Max Kistler
email: mkistler@univ-paris1.fr

According to Leibniz, every truth has an explanation, by virtue of the principle of sufficient reason (PSR), according to which "nothing happens without a reason why it should be so, rather than otherwise" (Monadology, §32, quoted by Vintiadis, p. 3). If everything has a "sufficient reason", and if a fact is brute if and only if it has no explanation, it might seem to follow that there are no brute facts. The 14 contributions to the collection Brute Facts, edited by Elly Vintiadis and Constantin Mekios, provide a wide range of deep and sophisticated philosophical reflections on the limits of scientific explanation. If, as John Symons argues in his contribution to the book, there is no good reason to accept the PSR as a universal metaphysical truth, although it is a useful methodological and heuristic principle, we are faced with the baffling and fascinating possibility that some facts are brute. Explanations are arguments produced by human subjects in particular circumstances. Facts that do not have an explanation in a given historical context, and are thus "epistemically brute", may nevertheless be explained later on, thanks to scientific progress. One is naturally led to ask whether some facts are absolutely brute rather than only relatively to a particular historical context. Are there "ontologically brute facts" (p. 2) that could not possibly ever be explained? While it may be unhappy to call them "ontologically unexplainable" (p. 2), the notion of such absolutely brute facts is explored in metaphysical terms by various authors in the book, by construing them (as Torin Alter) as ungrounded facts or (as Dana Goswick) as fundamental facts.

Here is an example of the sophisticated and deep philosophical thoughts provided by the authors of the book. It is often assumed that explanation is inseparable from understanding so that ontologically brute facts cannot possibly be understood. According to Hempel and Oppenheim's deductive-nomological model of explanation, an explanation provides understanding for a given fact by showing that there is a logically valid argument whose conclusion

is a proposition expressing this fact. On this model, brute facts cannot be explained because there are no more fundamental premises from which they could be derived. However, Peter Wyss challenges the equivalence of lack of explanation and lack of understanding: learning that something is a brute fact yields understanding. We know more when we know that there is no explanation to be found.

A large part of the book is dedicated to a particular sort of brute facts, i.e. emergent facts. This choice may surprise a reader who expects that metaphysicians and philosophers of science would first of all be interested in the brute character of fundamental facts of physics, both particular facts, such as the fact that the speed of light in the vacuum is c, and laws such as Maxwell's laws or Einstein's field equations of General Relativity. However, such facts and laws are hardly mentioned in the book (John Heil's chapter is an exception), whereas questions arising in the context of emergent properties and laws occupy centre stage in many chapters. A property of a complex system is said to be emergent if it (1) gives the system "novel" causal powers that none of the system's constituents has and (2) is irreducible in principle in terms of the properties of these constituents and their relations. It is controversial whether emergent facts fall in the domain of brute facts. Indeed, some authors, such as Bickhard, argue that emergence "does not require nor support brute facts" (p. 245).

The emergence of conscious states in humans, from physical states of their bodies, is the most widely discussed case of emergence. There are important challenges against the plausibility and even the coherence and intelligibility of the existence of emergent properties and facts, in the context of physicalism and such principles as the supervenience of all facts on the physical facts, and methodological naturalism, according to which there is no limit in principle to scientific explanation. Several chapters in the book address these challenges and argue for the compatibility of emergent brute facts with at least some of these principles.

The order of the chapters in the book follows a clear plan. Beginning with general metaphysical (Heil) and epistemological (Taylor) analyses of bruteness, the following chapters discuss particular sorts of brute facts: normative (Bickhard), moral (Levine), modal (van Cleve, Goswick), and emergent facts (or emergent laws, discussed by Vintiadis), in particular facts about phenomenal experience (Alter). Let me say a few words on each chapter.

In the opening chapter, John Heil ties the question of whether there are brute facts to the questions whether there are brute laws and whether laws flow from reality's brute powers, and ultimately to the issue of the bruteness of reality itself. Heil arrives at the conclusion that there is much less bruteness in reality than Humean metaphysics makes it appear. Reality itself might be the only brute thing there is; for all other apparently contingent facts, they might only appear as brute from the point of view of Humean metaphysics. Thus, the source of their apparent bruteness might lie "in ourselves" (p. 27).

Elanor Taylor shows that making the case for a fact's being ontologically brute requires showing (1) that there is no explanation of that fact, for even fundamental facts may have explanations, e.g. in terms of other fundamental facts, furthermore (2) that no explanation of any sort will be forthcoming in the future, and (3) that the absence of any explanation fits into a larger philosophical and scientific worldview. The metaphysical hypothesis that a given fact is ontologically brute must be justified by an abductive argument, in a way analogous to the justification of scientific theories.

Joseph Levine makes an illuminating case for the existence of brute moral facts. "There is something extra in the world, normative force, that is not guaranteed by the non-normative properties and logic" (p. 61). On the basis of a subtle argument, in which he compares moral facts with another important type of brute facts, i.e. facts of phenomenal experience, Levine concludes that the bruteness of the supervenience of phenomenal facts is only apparent (or epistemic), in the sense that it stems from our concepts, whereas the bruteness of the supervenience of moral facts is real or metaphysical.

James van Cleve explores the question whether there are brute modal facts, and in particular, necessary facts. Essentialists, such as Kit Fine and Gideon Rosen, propose to explain necessary truths by essences: if p is necessarily true, this necessity follows from the fact that it is in the nature of some object x that p is true. However, a consequence van Cleve draws from Fine's and Rosen's principles, namely, that those essences are themselves explained by higher order essences, leads to an infinite regress. In order to block this regress, Dasgupta has proposed that essences are autonomous. A fact is "autonomous" if it has no explanation but also need not any. For van Cleve, a fact is "brute" if it does not have any explanation although it would require one. Van Cleve accepts Gupta's proposal and adds that it might yield a reason to think that all necessary truths are autonomous.

The section in van Cleve's chapter on what is maybe the best known brute fact of all, the bruteness of the emergence of phenomenal facts from physical facts about the brain, is strangely inconclusive. It looks as if he did not use the results of the other sections to shed more light on this one. The section ends with the question whether an explanation of the emergence of phenomenal facts m from their physical grounds p by the identity of those facts (p = m) could make the mystery even bigger: identities may be unexplained and mysterious; therefore, the emergence of m from p might not really be explained by the fact that p = m.

Three other sections (on mathematical explanation, on brute necessity in ethics, where he argues that, on Moore's conception of ethics, there are necessary normative truths that are either autonomous or brute, and on the view that some necessary truths are either autonomous or intrinsically explained and that the rest are explained by necessary truths of these two sorts) of van Cleve's paper do not appear in the book-the reader is invited to read them in the longer version of the chapter that has been published as "Brute Necessity", Philosophy Compass 13, 9 (2018) and is available on the author's website.

Van Cleve's overview of various accounts of whether necessary truths have or need explanations is rich and subtle, but van Cleve's result might seem weak: "the mind-body problem and ethics (… ) (are) two arenas in which some philosophers may find and accept brute necessities" (p. 92). His exploration of the different ways in which the postulate of brute (or autonomous) necessary truths may be avoided is illuminating and indirectly sheds light on the concept of brute necessity itself: for example, views according to which every necessary truth can be explained by a higher order necessary truth lead to an infinite regress, which makes it doubtful whether there is any explanation in the first place. Thus, it turns out that on such a view, necessary truths are brute or autonomous after all.

Dana Goswick proposes a reductive account of de re modal facts. She elaborates an original strategy for reducing what she calls "modal objects". Instead of trying to reduce an object's modal properties to non-modal properties, Goswick construes modal objects as mereological sums of non-modal objects and sortal properties. She considers a realist and a non-realist version of such a view: according to her own non-realist version, a non-modal object and a sortal property s together compose a modal object if and only if a subject has the s-response to the non-modal object. According to this response-dependent theory of the existence of modal objects, it "isn't a deep objective, subject-independent feature of the world. It's a feature about us, our interests and our concerns" (p. 109). Goswick's thesis that de re modality is grounded in human responses may be challenged along the following lines. (1) The fact that a subject reacts to a given object or situation with an s-response is a fact about the subject's dispositions, which seems to make it a modal fact, just as the object's affordance for an s-response is dispositional and modal. Even if Goswick's account of modality is correct, it does not hold its promise of reducing modal facts to something non-modal, because it reduces modal facts about objects to modal facts about subjects. The modal fact that a subject has an s-response seems to raise the same challenge that her account was intended to meet, i.e. the challenge of reducing modal facts to something non modal. (2) Goswick judges that (a) "there is no room for primitive modality" (p. 102) in a worldview compatible with naturalism and that (b) modality is grounded in subjects' responses. However, the naturalist perspective Goswick endorses might warrant the opposite conclusion that modality is inseparable from the scientific worldview and thus that the acceptance of the existence of modality is part of naturalism.

Kevin Morris' chapter critically examines a recent attempt by Cameron and Barnes (Ross Cameron and Elizabeth Barnes, A Critical Study of John Heil's From an Ontological Point of View. SWIF Philosophy of Mind Review 6 (2007), p. 22-30) to make sense of emergence in terms of truthmaking. According to most interpretations of the concept of emergence, emergent properties are determined by more fundamental properties in a brute way, i.e. in a way that cannot be further illuminated, scientifically or otherwise. Such "brute determination"(p. 121) seems to be incompatible with naturalism, according to which everything nonfundamental can at least in principle be scientifically explained. Morris examines the attempt (by Cameron and Barnes) to escape this problem by showing that emergent properties are needed as truthmakers, thereby making the justification of the postulate of emergent properties independent from the relation between these emergent properties and more fundamental properties on which they may depend or on which they may supervene.

