Functionalized porphyrin derivatives for solar energy conversion

Accepted Manuscript Functionalized porphyrin derivatives for solar energy conversion Panagiotis Angaridis, Theodore Lazarides, Athanassios C. Coutsolelos PII: DOI: Reference:

S0277-5387(14)00250-2 http://dx.doi.org/10.1016/j.poly.2014.04.039 POLY 10683

To appear in:

Polyhedron

Received Date: Accepted Date:

28 February 2014 15 April 2014

Please cite this article as: P. Angaridis, T. Lazarides, A.C. Coutsolelos, Functionalized porphyrin derivatives for solar energy conversion, Polyhedron (2014), doi: http://dx.doi.org/10.1016/j.poly.2014.04.039

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Functionalized porphyrin derivatives for solar energy conversion Panagiotis Angaridis1*, Theodore Lazarides1*, Athanassios C. Coutsolelos2* 1

Department of Chemistry, Aristotle University of Thessaloniki, Thessaloniki, 54124, Greece. 2

Department of Chemistry, University of Crete, Voutes Campus, PO Box 2208, 71003 Heraklion, Crete, Greece.

[email protected] (pa) [email protected] (tl) [email protected] (agc)

Key words: porphyrins/electron and energy transfer/photocatalytic hydrogen production/dye-sensitized solar cells

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Abstract In this review, we discuss a collection of three sets of porphyrin-based systems recently synthesized in our group. In the first set we improve the light harvesting ability of porphyrins by preparing arrays in which strongly absorbing boron dipyrrin (BDP) chromophores are combined with porphyrins, either by covalent attachment to the periphery of the porphyrin ring or by axial coordination to a tin(IV) porphyrin. We observe that BDP increases significantly the light absorption capability of porphyrins by efficient BDP to porphyrin excitation energy transfer. Further functionalization of the arrays with redox active groups such as phenolate ligands or fullerenes gives rise to interesting electron transfer dynamics in the excited state. In the second set, we use porphyrins as sensitizers in photocatalytic hydrogen production schemes with cobaloxime catalysts either in supramolecular or diffusion controlled systems. Photocatalytic experiments, detailed spectroscopic studies on our systems and recent literature results show that attachment of a photosensitizer to a catalyst is in fact detrimental to H2 production. Diffusion controlled systems show considerably increased photocatalytic activity mainly because deleterious effects such as back electron transfer are avoided. In the final set, porphyrins are used as light harvesting dyes in dye-sensitized solar cells (DSSCs). Aiming to extend the light absorption range of the sensitizing dyes, to achieve enhanced solar cell performances, appropriately functionalized porphyrins with carboxylic acid anchoring groups are synthesized. These include porphyrins with π-conjugated groups at meso positions, “push-pull” porphyrins, porphyrins with bridges between the macrocycle and the anchoring group, and covalently linked arrays of porphyrins and other chromophores. Porphyrins with pyridyl anchoring groups are also utilized, in order to get insight into the effect of the nature and strength of the porphyrin binding on the solar cell efficiency. Devices with photovoltaic efficiencies higher than the average values of solar cells sensitized by simple porphyrin and other hybridporphyrin derivatives are obtained, particularly after reduction of dye-aggregation by means of chemical co-adsorbents, and by using co-sensitization, modified photoanodes, molten electrolytes and electrolyte additives.

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1. Introduction The harvesting and storage of solar energy by means of multichromophoric assemblies has been an active field of research over the last few decades [1]. The early stages of photosynthesis, where solar energy is absorbed and channeled to a reaction center through a series of energy and electron transfer steps, have been a major source of inspiration for the construction of artificial mimics [2]. Due to their importance in natural photosynthesis, porphyrins have been widely investigated as key constituents of artificial photosynthesis systems as they exhibit strong absorption in the visible region and highly redox active excited states. In the present review, we summarize recent results from our group on the use of porphyrins as light harvesting and energy storage agents in three different contexts: i) synthesis and photophysical study of multichromophoric systems involving porphyrins, ii) the use of porphyrins as sensitizers in photocatalytic hydrogen production schemes and iii) porphyrin derivatives as light harvesting dyes in dye sensitized solar cells (DSSCs).

2. Electron and energy transfer in porphyrin derivatives Despite of their favorable properties, one major drawback of porphyrin derivatives as light-harvesters is their relatively poor absorption cross section in the blue-green region of the spectrum (430-550 nm). This issue can be tackled by coupling porphyrins with chromophores, which absorb strongly in the desired spectral region and are thus able to sensitize the porphyrin excited state via efficient intramolecular energy transfer. Boron dipyrrin (BDP), see scheme 1, dyes are particularly attractive as “antenna” chromophres for porphyrins as they combine the following characteristics: i) BDPs are well-known for their high extinction coefficients in the 450-550 nm region ii) high fluorescence quantum yields and long-lived excited states and iii) excellent photostability, and stability in many solvents [3]. In addition, the absorption and emission properties of BDP dyes can be easily tuned in the 450-680 nm range owing to the ease of synthetic modification of the BDP core. Indeed, a wealth of studies exist in which, BDP chromophores transfer excitation energy to porphyrins thereby mimicking the photosynthetic antenna-reaction center [4]. In our

studies, we combined the BDP chromophore with porphyrins in two ways: i) covalent 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65

attachment of BDP to the periphery of the porphyrin ring and ii) axial attachment of BDP antennas on a porphyrin via coordination bonds. 2.1 Peripherally substituted porphyrin arrays 2.1.1 Porphyrin arrays linked via a cyanuric chloride bridge In this first example we exploited the favorable properties of cyanuric chloride, which allows the introduction of up to three different nucleophiles on an s-triazine unit through temperature-dependent sequential substitution of its three chlorine atoms. We thus prepared assemblies P1 and P2 in which a BDP chromophore is linked by a flexible cyanuric chloride bridge to either a free-base porphyrin (H2P) or a Zinc porphyrin (ZnP) [5]. In later work, P1 and P2 were further functionalized with an electron accepting fulleropyrrolidine unit [4a,6] through an amino substituted aliphatic chain to give electron donor/acceptor conjugates P3 and P4 (Scheme 1) [7]. Triads P3 and P4 show a triangular arrangement of three different photo- and redoxactive groups around the central s-triazine linker thereby permitting direct interaction of each chromophore with the other two.

