Photoinduced intramolecular reactions in triphenylamine–corrole dyads

Journal of Photochemistry and Photobiology A: Chemistry 296 (2014) 11–18

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Photoinduced intramolecular reactions in triphenylamine–corrole dyads Lingamallu Giribabu *, Kolanu Sudhakar Inorganic & Physical Chemistry Division, CSIR-Indian Institute of Chemical Technology, Uppal Road, Tarnaka, Hyderabad, 500007 (Telangana), India

A R T I C L E I N F O

A B S T R A C T

Article history: Received 10 July 2014 Received in revised form 26 August 2014 Accepted 16 September 2014 Available online 22 September 2014

Donor–Acceptor systems, in which a donor triphenylamine (TPA) directly connected to a corrole (Cor) acceptor (TPA-Cor) and separated by an ethynylphenyl bridge between TPA and Cor (TPA-E-Cor) have been designed, synthesized and fully characterized by elemental analysis, MALDI-MS, UV–Visible and 1H NMR spectroscopic methods. A comparison of the UV–Visible and 1H NMR features of these D–A systems with those of the corresponding individual model compounds (i.e., TPA and Cor) reveal that there exist minimum p–p interactions between triphenylamine and corrole p-planes. Quenched emission of triphenylamine (but not corrole) part of both the dyads have been observed in three different solvents. Excitation spectral data provides evidence for an intramolecular excitation energy transfer (EET) from the singlet triphenylamine to the corrole. Detailed analysis of the data suggests that Forster’s dipole–dipole mechanism does not adequately explain this energy transfer but, an electron exchange mediated mechanism can, in principle, contribute to the intramolecular EET. ã 2014 Elsevier B.V. All rights reserved.

Keywords: Corrole Triphenylamine Intramolecular Energy transfer Electron transfer Time-resolved

1. Introduction The conversion of solar light into valuable energy by natural or artificial photosynthesis is an appealing and intriguing process, making it an interesting topic for researchers in biology, physics and chemistry [1–4]. In natural photosynthesis, the initial charge separation reaction in the heart of the reaction center, the photogenerated positive and negative charges are moved apart to suppress the recombination reaction. The energy of the resulting long-lived charges is subsequently used to drive biochemical processes [5,6]. Photosynthesis represents a noteworthy and ubiquitous model that has motivated design of many elaborate assemblies to convert light energy into chemical potential. A great variety of donor–acceptor (D–A) systems have been reported in the literature in order to understand initial events of photosynthetic process [7–9]. Porphyrins almost monopolize the study of photoactive arrays, due to their easy availability and large body of information on their synthetic manipulations, electrochemical and photophysical processes [10–15]. In contrast, investigation of corroles, contracted porphyrin analogs lacking one meso-carbon atom has been overshadowed by porphyrin science, partly due to the challenging corrole synthesis. Recent increased interest in corrole-based

* Corresponding author. Tel.: +91 40 27191724; fax: +91 40 27160921. E-mail address: [email protected] (L. Giribabu). http://dx.doi.org/10.1016/j.jphotochem.2014.09.008 1010-6030/ ã 2014 Elsevier B.V. All rights reserved.

materials and their applications is in great part due to the impressive synthetic progress that has been made in the field over the last one and half decade, as catalysts for organic transformation, sensors and as sensitizers for photodynamic therapy as well as dye-sensitized solar cells applications [16–22]. We are particularly interested in the design of corrole based D–A systems. A few corrole based D–A systems have been reported in order to understand the initial events of natural photosynthesis [23–29]. For example, Nao et al. have constructed mixed corrole–porphyrin multi chromophoric systems and studied singlet–singlet energy transfer process by using transient absorption spectroscopy [25]. D’souza and co-workers have reported corrole–fullerene D–A system and studied the excited state dynamics [28]. It was found that the rate of charge separated state
1010  1011 s1. Recently, Brizet et al. have explained the modulation of singlet energy transfer in gallium corrole-BODIPY dyad systems in which the direction of energy transfer depends upon the type of substituents present on BODIPY [23]. As part of our continuing interest in studying the photoinduced processes of corrole based D–A systems, hereby, we report design, synthesis, spectral (UV–Visible, ESI-MS and 1H NMR) and electrochemical characterization of molecular dyads in which a donor triphenylamine has directly connected to a corrole (TPA-Cor) and an ethynylphenyl spacer in between (TPA-E-Cor). In addition, more emphasis focused on photophysical properties of a novel triphenylamine and corrole dyads (Scheme 1).

