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Synthesis and photophysical studies of covalently linked porphyrin-21-thiaporphyrin dyads
Inorganica Chimica Acta 360 (2007) 1731–1742 www.elsevier.com/locate/ica
Synthesis and photophysical studies of covalently linked porphyrin-21-thiaporphyrin dyads Iti Gupta, M. Ravikanth
*
Department of Chemistry, Indian Institute of Technology, Powai, Mumbai 400076, India Received 22 April 2006; received in revised form 30 July 2006; accepted 17 September 2006 Available online 29 September 2006
Abstract The synthesis and photophysical properties of four covalently linked unsymmetrical porphyrin dyads containing two different porphyrin cores such as N4 and N3S are reported. The covalently linked dyads were prepared by the coupling of appropriate porphyrin having ethynylphenyl functional group at meso-position with porphyrin having iodophenyl or bromo functional group at meso-position under mild palladium coupling conditions. The photophysical study indicated an intramolecular singlet–singlet energy transfer from N4/ ZnN4 porphyrin sub-unit to N3S porphyrin sub-unit in all four dyads with an efficiency of energy transfer process was typically P97%. To probe the role of linker in through bond electronic communication between the two porphyrin sub-units in dyads, the linker was varied from diphenylethyne to phenylethyne and the study revealed that the energy transfer rates and efficiencies were much higher for phenylethyne-bridged porphyrin dyads compared with diphenylethyne-bridged porphyrin dyads. 2006 Elsevier B.V. All rights reserved. Keywords: Porphyrin dyads; 21-Thiaporphyrins; Lifetimes; Energy transfer
1. Introduction Recently the design, synthesis and characterization of various porphyrin-metalloporphyrin and porphyrin-multipigment arrays are in a wide interest to mimic the photoinduced processes occurring in photosynthesis [1]. Many covalent [2,3] and non-covalent [4] multiporphyrin arrays have been synthesized and demonstrated as model light harvesting systems for artificial photosynthesis and are also found to be a prototype for the molecular-scale information-processing applications. However, most of the multiporphyrin arrays reported till date are symmetrical containing mostly N4 porphyrin cores [2,3]. Since the porphyrin units in symmetrical porphyrin arrays are identical, it is difficult to achieve the selective excitation of the porphyrin unit upon irradiation to study electron or energy transfer properties. The selective excitation of porphyrin *
Corresponding author. Tel.: +91 22 25767176; fax: +91 22 25767152. E-mail address: [email protected] (M. Ravikanth).
0020-1693/$ - see front matter 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.ica.2006.09.016
unit can be achieved in unsymmetrical arrays that consist of two or more different porphyrins or related units. This led to the synthesis of series of unsymmetrical porphyrin arrays [5] containing two different macrocycles such as porphyrin–chlorin, porphyrin–corrole, porphyrin–phthalocyanine. However, the reports on porphyrin arrays containing heteroatom substituted porphyrin as one of the sub-unit are very few [6] which may be because of synthetic inaccessibility of suitable heteroporphyrin building blocks. Recently, we developed [7] simple approaches to synthesize the desired functionalized heteroatom substituted porphyrins using easily available precursors under simple reaction conditions and showed their use in the construction of unsymmetrical porphyrin arrays. In continuation of our efforts in synthesizing unsymmetrical porphyrin arrays containing heteroporphyrin unit as one of the sub-unit, in this paper, we report the synthesis of four covalently linked unsymmetrical porphyrin dyads 2–5 (Chart 1) containing regular free base porphyrin sub-unit (N4 core) or its Zn(II) derivative (ZnN4) and 21-thiaporphyrin sub-unit (N3S
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N HN
N
N Zn N N
H
NH N
6
H
Zn6
OC8H17
N HN OC8H17
H S
N
N HN H S
N
OC8H17 8
7
N HN
N
N Zn N N
NH N 1 OC8H17
N
N
N HN
N
S
M N
N
OC8H17
M = Zn : 2 M = 2H : 3 OC8H17
N
N
N HN
N
S
M N
N
M = Zn : 4 M = 2H : 5 Chart 1. Structures of porphyrin monomers and porphyrin dyads 1–5.
core) by the coupling of appropriate porphyrin building blocks under Lindsey’s well-established palladium coupling conditions [8]. We synthesized two types of covalently linked unsymmetrical porphyrin dyads: (a) diphenylethyne-bridged covalent unsymmetrical porphyrin dyads 2 and 3; (b) phenylethyne-bridged covalent unsymmetrical
porphyrin dyads 4 and 5. The photophysical properties of dyads were studied and compared the properties of dyads 2 and 3 with the Lindsey’s covalently linked dyad 1 (Chart 1) containing free base and its Zn(II) derivative with N4 porphyrin core [2d]. The photophysical properties of the corresponding monomers such as 5-(p-ethynylphenyl)-
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10,15,20-tri(mesityl) porphyrin 6 and its Zn(II) derivative Zn6, 5-(p-ethynylphenyl)-10,15,20-tris(p-octyloxy phenyl)21-thiaporphyrin 7 and 5-ethynyl-10,15,20-tri-(p-tolyl)-21thiaporphyrin 8 were also studied as reference compounds. The studies indicated that all four dyads 2–5 showed energy transfer from regular porphyrin sub-unit (N4/ ZnN4) to 21-thiaporphyrin sub-unit on the selective excitation of N4/ZnN4 porphyrin sub-unit and the efficiencies of energy transfer were in the range of 97–99%. 2. Experimental 2.1. General Porphyrin dyad 1 [2d] and the desired porphyrin monomers 6 [9], Zn6 [9], 7 [7b], 8 [10], 5-bromo-10,15,20tri(p-tolyl)-21-thiaporphyrin (9) [10] and 5-(p-iodophenyl)10,15,20-tri(mesityl) Zn(II) porphyrin (Zn10) [9] were synthesized by the following literature methods. All general chemicals and solvents were obtained from SD fine chemicals, India. Pyrrole, aldehydes, thiophene, Pd2(dba)3, and AsPh3 were obtained from Lancaster. Column chromatography was performed on silica (60–120 mesh) and alumina obtained from Sisco Research laboratories, India. NMR spectra were recorded on a Varian 300 MHz spectrometer using tetramethylsilane as an internal standard and ES mass spectra were recorded with a Q-Tof micro (YA-105) mass spectrometer. The absorption and emission spectra were recorded in Perkin–Elmer Lambda 35 and Perkin– Elmer LS-55 spectrometers, respectively. The fluorescence quantum yields (/f) were estimated from the emission and absorption spectra by a comparative method [11] by the following equation: /f ¼ f½F ðsampleÞ½AðstandardÞ=½F ðstandardÞ ½AðsampleÞg/f ðstandardÞ
ð1Þ
where [F(sample)] and [F(standard)] are the integrated fluorescence intensities of the sample and the standard, [A(sample)] and [A(standard)] are the absorbance of sample and the standard at the excitation wavelength and /f (standard) represents the quantum yield of the standard sample. Free base tetraphenylporphyrin (H2TPP, /f = 0.11) and zinc(II) tetraphenylporphyrin (ZnTPP, /f = 0.033) were used as the standards [12]. The time resolved fluorescence decay measurements were carried out at a magic angle using a picosecond diode laser based time correlated single photon counting (TCSPC) fluorescence spectrometer from IBH, UK [13]. All the decays were fitted to single or multi-exponential functions using IBH software. The good fit criteria were low chi-square (1.0) and random distributions of residuals. 2.2. 4-[(5,10,15-Trimesityl-20-porphyrinato) zinc(II)]-4 0 [5,10,15-trioctyloxy-phenyl-21-monothia-20porphyrinyl]diphenylethyne (2) A solution of 5-(p-iodophenyl)-10,15,20-tri(mesityl) zinc(II) porphyrin (Zn10) (16.0 mg, 17.3 lmol) and 5-(p-ethy-