Scheme 1 In P1 and P2, steady-state and time-resolved fluorescence studies clearly show that initial excitation of the BDP unit at 490 nm is followed by rapid BDP to porphyrin energy transfer as shown by the greatly reduced BDP fluorescence lifetimes and quantum yields of P1 and P2 (Φ ≈ 0.01, τ ≈ 0.1 ns) with respect to the corresponding values of a free BDP control (Φ = 0.58, τ = 3.58 ns). Furthermore, ultrafast transient absorption studies showed that excitation of P1 and P2 into their BDP-based singlet π-π* excited states is followed by energy transfer to the lowest-lying porphyrin

based singlet excited states with rate constants of kP1 = 2.9×1010 sand kP2 = 2.2×1010 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65

s-1. Analysis of the experimental results with the assistance of DFT calculations on P1 and P2 suggested a Förster-type dipole-dipole interaction [8] as the most likely mechanism of BDP to porphyrin energy transfer. Turning to the transient absorption studies of triads P3 and P4, selective BDP excitation at 490 nm results in the immediate formation of the BDP singlet excited state which decays by energy transfer to the porphyrin singlet excited state with time constants of 32 and 26 ps respectively. In contrast to P1 and P2 where the porphyrin singlet excited state decays to the ground state by fluorescence and intersystem crossing to the triplet, in P3 and P4 it is followed by rapid intramolecular charge separation to yield the metastable radical ion pair states H2P∙+/C60∙ (P3) and ZnP∙+/C60∙ (P4), which decay back to the singlet ground state with lifetimes of 3000 and 900 ps respectively. It is worth noting, that no evidence of direct electron transfer from BDP to C 60 was seen in the transient spectra of P3 and P4, following excitation at 490 nm, even though charge separation with a lifetime of 80 ps to afford BDP ∙+ and C60∙ is confirmed as the dominant decay pathway of an analogous BDP-C60 dyad which was studied as a control. The absence of a BDP to C60 electron transfer pathway in P3 and P4 can be attributed to kinetic factors since BDP to H2P/ZnP (ca. 30-32 ps) energy transfer is faster than that of BDP to C60 electron transfer (80 ps). 2.1.2 Porphyrin arrays linked by an amino group In a next set of compounds (Scheme 2), we used a palladium–catalyzed coupling reaction to prepare triads P5 and P6 where two H2P or BDP units were linked to the meso positions of a central ZnP via amino groups [9]. In the UV-vis absorption spectra of P5 and P6 we observe features from all their constituent chromophores indicating weak interchromophoric interactions in the ground state (see Figure 1). Most notable in the UV-vis spectra of P5 and P6 is their wide range absorption over most of the visible region. When P5 is excited at either 517 nm (mostly peripheral H2P chromophores) or 467 nm (mostly central ZnP chromophore) an emission spectrum resembling that of a free base porphyrin is obtained with maxima at 661 and 719 nm, while the excitation spectrum of P5 shows absorption features from both the central and peripheral porphyrins when monitored at 661 nm. Time resolved emission studies with pulsed excitation of mostly the peripheral porphyrins

at 400 nm clearly shows emission from both the constituent chromophores of P5 as 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65

indicated by the dual exponential decay with lifetimes of 2.9 (ZnP) and 9.8 (H 2P) ns. Combination of the steady-state and time-resolved emission results shows P5 emits from both chromophores regardless of excitation wavelength as excitation of one chromophore results in partial energy transfer to the other. This is expected because the H2P and ZnP first singlet excited states of P5 are almost isoenergetic (1.88 and 1.82 eV respectively).

In the case of P6, both steady-state and time-resolved

emission spectra clearly show that excitation of the BDP units into their first singlet excited state at ca. 490 nm is followed by rapid energy transfer to the corresponding ZnP singlet excited state. Thus the BDP chromophores of P6, as in P1-4, are good sensitizers of the porphyrin singlet excited state.

Scheme 2

Figure 1. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65

UV-vis absorption spectrum of P6 showing features from both constituent chromophores

2.2 Axially linked porphyrin arrays After the study of peripherally substituted porphyrin-based arrays, we turned to the study of the axially bound systems P7 and P8 (Scheme 3) where a Sn(IV) porphyrin (SnP) is decorated with two BDP-functionalized phenolate and benzoate ligands, respectively [10]. Axial substitution offers the advantage of avoiding the usually lowyielding multistep procedures required for the synthesis of unsymmetrically peripherally functionalized porphyrins.

Scheme 3 Initial study of P7 and P8 by cyclic voltammetry and UV-vis absorption spectroscopy once again reveals weak ground state interchromophoric interactions as wellresolved signals, which can easily be attributed to each of the constituent chromophores, are seen. However, in the excited state P7 and P8 show distinctly different behavior. The room-temperature luminescence spectra of P7 show strong quenching of BDP fluorescence and an almost complete absence of SnP luminescence. The quenching of BDP fluorescence can be explained on the basis of energy transfer from the BDP π-π* singlet excited state to that of SnP. The absence of sensitized emission from SnP, upon photoexcitation of the BDP chromophore, in P7 was initially attributed to electron transfer quenching of the singlet SnP excited state by the phenolate ligand leading to the transient generation of a phenoxyl radical. Transient absorption measurements on P7 confirmed this explanation as

excitation of BDP was found to be followed by energy transfer to the SnP first singlet 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65

excited state with a rate constant of ca. 5.0×1011 s-1, which leads to the rapid formation of a charge separated (CS) state with a rate constant of ca. 2.5×1011 s-1. The CS state recombines back to the singlet excited state with a lifetime of 450 ps. The absence of an oxidizable phenolete moiety in P8 reflects its different dynamic behavior. Firstly, the steady state emission spectra of P8, upon selective BDP excitation, show strong quenching of BDP fluorescence and sensitized SnP emission with a lifetime of 1.3 ns. The transient absorption spectra of P8, show the initial BDP to SnP singlet excited energy transfer with a rate constant of ca. 2.9×1011 s-1. However, instead of a CS state, intersystem crossing from the SnP first singlet excited state to the corresponding triplet is observed with a rate constant of ca. 7.1×108 s-1. Triplet formation is confirmed by the relatively weak SnP phosphorescence at 780 nm observed in the low-temperature (77 K) emission spectrum of P8. As in the cases of P1 and P2, analysis of the spectra and DFT calculations results of P7 and P8 shows that BDP to SnP singlet energy transfer is proceeds predominantly via a coulombic Förster mechanism [8].