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L. Giribabu, K. Sudhakar / Journal of Photochemistry and Photobiology A: Chemistry 296 (2014) 11–18

Scheme 1. Synthetic scheme of TPA-Cor and TPA-E-Cor.

2. Experimental 2.1. General Commercially available reagents and chemicals were procured from sigma–Aldrich. A.R. grade solvents were used for synthesis. All solvents were distilled prior to use. Dichloromethane and N,N0 -dimethyl formamide were dried in the presence of calcium hydride under nitrogen atmosphere. ACME silica gel (100–200 mesh) was used for column chromatography. Thin-layer chromatography was performed on Merck-precoated silica gel 60-F254 plates. Either gravity or flash chromatography was performed for purification of all compounds. All the reactions were carried out under nitrogen or argon atmosphere using dry and degassed solvents in the absence of light. 2.2. Synthesis 5-Phenyldipyrromethane (DPM), 4-(diphenylamino) benzaldehyde (TPA-CHO), 4-((4-triphenylamino) ethynyl) benzaldehyde (TPA-E-CHO) and 5,10,15-triphenyl corrole (TPC) were synthesized as per reported in the literature [30–35]. 2.2.1. 10-Triphenylamine-5,15-diphenylcorrole (TPA-Cor) 4-(Diphenylamino) benzaldehyde (TPA-CHO) (1.5 g, 5.49 mmol) and 5-phenyldipyrromethane (DPM) (2.41 g, 10.98 mmol) were dissolved in 200 mL of methanol. Subsequently, solution of aq. HCl (36%, 1–5 mL) in H2O (200 mL) was added, and the reaction mixture was stirred 3 h at room temperature. The mixture was extracted with CHCl3, and the organic layer was washed twice with H2O, dried over anhydrous Na2SO4, filtered, and diluted to 400 mL with CHCl3. p-Chloranil (1.35 g, 5.49 mmol) was added, and the mixture was refluxed for 1 h. The reaction mixture was passed over a silica gel column (CHCl3), and all fractions containing corrole were combined and evaporated. The subsequent chromatography (silica gel) using CHCl3/hexane, 1:1 solvent mixture as eluent and crystallization (CHCl3/hexane) afforded pure corrole (1.2 g, 31%). Anal. calcd. for C49H35N5% (693.83): C, 84.82; H, 5.08; N,

10.09. Found C, 84.85; H, 5.04; N, 10.05. MS (MALDI-TOF) m/ z = 694.47 (40%), 693.83 calcd for C49H35N5. 1H NMR (CDCl3) d ppm 7.12 (s, 1H), 7.38–7.60 (m, 9H), 7.68–7.90 (m, 8H), 8.0–8.22 (d, 2H), 8.40 (d, 4H), 8.52–8.75 (d, 4H), 8.94 (d, 4H). 2.2.2. 10-(4-Ethynyltriphenylamino) phenyl-5,15-diphenylcorrole (TPA-E-Cor) 4-((4-Triphenylamino) ethynyl) benzaldehyde TPA-E-CHO (100 mg, 0.267 mmol) and 5-phenyldipyrromethane (DPM) (120 mg, 0.534 mmol) were added to 20 mL of the pre-prepared solution of TFA (0.01 mL in 20 mL of CH2Cl2). The reaction was left at room temperature. After 5 h, solution of DDQ (121 mg, 0.534 mmol) in toluene (2 mL) was added, and the reaction was stirred at room temperature for further 30 min. The solvent was removed under vacuo. The obtained solid material was subjected to silica gel column chromatography and eluted with hexane/ CH2Cl2 (2:8, v/v) solvent mixture. The solvent front running purple black color band was collected. Yield (21%). Anal. calcd. for C57H39N5% (793.95): C, 86.23; H, 4.95; N, 8.82. Found C, 86.25; H, 5.00; N, 8.80. MS (MALDI-TOF) m/z = 795 (100%), 793.93 calcd for C57H39N5. 1H NMR (CDCl3) d ppm 7.05–7.20 (m, 2H), 7.30 (m, 6H), 7.52 (d, 2H), 7.70–8.0 (m, 7H), 8.21 (m, 3H), 8.3–8.7 (m, 4H), 8.80–8.92 (m, 4H). 3. Methods 1