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nylphenyl) 10,15,20-tris(p-octyloxy phenyl)-21-monothiaporphyrin (7) (15.0 mg, 14.4 lmol) was dissolved in dry toluene/triethylamine (5:1, 18 ml) in a 25 ml round-bottomed flask. The flask was fitted with a reflux condenser and a gas inlet tube was inserted through the top of the condenser into the solution for argon purging. The reaction vessel was placed in an oil bath preheated to 35 C. The nitrogen was purged for 15 min. To this solution, Pd2(dba)3 (2.0 mg, 2.2 lmol), AsPh3 (5.4 mg, 17.5 lmol) were added and the reaction mixture was stirred at 35 C for 12 h. TLC analysis of the reaction mixture indicated the virtual disappearance of spots corresponding to the starting materials and the appearance of a new spot corresponding to the dyad. The solvents were removed in vacuo and the crude compound was passed through a small silica column using petroleum ether/dichloromethane (60:40) to remove the excess of AsPh3 and the crude mixture of small amounts of monomeric porphyrins along with the desired dyad was collected with petroleum ether/dichloromethane (20:80). The Pd species remained bound to the top of the column. The solution was concentrated in vacuo and the resulting crude mixture of porphyrinic monomers and dyad was dissolved in dichloromethane (3 ml) and dry slurry was prepared by adding neutral alumina powder. The slurry was loaded on neutral alumina column packed with petroleum ether. The small amounts of porphyrinic monomers were removed first with petroleum ether/dichloromethane and the desired dyad 2 was collected with petroleum ether/dichloromethane (20:80). The concentration of the solution gave porphyrin dyad as a violet solid in a 66% yield (12 mg). Mp > 300 C. dH (300 MHz, CDCl3; Me4Si) 2.60 (s, 1H, NH), 0.86–0.99 (m, 15H, CH3 and CH2), 1.25–1.66 (m, 24H, CH2), 1.87 (s, 18H, CH3), 1.99 (m, 6H, CH2), 2.64 (m, 9H, CH3), 4.25 (m, 6H, OCH2), 7.32 (m, 12H, Ar), 8.12 (m, 10H, Ar), 8.32 (m, 4H, Ar), 8.72 (m, 8H, b-pyrrole), 8.79 (d, J = 4.8 Hz, 2H, b-pyrrole), 8.93 (d, J = 4.8 Hz, 2H, b-pyrrole), 8.98 (s, 2H, b-pyrrole), 9.81 (m, 2H, b-thiophene). m/z (EI) 1843.9 (M+, C123H121N7SO3Zn requires 1842.7). 2.3. 4-[(5,10,15-Trimesityl-20-porphyrinyl)]-4 0 -[5,10,15tri(p-tolyl)-21-monothia-20-porphyrinyl]diphenylethyne (3) A solution of ZnN4–N3S dyad 2 (33.0 mg, 17.9 lmol) was treated in dichloromethane (20 ml) with trifluoroacetic acid (1 ml) under stirring at room temperature for 12 h. After this period, TLC analysis indicated the presence of a new spot. The reaction mixture was washed thoroughly with NaHCO3 solution (1 · 100 ml) followed by water (1 · 100 ml) and extracted with dichloromethane. The solvent was dried over anhydrous Na2SO4 and concentrated in vacuo under reduced pressure. The crude product was purified by neutral alumina column chromatography. The desired dyad 3 was collected using petroleum ether/dichloromethane (40:60) as a violet solid in a 63% yield (20 mg). Mp > 300 C. dH (300 MHz, CDCl3; Me4Si) 2.59 (s, 1H, NH), 2.52 (s, 2H, NH), 0.92–0.97 (m, 9H, CH3), 1.31–
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1.81 (m, 30H, CH2), 1.88 (s, 18H, CH3), 1.97 (m, 6H, CH2), 2.63 (m, 9H, CH3), 4.25 (m, 6H, OCH2), 7.28 (m, 12H, Ar), 8.12 (m, 10H, Ar), 8.31 (m, 4H, Ar), 8.68 (m, 8H, b-pyrrole), 8.74 (m, 2H, b-pyrrole), 8.86 (d, J = 4.8 Hz, 2H, bpyrrole), 8.98 (s, 2H, b-pyrrole), 9.81 (m, 2H, b-thiophene). m/z (EI) 1779.9 (M+, C123H123N7SO3, requires 1779.4). 2.4. 4-[(5,10,15-Trimesityl-20-porphyrinato) zinc(II)]-4[5,10,15-tri(p-tolyl)-21monothia-20-porphyrinyl]phenylethyne (4)
J = 4.5 Hz, 1H, b-pyrrole), 8.65 (m, 4H, b-pyrrole), 8.72 (d, J = 4.8 Hz, 1H, b-pyrrole), 8.77 (d, J = 4.8 Hz, 2H, bpyrrole), 8.88 (s, 2H, b-pyrrole), 8.95 (d, J = 4.8 Hz, 2H, b-pyrrole), 9.54 (d, J = 4.2 Hz, 1H,, b-pyrrole), 9.89 (d, J = 5.1 Hz, 1H, b-thiophene), 10.53 (d, J = 5.1 Hz, 1H, b-thiophene). m/z (EI) 1361.7 (M+, C96H77N7S requires 1360.8). 3. Results and discussion 3.1. Synthesis
A solution of 5,10,15-trimesityl-20-(4-ethynylphenyl) zinc (II) porphyrin (Zn6) (29.3 mg, 35.3 lmol) and 5-bromo-10,15,20-tris(p-tolyl)-21-thiaporphyrin (9) (20.0 mg, 29.4 lmol) in dry toluene/triethyl ammine (25:5 ml) was purged with nitrogen for 10 min. To this solution Pd2(dba)3 (4.0 mg, 4.4 lmol) and AsPh3 (10.8 mg, 35.3 lmol) were added to initiate the coupling. The reaction mixture was stirred at 45 C for 24 h. TLC analysis indicated the almost disappearance of starting material and the new spot corresponding to the required dyad. The solution was concentrated in vacuo and the resultant crude product was purified by neutral alumina column chromatography. The desired compound 4 was collected in petroleum ether/dichloromethane (50:50) as a violet solid in a 69% yield (29 mg). Mp > 300 C. dH (300 MHz, CDCl3; Me4Si) 2.17 (s, 1H, NH), 1.88 (s, 18H, CH3), 2.64 (s, 9H, CH3), 2.72 (m, 9H, CH3), 7.29 (m, 4H, Ar), 7.40 (m, 4H, Ar), 7.63 (m, 6H, Ar), 8.15 (m, 4H, Ar), 8.44 (m, 3H, b-pyrrole and Ar), 8.56 (d, J = 4.8 Hz, 1H, b-pyrrole), 8.64 (d, J = 4.2 Hz, 1H, b-pyrrole), 8.73 (m, 4H, b-pyrrole), 8.85 (m, 4H, b-pyrrole), 9.03 (d, J = 4.5 Hz, 2H, b-pyrrole), 9.55 (d, J = 4.5 Hz, 1H, b-pyrrole), 9.90 (d, J = 4.2 Hz, 1H, b-thiophene), 10.53 (d, J = 4.5 Hz, 1H, b-thiophene). m/z (EI) 1424.6 (M+, C96H75N7SZn requires 1424.1). 2.5. 4-[5,10,15-Trimesityl-20-porphyrinyl]-4-[5,10,15-tri(ptolyl)-21-monothia-20-porphyrinyl]phenylethyne (5) A solution of ZnN4–N3S dyad 4 (30.0 mg, 21.0 lmol) was treated with trifluoroacetic acid (1 ml) in dichloromethane (20 ml) at room temperature under stirring for 12 h. After this period, TLC analysis indicated the presence of a new spot. The solution was washed thoroughly with NaHCO3 solution (1 · 100 ml) and then with water (1 · 100 ml) and dried on anhydrous Na2SO4. The solvent was removed on rotary evaporator in vacuo under a reduced pressure and the resultant crude product was purified by neutral alumina column chromatography. The desired compound 5 was collected in petroleum ether/ dichloromethane (40:60) as a violet solid in a 55% yield (16 mg). Mp > 300 C. dH (300 MHz, CDCl3; Me4Si) 2.49 (s, 1H, NH), 2.14 (s, 1H, NH), 1.89 (s, 18H, CH3), 2.64 (s, 9H, CH3), 2.71 (br s, 9H, CH3), 7.31 (m, 4H, Ar), 7.44 (m, 4H, Ar), 7.65 (m, 6H, Ar), 8.12 (m, 4H, Ar), 8.42 (s, 3H, b-pyrrole and Ar), 8.56 (d,
The desired N3S monomeric porphyrin building blocks 7, 8 and 9 were synthesized as reported earlier [7] and the N4 porphyrin building blocks 6, Zn6 and Zn10 were synthesized by following the literature procedures [9]. The diphenylethyne and phenylethyne-bridged dyads were synthesized using Lindsey’s modified conditions [8] of Sonogashira coupling of an ethynylphenyl porphyrin with iodophenyl porphyrin and ethynylphenyl porphyrin with bromoporphyrin, respectively. Under these modified conditions [8], the coupling of an equimolar quantity of ethynylphenyl porphyrin with iodophenyl porphyrin or bromoporphyrin in dilute conditions can be carried out at a very low temperature and in the absence of copper. The low temperature conditions are needed to prevent side reactions such as the formation of ethyne oligomerization products and the absence of copper is important since the free base porphyrins are easily metalated. Thus, diphenylethyne-bridged dyad 2 containing ZnN4 and N3S porphyrin sub-units was synthesized in a 66% yield by the coupling of Zn10 and 7 in toluene/triethylamine at 35 C in the presence of catalytic amount of Pd2(dba)3/AsPh3 followed by column chromatographic purification (Scheme 1). Dyad 3 was synthesized in a 63% yield by the demetalation of dyad 2 on treatment with trifluoroacetic acid at room temperature for overnight (Scheme 1). The phenylethyne-bridged dyad 4 was prepared by the coupling of Zn6 and 9 under similar palladium coupling conditions [8] and afforded in a 69% yield. Dyad 5 was prepared in a 55% yield by the demetalation of dyad 4 with trifluoroacetic acid at room temperature for overnight. Dyads 2–5 are highly soluble in all common organic solvents and were characterized by NMR, mass spectrometry, absorption and fluorescence spectroscopic techniques. The mass spectrum of dyad 4 is presented in Fig. 1. The mass spectra of dyads 2–5 showed M+ ion peak confirming the identities of the compounds. The 1H NMR spectroscopy has been used to characterize dyads 2–5 in detail. The resonances of dyads 2–5 were assigned on the basis of the spectra observed for the two corresponding monomers taken independently. The 1H NMR spectra of dyads 2 and 5 are shown in Fig. 2 and the chemical shifts of the selected protons of dyads 2–5 are presented in Table 1. In dyad 2, considering the lower symmetry of N3S porphyrin subunit, one would expect one set of
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OC8H17
N
N Zn N N
I +
N HN
H
S
Toluene / Et3N 40oC , 12 h
Pd2 (dba)3 AsPh3
Zn10
OC8H17
N
7 OC8H17
OC8H17
N
N HN
N Zn N N
S
2
OC8H17
N
(66%) OC8H17 TFA/CH2Cl2 OC8H17
N HN
N HN
NH N
S
3
(63%)
N
OC8H17
OC8H17
Scheme 1. Synthetic scheme for dyads 2 and 3.
multiplet for two thiophene protons and three sets of signals for six pyrrole protons and also expect one signal for pyrrole protons of N4 subunit. As clearly evident in
Fig. 1. EI-MS mass spectrum of dyad 4.