To assess this claim, Morris distinguishes two sorts of emergent properties: (1) properties that are ontologically dependent on other properties, in the sense that they cannot be exemplified alone, but are neither supervenient on other properties nor metaphysically necessitated by other properties (having mass may be such a property) and (2) properties that are so necessitated. Morris argues that emergent properties of type (1) can indeed be defended against the challenges that have been raised against them, but that this defence is independent of their justification as truthmakers. Instead, what makes their defence possible is the fact that properties of type (1) are not metaphysically necessitated by (or supervenient on) more fundamental properties.

However, according to Morris, the construal of emergent properties in terms of truthmaking does not help overcoming the challenges that have been raised against emergent properties of type (2). In particular, their construal as truthmakers does not help understanding how such properties can make a distinctive causal contribution in addition to the properties by which they are metaphysically necessitated.

These are subtle matters, and Morris' analysis can be challenged. Kim (Philosophy of Mind, 2d edition. Cambridge, MA: Westview, 2006, p. 12) has shown that supervenience, if defined in terms of correlation, does not entail metaphysical necessitation. Supervenient properties need not be metaphysically determined by the properties in the supervenience base: the correlation between the supervenient properties and those on their base may be brute, or properties of both sets may be determined by properties in a third set. If Kim is right about this, there are 3 types of metaphysical dependence of properties on other properties. Emergent properties might fall in Morris' category of "dependence S " (dependence in the sense of having a supervenience base), without being metaphysically necessitated by other (more fundamental) properties. For emergent properties of that sort, construing them as truthmakers may be successful in showing how they can make a distinctive causal contribution (although they are supervenient on more fundamental properties) and maybe also escape other challenges that have been raised against emergent properties.

Torin Alter argues that we are not at present in a position to assess whether there are brute phenomenal facts. He shows that anti-materialist arguments such as Jackson's knowledge argument, even if sound, are not sufficient to establish the existence of brute phenomenal facts. The knowledge argument shows that comprehensive physical knowledge is incomplete: there are phenomenal facts that cannot be known on the mere basis of reasoning from (hypothetical) comprehensive physical knowledge. Alter's demonstration relies on a doctrine called "Russellian panprotopsychism" recently developed by David Chalmers. According to this doctrine, there are "intrinsic properties underlying basic structural-and-dynamic properties described by physics", called "quiddities", which are "protophenomenal" (p. 137). The doctrine rests on the hypothesis that these properties "collectively constitute phenomenal properties when organized in the appropriate way" (David J. Chalmers, The Character of Consciousness. New York: Oxford University Press, 2010, p. 151). The mere coherence of this doctrine is sufficient to show that even if the knowledge argument is sound, its soundness is not sufficient to show that there are brute phenomenal facts. The phenomenal facts that cannot, according to the knowledge argument, be derived from scientific knowledge may not be brute because they may be grounded on protophenomenal facts.

Alter's argument crucially depends on the coherence of Russellian panprotopsychism (or some other doctrine playing an equivalent logical role). He shows that two prominent critics of Russellian panprotopsychism (P. Goff and G. Strawson who defends panpsychism against panprotopsychism) do not succeed in establishing that it is incoherent. Alter's demonstration is impeccable according to which (1) Russellian protopsychism is "viable" (p. 148) and therefore (2) we should remain agnostic about the existence of brute phenomenal facts. However, I would challenge his claim that "we should not conclude that this view is implausible" (p. 144). The truth of Russellian protopsychism (or some other doctrine playing its role) seems to be a mere logical possibility. We have no positive reason to think it is plausible. If this is so, Alter's demonstration appears as a purely academic exercise of conceptual analysis, which has no bearing on the question whether complete scientific knowledge would be sufficient to understand phenomenal facts. Given that we have no positive reasons to believe that there are protophenomenal facts, their mere logical possibility gives us reasons neither for nor against the hypothesis that there are brute phenomenal facts. The hypothesis of Russellian protopsychism may turn out to be just a sophisticated way of expressing (but not justifying) the intuition that science is incomplete in principle, for the plausibility or truth of which we have been given no positive reasons.

Gerald Vision defends emergentism construed as the thesis that conscious states emerge "from wholly material circumstances" (p. 156), where emergence is taken to be a "brute connection" (p. 157). His strategy for defending the intelligibility and plausibility of this thesis consists in challenging panpsychism. Contemporary justifications of panpsychism, in particular by G. Strawson and D. Chalmers, start from the premise that the existence of conscious states can only be explained in two ways, either consciousness (or protoconsciousness, according to Chalmers) is among the fundamental properties of (at least some) fundamental particles or it is not. In the former case, the existence of conscious states in humans and certain animals can be explained as arising from the primitive consciousness of fundamental particles; in the latter case, it can only be explained with the help of emergence. Panpsychists argue that the former alternative is more plausible whereas the latter is either implausible or unintelligible. Emergentists argue on the contrary that panpsychism is at least implausible.

Panpsychists point out that emergentism requires the postulate of brute connections whereas panpsychism need not because it takes consciousness to be a brute property of fundamental particles. Given its existence, no brute connection between fundamental facts and nonfundamental facts needs to be postulated. Emergentism must postulate such brute connections in order to be able to explain the existence of conscious states in a world without any conscious (or protoconscious) fundamental states and properties. Vision argues that none of the reasons that have been offered for thinking that such brute connections are unintelligible or implausible can be derived "from an overview of scientific practice" (p. 166). On the contrary, current scientific knowledge warrants the emergentist thesis of the existence of brute connections between brain states and conscious states. Panpsychism faces its own difficulties. Chalmers' variant of panpsychism seems itself to require emergence: the relation between protophenomenal and phenomenal properties seems to be a relation of brute emergence, just as the relation between neural properties of the brain and phenomenal properties of experiences. This is related to the "combination problem", which has been raised by William James. How do the conscious states of the particles constituting us "coalesce into the transparent and simple experiences we undergo"? (p. 172).

The overall objective of John Symons' chapter is to show that the existence of strongly emergent properties is compatible with an appropriate interpretation of the principle of sufficient reason (PSR), according to which everything has an explanation. The acceptance of emergence is not necessarily incompatible with scientific rationality for two reasons. (1) There can be scientific reasons for accepting emergence, so that scientific rationality does not necessarily favour physicalism over emergentism. Here is an argument for the impossibility to reduce all chemical facts in physical terms. The chemical differences between isomers, such as methyl ether and ethanol, cannot be explained in terms of quantum mechanics, given that isomers share their Hamiltonian. Methyl ether and ethanol are isomers because they are "molecules with the same kinds and number of atoms, but with different molecular structures" (p. 183). So even if it is true that contemporary science makes the British Emergentists (Samuel Alexander, Lloyd Morgan, C.D. Broad) appear wrong about their examples of emergent properties, it warrants the characterization of other properties as emergent. (2) It is possible and indeed rational to accept the PSR as "a methodological heuristic for scientific inquiry" but not "as a metaphysical principle governing Being" (p. 178).

The common ground shared by physicalists and emergentists is the acceptance of the methodological principle that the existence of unexplained brute facts should be (just as anything else) accepted "only on the basis of empirical considerations" (p. 190). However, physicalism is usually defined with the help of metaphysical principles for which it is not obvious whether or how they can be justified by empirical or scientific considerations. How could science justify the principle "that the physical world is causally closed" (p. 190), given that causation is a metaphysical concept that is used in science but whose analysis cannot be found in science? The same question can be asked for the so-called "Hume's dictum", according to which "there are no metaphysically necessary connections between distinct, intrinsically typed entities" (Jessica Wilson, What is Hume's Dictum, and Why Believe It?, Philosophy and Phenomenological Research 80 (2010), p. 595-637). In fact, science seems to provide reasons for doubting that Hume's dictum is universally valid: it seems, e.g. to be contradicted by relations between parts of entangled quantum systems. Furthermore, the physicalist claim that the facts taken into account by present day physics, or even by an ideal physics of the future, are complete cannot possibly be warranted by (present day) science. Symons concludes that the best interpretation of the PSR is to accept it as a methodological heuristic that underlies und justifies a sceptical reaction to brute facts. However, only an interpretation of the PSR as a metaphysical principle would justify physicalism as a metaphysical doctrine, but such an interpretation is not justified.

Elly Vintiadis argues for the plausibility of the existence of emergent brute facts. A strategy similar to John Symons' leads her to a stronger conclusion. According to Vintiadis, we have empirical reasons for believing that there are irreducible brute trans-ordinal laws, in particular laws linking brain states to conscious states. If this is correct, the "explanatory gap" is not only epistemic as Joseph Levine has argued in "Materialism and Qualia: The Explanatory Gap", Pacific Philosophical Quarterly 64 (1983), p. 354-361, but corresponds to a "metaphysical gap" (Vintiadis, p. 210) between physical facts and consciousness, which is therefore a "nonphysical ontological brute fact" (p. 210). Vintiadis analyses three arguments that physicalists have put forward against the existence of emergent brute facts. One of them is the "coherence argument" according to which the emergentist position contains the incoherent claim that emergent facts are both dependent on other, more fundamental facts, and yet distinct from them. In her rebuttal of this argument, Vintiadis suggests that what is crucial for determining whether the dependence of emergent facts on more fundamental facts is coherent with their distinctness from those facts is the status of the laws linking the fundamental and emergent facts. She suggests that these interlevel or "trans-ordinal" laws may be themselves emergent and thus brute. This would make emergent bruteness acceptable insofar as everyone (including physicalists) has to accept the existence of brute fundamental laws (of physics).

The bruteness of trans-ordinal laws "is no worse than any other bruteness of fundamental facts that are accepted by physicalists" (p. 202). However, this claim may be challenged. The existence of brute trans-ordinal laws is certainly a logical possibility but it would require a different sort of justification than the existence of fundamental physical laws. In the case of the latter, the burden of proof seems to lie on those who deny their being fundamental, because we do not know any more fundamental facts and laws to which they might be reduced. In the case of trans-ordinal laws, the burden of proof seems to lie on the emergentist who holds that they are brute, i.e. can in principle not be reduced, because in the case of these laws, we know more fundamental facts and laws. The alternative to accepting fundamental physical laws is the hypothesis that there is no fundamental level at all but rather an infinite series of levels. It is hard to find positive reasons that would warrant the latter hypothesis. The alternative to the existence of brute trans-ordinal laws is the hypothesis that trans-ordinal laws can always be reductively derived from lower level laws. The existence of reductive explanations such as the explanation of the stability of the dihydrogen ion H 2 + from quantum mechanical principles gives some inductive support to the latter hypothesis, even if it is of course not conclusive.

