Émilie Caldeira

Anne-Marie Geourjon

Grégoire Rota-Graziosi

Taxing Aid: The end of a paradox?

Taxing aid was often discussed, but never implemented. This issue has now returned to the fore.

First, many developing countries reform their tax system becoming more "reasonable" and eliminating one of the main justifications for aid-related tax exemptions. Second, the International Conference on Financing for Development held in Addis Ababa in August 2015 emphasised domestic revenue mobilisation as the main source of development finance.

However, the broadening of tax base faces the proliferation of special tax arrangements, fuelled in part by the tax-exempt status of Official Development Assistance (ODA). Exemptions for project aid could represent as much as 3 percent of gross domestic product (GDP) in countries where tax revenues barely surpass 15 percent of GDP. In addition to loss of tax revenue, tax exemptions for project aid have particularly damaging effects on the formalisation of the economies of recipient countries. Moreover, systematic exemption reduces the credibility of the policies of donor countries and the consistency of their aid policy. Last, the taxation of aid meets the commitment made by donors in the Paris Declaration on Aid Effectiveness (2005) to use recipient countries' national Public Finance Management (PFM) systems. This note reviews tax exemptions of foreign aid-funded projects: their consequences in terms of domestic revenue mobilization in recipient countries, the induced inconsistency of foreign aid policy, their main historical justifications, and recent moves from some donor countries towards the taxation of their aid.

Introduction

The conference on development finance held in 2015 in Addis Ababa emphasized domestic resource mobilization as the largest source of development finance. 1 While ODA efforts have stagnated in most Development Assistance Committee (DAC) countries,2 domestic resource mobilisation remains insufficient (Keen et al., 2010). The tax reforms undertaken in developing countries since the 1980s have paved the way for a first-generation tax transition, consisting in offsetting the fall in external revenues from international trade under the effect of the dismantling of tariff barriers. However, volume targets for domestic resource mobilisation have not been met (Op de Beke, 2014). Despite an upward trend, Sub-Saharan African countries mobilised on average barely 15 percent of their GDP in tax revenue (excluding natural resources) in 2014. The corresponding figure for OECD member states was 33 percent. 3 Developing countries have continued to make efforts to improve their tax systems via tax legislation and capacity building among tax and customs administrations or authorities. These reforms have generally sought to broaden the tax base, lower tax rates, and restructure and modernise tax and customs administrations. However, special tax arrangements -tax exemptions, reduced rates, tax relief -have proliferated in developing countries, where tax exemptions have often seemed a necessary condition to attract foreign direct investment in light of infrastructure deficiencies and skilled labour shortages. 4 Like the informal sector, these special tax arrangements help to explain why developing countries are characterised by high tax rates, narrow bases, and low tax revenue to GDP ratios. Some developing countries have therefore made significant efforts to better identify and control these tax exemptions, which are sometimes granted outside of any legal framework (IMF, 2011). 5 Tax expenditures assessments and their publication in the appendices of finance laws have contributed to their rationalisation and have improved governments' fiscal transparency.

The elimination of some tax exemptions clearly helps improve domestic resource mobilisation.

Against this backdrop, tax expenditures related to project aid are paradoxical since they directly hamper developing countries' capacity to mobilise tax revenue. 6 For instance, a massive inflow of aid in the form of foreign-funded projects in a failed state will help improve its growth and therefore GDP but will have no positive effect on the country's tax revenue. Its tax revenue to GDP ratio will even decrease.

The vast literature on aid effectiveness (see Clemens et al., 2012 for a review) does not address the issue of aid tax exemptions. 7 However, Burnside and Dollar (2000) introduced the role of institutional quality as a condition for aid effectiveness. This paper triggered an academic debate and influenced multilateral development banks and some donor governments to review their aid allocation policy in favor of countries with stronger policies and institutions.