Fig. 2a, the multiplet resonance observed at 9.81 ppm is due to two thiophene protons and one broad singlet at 8.72 ppm and three doublets at 8.79, 8.93 and 8.98 ppm were due to six pyrrole protons of the N3S porphyrin sub-unit. The strong singlet observed at 8.72 ppm was due to eight pyrrole protons of the ZnN4 porphyrin subunit. The NH proton of the N3S subunit was observed at 2.60 ppm as a broad singlet. In dyad 3 containing N4 porphyrin sub-unit and N3S porphyrin sub-unit showed similar spectral features as that of 2. However, as expected, there are two signals of inner NH protons at 2.52 and 2.59 ppm corresponding to the N4 porphyrin sub-unit and the N3S porphyrin sub-unit, respectively. A comparison of chemical shifts of the bthiophene and NH protons of dyads 2 and 3 with those of monomer 7 indicates only minor differences suggesting that the two porphyrin subunits in the dyads interact very weakly.
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Fig. 2. 1H NMR spectra of dyads (a) 2 and (b) 5 recorded in CDCl3.
Table 1 1 H NMR chemical shift (d in ppm) of selected protons of porphyrin dyads 2–5 and N3S porphyrin 7 in CDCl3 Compound
b-Thiophene
NHa
7 2 3 4 5
9.67(d), 9.76(d) 9.81(m) 9.81(m) 9.90(d), 10.53(d) 9.89(d), 10.52(d)
2.64 2.60 2.59 2.17 2.14
a
The NH signal of N3S porphyrin.
The phenylethyne-linked dyad 4 exhibited two doublets for two thiophene protons, seven sets of signals for 14 pyrrole protons and one signal for inner NH proton. Dyad 5 showed two sets of signals for thiophene protons, eight sets of signals for 14 pyrrole protons and two signals for three inner NH protons of both the porphyrinic sub-units (Fig. 2b). Dyads 4 and 5 showed downfield shifts for the thiophene and the inner NH protons of the N3S porphyrin sub-unit as compared to 7 (Table 1) indicate a strong interaction between the two porphyrin sub-units.
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The fluorescence spectra of dyads 2 and 3 and the associated monomers were recorded in toluene at room temperature and the relevant data are presented in Table 3. The fluorescence peak maxima and the quantum yields of monomers are 6 (k = 650 nm, / = 0.12), Zn6 (k = 598 nm, / = 0.035) and 7 (k = 690 nm, / = 0.0184). These properties were in close match with the previously reported values of these monomers [2d,7b]. The fluorescence properties of dyads 2 and 3 showed very interesting features. Dyad 3 containing N4 and N3S porphyrin sub-units on excitation at 430 nm where N3S porphyrin sub-unit absorb strongly, showed a typical N3S porphyrin emission with quantum yield, / (0.0178 ) matching with that of monomer 7 (Fig. 3, inset). Interestingly, when dyad 3 was excited at 415 nm where N4 porphyrin sub-unit was the dominant absorber, the major emission was occurred from the N3S porphyrin sub-unit with a weak emission from the N4 porphyrin sub-unit (Fig. 3, inset). The quantum yield data calculated by comparative method [11] indicated that the N4 porphyrin sub-unit emission in dyad 3 was quenched by 94% (Table 3). However, when a 1:1 mixture of monomers 6 and 7 was excited at 415 nm, the major emission was observed mainly from 6 (Fig. 3, inset). Thus, the excitation of N4 porphyrin unit at 415 nm in dyad 3 yields the emission spectrum identical with that obtained upon the direct excitation of the N3S porphyrin sub-unit at 430 nm providing a clear evidence of highly efficient energy transfer operating in dyad 3. The same emission spectrum of the N3S porphyrin sub-unit in dyad 3 was noted when we excited at other wavelengths indicating that the emission spectrum
3.2. Photophysical studies 3.2.1. Diphenylethyne-bridged covalent unsymmetrical porphyrin dyads 2 and 3 The absorption spectra of dyads 2 and 3 and their corresponding monomers were recorded in toluene at room temperature and the data are presented in Table 2. The absorption spectra of monomers such as 6 with N4 core and 7 with N3S core exhibited four Q-bands and one Soret band. The absorption bands of 7 (682, 621, 554, 518, and 434 nm) were more bathochromically shifted as compared to 6 (648, 592, 551, 515, and 420 nm) due to the presence of one sulfur atom in the core. Monomer Zn6 showed two Q-bands (552 and 589 nm) and one Soret band (423 nm). A comparison of the absorption spectra of dyad 3 with a 1:1 mixture of the corresponding monomers 6 and 7 is shown in Fig. 3. Dyads 2 and 3 showed four Q-bands and a split Soret band. As clear from Fig. 3, the absorption spectra of dyads are approximately superposition of the spectra of the corresponding monomers with only minor differences in wavelength maxima and band shapes (Table 2). The Q-band absorption spectra of dyads 2 and 3 (Fig. 3) were almost identical to their corresponding 1:1 mixture of monomers. However, the Soret bands of dyads 2 and 3 were split unlike their corresponding 1:1 mixture of monomers, which showed a broad single Soret band. These observations suggest that weak electronic interactions occur only in the B-states of dyads 2 and 3, which are in agreement with the previous studies on similar type of diphenylethyne-bridged porphyrin arrays [1a].
Table 2 Absorption data of covalent dyads 1–5 and their corresponding monomers in toluene Compound
Soret band k (nm) e · 104 (dm3 mol1 cm1)
Zn6a 6a 7 8
423 420 434 437
2
Q-bands k(nm) (e · 103 dm3 mol1 cm1) I
II
III
515 518 524
552 551 554 561
589 592
424 (53.6) 435 (51.1)
518 (28.4)
552 (34.4)
588 (5.8)
621 (3.9)
(7.3)
3
422 (65.4) 435 (60.6)
518 (57.0)
553 (29.2)
593 (9.4)
618 (sh)
650 (6.9)
4
422 (25.4) 449 (19.4)
509 (sh)
552 (19.4) 592 (sh)
573 (22.9)
635 (sh)
5
420 (25.3) 448 (21.4)
516 (22.8)
548 (sh)
577 (26.1)
638 (sh)
a
Data taken from Ref. [2d].
IV
V
VI
648 621 628
682 691 683
684 (9.2)
700 (5.5)
650 (5.1)
699 (6.2)
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1.0
Absorbance
1:1
Intensity
600
1:1 Dimer
400
200
Dimer 0 600
0.5
700
800
Wavelength (nm)
x5 0.0 400
500
600
700
Wavelength (nm) Fig. 3. Comparison of absorption spectra of porphyrin dyad 3 and the 1:1 mixture of 6 and 7 in toluene. The fluorescence spectra (kex = 415 nm) of dyad 3 and 1:1 mixture of 6 and 7 are shown as an inset.
Table 3 Emission data of dyads 1–5 and corresponding monomers in toluene Compound
/f
s (ps)
K 1 ENT (ps)
/ENT
Zn6 6 7 8 1e 2 3 4 5
0.0350 0.1200 0.0184 0.0180 0.0017a 0.0015a 0.0063b 0.0057a 0.0009b
2258 12 600 1744 1859 24c 57c (1632)f 85d (1659)f 25c (1949)f 23d (1728)f
24 59 86 26 23
0.99 0.97 0.99 0.99 0.99
a b c d e f
/f of ZnN4 sub-unit at kex = 550 nm. /f of N4 sub-unit at kex = 415 nm. sf of ZnN4 sub-unit measured at 600 nm. sf of N4 sub-unit measured at 650 nm. Data taken from Ref. [2d]. sf of N3S sub-unit measured at 680 nm.
of dyad 3 was independent of the excitation wavelength. The excitation spectrum of dyad 3 recorded by scanning the excitation from 720 to 400 nm with fixed emission at 750 nm showed close matching with its absorption spectrum indicating a high yield of singlet-singlet energy transfer within dyad 3. If electron transfer is assumed to be occurring between the sub-units of dyad 3, then we do not expect to observe such a strong emission from the N3S porphyrin subunit on excitation at the N4 porphyrin subunit. Thus, the greatly diminished emission from the N4 porphyrin, the unaltered emission yield from the N3S porphyrin sub-unit and the predominant emission from the N3S porphyrin upon excitation of N4 porphyrin subunit are consistent with efficient energy transfer from the N4 porphyrin sub-unit to N3S porphyrin sub-unit in dyad
3. Similarly, in dyad 2 which contains ZnN4 and N3S porphyrin sub-units upon excitation at 550 nm where the ZnN4 porphyrin sub-unit absorbs very strongly, the emission was observed mainly from the N3S porphyrin sub-unit indicating an efficient energy transfer from ZnN4 porphyrin sub-unit to N3S porphyrin sub-unit. Time-resolved emission studies [13] also support the efficient energy transfer within dyads 2 and 3. The timeresolved fluorescence study helps to probe the dynamics of energy transfer and to obtain additional estimates for the energy transfer yields [2d]. Dyads 2 and 3 were excited at 406 nm and monitored at two different wavelengths corresponding to the emission peak maxima of donor (ZnN4 or N4 porphyrin sub-units) and acceptor (N3S porphyrin sub-unit). The fluorescence decay profiles and weighted residuals of dyads 2 and 3 are presented in Fig. 4 and the data are tabulated in Table 3. The fluorescence decays of Zn6, 6, and 7 were also excited at 406 nm and measured at their respective emission peak maxima 600, 650, and 678 nm, respectively. The fluorescence decays of Zn6, 6 and 7 were fitted to single exponential and the observed lifetimes were 2.25 ns for Zn6, for 12.6 ns 6 and 1.74 ns for 7 (Table 3). The fluorescence decays of dyads 2 and 3 observed at the N3S porphyrin sub-unit (680 nm) were fitted to a single exponential and s (1.63 and 1.66 ns, respectively) was matching closely with monomer 7. However, the fluorescence decays of dyads 2 and 3 monitored at 600 and 650 nm, respectively, corresponding to emission peak maxima of the ZnN4 and N4 porphyrin sub-units were fitted to two or three exponential with a dominant contribution from shorter component (>90%). The other one or two components of fluorescence decays of dyads 2 and 3 were attributed to the monomeric porphyrin impurities
I. Gupta, M. Ravikanth / Inorganica Chimica Acta 360 (2007) 1731–1742
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process is mediated by the donor–acceptor distance and orientation as well as their emission and absorption spectral overlap characteristics. In the Fo¨rster theory of energy transfer, the rate is given by 6 k TS ¼ ð8:8 1023 ÞK 2 /f Jn4 s1 D R
ð4Þ
where K2 is the orientation factor, /f is the fluorescence quantum yield of the donor in the absence of acceptor, J (in cm6 mmol1) is the spectral overlap integral, n is the solvent refractive index (1.49 for toluene), sD is the donor lifetime in the absence of acceptor and R is the centre-to˚ . The overlap intecentre distance of donor–acceptor in A gral J is calculated from the equation J ¼ F D ðmÞeðmÞm1 dm
Fig. 4. Fluorescence decays and weighted residuals of porphyrin dyads 2– 5.