Peter Wyss' contribution sheds original light on emergence by showing that "emergence can have a useful epistemic function" (p. 214). Against Hempel and Oppenheim's claim that emergentism "encourages an attitude of resignation which is stifling for research" (Carl G. Hempel and Paul Oppenheim, "Studies in the Logic of Explanation", Philosophy of Science 15 (1948), p. 135-75, p. 152), Wyss argues that "brute facts (…) can provide insights, understanding, and perhaps even explanations" (p. 215). He distinguishes two questions about emergence, to which correspond two types of informative answers. One can ask, first, why something emerges in a particular context, i.e. what are the sufficient conditions for its emergence. And one can ask, second, "why this emergent arises rather than another one" (p. 216), i.e. focus on the identity of what emerges. That the search for an answer to the first question "drives" (p. 220) research rather than "stifling" it can be seen from the fruitful research in neuroscience about the conditions for the emergence of consciousness. However, what makes the postulate of emergent properties positively explanatory is the way science explains their identity and their causal influence. Answers to the second question cannot be found at the level of properties and laws on which emergent properties are grounded: according to what Wyss calls "the identity approach" (p. 222) to emergence, emergent entities are "individuated through their active and passive powers, that is, by what they cause and what they are caused by" (p. 224). Emergent entities are postulated because they are required to account for "new systematic patterns of causality" (p. 224) that correspond to "distinctive laws associated with the emergents" (224). The causal powers and laws that come into being together with an emergent entity justify the idea that it belongs to a "higher ontological level" (224) than its basis.

The impossibility to derive an emergent entity from its basis provides an epistemic advantage: by acknowledging that a fact is brute, we obtain the epistemic gain of learning that there is no mystery left to be resolved. Wyss is led to a provocative re-evaluation of Samuel Alexander's idea that emergent entities have to be accepted with "natural piety". Rather than joining the choir of philosophers who have, beginning with Alexander's contemporaries and even C.D. Broad, blamed "natural piety" for being anti-scientific, Wyss recommends interpreting it as an expression of "epistemic modesty, or humility" (p. 228). "Natural piety" is not only compatible with a naturalistic attitude, but it is a helpful and constructive part of scientific method. "It is epistemically virtuous to stop unanswerable questions" (p. 229): once something has been recognized as emergent, waste of energy is avoided by redirecting research away from questions that are known to have no answer. Knowing that something is emergent is an illuminating second-order epistemic state, and "natural piety is (…) a meta-theoretical policy not to overreach our epistemic grasp" (p. 230). Not everything in Wyss' fascinating contribution is perfectly clear. One of his projects is to downplay the traditional distinction between epistemic and ontological emergence (p. 219), but even on this background, it is hard to make sense of what he calls "ontological reduction" (222) and "(ontological) irreducibility" (221). As long as Wyss does not provide a new definition of the concept of reduction, it is epistemic not ontological: reduction has been defined in different ways, among others by Ernest Nagel (The Structure of Science, London, Routledge and Kegan Paul, 1961) and Thomas Nickles, Two concepts of intertheoretic reduction, The Journal of Philosophy 70 (1973), p. 181-201) but always in epistemic terms. Mark Bickhard's ambitious contribution sketches a model of emergence that "does not require nor support brute facts" (245). He shows that Kim's famous argument against emergence relies on the assumption that the microphysical world is causally closed and that it is possible to account for causality at that level in terms of microphysical particles and their causal powers. This framework is not warranted by contemporary physics. Bickhard argues that a process metaphysics is more adequate to contemporary physics, according to which "there are no particles; everything is fields quantum fields" (237). Then he sketches a model for the emergence of normativity, which is the basis of the emergence of representation and cognition. Hume's argument against deriving norms from facts is valid but unsound: norms can be implicitly defined. Bickhard suggests that the fundamental normative asymmetry is between functional and dysfunctional and that it can be defined within process metaphysics in terms of the persistence or stability of far from equilibrium processes. Self-maintaining processes make functional contributions to their own persistence. A candle flame and a bacterium swimming up a sugar gradient provide illustrations of such processes. Representational normativity, which is the ground for all forms of cognition, arises from functional normativity. Bickhard's sketch of a model of emergence that does neither require nor support brute facts or brute laws is fascinating, but the reader is invited to consult other works of Bickhard's to learn about the details. His framework makes the issue of the existence of brute facts independent of the issue of emergence. Naturalism does not prejudge the question of brute facts in one sense or the other.

Arnellos and El-Hani sketch "a conception of ontological emergence according to which certain types of dynamical organizations possess irreducible properties that are nevertheless derivable from the substrate" (p. 249). They seem to hesitate whether their analysis should be interpreted in a realist way. On one hand, according to one of the authors (El-Hani), "ontological emergence" should be understood to be a feature not of the world but of scientific models (p. 249, note 1), but on the other hand, the cohesion of a persistent emergent system "is a causal property of the system itself, not merely a property of our description or conception of the system, i.e. it is an ontological rather than epistemological feature" (p. 260, note 16). Like Bickhard, they argue that emergence does not imply bruteness: "there is absolutely nothing brute in the emergence of such organizations" (p. 248). Arnellos and El-Hani make the surprising claim that emergent features of stable systems such as "constancy, persistence and resilience" (p. 250) are irreducible without being unexplainable and thus brute (p. 250). They work with a concept of reducibility as identity, so that irreducibility is compatible with explanation.

Arnellos and El-Hani argue that the supervenience of emergent properties of organized systems on the properties of the constituent parts may fail. In the case of fusion, the reason is that the constituent parts disappear during the appearance of the emergent property; in the case of organized systems, the reason is that the configuration of their parts is not part of the supervenience base. They distinguish the supervenience base from the emergence base (p. 257): the parts, together with their properties, are necessary but not sufficient for some emergent property. To reach a sufficient condition, the configuration of the parts must be added. It is a terminological choice to count the configuration as part of the emergence base but not as part of the supervenience base. However, the controversial result that emergents are not supervenient could be avoided by including the configuration of the parts in the supervenience base.

Arnellos and El-Hani justify the claim that organizational features of dynamical systems are emergent by their novel causal powers to act on the constitutive parts of the system by downward causation. Downward causation, which they call "medium DC" (p. 260), is explicitly distinguished from productive causation and conceived in terms of constraints. The organization constrains the number of degrees of freedom of the evolution of the constituent parts. Within these constraints, which exercise a "neo-Aristotelian (…) formal causal influence" (p. 261) on the parts and which they conceive in terms of probability raising, physical-level causation determines the evolution of the parts, so that the principle of the causal closure of the physical can be respected. Their choice of probability-raising as the appropriate model of causal influence is not explicitly motivated and seems independent of their analysis of DC. They could just as well have used other models, such as structural equations or counterfactuals.

The book is a collection of independent papers. In her very useful introduction, Vintiadis puts the papers collected in the volume in a common perspective, but the editors have not attempted more integration, by encouraging the authors of the chapters to engage in a debate with each other or by trying to impose a homogenous terminology. The word "brute" is a case in point: some authors distinguish "brute" from "fundamental", some do not. The former choice is explicitly adopted, e.g., by Taylor who justifies this distinction by pointing out that not all fundamental facts are brute: fundamental facts may have explanations, e.g. in terms of other fundamental facts, whereas brute facts have no explanation whatsoever. The latter terminological choice is explicitly adopted, e.g., by Alter: "I use 'brute ' and 'fundamental' interchangeably" (p. 131).

The authors disagree on many points. It would have been interesting to see the authors of the different chapters debate at least sometimes with each other. To mention just a few points of disagreement, Heil and Levine argue that only contingent facts can be brute whereas van Cleve argues that there may also be brute necessities, e.g. in mathematics, where they coexist with necessary truths that can be explained. Wyss argues that an attitude of natural piety towards emergent facts may be epistemically valuable, whereas Arnellos and El-Hani judge that emergence, at least in biology "cannot be accepted with 'natural piety'; rather it must be explained" (249). Most authors accept that emergent properties of some complex system supervene on the properties of the parts of the system, together with the relations between these parts, but Arnellos and El-Hani have "abandoned the conception of emergents as supervenients" (p. 266).

Points of convergence are not explicitly mentioned either. Wyss' suggestion that "one strategy to explain epistemic gaps is the appeal to metaphysical gaps" (p. 223) parallels Vintiadis' idea that "there is an explanatory gap because there is a metaphysical gap" (p. 210). Similarly, Wyss' observation that "we ought to postulate not as few entities as possible, say, but as many as necessary" (p. 230) might be compared to Vintiadis' remark that "the real question is not which view posits fewer brute facts, it is how many (or what kind of) brute facts we need to yield the best description and, when possible, explanation of a phenomenon" (p. 200). Bickhard (p. 241) and Arnellos and El-Hani (p. 260/1) both take the candle flame and a bacterium swimming up a sugar gradient (p. 241/2 and 265) as examples of two important types of self-maintaining emergent phenomena, but neither makes any reference to the other.