H NMR spectra were recorded on a 500 MHz INOVA spectrometer. Cyclic and differential-pulse voltammetric measurements were performed on a PC-controlled electrochemical analyzer (CH instruments model CHI620C). All these experiments were performed with 1 mM concentration of compounds in dichloromethane at a scan rate of 100 mV s1 in which tetrabutyl ammonium perchlorate (TBAP) is used as a supporting electrolyte, standard calomel as reference electrode, glassy carbon as working electrode and Pt-wire as counter electrode [36,37]. The optical absorption spectra were recorded on a Shimadzu (Model UV-3600) spectrophotometer. Concentrations of solutions are ca. to be 1 106 M (corrole Soret band) and 1 105 M

L. Giribabu, K. Sudhakar / Journal of Photochemistry and Photobiology A: Chemistry 296 (2014) 11–18

50 TPA TPC TPACor TPA-E-Cor

x103

(M-1cm-1)

40

30

20

10

0 300

400

500

600

700

Wavelength(nm) Fig. 1. UV–Visible absorption spectra in CH2Cl2.

(corrole Q-bands). Steady-state fluorescence spectra were recorded on a Fluorolog-3 spectrofluorometer (Spex model, JobinYvon) for solutions with optical density at the wavelength of excitation (lex)  0.05. Fluorescence quantum yields (F) were estimated by integrating the fluorescence bands and by using triphenylcorrole (F = 0.21 in toluene) as reference compound [38]. Fluorescence lifetime measurements were carried on a picosecond time-correlated single photon counting (TCSPC) setup (FluoroLog3-Triple Illuminator, IBH Horiba JobinYvon) employing a picosecond light emitting diode laser (NanoLED, lex = 405 nm) as excitation source. The decay curves were recorded by monitoring the fluorescence emission maxima of the triad (lem = 675 nm). Photomultiplier tube (R928P, Hamamatsu) was employed as the detector. The lamp profile was recorded by placing a scattered (dilute solution of Ludox in water) in place of the sample. The width of the instrument response function (IRF) was limited by the full width at half maxima (FWHM) of the excitation source,
625 ps at 405 nm. Decay curves were analyzed by nonlinear leastsquares iteration procedure using IBH DAS6 (version 2.3) decay analysis software. The quality of the fits was judged by the x2 values and distribution of the residuals. Full geometry optimization of the dyads TPA-Cor and TPA-E-Cor were carried out with the DFT-B3LYP method using 6-31G(d,p) basis set and frequency analysis confirmed that the obtained geometries are to be genuine global minimum structures. All calculations were performed with the Gaussian G03 (d01) package on a personal computer [39]. 4. Results and discussions 4.1. Ground state properties Both the dyads TPA-Cor and TPA-E-Cor were synthesized by condensation of 5-phenyl dipyrromethane (DPM) with