present in the dyads. The major short component for dyads 2 and 3 observed at 57 and 85 ps, respectively, were attributed to the ZnN4 porphyrin sub-unit in dyad 2 and the N4 porphyrin sub-unit in dyad 3. The quenched lifetimes of the ZnN4 and N4 porphyrin sub-units, respectively, in dyads 2 and 3 compared to their associated reference compounds Zn6 and 6 (Table 3) were due to the singlet–singlet excitation energy transfer from the ZnN4 or the N4 porphyrin sub-unit to the N3S porphyrin sub-unit. The rate of energy transfer (kENT) and the efficiency of energy transfer (/ENT) were calculated from the measured lifetimes (sDA) of the ZnN4 and the N4 porphyrin sub-units in dyads 2 and 3, respectively, and that of the corresponding monomers Zn6 and 6 (sD) using the following equations [2d]: k ENT ¼ 1=sDA 1=sD
ð2Þ
/ENT ¼ k ENT sDA
ð3Þ
The kENT and /ENT values calculated for dyads 2 and 3 were (59 ps1, 97%) and (86 ps1, 99%), respectively, (Table 3). Thus, the energy transfer efficiencies /ENT of dyads 2 and 3 were almost same as that of reported dyad 1 (99%) but the energy transfer rates kENT of 2 and 3 were found to be slower than that of 1 (24 ps1) [2d]. This may be due to the presence of sulfur atom in one of the porphyrin unit in the dyad, which enhances the contribution of non-radiative decay channels [14]. Furthermore, the extensive studies by Lindsey and co-workers on similar types of porphyrin arrays with diarylethyne linkers showed that the excited state energy transfer occurs mainly via through bond (TB) and the energy transfer contribution through space (TS) was relatively minor to the overall energy transfer rate. The TS energy transfer contribution can be evaluated in terms of Fo¨rster mechanism [15]. The Fo¨rster
ð5Þ
where FD(m) is the fluorescence intensity of the donor in wavenumber units with total intensity normalized to unity, e(m) is the absorption coefficient of the acceptor and m is the wavenumber in cm1. The calculated J values for dyads 2 and 3 are tabulated in Table 4. The centre-to-centre dis˚ based on tance of donor–acceptor was taken as 20 A MM+ force field calculations and 1.125 was used for K2. The relative contributions of TS (vTS) and TB (vTB) energy transfer to the overall rate were calculated using the following equations [2d]: k ENT ¼ k TB þ k TS
ð6Þ
vTS ¼ k TS =k ENT vTB ¼ k TB =k ENT
ð7Þ ð8Þ
The data are summarized in Table 4. An inspection of Table 4 reveals that the TS rates are much slower and TB contribution accounts for greater than 90% of the observed rate as observed previously for similar kind of diarylethyne-bridged dyadic systems [2d]. In addition, the electron transfer quenching interaction may also be possible in these systems which cannot be evaluated at this stage due to lack of electrochemical data. 3.2.2. Phenylethyne-bridged covalent unsymmetrical dyads 4 and 5 The absorption spectra of dyads 4 and 5 unlike 2 and 3 were quite different from their 1:1 ratio of the corresponding monomers Zn6 or 6 and 8 (Table 2). In dyads 4 and 5, Table 4 Through bond and through space energy transfer rates and contributions for covalently linked unsymmetrical dyads 1–5 Compound
a K 1 TS (ps)
b K 1 TB (ps)
vTS c
vTB d
1e 2 3 4 5
745 2388 551 641 628
25 89 66 25 27
0.04 0.04 0.11 0.04 0.04
0.96 0.96 0.89 0.96 0.96
a b c d e
Obtained from Fo¨rster energy transfer calculations. Obtained using Eq. (6). Obtained using Eq. (7). Obtained using Eq. (8). Data taken from Ref. [2d].
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I. Gupta, M. Ravikanth / Inorganica Chimica Acta 360 (2007) 1731–1742
the Q-bands were broad and ill-defined and the Soret band was clearly split into two peaks. Furthermore, the absorption bands of dyads 4 (Fig. 5) and 5 were bathochromically shifted compared to the corresponding monomers 6 or Zn6 and 8 (Table 2). These observations indicate that the different nature of the spacers affects the electronic communication between the macrocycles [16]. The steady state fluorescence studies on dyads 4 and 5 supported the energy transfer between the sub-units (Table 3). In dyad 5, on excitation at 415 nm where the N4 porphyrin sub-unit absorbs strongly, the emission of the N4 porphyrin sub-unit was quenched to 99% and the major emission was observed from the N3S porphyrin sub-unit supporting the energy transfer from the N4 porphyrin sub-unit to the N3S porphyrin sub-unit (Fig. 5, inset). Similarly, the energy transfer also occurred in dyad 4 from the ZnN4 porphyrin sub-unit to the N3S porphyrin sub-unit on selective excitation of the ZnN4 porphyrin sub-unit at 550 nm. The timeresolved fluorescence studies on dyads 4 and 5 were in agreement with the above observations (Fig. 4). Dyads 4 and 5 were excited at 406 nm and monitored the fluorescence decay at 680 nm corresponding to the N3S porphyrin sub-unit in both dyads and also at 600 and 650 nm, respectively, corresponding to the ZnN4 subunit in dyad 4 and the N4 porphyrin sub-unit in dyad 5. When fluorescence decay was monitored for dyads at 680 nm, it was fitted to single exponential with lifetime 1.94 ps for dyad 4 and 1.73 ps for dyad 5. This is in close match with the N3S porphyrin monomer 8 (1.86 ps). However, when the fluorescence decay (Fig. 4) was monitored for dyad 4 at 600 nm corresponding to the ZnN4 porphyrin sub-unit, the decay was fitted to two exponential with lifetimes 25 ps, and 2.2 ns with major contribution from short 25 ps component (Table 3). The minor lifetime component 2.2 ns was attrib-
200
Absorbance
0.24
Intensity
0.32 150
100
50
0
0.16
600
700
800
uted to monomeric impurity present in the dyad sample. The major 25 ps component of dyad 4 was attributed to the quenched lifetime of the ZnN4 porphyrin sub-unit of 4 because of excitation energy transfer from the ZnN4 porphyrin sub-unit to the N3S porphyrin sub-unit. Similarly, the fluorescence decay monitored for 5 at 650 nm corresponding to the N4 porphyrin sub-unit was fitted to three exponential with lifetimes 23 ps, 1.74 and 9.8 ns with major contribution from short 23 ps component (Table 3). The other two minor lifetime components 1.74 and 9.8 ns were attributed to the monomeric impurities present in the dyad sample. The major 23 ps component of dyad 5 was attributed to the quenched lifetime of the N4 porphyrin sub-unit of 5 supporting the energy transfer from the N4 porphyrin sub-unit to the N3S porphyrin sub-unit. The kENT and /ENT in both dyads 4 and 5 were in the range of 24– 26 ps1 and 99%, respectively, indicating that the energy transfer from one unit to another unit was very efficient and rapid (Table 3). However, the energy transfer rates of dyads 4 and 5 were much slower than that of phenylethyne-bridged dyad [16] with the ZnN4 and the N4 porphyrin subunits reported in literature. This is due to the presence of sulfur atoms in dyads 4 and 5 which enhances the other non-radiative decay channels [14]. Furthermore, the TS contribution estimated for dyads 4 and 5 (Table 4) were also found to be very minor supporting the predominant contribution of TB only. 4. Conclusions In conclusion, we studied the singlet-singlet energy transfer properties of four covalently linked unsymmetrical dyads containing ZnN4/N4 and N3S porphyrin sub-units connected via diphenylethyne and phenylethyne bridges. All four dyads exhibited an efficient energy transfer from the ZnN4/N4 porphyrin sub-unit to the N3S porphyrin sub-unit with an efficiency of 97–99%. However, the rate and efficiency of energy transfer of phenylethyne-bridged dyads 4 and 5 were higher than that of diphenylethynebridged dyads 2 and 3. This was due to the stronger interaction between the donor and acceptor units in dyads 4 and 5 resulted from their proximity because of shorter phenylethyne bridges. Acknowledgement
Wavelength (nm)
Financial assistance from CSIR, DST and BRNS, Government of India to M.R. is gratefully acknowledged. 0.08
x5 0.00 400
500
600
700
Wavelength (nm) Fig. 5. Absorption and fluorescence (inset) spectra of porphyrin dyad 4 recorded in toluene. The fluorescence spectrum was recorded at excitation wavelength 550 nm.