Moreover, several studies have shown a negative effect of ODA on domestic resource mobilisation, in particular in low-income countries with weak institutions (Benedek et al., 2012). 8 These studies have also underlined the importance of the composition of aid. Gupta et al. (2004) concluded that general budget support has a disincentive effect on domestic resource mobilisation. In the long term, aid reduces the accountability of governments in developing countries and affects governance in these countries (see Djankov et al. 2008). 9

In addition to the accounting effect, tax exemptions for public aid from institutions such as Non-Governmental Organizations (NGOs) may sustain informality in recipient countries and therefore their inability to mobilise more tax revenue. 10 Indeed, tax exemptions are particularly harmful regarding value added tax (VAT) as they break the payment-deduction chain of this tax and favour imports at the expense of locally supplied goods and services. A local supplier for a tax-exempt project can entirely bypass the tax authorities since their client (donor or NGO) does not carry over deductible VAT when purchasing of goods and services. Last, the proliferation of special tax arrangements significantly complicates the work of tax and customs administrations (or authorities) and significantly increases the risk of fraud and corruption. 11 The paper is structured as follows: Section 2 reviews the distortion costs of project aid tax exemption; Section 3 presents the policy initiative to remove project aid tax exemptions; Section 4 concludes.

7 A recent exception is Steel et al. (2018), who discuss the rationale for ODA tax exemptions through a survey covering 47 countries. 8 Such a crowding out effect of aid is not new in the literature. Focusing on the United States Public Law 480, which established the American food aid program in 1954, Schultz (1960) stressed the risk of a disincentive effect of this aid on domestic agricultural producers in recipient countries. 9 Nevertheless, this work has recently been the subject of debate, and new studies have showed that the negative relationship put forward is not so robust. In particular, Clist and Morrissey (2011) and Carter (2013) showed that this relationship is not robust relative to sample or specification changes and that it results from an endogeneity bias. Responding to the work of Benedek et al. (2012), Clist (2014) also concluded that these studies were not robust and attributed the negative effect of aid to poor econometric specifications and failure to take into account the obvious presence of endogeneity bias. 10 These exemptions even create the need for further aid, leading to the Samaritan dilemma described by Buchanan (1975). 11 For example, with the complicity of a customs officer, a merchant could claim a tax exemption granted to a donor and import goods free of any tax or duty without the donor knowing.

II.

The distortion costs of project aid tax exemption Project aid tax exemptions trigger several distortion costs in recipient countries. In particular, these donors' policies contribute to reduce tax revenue, fuel a culture of exemptions, favour imports at the expense of locally provided goods and services.

First, the proliferation of tax exemptions may erode significantly tax bases and deteriorates domestic resource mobilisation in developing countries. The non-taxation of aid is one component of this problem and can hardly be addressed in isolation. Another source of tax exemptions and even tax frauds is the poor implementation of the Vienna Convention on diplomatic relations of 1961, which specifies the privileges of diplomatic missions. Tax and tariffs exemptions are generally extended to service staff of diplomatic or consular missions in developing countries, even though this staff is not directly concerned by the Vienna Convention. 12 This generous interpretation of the Convention may be viewed as an additional tax expenditure. However, as ODA accounts for a sizeable share of GDP in most developing countries -especially in weak states -exempting it from tax leads to a more significant loss of tax revenue for these countries. Although low-income countries' reliance on ODA has tended to decline over the past 20 years, in 2015 ODA still represented on average 8.7 percent of gross national income (GNI). This figure was as high as 30 percent of GNI in the Central African Republic, 21 percent in Afghanistan, and 12 percent in Burundi and Gambia. 13 Further, project aid has increased in recent years, rising from 61 to 82 billion dollars between 2007 and 2015 at an annual average growth rate of 3.7 percent (see Figure 1).