3. Porphyrins as sensitizers for photocatalytic hydrogen production Photocatalytic production of hydrogen from water constitutes one of the main strategies for the storage of solar energy in the form a combustible fuel. The fact that environmental pollutants, such as carbon dioxide and particulate matter, are not released upon the combustion of hydrogen makes it an even more attractive energy source [11]. Hydrogen production in photocatalytic systems generally proceeds through sensitizer-mediated photoreduction of a catalyst (C) to an active form, while the sensitizer (PS) is continuously regenerated by an electron donor (D) (scheme 4). Hydrogen production catalysts include heterogeneous catalysts such as colloidal platinum and metal oxides [12] and homogeneous/molecular catalysts mostly based on first row transition metal complexes such as Co, Ni and Fe [13]. Of particular interest as hydrogen generation catalysts are the CoII or CoIII complexes belonging to the class of cobaloximes [14]. Hydrogen formation is generally accepted to proceed through the heterolytic decomposition of a CoII-hydride species. The latter is formed from a CoI intermediate through a series of protonation and

reduction steps [15]. In our group we studied two types of sensitizer-catalyst 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65

systems: i) supramolecularly attached porphyrin/corrole-cobaloxime systems and ii) a diffusion-controlled porphyrin-cobaloxime system.

Scheme 4 3.1 Study of porphyrin-cobaloxime and corrole-cobaloxime hybrids In our study of coupled sensitizer-catalyst systems we prepared the porphyrincobaloxime and corrole-cobaloxime conjugates P9-10 and P11 (scheme 5) [16]. The steady-state and time-resolved emission spectra of P9–11 show strong quenching of porphyrin and corrole fluorescence. This quenching was tentatively assigned to chromophore to cobalt electron transfer since an energy transfer pathway is not thermodynamically possible.

Scheme 5 Transient absorption spectra show that the singlet excited state features of the porphyrins and corrole in P9-11, formed after photoexcitation, decay with ultrafast kinetics (lifetimes of 380, 81 and 680 ps for P9, P10 and P11 respectively) without the formation of a photoproduct. This observation was explained on the basis of photoinduced chromophore to cobalt electron transfer followed by much faster

charge recombination. This assignment is further supported by the observation that 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65

considerably faster kinetics were observed for the ZnP derivative P10 (81 ps) compared to the H2P derivative P9 (380 ps) correlating with the more favorable thermodynamic driving force of photoinduced electron transfer predicted for the former. In experiments to access the photocatalytic activity of P9-11, the conjugates were irradiated with visible light in deoxygenated tetrahydrofuran/H2O 4:1 in the presence of triethylamine and only the ZnP derivative was found to be only modestly active producing about 3 turnovers of H2 after 24 h. This is in agreement with the transient absorption results, which suggest fast charge recombination thereby preventing the accumulation of sufficient amounts reduced cobalt species that would lead to H2 production. Recent literature results show that assembled sensitizer-catalyst systems tend to show poorer activity in comparison to diffusioncontrolled systems [14a,17]. A detailed mechanistic study performed by Iengo, Scandola and co-workers on similar porphyrin-cobaloxime hybrids confirms that association of donor sensitizer and catalyst is in fact detrimental to hydrogen production due to fast charge recombination [18]. This study also shows that hydrogen generation can be improved in solvent mixtures where bimolecular diffusion controlled mechanisms are favored due to dissociation of the components [18b]. 3.2 A diffusion controlled porphyrin – cobaloxime photocatalytic system Our studies on photocatalytic hydrogen production also included a diffusion controlled system consisting of the water soluble porphyrin zinc meso-tetrakis(1methylpyridinium-4-yl)porphyrin (P12) as sensitizer, a cobaloxime catalyst [Co(dmgH)2(py)Cl] and triethanolamine (TEOA) as a sacrificial electron donor (Scheme 6) [19].

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Scheme 6 Irradiation of systems containing the three components in water/acetonitrile 1:1 with visible light (λ > 440 nm) showed that highest photocatalytic activity is achieved at neutral to slightly basic pH values. At pH8, an initial rate of H 2 production of ca. 27 turnovers/h, with respect to photosensitizer, was observed. Photocatalytic activity decreases after ca. 5h of irradiation and ceases completely after about 6 hours producing in total 200 turnovers of H2. At pH 7 the initial rate of H2 production is about half that at pH 8 (ca. 13 turnovers/h), however the system shows considerably improved photostability as H2 production occurs for more than 20 h producing in total 280 turnovers of H2. Control experiments showed that the presence of all three components is essential in order to achieve H2 production. In addition, we showed that selective excitation of the porphyrin Q bands (at 572 nm using a 10 nm FWHM bandpass filter) also leads to H2 production thereby unambiguously confirming that P12 acts as a photosensitizer. UV-vis absorption studies on the photolysis solutions at pH 8 showed the appearance of a new broad signal at ca. 450 nm after 4 min of irradiation attributed to the accumulation of a CoII species. When the same experiment was performed at pH 11, where H2 production is greatly suppressed, a new broad absorption in the region of 500 – 600 nm appeared within 7 min due to the formation of a CoI species. To shed more light on the initial steps of the H2 production mechanism, transient absorption experiments were performed on the photocatalytic system at both the picosecond and microsecond timescales. Picosecond studies at pH7 show the formation and decay, with a lifetime of ca. 1.0 ns, of the P12 first singlet excited state. This indicates that the P12 singlet excited state does not participate in the photocatalytic process since its lifetime is similar to

that reported for P12 alone in aqueous solution [20]. Microsecond experiments 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65

show the formation of the first triplet excited state of P12, showing double exponential decay with lifetimes of 130 and 490 μs in the absence and single exponential decay with a lifetime of ca. 2.0 μs in the presence of *Co(dmgH) 2(py)Cl]. This observation indicates that the porphyrin first triplet excited state is quenched by the Co complex and therefore initiates the photocatalytic cycle. In summary, the combined spectroscopic results show that H2 is produced following the generally accepted mechanism involving a catalytically active CoI species produced by sequential reduction of the initial CoΙΙΙ complex mediated by the first triplet excited stet of P12.