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Synthesis and photophysical studies of covalently linked porphyrin-21-thiaporphyrin dyads Iti Gupta, M. Ravikanth
*
Department of Chemistry, Indian Institute of Technology, Powai, Mumbai 400076, India Received 22 April 2006; received in revised form 30 July 2006; accepted 17 September 2006 Available online 29 September 2006
Abstract The synthesis and photophysical properties of four covalently linked unsymmetrical porphyrin dyads containing two different porphyrin cores such as N4 and N3S are reported. The covalently linked dyads were prepared by the coupling of appropriate porphyrin having ethynylphenyl functional group at meso-position with porphyrin having iodophenyl or bromo functional group at meso-position under mild palladium coupling conditions. The photophysical study indicated an intramolecular singlet–singlet energy transfer from N4/ ZnN4 porphyrin sub-unit to N3S porphyrin sub-unit in all four dyads with an efficiency of energy transfer process was typically P97%. To probe the role of linker in through bond electronic communication between the two porphyrin sub-units in dyads, the linker was varied from diphenylethyne to phenylethyne and the study revealed that the energy transfer rates and efficiencies were much higher for phenylethyne-bridged porphyrin dyads compared with diphenylethyne-bridged porphyrin dyads. 2006 Elsevier B.V. All rights reserved. Keywords: Porphyrin dyads; 21-Thiaporphyrins; Lifetimes; Energy transfer
1. Introduction Recently the design, synthesis and characterization of various porphyrin-metalloporphyrin and porphyrin-multipigment arrays are in a wide interest to mimic the photoinduced processes occurring in photosynthesis [1]. Many covalent [2,3] and non-covalent [4] multiporphyrin arrays have been synthesized and demonstrated as model light harvesting systems for artificial photosynthesis and are also found to be a prototype for the molecular-scale information-processing applications. However, most of the multiporphyrin arrays reported till date are symmetrical containing mostly N4 porphyrin cores [2,3]. Since the porphyrin units in symmetrical porphyrin arrays are identical, it is difficult to achieve the selective excitation of the porphyrin unit upon irradiation to study electron or energy transfer properties. The selective excitation of porphyrin *
Corresponding author. Tel.: +91 22 25767176; fax: +91 22 25767152. E-mail address: [email protected] (M. Ravikanth).
0020-1693/$ - see front matter 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.ica.2006.09.016
unit can be achieved in unsymmetrical arrays that consist of two or more different porphyrins or related units. This led to the synthesis of series of unsymmetrical porphyrin arrays [5] containing two different macrocycles such as porphyrin–chlorin, porphyrin–corrole, porphyrin–phthalocyanine. However, the reports on porphyrin arrays containing heteroatom substituted porphyrin as one of the sub-unit are very few [6] which may be because of synthetic inaccessibility of suitable heteroporphyrin building blocks. Recently, we developed [7] simple approaches to synthesize the desired functionalized heteroatom substituted porphyrins using easily available precursors under simple reaction conditions and showed their use in the construction of unsymmetrical porphyrin arrays. In continuation of our efforts in synthesizing unsymmetrical porphyrin arrays containing heteroporphyrin unit as one of the sub-unit, in this paper, we report the synthesis of four covalently linked unsymmetrical porphyrin dyads 2–5 (Chart 1) containing regular free base porphyrin sub-unit (N4 core) or its Zn(II) derivative (ZnN4) and 21-thiaporphyrin sub-unit (N3S
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N HN
N
N Zn N N
H
NH N
6
H
Zn6
OC8H17
N HN OC8H17
H S
N
N HN H S
N
OC8H17 8
7
N HN
N
N Zn N N
NH N 1 OC8H17
N
N
N HN
N
S
M N
N
OC8H17
M = Zn : 2 M = 2H : 3 OC8H17
N
N
N HN
N
S
M N
N
M = Zn : 4 M = 2H : 5 Chart 1. Structures of porphyrin monomers and porphyrin dyads 1–5.
core) by the coupling of appropriate porphyrin building blocks under Lindsey’s well-established palladium coupling conditions [8]. We synthesized two types of covalently linked unsymmetrical porphyrin dyads: (a) diphenylethyne-bridged covalent unsymmetrical porphyrin dyads 2 and 3; (b) phenylethyne-bridged covalent unsymmetrical
porphyrin dyads 4 and 5. The photophysical properties of dyads were studied and compared the properties of dyads 2 and 3 with the Lindsey’s covalently linked dyad 1 (Chart 1) containing free base and its Zn(II) derivative with N4 porphyrin core [2d]. The photophysical properties of the corresponding monomers such as 5-(p-ethynylphenyl)-
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10,15,20-tri(mesityl) porphyrin 6 and its Zn(II) derivative Zn6, 5-(p-ethynylphenyl)-10,15,20-tris(p-octyloxy phenyl)21-thiaporphyrin 7 and 5-ethynyl-10,15,20-tri-(p-tolyl)-21thiaporphyrin 8 were also studied as reference compounds. The studies indicated that all four dyads 2–5 showed energy transfer from regular porphyrin sub-unit (N4/ ZnN4) to 21-thiaporphyrin sub-unit on the selective excitation of N4/ZnN4 porphyrin sub-unit and the efficiencies of energy transfer were in the range of 97–99%. 2. Experimental 2.1. General Porphyrin dyad 1 [2d] and the desired porphyrin monomers 6 [9], Zn6 [9], 7 [7b], 8 [10], 5-bromo-10,15,20tri(p-tolyl)-21-thiaporphyrin (9) [10] and 5-(p-iodophenyl)10,15,20-tri(mesityl) Zn(II) porphyrin (Zn10) [9] were synthesized by the following literature methods. All general chemicals and solvents were obtained from SD fine chemicals, India. Pyrrole, aldehydes, thiophene, Pd2(dba)3, and AsPh3 were obtained from Lancaster. Column chromatography was performed on silica (60–120 mesh) and alumina obtained from Sisco Research laboratories, India. NMR spectra were recorded on a Varian 300 MHz spectrometer using tetramethylsilane as an internal standard and ES mass spectra were recorded with a Q-Tof micro (YA-105) mass spectrometer. The absorption and emission spectra were recorded in Perkin–Elmer Lambda 35 and Perkin– Elmer LS-55 spectrometers, respectively. The fluorescence quantum yields (/f) were estimated from the emission and absorption spectra by a comparative method [11] by the following equation: /f ¼ f½F ðsampleÞ½AðstandardÞ=½F ðstandardÞ ½AðsampleÞg/f ðstandardÞ
ð1Þ
where [F(sample)] and [F(standard)] are the integrated fluorescence intensities of the sample and the standard, [A(sample)] and [A(standard)] are the absorbance of sample and the standard at the excitation wavelength and /f (standard) represents the quantum yield of the standard sample. Free base tetraphenylporphyrin (H2TPP, /f = 0.11) and zinc(II) tetraphenylporphyrin (ZnTPP, /f = 0.033) were used as the standards [12]. The time resolved fluorescence decay measurements were carried out at a magic angle using a picosecond diode laser based time correlated single photon counting (TCSPC) fluorescence spectrometer from IBH, UK [13]. All the decays were fitted to single or multi-exponential functions using IBH software. The good fit criteria were low chi-square (1.0) and random distributions of residuals. 2.2. 4-[(5,10,15-Trimesityl-20-porphyrinato) zinc(II)]-4 0 [5,10,15-trioctyloxy-phenyl-21-monothia-20porphyrinyl]diphenylethyne (2) A solution of 5-(p-iodophenyl)-10,15,20-tri(mesityl) zinc(II) porphyrin (Zn10) (16.0 mg, 17.3 lmol) and 5-(p-ethy-