13

4-(diphenylamino) benzaldehyde (TPA-CHO) and 4-((4-triphenylamino) ethynyl) benzaldehyde (TPA-E-CHO), respectively as shown in the scheme 1. Preliminary characterization of both the dyads was carried out by MALDI-MS and UV–Visible spectroscopic methods. The elemental analyses of the dyads were presented in the experimental section and it was found satisfactory. The mass spectrum of TPA-Cor showed a peak at m/z = 694.47 (C49H35N5) ascribable to the molecular ion peak. Similarly, the mass spectrum of TPA-E-Cor showed a peak at m/z = 795 (C57H39N5) ascribable to the molecular ion peak (See Supporting information). Fig. 1 depicts the electronic absorption spectra of TPA-Cor and TPA-E-Cor along with its monomeric constitutents i.e., triphenylamine (TPA) and 5,10,15-triphenyl corrole (TPC) in dichloromethane solvent. The corresponding wavelength of absorption maxima and molar extinction coefficients (e) are listed in Table 1. The monomeric component TPA shows an absorption band at 300 nm due to p–p* transitions whereas corrole has an intense Soret band at 416 nm, arised due to p–p* electronic transition from ground state to the second excited state (S2) and three less intense Q-bands, originated from p–p* electronic transitions, attributed to the first excited state (S1). But at 300 nm corrole also has some absorption as shown in Fig. 1 (
50%). A comparison of the UV–Visible spectrum of dyads with the spectra of the corresponding precursor units suggest that the lmax and e values of each dyad are found in the same range as those of the reference compounds. Thus, the spectrum of each dyad is more or less similar to the spectrum resulting from a combination (1:1 mole/mole ratio) (TPC) and (TPA) of the individual precursors. As a general observation the absorption data show a minimum p–p interactions and a localized description of the individual subunits can be adopted with confidence. 1 H NMR spectral data of both the dyads TAP-Cor and TPA-E-Cor are summarized in Section 2. From the experimental section, it is clear that the peaks in aromatic region consists of both corrole and triphenylamine peaks. The peak at 8.94 ppm belongs to the pyrrole-b protons of the corrole part of the dyad. In contrast, the peak at 7.12 ppm belongs to the triphenylamine. The inner imine protons of the dyad appeared at 2.70 ppm. All these peaks are suggesting the formation of dyad TPA-E-Cor molecule. A similar 1 H NMR was also observed in TPA-Cor dyad (See Supporting information). In contrast, cyclic voltammetric experiments have revealed that the electrochemical redox potentials of both dyads are in sensitive to its molecular structure. With a view to evaluate energies of the charge transfer states (ECT), which, as will be discussed in a later part of this manuscript, are useful quantities in analyzing the photochemical properties of these dyads, we have carried out electrochemical investigation. Fig. 2 shows the differential pulse voltammograms of the dyad, and Table 1 summarizes the redox potential data of both dyads along with that of the relevant monomeric analogues. Each dyad undergoes three oxidations and one reduction process under the experimental conditions employed in this study. Wave analysis suggested that, in general, while the first three reduction steps and first two oxidation steps are reversible (ipc/ipa = 0.9–1.0) and diffusion-controlled

Table 1 UV–Visible absorption and electrochemical data. Compound

TPA TPC TPA-Cor TPA-E-Cor a b

Absorption, lmax, nm (log e, M1 cm1)a TPA bands

Soret band

300 (4.36) – 300 (4.65) 296 (4.53)

– 414 (4.83) 417 (5.06) 418 (5.10)

366 (4.71)

Solvent CH2Cl2, error limits: lmax, 1 nm, log e, 10%. CH2Cl2, 0.1 M TBAP; glassy carbon working.

Potential V vs. SCEb Q-bands – 571 (4.08) 572 (4.13) 574 (4.32)

614 617 613 613

(3.99) (4.04) (4.18) (4.18)

647 (3.92) 648 (3.99) 649 (4.04) 649 (4.04)

Oxidation

Reduction

0.66, 0.58, 0.53, 0.36,

– 1.35 1.48 1.40

0.96 0.97, 1.48 0.85, 1.15 0.63, 1.05

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50

4

Flu.Intencity (a.u)

(M-1cm-1)

x103

30

20

10

400

500

600

(b)

TPC TPACor TPA-E-Cor

5

3 4 2

3 2

1

0 300

6 TPA TPACor TPA-E-Cor

TPA TPC TPACor TPA-E-Cor

40

(a)

1

700

Wavelength(nm)

0 0 300 400 500 600 700 800 600

Fig. 2. Differential pulse voltammograms dyads in CH2Cl2 with 0.1 M TBAP.

(ipc/n1/2 = constant in the scan rate (n) range 50–500 mV s1) oneelectron transfer (DEp = 60–70 mV; DEp = 65  3 mV for ferrocenium/ferrocene couple) reactions, the subsequent steps are, in general, either quasi-reversible (Epa – Epc = 90–200 mV and ipc/ipa = 0.5–0.8 in the scan rate (n) range 100–500 mV s1). The reduction peak is purely belongs to corrole macrocycle, whereas three oxidation peaks belongs to both TPA and corrole part of the dyads. The spectroscopic and electrochemical features described above suggested that in the ground state electronic communication between corrole and triphenylamine chromphores is quite negligible in these new D–A system. More importantly, one can exploit the excited state properties by selective excitation of the individual chromophoric units. 4.2. Singlet state properties Unlike the case with the ground state properties, major difference have been noticed in the singlet state activities of these dyads and its monomeric unit triphenylamine. The steady state luminescence properties of both the dyads were investigated upon excitation at 570 nm, where only corrole absorbs and upon excitation at 300 nm, where TPA and corrole moieties absorbs 50% of light. Fig. 3 illustrates the emission spectra of TPA-Cor, TPA-ECor and along with its individual constituent in dichloromethane solvent. The corresponding singlet state data are presented in Table 2. Upon excitation of both the dyads at 300 nm, compared with the model TPA absorbing the same number of photons in the dyads are shown in Fig. 3a. In both the dayds luminescence localized on the TPA is totaly quenched. The quenching of emission maxima were also collected in cyclohexane and acetonitrile solvents. However, selective excitation at 570 nm i.e., the lmax corresponds to the corrole absorption maxima, the resulted spectra are seen to be similar to that of the spectrum of TPC (Fig. 3b). The E0–0 (0–0 spectroscopic transition energy) values of the TPA (3.75  0.05 eV) and the corrole (1.88  0.05 eV for TPC) moieties of these dyads, as estimated from an overlap of their absorption and emission spectra, were found to be in the same range as the E0–0 values of TPA and TPC, respectively. A major difference between the fluorescence data of the dyads and those of TPC and TPA lie in the magnitude of Ff values (Table 2). Whereas Ff for an excitation into the corrole moiety was similar to that of TPC (lex = 570 nm, Ff = 0.21  0.19) in TPA-Cor. In contrast quantum yield values are slightly higher in TPA-E-Cor, when compared to TPC (Table 2). Fluorescence from