4. Porphyrin derivatives as light harvesting dyes in DSSCs Dye-sensitized solar cells (DSSCs), since their first report by O’Regan and Grätzel in 1991 [21], have received great attention as alternatives to the conventional p-n junction solar cells, owing to their low cost, ease of fabrication, and high efficiency in converting incident solar light into electricity [22]. A typical device of this type (shown in Figure 2) consists of three components: (a) a photoanode composed of a mesoporous, nanocrystalline semiconductor film (such as TiO2) on a transparent conductive oxide (TCO) glass substrate, on which a monolayer of a sensitizer has been attached; (b) a cathode, which is a platinum-coated TCO glass; (c) a molten electrolyte containing a redox couple (such as I/I3), that fills the space between the two electrodes. Upon light illumination, photo-excited electrons of the sensitizer are injected into the TiO2 conduction band (CB) and transferred through an external circuit to the platinum electrode, in which they react with I3 producing I and the reduced form of the sensitiser is regenerated. Overall, the device produces electricity from sunlight, without any permanent chemical transformation. The above processes are competed by recombination of the injected electrons, either with the oxidized dye or the electron acceptor of the redox couple (I3), resulting in the dark current of the solar cell, and by relaxation of the excited dye to its ground state by a non-radiative decay process.

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Figure 2.

Schematic diagram of a DSSC.

The sensitizer of a DSSC is of great importance, since it is responsible for absorbing light, generating excited electrons, and injecting them into the TiO2 CB. An efficient sensitizer should have suitable functional groups for anchoring onto the TiO2 film, exhibit strong and broad light absorption range (in the visible and nearinfrared regions of the solar spectrum), have frontier orbital energy levels that favor the electron injection and dye regeneration processes, and exhibit long-term photo and thermal stability [23]. Among the variety of molecular sensitizers that have been developed and tested, ruthenium polypyridyl complexes attain the best performance, in terms of efficiency and long-term stability, reaching overall power conversion efficiencies (PCEs) up to 11.5% [24]. However, due to their high cost, rarity, environmental concerns, and their insufficient light harvesting properties in the near IR region, much research interest has recently focused on the development of cheaper, environmentally friendly, and panchromatic sensitizers [25].

Porphyrins, owing to their excellent light harvesting potential in mimicking 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65

photosynthesis, stimulated significant interest as sensitizers for DSSCs [26]. By rational molecular design and synthesis, they can be appropriately functionalized at meso or β positions, at the periphery, or the central metal of the macrocycle, allowing fine control of their photophysical and electrochemical properties. In 2011, Diau, Grätzel and co-workers, used a functionalized porphyrin derivative, in combination with a Co2+/Co3+ containing electrolyte, and reported a DSSC with a PCE value of 12.3% [27]. More recently, a similar porphyrin that maximizes electrolyte compatibility and exhibits improved light harvesting efficiency resulted in an overall efficiency of 13% [28]. Considering the trend in the performance progress of porphyrin-based solar cells and the continuing development in synthetic chemistry of porphyrins, a potential advance is expected [26d]. Nevertheless, substantial progress is still needed in order to achieve higher conversion efficiencies and make porphyrin-sensitized solar cells suitable for industrial scale applications. A particular issue to be considered is the long-term stability of the solar cells. This is related to the instability not only of the liquid electrolyte (which is a general drawback of all DSSCs), but also to that of the substitutent groups of porphyrins. Another major concern is the relatively narrow light absorption range of porphyrins. In their UV-vis absorption spectra, they display a strong Soret band around 420 nm and moderate Q bands in the 500-650 nm region [29]. Their weak absorption in the red/near infrared region of the solar spectrum, where the solar flux of photons is maximum, significantly decreases their light harvesting ability. In the present review, we report the latest results obtained in our laboratory on the synthesis of new porphyrins, appropriately functionalized to be used in fabrication of DSSCs. Our studies are aiming to extend the light absorption range of porphyrin sensitizers with carboxylic acid anchoring groups by using suitable πconjugated groups at meso positions, “push-pull” porphyrins, porphyrins with bridges between the macrocycle and the anchoring group, and covalently linked arrays of porphyrins and other chromophores. Long alkoxyl group substituents have also been used to prevent porphyrin aggregation on the TiO2 surface. Furthermore,

we also synthesized porphyrin derivatives with pyridyl anchoring groups in order to 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65

elucidate the effect of the nature and strength of the binding of porphyrin onto the TiO2 surface, and the electron injection efficiency into the TiO2 CB. Other methods that we also used to enhance the performance of the studied DSSCs include the use of chemical co-adsorbents, co-sensitization, modified photoanodes, molten electrolytes, different redox couples in the electrolyte, and electrolyte additives. 4.1. porphyrins with carboxylic acid anchoring groups Carboxylic acids are the most extensively studied anchoring groups in porphyrinsensitized solar cells [30]. Synthetically, they can be easily introduced into porphyrin derivatives through a variety of readily accessible precursors, while in terms of their performance, they result in very efficient DSSCs, compared to other anchoring groups. By adopting a variety of binding modes [31], they ensure efficient adsorption of the sensitizer onto the metal oxide surface. Furthermore, when they are combined with appropriate bridges, they keep the porphyrin macrocycle at an optimum orientation and distance from the metal oxide surface. As a result, they promote strong electronic coupling between the energy levels of the excited sensitizer and the metal oxide, resulting in fast and efficient electron injection [32]. 4.1.1 porphyrins with π-conjugated groups at meso positions A factor that greatly affects the overall efficiency of a DSSC is the short-circuit current density, which is related to the light harvesting potential of the sensitizer. In general, porphyrins, due to their weak light harvesting ability in the green and red regions of solar spectrum, result in poor photovoltaic efficiencies. An effective approach that has been used in order to extend their light absorption to the near infrared region is the elongation of their π aromatic system by introduction of πconjugated substituent groups, either at meso [33] or β positions [34] of the porphyrin ring. The resulting removal of the symmetry of the porphyrin core leads to splitting of the porphyrin π and π* molecular orbital levels and decrease of the HOMO-LUMO energy gap, resulting in broadening and red shift of the Soret and Q absorption bands. Consequently, lower-energy photons are absorbed and excited into the LUMO energy levels, resulting in enhanced photocurrent generation and significantly improved overall device performances [35].