Brute facts are one important sort of limit to scientific explanation. All chapters of the book are original contributions to research in metaphysics and philosophy of science. The issue of the limits of science is very important for science education, although the relevance of the issues raised by brute facts for science education is not explicitly mentioned in the book. One of the strengths of science is its apparent universality. The scientific method seems to have a very wide scope: indeed, it can be used to address all factual questions, and it seems to provide us with the best possible means to answer them all. However, the scope of what can be scientifically explained today is certainly limited in many ways. There are many questions that have no scientific answer because there are not purely factual, such as questions about good policy or good medical therapies. Other questions, though purely factual, have at present no scientific answer because, among other reasons, (1) our ability of observation and description of facts is limited, (2) our theories and models are incorrect or incomplete, or (3) because our application of these models and theories requires more time or calculating power than is available. Our theories or models may be incomplete or incorrect because we lack time or money for acquiring or constructing the relevant experimental equipment or for improving the models and theories, or simply because our cognitive abilities are limited. The existence of brute facts would constitute an altogether different sort of limit of the scope of science. Brute facts can contribute to explaining other facts, but they cannot themselves be explained. They could not even be explained if all the before mentioned limitations had or could be overcome.

The contributions to the book clearly show that the existence of such brute facts, whether fundamental or emergent, does not threaten the unique power of science to explain all other, i.e. non-brute facts, and that a clear acknowledgment of the existence of brute facts itself enhances our understanding. The fact that some facts cannot be explained leaves untouched the status of science as uniquely rational method of inquiry.

A lucid evaluation of the limits of science is as important for science education as the analysis of what can be achieved by science. In order to defend the reliability of the scientific method against antiscientific challenges, it is important to reflect on the scope and limits of scientific explanation. By contributing to this task, the book will be useful and relevant as a background for science educators and science education research.

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Science & Education https://doi.org/10.1007/s11191-020-00169-9

Elly Vintiadis, Constantin Mekios (eds): Brute Facts, Emergence, and...

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Elly Vintiadis, Constantin Mekios (eds): Brute Facts, Emergence, and...


Stylianos Panagiotakis

Emmanouil Giannoudis

Asterios Charisiadis

Raphaella Paravatou

Maria-Eleni Lazaridi

Maria Kandyli

Kalliopi Ladomenou

Panagiotis A Angaridis

Hélène C Bertrand
email: helene.bertrand@ens.fr

Ganesh D Sharma
email: gdsharma273@gmail.com

Athanassios G Coutsolelos
email: coutsole@chemistry.uoc.gr

Increased Efficiency of Dye-Sensitized Solar Cells by Incorporation of a π Spacer in Donor-Acceptor Zinc Porphyrins Bearing Cyanoacrylic Acid as an Anchoring Group

Keywords: Photovoltaics, Porphyrins, Sensitizers, Solar cells, Zinc

à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

Introduction

The constantly growing consumption of global energy along with the depletion of fossil fuels have prompted scientists to explore clean and renewable energy sources. Among the various technologies that have been reported, photovoltaic devices and dye-sensitized solar cells (DSSCs), in particular, have drawn great attention as promising candidates for the utilization of regeneration. Solar cells sensitized by ZnP(SP)CNCOOH and ZnPCNCOOH show power conversion efficiencies of 7.61 and 5.02 %, respectively. Photovoltaic measurements (J-V curves and incident photon to current conversion efficiency spectra) show that higher short-circuit current (J sc ) and open-circuit voltage (V oc ) values are reached for the solar cell based on ZnP(SP)CNCOOH. This can be mainly ascribed to suppressed charge recombination, as indicated by their electrochemical impedance spectra. solar energy leading to a sustainable society worldwide. [1] Since the first report of DSSCs, [2] numerous inorganic and organic compounds have been studied as sensitizers. [3] In photosynthetic organisms, solar irradiation is collected by chlorophylls and is consequently converted into chemical energy. Intrigued by that, a wide portion of chemists have studied artificial photosynthetic schemes based on porphyrins, which are the synthetic analogues of chlorophyll. [4] What is more, porphyrins have been proven to be ideal candidates for countless applications, as they possess several needed properties, such as strong absorption, great stability, tunable redox potentials, and easy modification and metal-ion insertion. [5] During the last decades, porphyrin derivatives have been extensively studied in DSSCs, [6] and the best results have been achieved upon using push-pull-type porphyrins as sensitizers. [7] Up to date, the most efficient push-pull porphyrin-based photosensitizer was reported by Grätzel and co-workers, and it reached an efficiency of approximately 13 %. [8] Despite the fact that such push-pull sensitizers are highly efficient, their synthesis normally requires multistep routes with low overall yields, thus limiting their use in large-scale applications. [9] On the contrary, the synthesis of tetraarylporphyrins by condensation of pyrrole with an aldehyde requires fewer synthetic steps, and higher yields are achieved. [10] A suitable anchoring group at the periphery of the macrocycle is necessary for the successful attachment of the sensitizer onto the semiconductor (i.e., TiO 2 ) surface. Porphyrins that contain carboxylic or cyanoacrylic acid as anchoring groups are remarkably efficient sensitizers in DSSCs. [11] Another key factor is the incorporation of an electron-withdrawing spacer between the porphyrin ring and the anchoring group. This can alter the properties of the chromophore (i.e., lower HOMO-LUMO gap, wider absorption), resulting in better device performance. [9c,10b,11e,12] Dye aggregation is frequently observed in porphyrin-based DSSCs, and it is detrimental to device performance, as it enables intermolecular charge recombination. Numerous studies reported that porphyrins containing long alkoxy chains, either at the periphery or at the electron-withdrawing spacer, were capable of preventing the formation of aggregates. [11c,13] Thus, the introduction of an anchoring group and alkoxy chains along with the incorporation of a spacer promoting the electron-transfer process could significantly enhance the performance of porphyrin derivatives in DSSCs.

To this end, we synthesized two novel Zn II -porphyrin sensitizers, namely, ZnP(SP)CNCOOH and ZnPCNCOOH, as shown in Scheme 1. Synthetic procedure for compound ZnP(SP)CNCOOH. Scheme 2. Synthetic procedure for compound ZnPCNCOOH.

Schemes 1 and 2, respectively. Both porphyrins contain cyanoacrylic acid as the anchoring group for efficient attachment onto the semiconductor (TiO 2 ) surface. In the case of ZnPCNCOOH, the anchoring group is directly linked to one of the macrocycle's four meso-phenyl rings, whereas in ZnP(SP)CNCOOH a π spacer (SP) bearing two polyethylene glycol chains is inserted between the macrocycle and the anchoring group. The π spacer allows better electronic communication between the macrocycle and the anchoring group, whereas the polyethylene glycol chains are used to prevent dye aggregation. This hypothesis is supported by density functional theory (DFT) calculations, which also indicate that ZnP(SP)CNCOOH is expected to act as a more efficient photosensitizer owing to its large dipole moment. DSSCs sensitized by both porphyrins were fabricated, and the photovoltaic performance (power conversion efficiency, PCE) of ZnP(SP)CNCOOH is found to be twofold higher than that of solar cells based on ZnPCNCOOH.

Results and Discussion

Synthesis and Characterization

The procedures followed for the preparation of compounds ZnP(SP)CNCOOH and ZnPCNCOOH are outlined in Schemes 1 and 2, respectively. For the synthesis of ZnP(SP)CNCOOH, porphyrin derivative 1 was initially prepared by condensation of methyl 4-formylbenzoate, benzaldehyde, and pyrrole in propionic acid. Then, reduction with LiAlH 4 produced compound 2, which was subsequently treated with N-bromosuccinimide (NBS) to give bromo-substituted porphyrin 3. Compound 3 was heated at reflux with triphenylphosphine to produce phosphonium ylide porphyrin 4, which then underwent a Wittig reaction along with aldehyde 5 [14] to yield derivative 6. Afterwards, the porphyrin ring was metalated with zinc to form derivative 7, and finally, a Knoevenagel condensation was performed with 2-cyanoacetic acid to obtain the desired product, ZnP(SP)CNCOOH, in 43 % yield.

Accordingly, for the synthesis of porphyrin ZnPCNCOOH, the first step involved the metalation of formyl-substituted porphyrin 8 [15] with zinc to yield corresponding zinc-porphyrin 9. Finally, a Knoevenagel condensation between derivative 9 and 2-cyanoacetic acid was performed to afford ZnPCNCOOH in 56 % yield.

All compounds were fully characterized by 1 H NMR and can be attributed to the protons of the double bonds, whereas in the 13 C NMR spectrum of ZnPCNCOOH the characteristic resonances of the -CN (δ = 117.8 ppm) and -COOH (δ = 163.2 ppm) groups are present.

Single-Crystal X-ray Diffraction Study

Single crystals, suitable for X-ray crystallographic analysis, of intermediate porphyrin 4 were obtained by slow evaporation of a dichloromethane/hexane (1:1, v/v) solution of the compound. The crystal structure and refinement parameters are given in the Supporting Information (text and Table S1), whereas a view of the molecular structure, together with selected bond lengths and angles, are given in Figure S26 (full list of bond lengths and angles are provided in Tables S2-S5).

Photophysical Properties

The photophysical characterization of the final compounds [ZnP(SP)CNCOOH and ZnPCNCOOH] was performed by absorption spectroscopy both in solution and in films adsorbed onto TiO 2 . In Figure 1, the UV/Vis absorption spectra of the final dyes are presented, and typical features for zinc-porphyrin derivatives are observed for both compounds. [16] For both porphyrin dyes one strong absorption band (Soret band) is present at 428 nm in the case of ZnPCNCOOH and at 422 nm for ZnP(SP)CNCOOH, along with two Q bands located in the 540-610 nm region. Particularly, ZnPCNCOOH exhibits two Q bands at 561 and 601 nm, whereas ZnP(SP)CNCOOH exhibits these bands at 548 and 587 nm. A summary of the absorption data along with the absorption coefficients for both porphyrin derivatives is provided in Table S6.

Upon adsorption of the ZnP(SP)CNCOOH and ZnPCNCOOH porphyrins onto TiO 2 films, their corresponding UV/Vis absorption bands appear to be significantly broader and redshifted relative to those in solution (Figure 1a,b). This broadening may be attributed to intra-or intermolecular interactions between the porphyrin dyes and TiO 2 , which enhance light harvesting in the far near red region. [17] Using the onset absorption edge, λ onset , of the Q band of the absorption spectra in films and the expression E gopt = 1240/ λ onset , the optical band gaps of ZnP(SP)CNCOOH and ZnPCNCOOH were found to be 1.90 and 1.94 eV, respectively.