650

700

750

800

Wavelength (nm) Fig. 3. Fluorescence spectra equiabsorbing solutions (OD at lex = 0.14) in CH2Cl2 (a) lex = 300 nm (b) lex = 570 nm.

triphenylamine part of the dyads was found to be strongly quenched in comparison with the fluorescence of TPA in all the three investigated solvents. The quenching efficiency Q, Q¼

FðTPAÞ  FððTPA  CorðorÞTPA  E  CorÞ FðTPAÞ

(1)

and kobs, kobs ¼

Q=ð1  QÞ t ðTPAÞ

(2)

Values as evaluated using the fluorescence data are given in Table 2. In Eqs. (1) and (2), F(TPA) and F((TPA-Cor or TPA-E-Cor) refer to the fluorescence quantum yields for triphenylamine and the dyads (lex = 570 nm), respectively, and t (TPA) is the singletstate life time of triphenylamine (2.23 ns, 1.02 ns, and 1.97 ns, in cyclohexane, CH2Cl2, and CH3CN, respectively) [27,40]. There exists a strong overlap between the emission of triphenylamine and the absorption of corrole in both the dyads and, this suggests that quenching of the fluorescence of triphenylamine in these D–A systems can be due to an intramolecular EET from the singlet state of triphenylamine to the corrole. Indeed, excitation of approximately 107 M solution of both the dyads at 300 nm resulted in the appearance of well-defined corrole emission bands in all the three solvents (Fig. 3a). In contrast, fluorescence was also detected when a solution of TPC having the same concentration was excited at the same wavelength (See Supporting information). Furthermore, when the fluorescence was monitored at the corrole emission maximum (lem = 680 nm), the excitation spectrum taken for TPA-Cor and TPA-E-Cor showed bands characteristic of triphenylamine absorption. Collectively, these observations provide evidence for intramolecular EET in these bichromophoric dyads. It should be noted here that a solution containing 1:1 (mole/mole) intermolecular mixture of TPC and TPA did not show quenching of the fluorescence due to triphenylamine (lex = 300 nm), nor did it suggest energy transfer in the excitation spectrum. All these evidences suggest, that there is an efficient EET from singlet state of TPA to TPC in an intramolecular manner even though TPC has absorption at this wavelength. Energy transfer efficiencies, Tobs, that are obtained by comparing an overlap of the excitation and absorption spectra in each

L. Giribabu, K. Sudhakar / Journal of Photochemistry and Photobiology A: Chemistry 296 (2014) 11–18

15

Table 2 Fluorescence data.a

lem, nm (F, %Q)b lex = 300 nm

lex = 570 nm

Solvent

TPA

TPA-Cor

TPA-E-Cor

TPC

TPA-Cor

TPA-E-Cor

Cyclohexane CH2Cl2 CH3CN

354 (0.030) 360 (0.034) 360 (0.059)

427 (0.003, 90) 439 (0.003, 92) 432 (0.004, 94)

403 (0.009, 70) 495 (0.009, 74) 542 (0.01, 83)

666 (0.193) 670 (0.190) 649 (0.290)

674 (0.192) 678 (0.195) 651 (0.280)