Along these lines, we synthesized a zinc-metallated A2B2-type porphyrin P13 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65

(Scheme 7) with two phenylcarboxylic acid groups at two opposite meso-positions of the porphyrin ring, while the other two meso positions are substituted by two πconjugated para-phenylene vinylene units, terminated by 4-nitro-α-cyanostilbene groups [36]. Photophysical measurements of P13 showed that the presence of two para-phenylene vinylene substituents results in broadening and strengthening of the Soret and Q absorption bands, consistent with the extension of the π-conjugation of the porphyrin ring. This absorption broadening has a profound effect on the HOMOLUMO gap of P13, found to be 1.90 eV, as indicated by electrochemistry data. The IPCE spectrum of P13 displayed very broad Soret and Q band absorptions with maximum values of 65 and 60%, respectively. However, its overall DSSC performance, using a quasi solid state electrolyte, was found to be 2.90%.

Scheme 7 A major reason for the low conversion efficiency of the P13-based solar cell was porphyrin aggregation [37]. In general, the flat, wide and electron rich surfaces of porphyrin units facilitate their close proximity, through a combination of van der Waals, π-π stacking and charge-transfer interactions, and the formation of aggregates on the TiO2 surface. When this phenomenon happens, there is an increased probability of relocation of the porphyrin excited states in neighboring porphyrin units (exciton quenching), leading to decrease of the electron injection efficiency and increase of the charge recombination efficiency. An effective way to overcome this problem is the use of chemical co-adsorbents, i.e. compounds that strongly bind onto the TiO2 surface, in competition with the porphyrin molecules, and break porphyrin aggregation. As a consequence, the electron injection efficiency into the TiO2 CB is increased, while the recombination of the injected electrons with

the redox electrolyte is decreased, leading to the enhancement of the short-circuit 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65

current density and open-circuit voltage values [38]. Indeed, the use of deoxycholic acid (DCA) as co-adsorbent in a P13 sensitized solar cell resulted in the enhancement of its photovoltaic parameters and its overall efficiency to 4.22%, despite the concurrent reduction of the amount of dye adsorbed onto the TiO2 surface. 4.1.2. “push-pull” porphyrins An increase of the short-circuit current density of porphyrin based solar cells can also be achieved by utilizing porphyrins with the “push-pull” molecular structure. [39,40]. This type of porphyrins are based on a donor-π-acceptor (D-π-A) architecture, in which π represents the π conjugated porphyrin macrocycle (the light harvesting center), D is an electron donating group, and A is the electron accepting group (usually, the highly polar carboxylic acid anchoring group). For this purpose, amines have been proved to be very effective donor groups through resonance effects [41]. The electronegativity difference between the donor and acceptor groups gives rise to an intramolecular dipole moment, which facilitates an effective intramolecular separation of photoexcited electrons and holes. This induced charge transfer directionality decreases the recombination rate, and enhances the observed photocurrent [42]. Indeed, the utilization of “push-pull” porphyrin derivatives in DSSCs allowed for the fabrication of solar cells with increased overall efficiencies [43]. We synthesized two “push-pull” porphyrin derivatives, namely P14 and P15 (Scheme 8) [44]. Both are of the A2B2-type containing two N,N΄-dimethylphenyl groups at 5- and 15-positions of the porphyrin ring, acting as electron donating groups, and two phenylcarboxylic acid groups at 10- and 20-positions, serving as electron accepting and anchoring groups. Photovoltaic measurements of the DSSCs sensitized by P14 and P15 revealed enhanced photovoltaic parameters and higher PCE value for the latter solar cell (3.80 vs 4.90%). This was attributed to a higher amount of P15 adsorbed onto the TiO2 surface, arising from the stronger affinity of P15 for TiO2, and to a better electronic coupling with the TiO2 surface. An additional reason for the enhanced performance of the solar cell based on the zinc-metallated porphyrin is its longer electron lifetime, as it is revealed by electrochemical impedance spectra (EIS). This is in agreement with the observation that zinc-

metallated porphyrins are, in general, better photosensitizers than their free-base 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65

analogues, due to their higher singlet excited states [45]. Therefore, P15 has a greater driving force for injection of the photogenerated electrons into the TiO 2 CB, resulting in faster injection dynamics, and better photovoltaic response. In general, both P14 and P15 exhibit enhanced photovoltaic parameters; however, they are not as efficient as other D-π-A porphyrin derivatives, and ruthenium polypyridyl dyes. This can be explained by their relatively small intramolecular dipole moments, due to the relative positions of the D- and A-groups (compared to the unsymetrically substituted porphyrin derivatives of the AB2C-type [46]), and due to their tendency for aggregation.

Scheme 8 The efficiency of the P15 sensitized solar cell was further optimized by means of co-sensitization. According to this method, two or more molecular sensitizers with strong and complementary light absorption profiles are combined onto the TiO 2 film, resulting in extension of the light harvesting ability and increase of the resulting photocurrent density of the solar cell [47]. Co-sensitization of the P15 based solar cell with the metal-free dye shown below resulted in a significant enhancement of its light harvesting ability and decrease of charge recombination, giving a PCE value to 6.25% [48]. Further enhancement of the overall performance of the solar cell was achieved by co-adsorption of DCA which increased the PCE value to 7.46%.