Electrochemical Studies

The electrochemical properties of ZnP(SP)CNCOOH and ZnPCNCOOH were investigated by cyclic and square-wave voltammetry measurements (Figure S27). Both dyes exhibit two reversible oxidations and two reversible reductions, and their potentials are presented in Table 1. The first oxidation potentials of ZnP(SP)CNCOOH and ZnPCNCOOH, corresponding to their HOMO energy levels, are found at 1.06 and 1.07 V (vs. SCE), respectively. The HOMOs of both photosensitizers are more positive than the redox potential of I 3 -/I -. Consequently, all oxidized dyes can be regenerated by the reducing species of the electrolyte, considering that the reduction potential for the I 3 -/I -redox couple is 0.40 V (vs. SCE). In addition, the LUMO energy levels of ZnP(SP)CNCOOH and ZnPCNCOOH, corresponding to their first reduction potentials, are at -1.28 and -1.29 V (vs. SCE), respectively. The reduction potentials are more negative than the conduction band (CB) of TiO 2 (-0.74 V vs. SCE). This implies an effective electron-injection process from the excited dyes into the CB of TiO 2 .

DFT Studies

Density functional theory (DFT) calculations were performed to shed light onto the molecular structures and the electronic [a] All potentials are reported vs. SCE, and ferrocene/ferrocenium (Fc/Fc + ) was used as the internal standard (E 1/2 ox = 0.60 V).. properties of the two final porphyrin dyes. The gas-phase-optimized coordinates of ZnPCNCOOH and ZnP(SP)CNCOOH are listed in Tables S7 andS8, respectively, whereas two different views of their corresponding structures are illustrated in Figures 2 and S28. In both cases, it can be noted that all four peripheral phenyl rings are almost perpendicular to the porphyrin macrocycle, whereas the central zinc ion is coplanar to the four pyrrole rings. Furthermore, in the optimized geometry we observe that the phenyl rings of the π-spacer group are nearly perpendicular to the porphyrin core, and they simultaneously adopt a coplanar orientation to each other. The polyethylene glycol chains on the second phenyl substituent of the π-spacer are extended above and under the plane of the dye, which thus reduces the probability of dye aggregation onto the TiO 2 surface as a result of π-π stacking. The frontier molecular orbitals (FMOs) as well as the energy contributions corresponding to ZnPCNCOOH and ZnP(SP)CNCOOH are depicted in Fig- ures S29 andS30, respectively. As it can be clearly seen, in both derivatives the electron-density distributions of the HOMOs are predominantly spread over the porphyrin macrocycle. In ZnPCNCOOH, some additional contributions can be found on the peripheral phenyl rings, whereas in the case of ZnP(SP)CNCOOH part of the electron density is located at the spacer's phenyl unit bearing the polyethylene glycol chains (HOMO-2). In a similar manner, the electron densities of the LUMO orbital of both final compounds are localized on the anchoring group, whereas in the other two orbitals (LUMO+1 and LUMO+2) the electron densities are mainly spread over the porphyrin macrocycle and the peripheral units with minor contributions to the cyanoacrylic acid. On the basis of the aforementioned findings, it is safe to assume that intramolecular electron transfer is favored in both ZnPCNCOOH and ZnP(SP)CNCOOH and that they can be considered as promising sensitizers for DSSC applications. Noteworthy, the dipole moment (μ) of ZnP(SP)CNCOOH (13.51 D) is considerably larger than that of ZnPCNCOOH (7.40 D). According to previously published studies, dyes that present larger μ values normally reach higher power conversion efficiencies (PCEs) if applied in DSSCs. [16a,18] This hypothesis is further supported by the photovoltaic parameters recorded for the cells based on both sensitizers. More specifically, the PCE value is significantly higher in the case of ZnP(SP)CNCOOH (7.61 %) than in the case of ZnPCNCOOH (5.02 %). Moreover, the calculated HOMO and LUMO energy values, along with the theoretically calculated HOMO-LUMO (H-L) gap and the dipole moment (μ) of the two final porphyrin derivatives are summarized in Table S9. The theoretically calculated H-L gaps [2.344 eV for ZnPCNCOOH and 2.173 eV for ZnP(SP)CNCOOH] were determined by using THF as the solvent, and they were found to be in close agreement with the corresponding experimentally obtained H-L gaps that were derived through electrochemical studies (Table 1).

DSSC Performance

The photovoltaic performance of DSSCs sensitized by ZnP(SP)CNCOOH and ZnPCNCOOH were investigated under illumination (AM 1.5G, 100 mW cm -2 ). An electrolyte consisting of 0.3 M 1,2-dimethyl-3-propylimidazolium iodide (DMPII), 0.1 M LiI, 0.05 M I 2 , and 0.5 M 4-tert-butylpyridine (4-TBP) in acetonitrile was used. Chenodeoxycholic acid (CDCA) (2 mM) was also added as a co-adsorbent, as it can hinder dye aggregation onto the TiO 2 surface. The current-voltage (J-V) characteristics of the two DSSCs are shown in Figure 3a, whereas the corresponding photovoltaic parameters are summarized in Table 2. The solar cells sensitized by ZnPCNCOOH reach an overall PCE value of 5.02 % with a short-circuit current (J sc ) of 11.04 mA cm -2 , an open-circuit voltage (V oc ) of 0.67 V, and a fill factor (FF) of 0.68, whereas the solar cells based on ZnP(SP)CNCOOH display a higher PCE of 7.61 % and enhanced photovoltaic characteristics, that is, J sc = 14.49 mA cm -2 , V oc = 0.73 V, and FF = 0.72. This clearly indicates that the incorporation of the π-spacer between the porphyrin (donor) unit and the cyanoacrylic acid (acceptor) group significantly improves the overall performance of ZnP(SP)CNCOOH. The effect of the π-conjugated bridge on the DSSC performance was further investigated by measuring the incident photon to current conversion efficiency (IPCE) spectra. As shown in Figure 3b, both porphyrins can effectively convert light into a photocurrent over a wide UV/Vis spectral region. The IPCE spectra of the two solar cells closely resemble the absorption spectra of the corresponding porphyrin sensitizers. The band that is located in the 400-440 nm range in both DSSCs reaches a similar peak value of approximately 63 %. On the other hand, the solar cell sensitized by ZnP(SP)CNCOOH achieves higher peak values in the two bands in the 520-620 nm region. In general, the IPCE spectrum of the ZnP(SP)CNCOOH-based device is broader than that of the ZnPCNCOOH-based device, which leads to a significantly higher J sc value. The integrated values of J sc for the solar cells based on ZnPCNCOOH and ZnP(SP)CNCOOH, as calculated from their corresponding IPCE spectra, are 10.91 and 14.36 mA cm -2 , respectively, which are consistent with the values measured in their experimental J-V characteristics.

To obtain more information about the differences in the J sc values and the IPCE responses of the two solar cells, the lightharvesting efficiencies (LHEs) were evaluated. The transmittance spectra of their corresponding TiO 2 films with a thickness of 8 μm, upon using 0.2 mM solutions of the dyes, were measured. As shown in Figure 4, in the case of ZnP(SP)CNCOOH a higher LHE was calculated covering almost 90 % of the entire wavelength region (400-670 nm). On the contrary, ZnPCNCOOH adsorbed onto TiO 2 exhibited lower LHE with a narrower coverage window (400-620 nm), which is in agreement with the observed J sc values and the IPCE spectra. [19] Figure 4. LHE spectra of dye-loaded TiO 2 films.

In general, the LHE can be enhanced either by increasing the dye loading onto TiO 2 or by increasing the molar extinction coefficient of the dye. [20] Dye loading values for the two devices based on either ZnPCNCOOH or ZnP(SP)CNCOOH were calculated by measuring the absorption spectra of solutions containing amounts of unbound dye molecules that were detached from the semiconductor surface. Particularly, after the attachment of porphyrins onto TiO 2 , the modified films were washed to remove the unbound porphyrin derivatives. The dye loading values were found to be 2.14 × 10 -7 and 2.62 × 10 -7 mol cm -2 for ZnPCNCOOH and ZnP(SP)CNCOOH, respectively. Given that there is a very small difference between these values, it can be concluded that the high LHE of the ZnP(SP)CNCOOH-sensitized DSSC originates from the strong light-absorbing ability of the dye. [21] Another important parameter that affects the LHE of a porphyrin-sensitized solar cell is the total area of the Q-absorption band estimated by the integral ∫ε(v)dv, in which ε is the molar extinction coefficient and ν is the wavenumber. Indeed, the integrals ∫ε(v)dv for DSSCs based on ZnPCNCOOH and ZnP(SP)CNCOOH were found to be 28.6 × 10 6 and 19.5 × 10 6 M -1 cm -1 , respectively. To gain insight into the charge-transfer processes and recombination dynamics of the constructed DSSCs, electrochemical impedance spectroscopy (EIS) in the dark was employed. Also, to determine the interfacial charge recombination resistance (R cr ), effective capacitance (C μ ), and electron lifetime (τ n ), measurements were conducted. [19][20][21] The applied voltage was around -0.7 V and was scanned from 10 5 to 0.1 Hz. The Nyquist and EIS plots of the DSSCs are shown in Figure 5a. In general, these plots consist of three semicircles that are associated with charge transfer at the counter electrode/electrolyte interface (high-frequency region), electron-transfer kinetics at the TiO 2 / dye/electrolyte interface (middle-frequency region), and Nernstian diffusion of I 3 -/I -(low-frequency region). [22] The radius of the semicircle of the Nyquist plots in the high-frequency region is larger for the DSSC based on ZnP(SP)CNCOOH than the DSSC based on ZnPCNCOOH, indicating a larger resistance of the charge-transfer process at the counter electrode/electrolyte interface (R ce ) in the case of ZnP(SP)CNCOOH. [23] This can be attributed to the different distribution and velocity of the electrolyte as well as the different molecular geometry and intermolecular packing of the dye onto TiO 2 . [21,24] The large semicircle observed in the middle-frequency region represents the charge recombination resistance (R rec ) at the TiO 2 /dye/electrolyte interface, which is inversely proportional to the rate of recombination. The radius of this circle is larger for ZnP(SP)CNCOOH than for ZnPCNCOOH, indicating an increased R rec value, which suppresses the recombination rate. Moreover, the C μ value of ZnP(SP)CNCOOH (342 μF cm -2 ) is lower than that of ZnPCNCOOH (315 μF cm -2 ), and this suggests a possible positive shift in the TiO 2 conduction band edge. In addition, the τ n values can be calculated from the peak frequency (f max ) in the lower-frequency region from the Bode phase plots (Figure 5b) of the two DSSCs, according to the expression [25] τ n = 1/(2πf max ), and they were found to be 29.62 and 19.13 ms for ZnP(SP)CNCOOH and ZnPCNCOOH, respectively. The higher R rec and τ n values for the DSSC based on ZnP(SP)CNCOOH indicate that charge transfer from TiO 2 back to the oxidized dye and the redox couple is slower. These contradictory sequences (a positive shift in the TiO 2 conduction band edge leads to a decrease in V oc ) reveal that the relatively high V oc value for the ZnP(SP)CNCOOH-sensitized DSSC is attributed to suppressed charge recombination rather than a shift in the TiO 2 conduction band. [19] The higher R rec and τ n values for the ZnP(SP)CNCOOHsensitized DSSC indicate that its greater J sc and PCE values may arise from the considerably suppressed charge recombination.