666 (0.247) 670 (0.235) 650 (0.305)

a b

Spectra were measured at 293  3 K. Error limits: lem, 1 nm; Ff, 10%; t , 10%.

investigated solvent are collected in Table 3 and spectra showing this overlap for both the dyads are illustrated in Fig. 4. Table 3 also contains data on kEN(obs) values where T =ð1  T obs Þ kEN ðobsÞ ¼ obs t ðTPAÞ

(3)

In what follows now we shall try to arrive at a mechanistic interpretation of the EET occurring in these dyads by employing the obtained data. EET reactions in bichromophoric D–A systems can proceed mainly through two types of mechanisms [41]: (i) over large D–A separations and in the absence of any interchromophore interactions, they may occur via a Columbic interaction between the transition dipoles of excited donor and ground state acceptor (Forster mechanism) [42] and (ii) in situations where there is some degree of interchromophore orbital overlap they can also be mediated by overlap dependent electronic coupling, one of which is the quantum mechanical exchange interaction (Dexter mechanism) [43]. Both the Forster and the Dexter mechanisms require, among other things, that the rate of energy transfer be proportional to spectral overlap J of the donor emission and the acceptor absorption. Overlap integrals JForster (Eq. (4) and JDexter (eq. (5) for the present dyads in the three investigated solvents have been estimated to be (2.39  0.02)  1016 cm6 mmol1 and (3.27  0.02)  104 cm4 respectively, and in both the cases, no direct correlation of these integrals could be made with the kEN(obs) (or Tobs) values R FðnÞeðnÞe4 dn R JForster ¼ (4) FðnÞdn

kForster ¼

8:8  1023 k2 FD JForster n 4 t D R6

(6)

Here n, is the solvent refractive index, FD and t are the fluorescence quantum yield and lifetime of the isolated donor, k2 is an orientation factor takes into account the relative orientation of the transition dipole moments of the donor and the acceptor and can be simplified to the statistical value, 2/3 and R (cm) is the donor–acceptor center-to-center distance. It can be easily seen from the data given in Table 3 that no correlation between the theory and experiment can be done in these donor–acceptor systems. That is, kEN(obs) values do not show a good correlation with the kForster values calculated for EET. Collectively, these observations clearly suggest that the rates of EET in this D–A system are inadequately explained by the Forster mechanism as is the case with the 1,8-naphthalimide-corrole dyads and porphyrin-anthracene dyads [29,44].

R JDexter ¼ R

FðnÞeðnÞdn FðnÞdneðnÞdn

(5)

(F(n) is the normalized fluorescence intensity of the energy donor at wave number n (cm1), and e (mmol1 cm1) is the molar extinction coefficient of energy acceptor). The Forster mechanism predicts the rate constant kForster for the EET to follow Table 3 Energy transfer data.a Compound

Solvent

%Q

kobs (109 s1)

kEN(obs) (109 s1)

kForster (109 s1)c

TPA-Cor TPA-E-Cor TPA-Cor TPA-E-Cor TPA-Cor TPA-E-Cor

Cyclohexane (n = 1.426, e = 2.04)b CH2Cl2 (n = 1.452, e = 8.93)b CH3CN (n = 1.344, e = 37.50)b

90 74 92 74 94 83

4.03 1.27 11.27 2.79 7.95 2.47

3.62 0.80 18.62 2.28 9.64 2.03

1.05 0.03 2.69 0.08 2.96 0.08

a b

Error limits: %Q,kobs 8%, %T, kEN(obs): 15%. n and e refer to refractive index and dielectric constant of the solvents.

Fig. 4. Overlay of the excitation (- - - -) and absorption (________) spectra of (a) TPACor and (b) TPA-E-Cor in CH2Cl2 solvent (lem = 680 nm). The excitation spectra were corrected for the instrument response function and were normalized with respect to the absorption spectra as described in Ref. [44].

