of porphyrins with the light-harvesting in a lot of optical applications.

Acknowledgements

We thank the Research Foundation of The Selcuk University (BAP) and Tübitak (114Z095) for financial support of this work. The author expresses his appreciation to Prof. Ross W. Boyle for helpful discussions.

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