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4.1.3 porphyrins with bridges between the macrocycle and the anchoring group Another factor of great importance for efficient DSSCs is the coordination geometry of the sensitizer on the TiO2 surface of the photoanode [26a]. By introducing appropriate bridges (B) between the anchoring acceptor group (A) and the πconjugated porphyrin unit, as in the case of porphyrins with the D-π-B-A architecture, an optimal orientation and distance of the porphyrin macrocycle from the conducting electrode can be achieved that can enhance interfacial charge separation and retard charge recombination [26a,49]. Among the variety of functional groups that have been tested as bridges [50], ethynyl and ethynylphenyl groups attached to meso positions of porphyrin units have been proved to be very efficient. Not only they maintain the sensitizer at an appropriate orientation and distance, but also they enhance its light absorption efficiency to the near infrared region, resulting in significantly enhanced DSSC performances [51]. We have synthesized two porphyrin derivatives of the D-π-B-A type, namely the compounds P16 and P17 (Scheme 9), in which B represents an ethynyl bridge attached to two opposite meso positions of the porphyrin macrocycle, terminated by a carboxylic acid anchoring group (A) and a donor group (D) [52]. The other two meso positions are occupied by a tert-butyl-phenyl group and a pyridyl group. The latter can serve either as an additional anchoring group, or as an additional way to reduce dye aggregation, as the porphyrin units could be able to self-organized onto the TiO2 surface through the formation of coordination bonds between Zn and N(pyridine) atoms of different porphyrin units. However, despite the promising D-πB-A molecular architecture of the P16 and P17 sensitizers, their corresponding solar cells were found to exhibit poor photovoltaic parameters and overall efficiencies of 1.58 and 2.84%, respectively. This can be attributed to porphyrin aggregation, due to the flat and electron rich porphyrin surfaces of both dyes. The slightly better performance of P17 might be ascribed to the stronger amine electron donating group opposite to the carboxylic acid anchoring group.

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Scheme 9 The use of appropriate bridges (B) has also been employed by our group as a method to control porphyrin aggregation. In many cases, this phenomenon can be reduced by increasing the steric hindrance between the porphyrin molecules, through the introduction of non-planar and bulky substituents into their structural frameworks. In particular, the employment of long alkoxy chains as side-groups of phenyl substituents has been proved to be very successful, giving improved solar cells performances [53]. We synthesized the series of porphyrin derivatives P18-21 (Scheme 10), in which long chain alkoxy group bridges (B) have been introduced between the porphyrin macrocycle and terminal carboxylic acid groups [54]. The simpler analogues of P18-21, meso tetra-benzoic acid porphyrins, had been reported to provide efficient charge injection into the TiO2 CB, but very low solar energy conversion efficiencies [55]. Similarly, photovoltaic measurements of the P18-21 sensitized solar cells revealed very poor overall performances. This was attributed to the unfavorable energetic positioning of their excited states. As shown by cyclic voltammetry measurements, due to the electron donating properties of the long alkoxy chains, the potential of the first oxidation of the dyes is shifted to more positive values (actually, the compounds P18-21 are more easily oxidized than their meso tetra-phenyl porphyrin analogues). The resulting lowering of the porphyrins’ singlet excited states has a negative effect on the injection dynamics of the photogenerated electrons into the TiO2 CB and leads to a decreased photocurrent response [45]. By using a photoanode based on a semiconductor with a lower CB (e.g. SnO2) or an electrolyte with a lower redox potential (e.g. Co2+/Co3+) improvement of the P18-21 sensitized solar cells performance might be expected

[56]. An additional reason for the poor performance of the P18-21 sensitizers might 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65

be that the alkoxy bridges do not facilitate efficient coupling between the porphyrin macrocycle and the semiconductor surface, due to their elongated and insulating nature, as observed in similar phthalocyanin systems [57].

Scheme 10 4.1.4 covalently linked arrays of porphyrins and other chromophores Another effective approach to enhance the overall performance of porphyrin based solar cells is to employ as sensitizers covalently linked arrays of porphyrins and other chromophores [58]. Such systems can act as molecular antennae, in which the higher energy chromophore transfers intramolecularly excitation energy to the lower energy chromophore, which finally injects electrons to the TiO2 CB. The efficiency of the energy and electron transfer processes depends on the extent of electronic coupling between the chromophores, which can be modulated by the type of bridge connecting the chromophore units and their relative spatial orientation. Arrays of porphyrin units, covalently linked either through π-conjugated bridges [59] or directly [60], exhibit strong electronic coupling between porphyrin units, forming extended π-conjugated systems, in which the splitting of the porphyrins’ HOMO and LUMO levels leads to broadening of the Soret and Q absorption bands. Consequently, such systems display improved overall harvesting efficiencies, and lead to better photovoltaic performances relative to the corresponding monomeric porphyrin sensitizers [61].

Following this strategy, we synthesized a series of covalently linked porphyrin 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65

dyads P22-25 (Scheme 11), in which two porphyrin units (either in their zincmetallated or free-base forms) are covalently linked through peripheral aryl-amino groups by the π-conjugated unit of s-triazine. The compounds are functionalized by terminal carboxylic acid groups, either on the central triazine unit or on a peripheral aryl group. The P22 and P23 sensitized solar cells were found to exhibit PCE values of 3.61 and 4.46%, respectively [62]. The higher PCE value of the latter is attributed to the enhanced photovoltaic solar cell parameters and the higher loading of P23 onto the TiO2 surface. In addition, ESI measurements revealed shorter electron transport time, longer electron lifetime, and high charge recombination resistance for the P23 sensitized solar cell. The superior performance of P23 can be explained on the basis of its unsymmetrical structure. It was proposed that the presence of a free-base and a zinc-metallated unit in P23 might cause a polarizing and cooperating effect that directs the electron transfer to the TiO2 CB, and leads to more efficient electron injection, while in the symmetrical P22 there is no directionality in the electron transfer process. However, the P23 based solar cell is not as efficient as the multiporphyrin arrays with the “push-pull” architecture [60]. This is ascribed to the fact that in P23 the carboxylic acid anchoring group resides on the triazine linker, while in “push-pull” systems it is located on the electron acceptor unit and, hence, results in very efficient charge transfer directionality and enhanced photocurrent. Comparison of the solar cells based on P24 and P25 revealed enhanced photovoltaic parameters and PCE value for the former (5.28 vs 3.50%, respectively) [63]. Apparently, the presence of two carboxylic acid groups in the molecular structure of P24, in contrast to P25, results in a more effective binding onto the TiO2 surface and, therefore, in the formation of a more compact dye layer on the TiO2 surface, which is manifested by the increased dye loading of the P24 sensitized solar cell. Furthermore, it produces a cooperative effect in the electron transfer into the TiO2 CB, which gives a faster and more efficient electron injection process. This is evidenced by ESI measurements revealing longer electron lifetime, and more effective suppression of charge recombination for the P24 based solar cell.