Table 1 displays a simple computation of tax revenues losses resulting from aid exemptions for 2016. 14 We notice that the tax exemption of foreign aid is important in more than 20 countries, in which revenue losses exceed one percentage point of GDP. In Malawi, Mozambique, and Rwanda this figure is around 3 percent of GDP or above. 15 If we consider only indirect taxes (tariffs included), 16 revenue losses remain large in ten countries. This second estimation 12 The reciprocity principle may allow an extension of some tax advantages. However, this principle does not seem relevant when it is applied between developed and developing countries. Moreover, numerous scandals of tax abuse prejudice current practices. 13 Authors' calculation based on World Bank data. 14 We compile the datasets on tax revenues provided from the UNU-Wider (https://www.wider.unu.edu/project/governmentrevenue-dataset) and on aid from the OECD (https://data.oecd.org/oda/net-oda.htm). We multiply the total tax and indirect taxes revenue to GDP ratios by the aid to GDP ratio. We deduce then an estimate of total tax and indirect taxes revenue losses resulting from the tax exemption of aid.

15 For Liberia, this figure rises even to 7.43 percent but we should be cautious with the accuracy of used data since the UNU-Wider tax revenue to GDP ratio is equal to 31.04 percent, this same ratio is equal 12.9 percent in 2013 in the Government Finance Statistics of the International Monetary Fund. 16 Steel et al (2018) stressed that VAT and duties exemptions are far more frequent than other tax exemptions.

provides a lower limit of revenue losses. These estimations remain obviously very preliminary, but highlight the potential impact of tax exemption of aid.

For MAEE (2010), tax exemptions for project aid may represent a tax revenue shortfall around 2 percent to 3 percent of GDP depending on the country. In 2009, the OECD's Task Team on Taxation and Accountability and ATAF (OECD, 2009) estimated that revenue losses represented 18 percent of funded projects and 10 percent of overall tax revenue in Niger. In Tanzania, exemptions on customs duties for donors amounted to 17 percent of the gross value of imports. The tax administration in Madagascar includes the taxation of aid in the benchmark tax system and provides estimates of the cost of these exemptions (Ministry of Finance and the Budget, 2017). Among the identified 243 tax expenditures, 34 percent of these concerns foreign aid, ahead of the mining (11 percent) and healthcare (5 percent) sectors. 17 In Burkina Faso, exemptions for donor-financed projects (excluding NGOs) resulted in a loss of revenue of CFAF 13 billion in 2016. Between January and October 2017, the loss of revenue reached CFAF 19 billion, or 20 percent of total customs revenue losses (all taxes and duties combined) related to exempt imports (all categories) and 4 percent of the total amount of taxes and duties paid over the same period for imports. 18 Beyond the resulting direct loss of revenue, non-taxation of project aid encourages the development of a governance culture of exemptions. It incentivises claims for exemptions, resulting in the multiplication of legal texts, a lack of transparency over tax law, and reduced the effectiveness of tax audits. It also runs counter to the main objective of the tax system reforms encouraged by donors, namely the widespread application of ordinary tax law. In particular, emerging countries -especially China -seek to benefit from the same treatment, while it can be difficult in practice to distinguish between what is and is not ODA. NGOs, which are often involved in implementing externally funded projects, enjoy the same exemptions.

Given their number and diversity, the risk of fraud is considerable. 19

17 As in many developing countries, it is impossible to estimate the cost of customs exemptions granted in Madagascar to project aid from customs data: 1) the codes used for exemptions in the computer system do not separate them; and 2) they are sometimes considered fully taxed imports if taxes and duties are collected as "balancing transactions." 18 Authors' calculation. 19 For example, apart from frequent and practically uncontrollable misappropriations, exemptions create opportunities for customs evasion through the fraudulent use of the tax identification number of the development partner without its knowledge. Exempting aid leads to distortions in favour of imports at the expense of locally provided goods and services. Non-taxation of aid mechanically modifies the structure of incentives and gives rise to distortions. As products imported under such aid are not subject to the same tax regime as comparable products in the national market outside the scope of the aid, it is less attractive to buy locally. 20 This import-favouring bias of aid is not new. For instance, the aim of the Bellmon amendment to the Public Law 480 was to protect the production of local farmers (see Deaton, 1980). Moreover, as mentioned in the previous point, some firms may try to take advantage of the exemptions granted to foreign aid activities for other ends, giving rise to unfair competition with other local or foreign firms.