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nylphenyl) 10,15,20-tris(p-octyloxy phenyl)-21-monothiaporphyrin (7) (15.0 mg, 14.4 lmol) was dissolved in dry toluene/triethylamine (5:1, 18 ml) in a 25 ml round-bottomed flask. The flask was fitted with a reflux condenser and a gas inlet tube was inserted through the top of the condenser into the solution for argon purging. The reaction vessel was placed in an oil bath preheated to 35 C. The nitrogen was purged for 15 min. To this solution, Pd2(dba)3 (2.0 mg, 2.2 lmol), AsPh3 (5.4 mg, 17.5 lmol) were added and the reaction mixture was stirred at 35 C for 12 h. TLC analysis of the reaction mixture indicated the virtual disappearance of spots corresponding to the starting materials and the appearance of a new spot corresponding to the dyad. The solvents were removed in vacuo and the crude compound was passed through a small silica column using petroleum ether/dichloromethane (60:40) to remove the excess of AsPh3 and the crude mixture of small amounts of monomeric porphyrins along with the desired dyad was collected with petroleum ether/dichloromethane (20:80). The Pd species remained bound to the top of the column. The solution was concentrated in vacuo and the resulting crude mixture of porphyrinic monomers and dyad was dissolved in dichloromethane (3 ml) and dry slurry was prepared by adding neutral alumina powder. The slurry was loaded on neutral alumina column packed with petroleum ether. The small amounts of porphyrinic monomers were removed first with petroleum ether/dichloromethane and the desired dyad 2 was collected with petroleum ether/dichloromethane (20:80). The concentration of the solution gave porphyrin dyad as a violet solid in a 66% yield (12 mg). Mp > 300 C. dH (300 MHz, CDCl3; Me4Si) 2.60 (s, 1H, NH), 0.86–0.99 (m, 15H, CH3 and CH2), 1.25–1.66 (m, 24H, CH2), 1.87 (s, 18H, CH3), 1.99 (m, 6H, CH2), 2.64 (m, 9H, CH3), 4.25 (m, 6H, OCH2), 7.32 (m, 12H, Ar), 8.12 (m, 10H, Ar), 8.32 (m, 4H, Ar), 8.72 (m, 8H, b-pyrrole), 8.79 (d, J = 4.8 Hz, 2H, b-pyrrole), 8.93 (d, J = 4.8 Hz, 2H, b-pyrrole), 8.98 (s, 2H, b-pyrrole), 9.81 (m, 2H, b-thiophene). m/z (EI) 1843.9 (M+, C123H121N7SO3Zn requires 1842.7). 2.3. 4-[(5,10,15-Trimesityl-20-porphyrinyl)]-4 0 -[5,10,15tri(p-tolyl)-21-monothia-20-porphyrinyl]diphenylethyne (3) A solution of ZnN4–N3S dyad 2 (33.0 mg, 17.9 lmol) was treated in dichloromethane (20 ml) with trifluoroacetic acid (1 ml) under stirring at room temperature for 12 h. After this period, TLC analysis indicated the presence of a new spot. The reaction mixture was washed thoroughly with NaHCO3 solution (1 · 100 ml) followed by water (1 · 100 ml) and extracted with dichloromethane. The solvent was dried over anhydrous Na2SO4 and concentrated in vacuo under reduced pressure. The crude product was purified by neutral alumina column chromatography. The desired dyad 3 was collected using petroleum ether/dichloromethane (40:60) as a violet solid in a 63% yield (20 mg). Mp > 300 C. dH (300 MHz, CDCl3; Me4Si) 2.59 (s, 1H, NH), 2.52 (s, 2H, NH), 0.92–0.97 (m, 9H, CH3), 1.31–
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1.81 (m, 30H, CH2), 1.88 (s, 18H, CH3), 1.97 (m, 6H, CH2), 2.63 (m, 9H, CH3), 4.25 (m, 6H, OCH2), 7.28 (m, 12H, Ar), 8.12 (m, 10H, Ar), 8.31 (m, 4H, Ar), 8.68 (m, 8H, b-pyrrole), 8.74 (m, 2H, b-pyrrole), 8.86 (d, J = 4.8 Hz, 2H, bpyrrole), 8.98 (s, 2H, b-pyrrole), 9.81 (m, 2H, b-thiophene). m/z (EI) 1779.9 (M+, C123H123N7SO3, requires 1779.4). 2.4. 4-[(5,10,15-Trimesityl-20-porphyrinato) zinc(II)]-4[5,10,15-tri(p-tolyl)-21monothia-20-porphyrinyl]phenylethyne (4)
J = 4.5 Hz, 1H, b-pyrrole), 8.65 (m, 4H, b-pyrrole), 8.72 (d, J = 4.8 Hz, 1H, b-pyrrole), 8.77 (d, J = 4.8 Hz, 2H, bpyrrole), 8.88 (s, 2H, b-pyrrole), 8.95 (d, J = 4.8 Hz, 2H, b-pyrrole), 9.54 (d, J = 4.2 Hz, 1H,, b-pyrrole), 9.89 (d, J = 5.1 Hz, 1H, b-thiophene), 10.53 (d, J = 5.1 Hz, 1H, b-thiophene). m/z (EI) 1361.7 (M+, C96H77N7S requires 1360.8). 3. Results and discussion 3.1. Synthesis
A solution of 5,10,15-trimesityl-20-(4-ethynylphenyl) zinc (II) porphyrin (Zn6) (29.3 mg, 35.3 lmol) and 5-bromo-10,15,20-tris(p-tolyl)-21-thiaporphyrin (9) (20.0 mg, 29.4 lmol) in dry toluene/triethyl ammine (25:5 ml) was purged with nitrogen for 10 min. To this solution Pd2(dba)3 (4.0 mg, 4.4 lmol) and AsPh3 (10.8 mg, 35.3 lmol) were added to initiate the coupling. The reaction mixture was stirred at 45 C for 24 h. TLC analysis indicated the almost disappearance of starting material and the new spot corresponding to the required dyad. The solution was concentrated in vacuo and the resultant crude product was purified by neutral alumina column chromatography. The desired compound 4 was collected in petroleum ether/dichloromethane (50:50) as a violet solid in a 69% yield (29 mg). Mp > 300 C. dH (300 MHz, CDCl3; Me4Si) 2.17 (s, 1H, NH), 1.88 (s, 18H, CH3), 2.64 (s, 9H, CH3), 2.72 (m, 9H, CH3), 7.29 (m, 4H, Ar), 7.40 (m, 4H, Ar), 7.63 (m, 6H, Ar), 8.15 (m, 4H, Ar), 8.44 (m, 3H, b-pyrrole and Ar), 8.56 (d, J = 4.8 Hz, 1H, b-pyrrole), 8.64 (d, J = 4.2 Hz, 1H, b-pyrrole), 8.73 (m, 4H, b-pyrrole), 8.85 (m, 4H, b-pyrrole), 9.03 (d, J = 4.5 Hz, 2H, b-pyrrole), 9.55 (d, J = 4.5 Hz, 1H, b-pyrrole), 9.90 (d, J = 4.2 Hz, 1H, b-thiophene), 10.53 (d, J = 4.5 Hz, 1H, b-thiophene). m/z (EI) 1424.6 (M+, C96H75N7SZn requires 1424.1). 2.5. 4-[5,10,15-Trimesityl-20-porphyrinyl]-4-[5,10,15-tri(ptolyl)-21-monothia-20-porphyrinyl]phenylethyne (5) A solution of ZnN4–N3S dyad 4 (30.0 mg, 21.0 lmol) was treated with trifluoroacetic acid (1 ml) in dichloromethane (20 ml) at room temperature under stirring for 12 h. After this period, TLC analysis indicated the presence of a new spot. The solution was washed thoroughly with NaHCO3 solution (1 · 100 ml) and then with water (1 · 100 ml) and dried on anhydrous Na2SO4. The solvent was removed on rotary evaporator in vacuo under a reduced pressure and the resultant crude product was purified by neutral alumina column chromatography. The desired compound 5 was collected in petroleum ether/ dichloromethane (40:60) as a violet solid in a 55% yield (16 mg). Mp > 300 C. dH (300 MHz, CDCl3; Me4Si) 2.49 (s, 1H, NH), 2.14 (s, 1H, NH), 1.89 (s, 18H, CH3), 2.64 (s, 9H, CH3), 2.71 (br s, 9H, CH3), 7.31 (m, 4H, Ar), 7.44 (m, 4H, Ar), 7.65 (m, 6H, Ar), 8.12 (m, 4H, Ar), 8.42 (s, 3H, b-pyrrole and Ar), 8.56 (d,
The desired N3S monomeric porphyrin building blocks 7, 8 and 9 were synthesized as reported earlier [7] and the N4 porphyrin building blocks 6, Zn6 and Zn10 were synthesized by following the literature procedures [9]. The diphenylethyne and phenylethyne-bridged dyads were synthesized using Lindsey’s modified conditions [8] of Sonogashira coupling of an ethynylphenyl porphyrin with iodophenyl porphyrin and ethynylphenyl porphyrin with bromoporphyrin, respectively. Under these modified conditions [8], the coupling of an equimolar quantity of ethynylphenyl porphyrin with iodophenyl porphyrin or bromoporphyrin in dilute conditions can be carried out at a very low temperature and in the absence of copper. The low temperature conditions are needed to prevent side reactions such as the formation of ethyne oligomerization products and the absence of copper is important since the free base porphyrins are easily metalated. Thus, diphenylethyne-bridged dyad 2 containing ZnN4 and N3S porphyrin sub-units was synthesized in a 66% yield by the coupling of Zn10 and 7 in toluene/triethylamine at 35 C in the presence of catalytic amount of Pd2(dba)3/AsPh3 followed by column chromatographic purification (Scheme 1). Dyad 3 was synthesized in a 63% yield by the demetalation of dyad 2 on treatment with trifluoroacetic acid at room temperature for overnight (Scheme 1). The phenylethyne-bridged dyad 4 was prepared by the coupling of Zn6 and 9 under similar palladium coupling conditions [8] and afforded in a 69% yield. Dyad 5 was prepared in a 55% yield by the demetalation of dyad 4 with trifluoroacetic acid at room temperature for overnight. Dyads 2–5 are highly soluble in all common organic solvents and were characterized by NMR, mass spectrometry, absorption and fluorescence spectroscopic techniques. The mass spectrum of dyad 4 is presented in Fig. 1. The mass spectra of dyads 2–5 showed M+ ion peak confirming the identities of the compounds. The 1H NMR spectroscopy has been used to characterize dyads 2–5 in detail. The resonances of dyads 2–5 were assigned on the basis of the spectra observed for the two corresponding monomers taken independently. The 1H NMR spectra of dyads 2 and 5 are shown in Fig. 2 and the chemical shifts of the selected protons of dyads 2–5 are presented in Table 1. In dyad 2, considering the lower symmetry of N3S porphyrin subunit, one would expect one set of
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OC8H17
N
N Zn N N
I +
N HN
H
S
Toluene / Et3N 40oC , 12 h
Pd2 (dba)3 AsPh3
Zn10
OC8H17
N
7 OC8H17
OC8H17
N
N HN
N Zn N N
S
2
OC8H17
N
(66%) OC8H17 TFA/CH2Cl2 OC8H17
N HN
N HN
NH N
S
3
(63%)
N
OC8H17
OC8H17
Scheme 1. Synthetic scheme for dyads 2 and 3.
multiplet for two thiophene protons and three sets of signals for six pyrrole protons and also expect one signal for pyrrole protons of N4 subunit. As clearly evident in
Fig. 1. EI-MS mass spectrum of dyad 4.