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Scheme 11 Along the same lines, we synthesized the propeller-shaped, porphyrin triad P26 (Scheme 12) [64]. The compound consists of one free-base porphyrin unit functionalized with a carboxylic acid anchoring group, and two zinc-metallated porphyrin units linked by an s-triazine unit. The PCE value of the P26 sensitized solar cell was found to be 3.80%. Attempts to improve its performance directed us to use

a DSSC with a modified photoanode. When the TiO2 film of the anode was processed 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65

by electrophoretic deposition (EDP) method instead of the traditional paste coating (PC) method, the overall efficiency of the cell increased up to 4.91%, which is higher than the average efficiencies of solar cells sensitized by simple porphyrin and other hybrid-porphyrin derivatives. As revealed by powder X-ray powder diffraction and scanning electron microscopy measurements, the TiO2 layer of the latter photoanode exhibits a more compactly packed surface morphology that causes a larger amount of P26 to be adsorbed onto TiO2 surface (giving a higher short circuit current density). The longer electron lifetime and higher charge recombination resistance that is observed also trigger its high performance. Further improvement of the solar cell efficiency to 5.56% was achieved by the use chenodeoxychlolic acid (CDCA) as co-asdorbent.

Scheme 12 Aiming to extend the light absorption of porphyrin-based sensitizers to even wider spectral regions, we combined porphyrins with other types of chromophores, as additional antenna components, and synthesized the triads P27 and P28 (Scheme 13) [65]. In the former, two [Ru(bipy)3]2+ (bipy = 2,2΄-bipyridine) units are linked through amide bonds to the peripheral aryl groups at opposite meso positions of a free-base porphyrin unit, while in the latter, two boron dipyrrine (BDP) moieties are connected through ethynyl groups at opposite meso positions of a zinc-metallated porphyrin unit. P27 and P28 are also functionalized by two benzoic acid anchoring

groups at the other two meso positions. Both triads in their UV-vis absorption 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65

spectra exhibit absorption features from their constituent chromophores, i.e. porphyrin, [Ru(bipy)3]2+ and BDP, and hence wider spectral absorptions than the corresponding porphyrins without the “auxiliary” chromophores are obtained. However, they resulted in very poor photovoltaic efficiencies (0.3 and 0.6%, respectively). As suggested by DFT calculations, the low performance of the P27based cell could be attributed to the unsuitable positioning of the frontier molecular orbitals of the triad, since its LUMO electronic density is primarily localized over the two [Ru(bipy)3]2+ units, and not on the carboxylic acid anchoring groups, as it should be expected for efficient electron injection into the TiO2 CB. In addition, photophysical studies revealed a very poor electronic communication between the two [Ru(bipy)3]2+ units and the central porphyrin unit with the carboxylic acid anchoring groups in the ground state of the triad. Similarly, the low performance of the P28-sensitized solar cell was justified by the same electronic factors.

Scheme 13

4.2 porphyrins with pyridyl anchoring groups As mentioned above, the type of the anchoring group of the sensitizer is of great importance for efficient DSSCs, since it affects the electron injection process into the

TiO2 CB and the long-term stability of the photoanode. By far, carboxylic acids have 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65

been proved the most appropriate anchoring groups. However, under particular conditions (e.g. in aqueous alkaline solutions [66]) they are not stable and tend to dissociate from the TiO2 surface, a phenomenon that led to other anchoring groups to be tested [67]. Recently, porphyrin sensitizers with a pyridine electron withdrawing anchoring group have been synthesized and tested, giving electron injection efficiencies superior to those with the corresponding porphyrin sensitizers with carboxylic acid anchoring groups [68]. This is attributed to the nature of the coordinate bond between pyridine and the Lewis acid sites of the TiO2 surface, which provides better electronic communication than the ester linkage of the carboxylic acid groups. Aiming to evaluate the efficiency of simple porphyrin sensitizers with meso substituted pyridyl anchoring groups, we synthesized a series of porphyrin derivatives with one, two, and four pyridyl groups, respectively, designated below P29, P30, and P31 (Scheme 14) [69]. Photovoltaic measurements of the corresponding DSSCs gave PCE values in the range of 2.5-3.9%, with the porphyrin derivative with two pyridyl groups, P30, displaying the most enhanced photovoltaic parameters and PCE value. In addition, EIS of the P30-sensitized solar cell revealed a longer electron lifetime. This was rationalized on the basis of a larger dye amount of P30 adsorbed on the TiO2 surface of the solar cell. Furthermore, DFT calculations suggested the presence of more suitable frontier molecular orbitals in P30 that promote efficient electron injection and dye regeneration. However, the performance of the latter cell is still poorer than the average performance of other porphyrin sensitized solar cells reported in literature, ascribed to improper sensitization (due to poor dye loading) and increased dye aggregation. Indeed, treatment of the photoanode of the P30-sensitized solar cell with formic acid resulted in a higher amount of dye adsorbed onto TiO2 (which improved the light harvesting efficiency of the cell) and higher charge collection efficiency, yielding an increased PCE value of 5.23%. Enhancement of the solar cell efficiency (6.12%) was achieved by reduction of dye aggregation, by means of deoxycholic acid (DCA) as coadsorbent.