We also measured the charge-transport resistance (R The faster electron transport is associated with its higher J sc and indicates that the electrons are collected at the photoanode at a faster rate. Finally, the J sc of a DSSC is highly dependent on its charge-collection efficiency (η cc ), which is calculated according to the relationship: [26] The η cc value for the solar cell sensitized with ZnP(SP)CNCOOH (69 %) is higher than that for the solar cell sensitized with ZnPCNCOOH (54 %), and ultimately, this leads to improved J sc and PCE values. These results indicate that the porphyrin dye bearing polyethylene glycol side chains favors slow charge recombination along with faster charge extraction.

It is evident from the IPCE spectra (Figure 3b) that the IPCE values for the DSSC based on ZnP(SP)CNCOOH are higher than those recorded for the DSSC based on ZnPCNCOOH, particularly in the Q bands. The IPCE of the DSSC is the product of three parameters, namely, LHE, electron-injection efficiency from the excited dye into the CB of the TiO 2 (φ inj ), and η cc , described as: IPCE (λ) = LHE × φ inj × η cc As discussed above, ZnP(SP)CNCOOH-based DSSCs present higher LHE. The φ inj in DSSCs depends on the energy difference between the LUMO of the dye and the conduction band of the semiconductor. As can be seen from the energy-band diagram (Figure 6), this difference is similar for both cells; hence, we can assume that the electron-injection efficiency is almost the same. Owing to the higher LHE for the ZnP(SP)CNCOOH-based DSSC (Figure 4), more electrons are probably injected into the conduction band of TiO 2 from the excited state of ZnP(SP)CNCOOH. The value of η cc for the ZnP(SP)CNCOOHbased DSSC is higher than that for the ZnPCNCOOH-based DSSC, resulting in improved IPCE values.

Conclusions

In this report, the synthesis of two new porphyrin dyes, namely, ZnPCNCOOH and ZnP(SP)CNCOOH, and their study as sensitizers in DSSCs were presented. Both porphyrins were adsorbed onto the TiO 2 surface through cyanoacrylic acid anchoring groups. Moreover, in the case of ZnP(SP)CNCOOH a π-conjugated spacer was introduced between the porphyrin and the anchoring group, which improved their electronic communication and suppressed their undesired aggregation on the TiO 2 surface. Both derivatives were fully characterized by NMR and UV/Vis spectroscopy and MALDI-TOF mass spectrometry. DFT calculations along with cyclic voltammetry experiments showed that both dyes possess appropriate HOMO and LUMO levels for their application in DSSCs. ZnP(SP)CNCOOH-and ZnPCNCOOHbased solar cells were constructed, and they showed PCE values of 7.61 and 5.02 %, respectively. The higher PCE value of ZnP(SP)CNCOOH was attributed to its enhanced short-circuit current (J sc ) under illumination, its longer electron lifetime (τ n ), and higher charge recombination resistance (R rec ).

Experimental Section

Materials and Techniques: Reagents and solvents were purchased as reagent grade from usual commercial sources and were used without further purification, unless otherwise stated. Thin-layer chromatography was performed on silica gel 60 F 254 plates, whereas chromatographic separations were performed by using silica gel 60, SDS, 70-230 mesh ASTM. 1 H NMR and 13 C NMR spectra were recorded with Bruker AMX-500 MHz and Bruker DPX-300 MHz spectrometers as solutions in deuterated solvents by using the solvent signal as the internal standard. High-resolution mass spectra were recorded with a Bruker ultrafleXtreme MALDI-TOF/TOF spectrometer by using trans-2-[3-(4-tert-butylphenyl)-2-methyl-2-propenylidene]malononitrile as the matrix.

X-ray Crystallography

Suitable single crystals of compound 4 were protected with paratone-N and mounted for data collection on a STOE IPDS II diffractometer equipped with a Mo-K α sealed-tube X-ray source (λ = 0.71073 Å, graphite monochromated) and an image plate detector. Data collection, data reduction, integration, and cell-parameter determination were performed by using the STOE X-AREA package software, [START_REF]Program for the Acquisition and Analysis Data (version 1.30[END_REF] whereas a numerical absorption correction was also applied by using the STOE X-RED [START_REF]Program for Data Reduction and Absorption Correction (version 1.28b[END_REF] and X-SHAPE software packages. [START_REF]Program for Crystal Optimization for Numerical Absorption Correction (version 2.05[END_REF] The structure was solved by direct methods and was implemented in SHELXS-2014, [30] which allowed immediate location of most of the heaviest atoms. The remaining non-hydrogen atoms were located from difference Fourier maps calculated from succes-sive full-matrix least-squares refinement cycles on F 2 by using SHELXL-2014. [START_REF] Sheldrick | Program for Crystal Structure Refinement (version 2014/ 7)[END_REF] All structural refinements were performed by using the graphical interface ShelXle. [30a] All non-hydrogen atoms were successfully refined by using anisotropic displacement parameters. The PLATON SQUEEZE [32] procedure was applied to recover 64 electrons per unit cell in one void (total volume 576 Å 3 ), that is, 16 electrons per formula unit. Lattice water (10 electrons/H 2 O) was present, and therefore, the electrons recovered by SQUEEZE were assigned as 1.5 H 2 O molecules per formula unit.

CCDC 1819581 (for 4) contains the supplementary crystallographic data for this paper. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre.

Photophysical Measurements: UV/Vis absorption spectra were measured with a Shimadzu UV-1700 spectrophotometer by using 10 mm path-length cuvettes.

Electrochemistry: Cyclic and square-wave voltammetry experiments were performed at room temperature by using an AutoLab PGSTAT20 potentiostat and appropriate routines available in the operating software (GPES version 4.9). All measurements were performed in freshly distilled and deoxygenated tetrahydrofuran (THF) with a solute concentration of about 1.0 mM in the presence of tetrabutylammonium tetrafluoroborate (0.1 M) as a supporting electrolyte. A three-electrode cell setup was used with a platinum working electrode, a saturated calomel electrode (SCE) as the reference electrode, and a platinum wire as the counter electrode. All potentials are reported versus the ferrocene/ferrocenium couple (0.60 V vs. SCE under the above conditions).

Computational Details: Density functional theory (DFT) calculations [33] were performed with the B3LYP/6-31G(d) [34] level of theory by using the GAUSSIAN 03 program suite. [START_REF] Frisch | Gaussian 03, Revision D.01[END_REF] For all the gas-phase geometry optimizations, the LANL2DZ basis set was selected for Zn atoms and the 6-31G(d) basis set was used for lighter atoms. To guarantee the absence of imaginary frequencies, the optimized minimum-energy structures were subjected to vibrational frequency analysis calculations. Tomasi's polarizable continuum model (PCM) [START_REF] Cossi | [END_REF] was applied to describe the solvent effect (tetrahydrofuran) with standard dielectric constant e = 7.426. The output geometries and molecular orbitals of ZnPCNCOOH and ZnP(SP)CNCOOH were visualized by using ChemCraft software (version 1.6). [START_REF] Zhurko | ChemCraft 1.6[END_REF]

DSSC Fabrication and Characterization

For fabrication of the DSSCs, the transparent fluorine-doped tin oxide (FTO) glass substrates (10 Ω sq -1 ) were first successively cleaned in detergent solution, deionized water, ethanol, and acetone by using an ultrasonic bath for 20 min and were then dried under ambient conditions. Details of the fabrication of the photoanode are described in our earlier work. [16a] The adsorption of the porphyrin dyes on TiO 2 was performed with 0.4 mM dye solution in THF/EtOH (1:4) for 12 h at room temperature. The counter electrode was prepared by spin coating of a H 2 PtCl 4 solution (0.002 g in 1 mL iPrOH) onto a precleaned FTO-coated glass substrate and then heating at 450 °C for 15 min in air. The sensitized working electrode was assembled with the Pt-coated FTO electrode into a sandwich-type cell and was sealed with the hot-melt polymer Surlyn as a spacer between the electrodes. To complete the fabrication of the DSSC, the electrolyte composed of 0.3 M DMPII, 0.1 M LiI, 0.05 M I 2 , and 0.5 M 4-tert-butylpyridine in CH 3 CN was introduced into the space between the two electrodes through a predrilled hole in the platinumcoated FTO by vacuum backfilling. The amounts of dye loaded were determined by dye desorption in a basic solution (0.1 M NaOH in THF/H 2 O = 1:1) followed by spectroscopic measurements.