Non-taxation of project aid encourages tax fraud and corruption by offering a path for circumventing the rules. Like any exemption, it breaks the tax chain -especially for VATcomplicating the task of tax auditing. Maintaining the system of exemptions amounts to encouraging the growth of the informal sector. For tax and customs authorities in recipient countries, managing, monitoring, and auditing exemptions for project aid against the backdrop of a significant risk of fraud constitute a considerable additional workload (Orlowski, 2007). In addition to the loss of revenue, tax exemptions for aid hamper already-limited human and financial capacities (International Tax Dialogue, 2005, 2006). For instance, it is difficult to trace exempted goods. Authorities must ensure that these goods are indeed intended for the projects for which they have been allocated and not sold in the domestic market in competition with firms subject to the ordinary tax code. In developing countries, moreover, the categorisation and codification of customs exemptions are not conducive to rigorous monitoring or to estimations of their budgetary impact.

Tax exemptions of aid led some recipient countries to record revenue losses as their own contributions to the project. For instance, the Togolese Government adopted Order No. 93/005,21 which defined the tax regime of project aid. Similarly, Guinea establishes the Instruction N° 196/414/PM/MBRSP. In these legal texts the State commits to bear the financial part of the tax burden (direct and indirect taxes and duties) of the operation. After a series of administrative procedures, the contractor receives Treasury cheques, which allow it to pay relevant taxes and tariffs. Treasury cheques induce fictitious collected tax revenues as 20 For example, a local supplier will not be able to collect VAT and therefore to deduce VAT from its inputs. Instead, it will be forced to carry over some of this VAT in the price of its service or reduce its profit margin. Exemptions for the suppliers of the donor's supplier could also be considered. This solution, which has been adopted in the agricultural, mining, and oil sectors, is particularly costly in terms of tax revenue and does not favor the formalization of the economy.

"balancing transactions" equal to the granted exemptions. Such behaviour may have a macroeconomic impact on the ratio of tax revenue to GDP and leads to significant and damaging overestimates of countries' capacity to mobilise domestic tax revenue.

Finally, tax aid exemptions may affect the effectiveness of donors' assistance policy and their respective credibility. First, exemptions for project aid involve significant transaction costs for donor countries. Understanding texts and procedures can be a sizeable administrative burden, especially in the presence of a large number of bilateral agreements. Procedures for granting exemptions are generally time-consuming, requiring many cumbersome and repetitive processes that involve different departments and delay clearance times. The mechanisms introduced by recipient countries in recent years to better monitor and control exemptions (treasury cheques, lists, quotas) make these procedures even more cumbersome.

Second, exempting aid erodes donors' credibility and consistency between statements and actions. All donors and financial partners -with the IMF at the fore -encourage the fight against the proliferation of exemptions. Recognised as a barrier to domestic resource mobilisation, their economic efficiency in attracting FDI or reducing inequality is also controversial. In the interests of consistency and exemplarity, these same donors can hardly seek special tax arrangements for their foreign aid activities. There are no obvious reasons why tax arrangements regarding aid-funded goods and services should not be debated like the rest of tax expenditure. It seems problematic for donors to both make domestic resource mobilisation in developing countries a high priority and continue to benefit from tax exemptions on their project aid.

Moreover, the maintenance of tax aid exemption runs counter to donors' discourse on strengthening the capacities of tax authorities and the commitments made in the Paris Declaration on Aid Effectiveness ( 2005) and the Monterrey Consensus ( 2002) on Financing for Development. Indeed, donor countries have committed to "reduce transaction costs" and to "increase alignment of aid with partner countries' priorities, systems, and procedures and help to strengthen their capacities" (United Nations, 2002). Keeping the taxation of ODA outside the scope of ordinary law runs counter to these commitments. It increases the workload of tax and customs administrations and diverts their resources from tax collection objectives.