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65

Scheme 14 An enhancement of the performance of the P30 sensitized solar cell was achieved by co-sensitization with a mononuclear ruthenium(II) complex with 2,6bis(1-methylbenzimidazol-2-yl)pyridine and terpyridine tricarboxylic acid as ligands (its structure is shown below), giving a PCE value of 7.35% [70]. The efficiency of this co-sensitized solar cell was further improved up to 8.15% by using a graphene modified TiO2 photoanode, instead of a pure TiO2 photoanode. Suggested by EIS and dark current measurements, the graphene modified photoanode results in suppression of charge recombination at the photoanode/dye/electrolyte interface, and enhancement of electron transfer between the photoanode and the collecting electrode.

Attempting to extend the light absorption of pyridine-containing porphyrin sensitizers to the near infrared region, we synthesized P32 (Scheme 15) which bears a π-conjugated substituent, a 3-pyridinylethynyl-phenyl group [71]. Nevertheless, the light harvesting ability was not improved, as indicated by the relatively low PCE value of 3.36%. Apparently, the twisted phenyl ring connecting the pyridinylethynyl substituent does not elongate the π aromatic porphyrin system, as it is also confirmed by the UV-vis absorption spectrum of P32 in solution, which shows neither broadening nor red shift of the Soret and Q absorption bands. Dye

aggregation is an additional reason for the low efficiency of the P32-based solar cell, 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65

which however, was reduced by CDCA as co-adsorbent that reduced charge recombination, yielding an increased solar cell efficiency of 4.56%. A modified photoanode incorporating Ag nanoparticles further increased the performance of the cell to 5.66%, by enhancing its light harvesting efficiency in the visible region of the solar spectrum, and, hence, its short circuit current density, and improving charge transport and electron lifetime.

Scheme 15 In general, electrolyte additives can be used to improve the efficiency of DSSCs by affecting the short circuit current and open circuit voltage of a DSSC, by shifting positively or negatively the TiO2 CB edge, and by altering the recombination kinetics at the TiO2/electrolyte interface [72]. Usual electrolyte additives include 4tert-butylpyridine, 4-methylbenzimidazole, guanidinium thiocyanate, and thiourea [73]. The PCE value of the P32-based solar cell with a thiourea containing electrolyte was increased to 5.34% [74]. The enhancement of the solar cell efficiency was attributed to an increase of the short circuit current density, which was due to a positive shift of the TiO2 CB edge, and suppression of charge recombination. UV-vis absorption spectra and X-ray powder diffraction patterns of pure and thioureacontaining TiO2 films also suggest that thiourea molecules are chemisorbed onto TiO2, and passivize its surface recombination sites between the injected electrons and the I3- ions of the electrolyte. The resulting slower recombination kinetics (which is documented by a longer electron lifetime) is also suggested to as a reason for the slight increase of the open circuit voltage of the solar cell, which compensates for the opposing effect of the positive shift of the TiO2 CB edge.

Conclusions 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65

In this collection of studies, we show that porphyrin-based excited states are efficiently sensitized by the presence of strongly absorbing boron dipyrrin (BDP) chromophores both as substituents at the periphery of the porphyrin ring and as axial ligands through rapid BDP to porphyrin excitation energy transfer. This provides a possible strategy for the enhancement of the light harvesting ability of porphyrins thereby making them more attractive for solar energy conversion applications. In addition, the excited state of porphyrins can efficiently transfer electrons to cobaloxime catalysts and initiate a hydrogen production catalytic cycle. We have confirmed that diffusion controlled photocatalytic systems show far greater activity than supramolecular ones because the latter suffer from rapid back electron transfer which prevents the, essential for H2 production, accumulation of a reduced catalytic species. In an effort to obtain suitable porphyrin-based sensitizers for DSSCs, a variety of functionalized porphyrins with carboxylic acid anchoring groups were designed and synthesized. Porphyrins with π-conjugated groups at meso positions, “push-pull” porphyrins, porphyrins with bridges between the macrocycle and the anchoring group, and covalently linked arrays of porphyrins and other chromophores were used for the fabrication of DSSCs. Porphyrins with pyridyl anchoring groups have also been utilized. However, the lack of absorption in the red/near infrared region of the visible spectrum of these sensitizers and dye aggregation phenomena on the TiO2 surface resulted in devices with moderate performances. Nevertheless, when these sensitizers were used in combination with chemical co-adsorbents (which reduce dye aggregation on the TiO2 surface), co-sensitizers, modified photoanodes, molten electrolytes or electrolyte additives, the efficiencies of the resulting photovoltaic devices were significantly increased, reaching values exceeding the average efficiencies of solar cells based on simple porphyrin and other hybrid-porphyrin derivatives were obtained.

Acknowledgments: The authors wish to thank all coworkers and collaborators who are mentioned in the reference list. In particular, this work was made possible through collaboration with the groups of Prof. Dirk M. Guldi (University of Erlangen-

Nuremberg, Germany) and Dr. Julia A. Weinstein (University of Sheffield, UK) who 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65

performed detailed spectroscopic studies, and the group of Ganesh D. Sharma (Jaipur Engineering College, India) who performed photocurrent measurements on DSSCs. Financial support from the European Commission (FP7-REGPOT-2008-1, Project BIOSOLENUTI No 229927), is greatly acknowledged. Finally Special Research account of the University of Crete is also acknowledged.

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*Graphical Abstract - Pictogram (for review)

*Graphical Abstract - Synopsis (for review)

In this review, we report the collection of three sets of porphyrin-based systems recently synthesized in our group. In the first set we improve the light harvesting ability of porphyrins by preparing arrays in

which strongly absorbing boron dipyrrin (BDP) chromophores are combined with porphyrins, either by covalent attachment to the periphery of the porphyrin ring or by axial coordination to a tin(IV)

porphyrin. In the second set, we use porphyrins as sensitizers in photocatalytic hydrogen production schemes with cobaloxime catalysts either in supramolecular or diffusion controlled systems. In the final set, porphyrins are used as light harvesting dyes in dye-sensitized solar cells (DSSCs).