Fig. 2a, the multiplet resonance observed at 9.81 ppm is due to two thiophene protons and one broad singlet at 8.72 ppm and three doublets at 8.79, 8.93 and 8.98 ppm were due to six pyrrole protons of the N3S porphyrin sub-unit. The strong singlet observed at 8.72 ppm was due to eight pyrrole protons of the ZnN4 porphyrin subunit. The NH proton of the N3S subunit was observed at 2.60 ppm as a broad singlet. In dyad 3 containing N4 porphyrin sub-unit and N3S porphyrin sub-unit showed similar spectral features as that of 2. However, as expected, there are two signals of inner NH protons at 2.52 and 2.59 ppm corresponding to the N4 porphyrin sub-unit and the N3S porphyrin sub-unit, respectively. A comparison of chemical shifts of the bthiophene and NH protons of dyads 2 and 3 with those of monomer 7 indicates only minor differences suggesting that the two porphyrin subunits in the dyads interact very weakly.
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Fig. 2. 1H NMR spectra of dyads (a) 2 and (b) 5 recorded in CDCl3.
Table 1 1 H NMR chemical shift (d in ppm) of selected protons of porphyrin dyads 2–5 and N3S porphyrin 7 in CDCl3 Compound
b-Thiophene
NHa
7 2 3 4 5
9.67(d), 9.76(d) 9.81(m) 9.81(m) 9.90(d), 10.53(d) 9.89(d), 10.52(d)
2.64 2.60 2.59 2.17 2.14
a
The NH signal of N3S porphyrin.
The phenylethyne-linked dyad 4 exhibited two doublets for two thiophene protons, seven sets of signals for 14 pyrrole protons and one signal for inner NH proton. Dyad 5 showed two sets of signals for thiophene protons, eight sets of signals for 14 pyrrole protons and two signals for three inner NH protons of both the porphyrinic sub-units (Fig. 2b). Dyads 4 and 5 showed downfield shifts for the thiophene and the inner NH protons of the N3S porphyrin sub-unit as compared to 7 (Table 1) indicate a strong interaction between the two porphyrin sub-units.
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The fluorescence spectra of dyads 2 and 3 and the associated monomers were recorded in toluene at room temperature and the relevant data are presented in Table 3. The fluorescence peak maxima and the quantum yields of monomers are 6 (k = 650 nm, / = 0.12), Zn6 (k = 598 nm, / = 0.035) and 7 (k = 690 nm, / = 0.0184). These properties were in close match with the previously reported values of these monomers [2d,7b]. The fluorescence properties of dyads 2 and 3 showed very interesting features. Dyad 3 containing N4 and N3S porphyrin sub-units on excitation at 430 nm where N3S porphyrin sub-unit absorb strongly, showed a typical N3S porphyrin emission with quantum yield, / (0.0178 ) matching with that of monomer 7 (Fig. 3, inset). Interestingly, when dyad 3 was excited at 415 nm where N4 porphyrin sub-unit was the dominant absorber, the major emission was occurred from the N3S porphyrin sub-unit with a weak emission from the N4 porphyrin sub-unit (Fig. 3, inset). The quantum yield data calculated by comparative method [11] indicated that the N4 porphyrin sub-unit emission in dyad 3 was quenched by 94% (Table 3). However, when a 1:1 mixture of monomers 6 and 7 was excited at 415 nm, the major emission was observed mainly from 6 (Fig. 3, inset). Thus, the excitation of N4 porphyrin unit at 415 nm in dyad 3 yields the emission spectrum identical with that obtained upon the direct excitation of the N3S porphyrin sub-unit at 430 nm providing a clear evidence of highly efficient energy transfer operating in dyad 3. The same emission spectrum of the N3S porphyrin sub-unit in dyad 3 was noted when we excited at other wavelengths indicating that the emission spectrum
3.2. Photophysical studies 3.2.1. Diphenylethyne-bridged covalent unsymmetrical porphyrin dyads 2 and 3 The absorption spectra of dyads 2 and 3 and their corresponding monomers were recorded in toluene at room temperature and the data are presented in Table 2. The absorption spectra of monomers such as 6 with N4 core and 7 with N3S core exhibited four Q-bands and one Soret band. The absorption bands of 7 (682, 621, 554, 518, and 434 nm) were more bathochromically shifted as compared to 6 (648, 592, 551, 515, and 420 nm) due to the presence of one sulfur atom in the core. Monomer Zn6 showed two Q-bands (552 and 589 nm) and one Soret band (423 nm). A comparison of the absorption spectra of dyad 3 with a 1:1 mixture of the corresponding monomers 6 and 7 is shown in Fig. 3. Dyads 2 and 3 showed four Q-bands and a split Soret band. As clear from Fig. 3, the absorption spectra of dyads are approximately superposition of the spectra of the corresponding monomers with only minor differences in wavelength maxima and band shapes (Table 2). The Q-band absorption spectra of dyads 2 and 3 (Fig. 3) were almost identical to their corresponding 1:1 mixture of monomers. However, the Soret bands of dyads 2 and 3 were split unlike their corresponding 1:1 mixture of monomers, which showed a broad single Soret band. These observations suggest that weak electronic interactions occur only in the B-states of dyads 2 and 3, which are in agreement with the previous studies on similar type of diphenylethyne-bridged porphyrin arrays [1a].
Table 2 Absorption data of covalent dyads 1–5 and their corresponding monomers in toluene Compound
Soret band k (nm) e · 104 (dm3 mol1 cm1)
Zn6a 6a 7 8
423 420 434 437
2
Q-bands k(nm) (e · 103 dm3 mol1 cm1) I
II
III
515 518 524
552 551 554 561
589 592
424 (53.6) 435 (51.1)
518 (28.4)
552 (34.4)
588 (5.8)
621 (3.9)
(7.3)
3
422 (65.4) 435 (60.6)
518 (57.0)
553 (29.2)
593 (9.4)
618 (sh)
650 (6.9)
4
422 (25.4) 449 (19.4)
509 (sh)
552 (19.4) 592 (sh)
573 (22.9)
635 (sh)
5
420 (25.3) 448 (21.4)
516 (22.8)
548 (sh)
577 (26.1)
638 (sh)
a
Data taken from Ref. [2d].
IV
V
VI
648 621 628
682 691 683
684 (9.2)
700 (5.5)
650 (5.1)
699 (6.2)
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1.0
Absorbance
1:1
Intensity
600
1:1 Dimer
400
200
Dimer 0 600
0.5
700
800
Wavelength (nm)
x5 0.0 400
500
600
700
Wavelength (nm) Fig. 3. Comparison of absorption spectra of porphyrin dyad 3 and the 1:1 mixture of 6 and 7 in toluene. The fluorescence spectra (kex = 415 nm) of dyad 3 and 1:1 mixture of 6 and 7 are shown as an inset.
Table 3 Emission data of dyads 1–5 and corresponding monomers in toluene Compound
/f
s (ps)
K 1 ENT (ps)
/ENT
Zn6 6 7 8 1e 2 3 4 5
0.0350 0.1200 0.0184 0.0180 0.0017a 0.0015a 0.0063b 0.0057a 0.0009b
2258 12 600 1744 1859 24c 57c (1632)f 85d (1659)f 25c (1949)f 23d (1728)f
24 59 86 26 23
0.99 0.97 0.99 0.99 0.99
a b c d e f
/f of ZnN4 sub-unit at kex = 550 nm. /f of N4 sub-unit at kex = 415 nm. sf of ZnN4 sub-unit measured at 600 nm. sf of N4 sub-unit measured at 650 nm. Data taken from Ref. [2d]. sf of N3S sub-unit measured at 680 nm.
of dyad 3 was independent of the excitation wavelength. The excitation spectrum of dyad 3 recorded by scanning the excitation from 720 to 400 nm with fixed emission at 750 nm showed close matching with its absorption spectrum indicating a high yield of singlet-singlet energy transfer within dyad 3. If electron transfer is assumed to be occurring between the sub-units of dyad 3, then we do not expect to observe such a strong emission from the N3S porphyrin subunit on excitation at the N4 porphyrin subunit. Thus, the greatly diminished emission from the N4 porphyrin, the unaltered emission yield from the N3S porphyrin sub-unit and the predominant emission from the N3S porphyrin upon excitation of N4 porphyrin subunit are consistent with efficient energy transfer from the N4 porphyrin sub-unit to N3S porphyrin sub-unit in dyad
3. Similarly, in dyad 2 which contains ZnN4 and N3S porphyrin sub-units upon excitation at 550 nm where the ZnN4 porphyrin sub-unit absorbs very strongly, the emission was observed mainly from the N3S porphyrin sub-unit indicating an efficient energy transfer from ZnN4 porphyrin sub-unit to N3S porphyrin sub-unit. Time-resolved emission studies [13] also support the efficient energy transfer within dyads 2 and 3. The timeresolved fluorescence study helps to probe the dynamics of energy transfer and to obtain additional estimates for the energy transfer yields [2d]. Dyads 2 and 3 were excited at 406 nm and monitored at two different wavelengths corresponding to the emission peak maxima of donor (ZnN4 or N4 porphyrin sub-units) and acceptor (N3S porphyrin sub-unit). The fluorescence decay profiles and weighted residuals of dyads 2 and 3 are presented in Fig. 4 and the data are tabulated in Table 3. The fluorescence decays of Zn6, 6, and 7 were also excited at 406 nm and measured at their respective emission peak maxima 600, 650, and 678 nm, respectively. The fluorescence decays of Zn6, 6 and 7 were fitted to single exponential and the observed lifetimes were 2.25 ns for Zn6, for 12.6 ns 6 and 1.74 ns for 7 (Table 3). The fluorescence decays of dyads 2 and 3 observed at the N3S porphyrin sub-unit (680 nm) were fitted to a single exponential and s (1.63 and 1.66 ns, respectively) was matching closely with monomer 7. However, the fluorescence decays of dyads 2 and 3 monitored at 600 and 650 nm, respectively, corresponding to emission peak maxima of the ZnN4 and N4 porphyrin sub-units were fitted to two or three exponential with a dominant contribution from shorter component (>90%). The other one or two components of fluorescence decays of dyads 2 and 3 were attributed to the monomeric porphyrin impurities
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process is mediated by the donor–acceptor distance and orientation as well as their emission and absorption spectral overlap characteristics. In the Fo¨rster theory of energy transfer, the rate is given by 6 k TS ¼ ð8:8 1023 ÞK 2 /f Jn4 s1 D R
ð4Þ
where K2 is the orientation factor, /f is the fluorescence quantum yield of the donor in the absence of acceptor, J (in cm6 mmol1) is the spectral overlap integral, n is the solvent refractive index (1.49 for toluene), sD is the donor lifetime in the absence of acceptor and R is the centre-to˚ . The overlap intecentre distance of donor–acceptor in A gral J is calculated from the equation J ¼ F D ðmÞeðmÞm1 dm
Fig. 4. Fluorescence decays and weighted residuals of porphyrin dyads 2– 5.