The current-voltage (J-V) characteristics of the DSSCs under illumination were measured by a Keithley source meter, a solar simulator coupled with a 150 W xenon lamp, and an AM optical filter to give 100 mW cm -2 illumination at the DSSC surface. The active area of the DSSCs was 0.20 cm 2 . Incident photon to current conversion efficiency (IPCE) data were obtained as a function of different wavelengths by using a xenon lamp, a monochromator, and a Keithley source meter under constant illumination intensity at each wavelength. Intensity calibration for IPCE data was performed by using a standard silicon photodiode. Current was measured under shortcircuit conditions. Electrochemical impedance (EI) spectra, in the dark, were recorded by using an electrochemical workstation (Autolab PGSTAT) with a frequency response analyzer. A frequency range of 10 mHz to 100 kHz and an alternating current potential of 10 mV were used. A direct-current bias equivalent to the open-circuit voltage of the DSSC was applied. Electrochemical impedance spectroscopy (EIS) data were analyzed by using Z-View software with an appropriate equivalent circuit.

5-[4-(Methoxycarbonyl)phenyl]-10,15,20-triphenylporphyrin (1):

A propionic acid (40.0 mL) solution of benzaldehyde (0.70 mL, 6.9 mmol) and methyl 4-formylbenzoate (377 mg, 2.3 mmol) was added to a round-bottomed flask. The mixture was heated to 100 °C and then pyrrole (0.64 mL, 9.20 mmol) was added dropwise. The mixture was heated at reflux whilst stirring for 3 h, protected from light. The solution was cooled to room temperature, and then H 2 O was added. The mixture was filtered, and the precipitate was washed with water (100 mL). The crude residue was purified by column chromatography (silica gel, CH 2 Cl 2 /hexane, 2:3) to obtain porphyrin 1 as a dark purple solid. Yield: 240 mg (16 %). 1 H NMR (300 MHz, CDCl 3 ): δ = 8.87 (m, 6 H), 8.80 (d, J = 4.8 Hz, 2 H), 8.45 (d, J = 8.1 Hz, 2 H), 8.32 (d, J = 8.1 Hz, 2 H), 8.23 (m, 6 H), 7.76 (m, 9 H), 4.12 (s, 3 H), -2.76 (s, 2 H) ppm. 13 C NMR (75 MHz, CDCl 3 ): δ = 167. 5, 147.2, 142.18, 142.15, 134.72, 134.68, 131.4, 129.7, 128.0, 127.9 126.86, 126.85, 120.7, 120.5, 118.7, 52.6 (18 mg, 0.38 mmol) was added to a solution of 1 (65 mg, 0.096 mmol) in anhydrous THF (10 mL), and the mixture was stirred at room temperature. After stirring for 30 min, H 2 O and then an aqueous solution of NaOH (15 %, w/v) was very carefully poured into the mixture to neutralize LiAlH 4 . CH 2 Cl 2 was added, and the organic layer was washed with saturated NaHCO 3 solution. The organic solvents were evaporated under reduced pressure, and the product was purified by column chromatography (silica gel, CH 2 Cl 2 ) to obtain porphyrin 2 as a purple solid. Yield: 43 mg (70 %). 1 H NMR (500 MHz, CDCl 3 ): δ = 8.86 (s, 8 H), 8.22 (m, 8 H), 7.77 (m, 11 H), 5.05 (s, 2 H), -2.76 (s, 2 H) ppm. 13 C NMR (75 MHz, CDCl 3 ): δ = 142. 3, 141.6, 140.5, 134.9, 134.7, 131.2, 127.9, 126.8, 125.4, 120.3, 119.9, 65.4 (10 mL) was cooled to 0 °C. Then, NBS (36 mg, 0.200 mmol) was added, and the mixture was stirred at 0 °C for 3 h, protected from light. The green solution was washed with saturated NaHCO 3 solution until it became red and once with H 2 O. The organic layer was then dried (Na 2 SO 4 ), and the solvent was evaporated under reduced pressure. The product was purified by column chromatography (silica gel, CH 2 Cl 2 /hexane, 3:2) to obtain purple porphyrin 3. Yield: 30 mg (85 %). 1 H NMR (300 MHz, CDCl 3 ): δ = 8.86 (s, 8 H), 8.22 (m, 8 H), 7.77 (m, 11 H), 4.85 (s, 2 H), -2.78 (s, 2 H) ppm. 13 C NMR (75 MHz, CDCl 3 ): δ = 142.5, 142. 2, 137.4, 135.1, 134.1, 131.2, 127.9, 127.6, 126.8, 120.5, 120.4, 119.3, 33.7 (33 mg, 0.126 mmol), and toluene (10 mL) were added into a round-bottomed flask. The mixture was heated at reflux for 3 h. Then, it was cooled to 0 °C for 1 h; this caused precipitation of the product, which was filtered and concentrated. The filtrate (5 mL) and PPh 3 (11 mg, 0.042 mmol) were added into a round-bottomed flask. The mixture was heated at reflux for another 3 h. Then, it was cooled to 0 °C for 1 h; this caused precipitation of the product, which was filtered. The precipitate was washed with toluene (25 mL) 54, 142.48, 142.2, 135.31, 135.29, 135.01, 134.98, 134.8, 134.6, 131.5, 131.4, 131.3, 130.8, 130.6, 130.4, 130.0, 129.9, 127.9, 127.3, 127.2, 126.8, 120.6, 120.4, 118.8, 117.7, 31.3, 30.7

5-{(E)-4-[4-({4-[(4-Formylphenyl)ethynyl]-2,5-bis[2-(2-methoxyethoxy)ethoxy]phenyl}ethynyl

)styryl]phenyl}-10,15,20-triphenylporphyrin ( 6): Porphyrin 4 (30 mg, 0.031 mmol) and 4,4′-({2,5bis [2-(2-methoxyethoxy)ethoxy]-1,4-phenylene}bis(ethyne-2,1diyl))dibenzaldehyde (5; 21 mg, 0.037 mmol) were dissolved in dry CH 2 Cl 2 (40 mL). LiOEt (1 M in EtOH, 375 μL, 0.116 mmol) was added dropwise, and the mixture was stirred at room temperature, protected from light. After 24 h, the reaction was quenched by the addition of dilute aqueous HCl (1 M). The organic layer was separated, washed with water, dried (Na 2 SO 4 ), filtered, and concentrated. The product was purified by column chromatography (silica gel, CH 2 Cl 2 /EtOH, 98:2) to obtain purple porphyrin 6. Yield: 12 mg (32 %). 1 H NMR (500 MHz, CDCl 3 ): δ = 10.03 (s, 1 H), 8.91 (d, J = 5 Hz, 2 H), 8.87 (m, 6 H), 8.23 (m, 8 H), 7.92 (d, J = 8 Hz, 2 H), 7.88 (d, J = 8 Hz, 2 H), 7.77 (m, 9 H), 7.69 (d, J = 8.5 Hz, 2 H), 7.67 (d, J = 8 Hz, 2 H), 7.61 (d, J = 8.5 Hz, 2 H), 7.49 (d, J = 16.5 Hz, 1 H), 7.42 (d, J = 16 Hz, 1 H), 7.12 (s, 1 H), 7. 10 (s, 1 H), 4.27 (m, 4 H), 3.98 (m, 4 H), 3.87 (m, 2 H), 3.83 (m, 2 H), 3.59 (m, 4 H), 3.42 (s, 3 H), 3.39 (s, 3 H), -2.74 (s, 2 H) ppm. 13 C NMR (75 MHz, CDCl 3 ): δ = 191. 6, 154.0, 153.7, 142.3, 142.0, 137.7, 136.5, 135.5, 135.3, 134.7, 132.2, 131.3, 129.7, 129.6, 128.8, 128.5, 127.9, 126.8, 125.1, 122.5, 120.4, 119.8, 117.6, 117.4, 117.3, 115.3, 113.4, 96.0, 94.3, 90.2, 86.9, 77.6, 77.2, 76.7, 72.2, 71.3, 71.2, 69.9, 69.8, 69.6, 59.2 (d, J = 16.4 Hz, 1 H), 7.40 (d, J = 16.4 Hz, 1 H), 6.99 (s, 1 H), 6.98 (s, 1 H), 4.12 (m, 2 H), 4.00 (m, 2 H), 3.82 (m, 2 H), 3.67 (m, 4 H), 3.51 (m, 2 H), 3.40 (m, 2 H), 3.28 (m, 2 H), 3.26 (s, 3 H), 3.21 (s, 3 H) ppm. 13 C NMR (75 MHz, CDCl 3 ): δ = 191. 6, 153.8, 153.5, 150.3, 150.2, 143.0, 142.8, 137.7, 136.2, 135.4, 135.1, 134.6, 132.1, 129.7, 128.6, 127.6, 126.7, 125.0, 122.4, 121.3, 120.7, 117.5, 117.2, 115.2, 113.2, 95.9, 94.2, 90.0, 86.8, 72.0, 71.9, 71.1, 70.8, 69.72, 69.67, 69.6, 69.4, 59.11, 59.06 (5 μL) were added to a solution of 7 (21 mg, 0.017 mmol) in THF (2 mL), and the mixture was heated to 65 °C under N 2 for 24 h. The solvents were evaporated under reduced pressure. The solid was dissolved in CH 2 Cl 2 (10 mL) and washed with 0.5 M H 3 PO 4 (2 × 20 mL). CH 2 Cl 2 was removed under vacuum, and the product was purified by column chromatography (silica gel, CH 2 Cl 2 /MeOH, 9:1) to obtain dark red-purple porphyrin ZnP(SP)CNCOOH. Yield: 10 mg (43 %). 1 H NMR (500 MHz, DMSO): δ = 8.86 (d, J = 4.6 Hz, 2 H), 8.78 (m, 6 H), 8.20 (m, 8 H), 8.14 (s, 1 H), 8.04 (m, 4 H), 7.82 (m, 11 H), 7.72 (d, J = 16.3 Hz, 1 H), 7.68 (d, J = 8.2 Hz, 2 H), 7.64 (m, 3 H), 7.28 (s, 2 H), 4.23 (m, 4 H), 3.84 (m, 4 H), 3.72 (m, 4 H), 3.49 (m, 4 H), 3.28 (s, 3 H), 3.24 (s, 3 H) ppm. Due to solubility reasons, we were unable to record a 13 C NMR spectrum for ZnP(SP)CNCOOH. UV/Vis (CH 2 Cl 2 ): λ max (ε, mM -1 cm -1 ) = 422 (275.0), 548 (13.1), 587 nm (3.7). HRMS (MALDI-TOF): calcd. for C 8 2 H 6 3 N 5 O 8 Zn 1309.3968 [M] + ; found 1309.3961. 150.6, 150.5, 150.3, 149.6, 149.5, 142.8, 135.5, 135.2, 134.6, 132.6, 132.4, 132.3, 131.5, 128.1, 127.7, 126.7, 121.8, 121.6, 119. (15 mg, 0.18 mmol) and piperidine (9 μL) were added to a solution of porphyrin 9 (25 mg, 0.03 mmol) in THF (4 mL), and the mixture was heated to 65 °C under a N 2 atmosphere overnight. The solvents were evaporated under reduced pressure, and the product was purified by column chromatography (silica gel, CH 2 Cl 2 /EtOH, 97:3) to obtain porphyrin ZnPCNCOOH as a green-purple solid. Yield: 15 mg (56 %). 1 H NMR (500 MHz, DMSO): δ = 8.82 (d, J = 4.5 Hz, 2 H), 8.79 (d, J = 4.5 Hz, 2 H), 8.78 (m, 4 H), 8.53 (s, 1 H), 8.41 (d, J = 8.0 Hz, 2 H), 8.36 (d, J = 8.0 Hz, 2 H), 8.19 (m, 6 H), 7.83 (m, 9 H) ppm. 13 C NMR (75 MHz, DMSO): δ = 163. 2, 157.9, 149.5, 149.3, 148.9, 146.7, 142.7, 135.0, 134.2, 132.0, 131.7, 131.5, 128.5, 127.6, 126.7, 120.8, 120.6, 119.0, 117.8 ppm. UV/Vis (DMSO): λ max (ε, mM -1 cm -1 ) = 428 (230.0), 561 (11.0), 601 nm (5.5). HRMS (MALDI-TOF): calcd. for C 48 H 29 N 5 O 2 Zn 771.1613 [M] + ; found 771.1619.