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In contrast, participation of the Dexter mechanism in the EET occurring in TPA-Cor and TPA-E-Cor is indicated by arguments based on D–A distance (DFT-B3LYP method using 6-31G(d,p)) and thermodynamic (redox data) criteria. The estimated edge-to-edge approach distances (Re) between triphenylamine and the corrole in TPA-Cor and TPA-E-Cor are 1.493 Å and 8.395 Å, respectively (for energy minimization structures of TPA-Cor & TPA-E-Cor, See supporting information). These Re distances may facilitate D–A orbital interactions and promote electron exchange process. In addition, Dexter’s mechanism involves both the lowest unoccupied molecular orbital (LUMO) and highest occupied molecular orbital (HOMO) of D and A, and electron exchange can occur in either a concerted process or via stepwise electron transfer reactions involving radical intermediates of the type D+A and DA+. In this regard, it can be argued that if neither electron transfer step can occur to an orbital at a higher energy level, the HOMO and LUMO of the acceptor must be either iso-energetic with or sandwiched between the levels of the HOMO and the LUMO of the donor [45]. The latter situation holds good for the dyads under investigation as shown by the energy level diagram shown in Fig. 5a. It can be seen that the corrole energy levels (Cor+ and Cor) are sandwiched between those of triphenylamine (TPA+ and TPA) and that electron exchange energy transfer from the singlet triphenylamine to the corrole subunit in these dyad systems are possible on thermodynamic grounds. Thus, both the distance and thermodynamic criteria indicate the possibility of the involvement of electron exchange mediated energy transfer in these D–A systems. Despite these, we believe that it is not generally correct to consider exclusively a Forster or a Dexter mechanism for a singlet-singlet interchromophore EET and that this is especially so for the bichromophoric D–A systems investigated in this study which has an intermediate D–A separation distance. In addition to the EET reaction explained above, a PET reaction is thermodynamically feasible in these D–A systems. The free-energy change for an electron transfer (DGPET) from the singlet triphenylamine to the corrole in these systems as calculated by employing the redox potential and E0–0 data by using following equation.

DGPET ¼ E1=2 ðTPAþ =TPAÞ  E1=2 ðCor=Cor Þ  E00  X

(7)

where the term X accounts for the finite donor–acceptor seperation (Rc), ionic radii (r+, r) and solvent dielectric constant (es), X¼

e2 e2 1 1 1 1 þ ð þ Þð  Þ 4pe0 eS RC 8pe0 rþ r eref eS

(8)

The radii of the molecular ions used were r+ = 4.23 Å (TPA+) and r = 7.53 Å (Cor) from calculations. The center-to-center distance between donor acceptor was found to be 8.69 and 15.59 Å for TPA-Cor and TPA-E-Cor, respectively calculated from energy minizations studies. (E1/2(TPA+/TPA) and E1/2(Cor/Cor) refer to the oxidation potential of triphenylamine and the reduction potential of the corrole, respectively, vide supra). DGPET of TPA-Cor was found 1.66  0.05, 1.78  0.05 and 1.67  0.05 eV for solvent cyclohexane, DCM and acetonitrile, respectively. In contrast, DGPET of TPA-E-Cor was found 1.64  0.05, 1.77  0.05 and 1.67  0.05 eV for solvent cyclohexane, DCM and acetonitrile, respectively. Fig. 5b, which is based on Eq. (7), clearly illustrates the possibility of a PET from the singlet triphenylamine to corrole in these D–A systems. However, considering the fact that contribution of EET in each investigated solvent is >90% for TPA-Cor and >70% for TPA-E-Cor, quenching of fluorescence seen for these D–A systems via a PET mechanism is assumed to be not so important. In contrast, Tasior et al. has observed 99% energy transfer from donor triphenylamine to acceptor corrole. The energy transfer efficiency is higher in these D–A systems than the present D–A systems, probably due to electron withdrawing element fluorine present on the phenyl group of corrole macrocycle [27]. Similar photoinduced reactions were observed by Gryko and co-workers in 1,8-naphthalimidecorrole dyads in which the energy transfer efficiency decreases from 100% in the dyads containing the corrole more difficult to oxidize to 65% and 15% in the dyads containing corrole with progressively lower oxidation potentials whereas electron transfer efficiency follows the reverse pattern [29]. On the other hand, the emission quench was explained by purely photoinduced electron transfer in corrole-anthraquinone dayd systems [46]. Fig. 6 illustrates excited state fluorescence decay profile of both dyads collected in cyclohexane solvent, by selectively exciting sample with two different excitation sources (lex = 300 and 405 nm), where the individual subunits (TPA and corrole) absorb predominantly at one wavelength. As expected, when we monitored the corrole emission (lem = 700 nm), the fluorescence

10000 Prompt TPA TPACor TPA-E-Cor

Counts

1000

100

10 10 Fig. 5. (a) Energy levels of corrole (Cor) and triphenylamine (TPA) as derived from the electrochemical redox potential data. (b) Energy level diagram showing both energy (EET) and electron (PET) transfer reactions from the singlet excited state of triphenylamine moieties of TPA-Cor and TPA-E-Cor.