present in the dyads. The major short component for dyads 2 and 3 observed at 57 and 85 ps, respectively, were attributed to the ZnN4 porphyrin sub-unit in dyad 2 and the N4 porphyrin sub-unit in dyad 3. The quenched lifetimes of the ZnN4 and N4 porphyrin sub-units, respectively, in dyads 2 and 3 compared to their associated reference compounds Zn6 and 6 (Table 3) were due to the singlet–singlet excitation energy transfer from the ZnN4 or the N4 porphyrin sub-unit to the N3S porphyrin sub-unit. The rate of energy transfer (kENT) and the efficiency of energy transfer (/ENT) were calculated from the measured lifetimes (sDA) of the ZnN4 and the N4 porphyrin sub-units in dyads 2 and 3, respectively, and that of the corresponding monomers Zn6 and 6 (sD) using the following equations [2d]: k ENT ¼ 1=sDA 1=sD
ð2Þ
/ENT ¼ k ENT sDA
ð3Þ
The kENT and /ENT values calculated for dyads 2 and 3 were (59 ps1, 97%) and (86 ps1, 99%), respectively, (Table 3). Thus, the energy transfer efficiencies /ENT of dyads 2 and 3 were almost same as that of reported dyad 1 (99%) but the energy transfer rates kENT of 2 and 3 were found to be slower than that of 1 (24 ps1) [2d]. This may be due to the presence of sulfur atom in one of the porphyrin unit in the dyad, which enhances the contribution of non-radiative decay channels [14]. Furthermore, the extensive studies by Lindsey and co-workers on similar types of porphyrin arrays with diarylethyne linkers showed that the excited state energy transfer occurs mainly via through bond (TB) and the energy transfer contribution through space (TS) was relatively minor to the overall energy transfer rate. The TS energy transfer contribution can be evaluated in terms of Fo¨rster mechanism [15]. The Fo¨rster
ð5Þ
where FD(m) is the fluorescence intensity of the donor in wavenumber units with total intensity normalized to unity, e(m) is the absorption coefficient of the acceptor and m is the wavenumber in cm1. The calculated J values for dyads 2 and 3 are tabulated in Table 4. The centre-to-centre dis˚ based on tance of donor–acceptor was taken as 20 A MM+ force field calculations and 1.125 was used for K2. The relative contributions of TS (vTS) and TB (vTB) energy transfer to the overall rate were calculated using the following equations [2d]: k ENT ¼ k TB þ k TS
ð6Þ
vTS ¼ k TS =k ENT vTB ¼ k TB =k ENT
ð7Þ ð8Þ
The data are summarized in Table 4. An inspection of Table 4 reveals that the TS rates are much slower and TB contribution accounts for greater than 90% of the observed rate as observed previously for similar kind of diarylethyne-bridged dyadic systems [2d]. In addition, the electron transfer quenching interaction may also be possible in these systems which cannot be evaluated at this stage due to lack of electrochemical data. 3.2.2. Phenylethyne-bridged covalent unsymmetrical dyads 4 and 5 The absorption spectra of dyads 4 and 5 unlike 2 and 3 were quite different from their 1:1 ratio of the corresponding monomers Zn6 or 6 and 8 (Table 2). In dyads 4 and 5, Table 4 Through bond and through space energy transfer rates and contributions for covalently linked unsymmetrical dyads 1–5 Compound
a K 1 TS (ps)
b K 1 TB (ps)
vTS c
vTB d
1e 2 3 4 5
745 2388 551 641 628
25 89 66 25 27
0.04 0.04 0.11 0.04 0.04
0.96 0.96 0.89 0.96 0.96
a b c d e
Obtained from Fo¨rster energy transfer calculations. Obtained using Eq. (6). Obtained using Eq. (7). Obtained using Eq. (8). Data taken from Ref. [2d].
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I. Gupta, M. Ravikanth / Inorganica Chimica Acta 360 (2007) 1731–1742
the Q-bands were broad and ill-defined and the Soret band was clearly split into two peaks. Furthermore, the absorption bands of dyads 4 (Fig. 5) and 5 were bathochromically shifted compared to the corresponding monomers 6 or Zn6 and 8 (Table 2). These observations indicate that the different nature of the spacers affects the electronic communication between the macrocycles [16]. The steady state fluorescence studies on dyads 4 and 5 supported the energy transfer between the sub-units (Table 3). In dyad 5, on excitation at 415 nm where the N4 porphyrin sub-unit absorbs strongly, the emission of the N4 porphyrin sub-unit was quenched to 99% and the major emission was observed from the N3S porphyrin sub-unit supporting the energy transfer from the N4 porphyrin sub-unit to the N3S porphyrin sub-unit (Fig. 5, inset). Similarly, the energy transfer also occurred in dyad 4 from the ZnN4 porphyrin sub-unit to the N3S porphyrin sub-unit on selective excitation of the ZnN4 porphyrin sub-unit at 550 nm. The timeresolved fluorescence studies on dyads 4 and 5 were in agreement with the above observations (Fig. 4). Dyads 4 and 5 were excited at 406 nm and monitored the fluorescence decay at 680 nm corresponding to the N3S porphyrin sub-unit in both dyads and also at 600 and 650 nm, respectively, corresponding to the ZnN4 subunit in dyad 4 and the N4 porphyrin sub-unit in dyad 5. When fluorescence decay was monitored for dyads at 680 nm, it was fitted to single exponential with lifetime 1.94 ps for dyad 4 and 1.73 ps for dyad 5. This is in close match with the N3S porphyrin monomer 8 (1.86 ps). However, when the fluorescence decay (Fig. 4) was monitored for dyad 4 at 600 nm corresponding to the ZnN4 porphyrin sub-unit, the decay was fitted to two exponential with lifetimes 25 ps, and 2.2 ns with major contribution from short 25 ps component (Table 3). The minor lifetime component 2.2 ns was attrib-
200
Absorbance
0.24
Intensity
0.32 150
100
50
0
0.16
600
700
800
uted to monomeric impurity present in the dyad sample. The major 25 ps component of dyad 4 was attributed to the quenched lifetime of the ZnN4 porphyrin sub-unit of 4 because of excitation energy transfer from the ZnN4 porphyrin sub-unit to the N3S porphyrin sub-unit. Similarly, the fluorescence decay monitored for 5 at 650 nm corresponding to the N4 porphyrin sub-unit was fitted to three exponential with lifetimes 23 ps, 1.74 and 9.8 ns with major contribution from short 23 ps component (Table 3). The other two minor lifetime components 1.74 and 9.8 ns were attributed to the monomeric impurities present in the dyad sample. The major 23 ps component of dyad 5 was attributed to the quenched lifetime of the N4 porphyrin sub-unit of 5 supporting the energy transfer from the N4 porphyrin sub-unit to the N3S porphyrin sub-unit. The kENT and /ENT in both dyads 4 and 5 were in the range of 24– 26 ps1 and 99%, respectively, indicating that the energy transfer from one unit to another unit was very efficient and rapid (Table 3). However, the energy transfer rates of dyads 4 and 5 were much slower than that of phenylethyne-bridged dyad [16] with the ZnN4 and the N4 porphyrin subunits reported in literature. This is due to the presence of sulfur atoms in dyads 4 and 5 which enhances the other non-radiative decay channels [14]. Furthermore, the TS contribution estimated for dyads 4 and 5 (Table 4) were also found to be very minor supporting the predominant contribution of TB only. 4. Conclusions In conclusion, we studied the singlet-singlet energy transfer properties of four covalently linked unsymmetrical dyads containing ZnN4/N4 and N3S porphyrin sub-units connected via diphenylethyne and phenylethyne bridges. All four dyads exhibited an efficient energy transfer from the ZnN4/N4 porphyrin sub-unit to the N3S porphyrin sub-unit with an efficiency of 97–99%. However, the rate and efficiency of energy transfer of phenylethyne-bridged dyads 4 and 5 were higher than that of diphenylethynebridged dyads 2 and 3. This was due to the stronger interaction between the donor and acceptor units in dyads 4 and 5 resulted from their proximity because of shorter phenylethyne bridges. Acknowledgement
Wavelength (nm)
Financial assistance from CSIR, DST and BRNS, Government of India to M.R. is gratefully acknowledged. 0.08
x5 0.00 400
500
600
700
Wavelength (nm) Fig. 5. Absorption and fluorescence (inset) spectra of porphyrin dyad 4 recorded in toluene. The fluorescence spectrum was recorded at excitation wavelength 550 nm.
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