Supporting Information (see footnote on the first page of this article): Tables with X-ray crystallographic information for 4•1.5H 2 O, additional photophysics measurements and spectroscopic data, coordinates of the gas-phase-optimized structures of ZnPCNCOOH and ZnP(SP)CNCOOH, as calculated by DFT.

13C

  Figure 1. UV/Vis absorption spectra of the (a) ZnPCNCOOH (1) and (b) ZnP(SP)CNCOOH (2) porphyrin dyes in solution (black) and adsorbed onto TiO 2 films (red).

Figure 2 .

 2 Figure 2. Gas-phase-geometry-optimized structures of ZnPCNCOOH (left) and ZnP(SP)CNCOOH (right). Carbon, nitrogen, hydrogen, oxygen, and zinc correspond to gray, blue, white, red, and green spheres, respectively.

Figure 3 .

 3 Figure 3. (a) Current-voltage (J-V) characteristics under illumination and (b) IPCE spectra of DSSCs based on ZnP(SP)CNCOOH (2) and ZnPCNCOOH (1).

Figure 5 .

 5 Figure 5. (a) Nyquist plots and (b) Bode phase plots of DSSCs based on the ZnPCNCOOH (1) and ZnP(SP)CNCOOH (2) porphyrin dyes.

  d ) for both DSSCs from the Nyquist and EIS plots under illumination, and the values are 25.04 Ω cm 2 for ZnPCNCOOH and 20.16 Ω cm 2 for ZnP(SP)CNCOOH. The R d and R rec values of a DSSC are related to the τ n and electron-transport time (τ d ) [which is a measure of the average time taken by the injected electron to reach the fluorine-doped tin oxide (FTO) electrode] according to the relationship: The τ d value of the ZnP(SP)CNCOOH-based DSSC (12.14 ms) is lower than that of the ZnPCNCOOH-based DSSC (16.39 ms).

Figure 6 .

 6 Figure 6. Electron injection into TiO 2 and regeneration in solar cells based on the ZnPCNCOOH and ZnP(SP)CNCOOH dyes are presented in the electrochemical potential diagram [V].

  A solution of Zn(CH 3 COO) 2 •2H 2 O (45 mg, 0.203 mmol) in CH 3 OH (3 mL) was added to a solution of porphyrin 6 (24 mg, 0.020 mmol) in CH 2 Cl 2 (15 mL), and the mixture was stirred at room temperature overnight. The crude residue was purified by column chromatography (silica gel, CH 2 Cl 2 /EtOH, 98:2) to obtain porphyrin 7 as a light-purple solid. Yield: 24 mg (96 %). 1 H NMR (500 MHz, CDCl 3 ): δ = 9.97 (s, 1 H), 9.01(d, J = 4.5 Hz, 2 H), 8.95 (m, 6 H), 8.23 (m, 8 H), 7.91 (d, J = 8.0 Hz, 2 H), 7.82 (d, J = 8.0 Hz, 2 H), 7.77 (m, 9 H), 7.64 (d, J = 8.2 Hz, 2 H), 7.61 (d, J = 8.0 Hz, 2 H), 7.57(d, J = 8.1 Hz, 2 H), 7.47

2 -

 2 Cyanocetic acid (9 mg, 0.102 mmol) and piperidine

  Zn(CH 3 COO) 2 •2H 2 O (163 mg, 0.745 mmol) in CH 3 OH (10 mL) was added to a solution of por phyr in 8 (48 mg, 0.0745 mmol) in CH 2 Cl 2 (50 mL), and the mixture was stirred at room temperature overnight. The crude residue was purified by column chromatography (silica gel, CH 2 Cl 2 ) to obtain porphyrin 8 as a red-purple solid. Yield: 52 mg (98 %). 1 H NMR (500 MHz, CDCl 3 ): δ = 10.34 (s, 1 H), 8.98 (d, J = 5.0 Hz, 2 H), 8.96 (s, 4 H), 8.88 (d, J = 4.5 Hz, 2 H), 8.41 (d, J = 8.0 Hz, 2 H), 8.27 (d, J = 8.0 Hz, 2 H), 8.22 (m, 6 H), 7.77 (m, 9 H) ppm. 13 C NMR (75 MHz, CDCl 3 ): δ = 192.7,

  2 ppm. HRMS (MALDI-TOF): calcd. for C 45 H 28 N 4 OZn 704.1555 [M] + ; found 704.1562. {5-[(Z)-4-(2-Carboxy-2-cyanovinyl)phenyl]-10,15,20-triphenyl-porphyrinato}zinc(II) (ZnPCNCOOH): 2-Cyanoacetic acid

Table 1 . Electrochemical redox data for ZnP(SP)CNCOOH and ZnPCNCOOH in THF. [a]

 1

	Dye	E ox 1	E ox 2	E red 1	E red 2	H-L gap (expt)
		(V vs. SCE)	(V vs. SCE)	(V vs. SCE)	(V vs. SCE)	(eV)
	ZnP(SP)CNCOOH	1.06	1.32	-1.28	-1.46	2.34
	ZnPCNCOOH	1.07	1.32	-1.29	-1.61	2.36

Table 2 .

 2 Photovoltaic parameters of DSSCs sensitized with ZnP(SP)CNCOOH and ZnPCNCOOH dyes.

	Sensitizer	J sc (mA cm -2 )	V oc (V)	FF	PCE (%)
	ZnP(SP)CNCOOH (2)	14.49	0.73	0.72	7.61
	ZnPCNCOOH (1)	11.04	0.67	0.68	5.02

-(Hydroxymethyl)phenyl]-10,15,20-triphenylporphyrin (2): LiAlH 4

	ppm. HRMS (MALDI-
	TOF): calcd. for C 46 H 32 N 4 O 2 672.2525 [M] + ; found 672.2532.
	5-[4

  ppm. HRMS (MALDI-TOF): calcd. for C 45 H 32 N 4 O 644.2576 [M] + ; found 644.2568.

	5-[4-(Bromomethyl)phenyl]-10,15,20-triphenylporphyrin (3): A
	solution of porphyrin 2 (32 mg, 0.050 mmol) and PPh 3 (54 mg,
	0.205 mmol) in CH 2 Cl 2

  ppm. HRMS (MALDI-TOF): calcd. for C 45 H 31 BrN 4 706.1732 [M] + ; found 706.1741. Porphyrin 3 (30 mg, 0.042 mmol), PPh 3

	Porphyrin 4:

  ppm. HRMS (MALDI-TOF): calcd. for C 63 H 46 N 4 P 889.3460 [M -Br] + ; found 889.3468.

  ppm. HRMS (MALDI-TOF): calcd. for C 79 H 65 N 4 O 7 1181.4853 [M + H] + ; found 1181.4859.

  ppm. HRMS (MALDI-TOF): calcd. for C 79 H 62 N 4 O 7 Zn 1242.3910 [M] + ; found 1242.3918.

Acknowledgments

This work was supported by the European Commission's Seventh Framework Programme (FP7/2007-2013) under grant agreement no. 229927 (FP7-REGPOT-2008-1, Project BIOSOLE-NUTI). The Special Research Account of the University of Crete is also acknowledged.