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Time (ns) Fig. 6. Fluorescence decay curves in cyclohexane.

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lifetime (t ) of the both dyads TPA-Cor and TPA-E-Cor were not quenched as compared to TPC (See Supporting information). On the other hand, when the TPA emission is monitored at lem = 360 nm and excited at 300 nm, the fluorescence lifetime of the TPA-Cor in cyclohexane is found to be 0.82 ns, while TPA-E-Cor lifetime is found to be 1.26 ns. The excited state decay studies further support to this intramolecular photoinduced reactions of these dayd systems. 5. Conclusions In conclusion, we have designed and synthesized D–A corrole conjugates having either donor triphenylamine connected directly to the acceptor corrole macrocycle or an ethynylphenyl bridge in between them. Both dyads have been characterized by various spectroscopic techniques. All ground state properties indicate that there exist minimum p–p interactions between the aromatic p-planes. This has been supported by energy minimization by DFT calculations. Photophysical studies of these dyads and the reference compound showed that whereas upon selective excitation of the corrole component no photo-induced process occurs, excitation of the triphenylamine moiety results in very efficient energy transfer process. However, the photoinduced electron transfer reaction cannot be ignored based on thermodynamic consideration. The photoinduced processes are discussed in the frame of current theories. Acknowledgements We are grateful to the Department of Science and Technology (DST, SB/S1/IC-14/2014) and CSC-0134 (“Molecules to Materials and Devices M2D”) for financial support of this work. The author KS acknowledges University Grants Commission (UGC) for Senior Research Fellowship (SRF). Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.jphotochem.2014.09.008. References [1] S. Beradi, S. Drouet, L. Francas, C. Gimbert-Surinach, M. Guttentag, C. Richmond, T. Stoll, A. Llobet, Molecular artificial photosynthesis, Chem. Soc. Rev. (2014) , doi:http://dx.doi.org/10.1039/C3CS60405E. [2] V.K. Singh, R.K. Kanaparthi, L. Giribabu, Emerging molecular engineering strategies of unsymmetrical phthalocyanines for dye-sensitized solar cell applications, RSC Adv. 4 (2014) 6970–6984. [3] M.J. Leonardi, M.R. Topka, P.H. Dinolfo, Efficient förster resonance energy transfer in 1,2,3-triazole linked BODIPY-Zn(II) meso-tetraphenylporphyrin donor–acceptor arrays, Inorg. Chem. 51 (2012) 13114–13122. [4] G.D. Scholes, G.R. Flemming, A. Olaya-Castro, R. vanGrondelle, Lessons from nature about solar light harvesting, Nat. Chem. 3 (2011) 763–774. [5] A. Harriman, Photo-oxidation of water under ambient conditions – the search for effective oxygen – evolving catalysts, Euro. J. Inorg. Chem. (2014) 573–580. [6] R.E. Blankenship, D.M. Tiede, J. Barber, G.W. Brudvig, G. Fleming, M. Ghirardi, M.R. Gunner, W. Junge, D.M. Kramer, A. Melis, T.A. Moore, C.C. Moser, D.C. Nocera, A.J. Nozik, D.R. Ort, W.W. Parson, R.C. Prince, R.T. Sayre, Comparing photosynthetic and photovoltaic efficiencies and recognizing the potential for improvement, Science 332 (2011) 805–809. [7] X.-F. Wang, H. Tamaiki, Cyclic tetrapyrrole based molecules for dye-sensitized solar cells, Energy Environ. Sci. 3 (2010) 94–106. [8] L. Flamigni, D. Gryko, Photoactive corrole-based arrays, Chem. Soc. Rev. 38 (2009) 1635–1646. [9] R. Chitta, F. D’souza, Self-assembled tetrapyrrole-fullerene and tetrapyrrole carbon nanotube donor–acceptor hybrids for light induced electron transfer applications, J. Mater. Chem. 18 (2008) 1440–1471. [10] S.G. DiMagno, V.S.-Y. Lin, M.J. Therien, Facile elaboration of porphyrins via metal-mediated cross-coupling, J. Org. Chem. 58 (1993) 5983–5993.

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