III. The policy initiative

The practice of exempting ODA-financed projects from tax is as old as foreign aid itself (1950-1960). Wary of the quality of recipient countries' financial and other institutions, donors preferred to circumvent national systems by setting up their own agencies or institutions. 22 Tax and customs exemptions for these aid programs naturally followed. Donors justified their demands for exemptions by citing the "unreasonable" nature of the tax systems of most developing countries (ITD, 2006) with regard to both tax policy and administration: high rates, myriad taxes or rates, tax law lacking transparency, abusive interpretation of legislation, and risk of discrimination and corruption. In particular, before the reforms undertaken as part of structural adjustment programs, customs tariffs were characterised by a large number of taxes and different rates for similar products. 23 Moreover, the use of non-tariff instruments was widespread.

Some donors remain opposed to the taxation of aid, thus contributing to the status quo. There are at least four explanations: (1) Taxing aid reduces the amount of aid allocated directly to projects (in the order of 10 percent to 20 percent of the total amount allocated); (2) It runs the risk that collected taxes and duties might be poorly used or even have a negative long-term impact on governance. This second argument is particularly relevant in the case where recipient countries display such low governance indicators that they are not eligible to budget support (e.g. Liberia, Somalia, and Zimbabwe); (3) Project aid also gives greater visibility to donors' actions than general budget support; (4) There is also consideration of the risk of unequal treatment between the 30 DAC donors and non-member countries -China, in particular-, if the former opt for the taxation of aid.

In addition, donors preferred -and continue to prefer -project aid, which is more visible and comes with targets, over general budget support because of common perceptions of inefficient public spending management. Figure 1 displays the evolution of the composition of foreign aid over the period 2007-2015 and stresses the preponderance of project aid over other types of aid.

In cases of recipient countries considered to be poorly governed or corrupt, donor countries would seek to avoid directly feeding their budgets. Last, as ODA replaces actions the failed state should have financed by mobilising its own resources, donors have had difficulty justifying paying taxes or duties on these activities. On the contrary, exemptions came to be seen either implicitly or explicitly as recipient countries' contribution to project financing.

22 Knack (2013) studied decisions to bypass recipient country institutions. The author highlighted the role of donors' trust in these institutions, their quality, and risk aversion on the part of donor countries. 23 These rates could vary very significantly depending on whether or not the good could be produced locally.

Further, recipient countries had not previously demonstrated any concerted initiative to tax foreign aid. Competition among them to attract donors is a potential explanation. Another is the ability of political leaders in some recipient countries to divert more easily foreign aid than taxation. While tax participates to modern state building (Brautigam et al., 2008), Djankov et al. (2008) show empirically that foreign aid damages democratic rules in recipient countries and that this aid curse is larger than resource (oil) curse. 24 If donor countries initially justified exemptions by citing the inefficiency of recipient countries' tax systems and administrations, this argument has now lost its potency following the numerous reforms that have been implemented to improve tax systems (such as rate cuts and simplification) and to modernise tax departments in most recipient countries.

Some donor countries have programs in place to provide tax-exempt project aid and budget support for the same country. This may result from the multiplicity of institutions, agencies, and ministries involved in donor countries' ODA policy, but it does underline some inconsistency in ODA at the level of the recipient country. Since the discussion began on the taxation of aid, the DAC has ensured that if project aid were to be taxed, it would be accounted for as tax-inclusive. Taxation of ODA-financed projects would then have no impact on the total amount of aid allocated by donors. The loss of resources for each project due to taxation would be offset by the payment of the tax, amounting to general budget support. Thus, the amount of aid allocated to projects would be reduced by the amount of tax paid, but the volume of total aid would remain unchanged since the recipient country would receive the income from these taxes. If donor countries wished to continue to provide project aid and budget support, they would simply have to offset the automatic allocation of a portion of project aid to budget support by rebalancing the two instruments so that the share of project aid is increased in the total volume of aid. However, when the country in question does not provide budget support, the issue remains unsolved. This is the case for most DAC member countries, where budget support represented less than 1 percent of total ODA in 2015 compared with 3.23 percent in 2007 (see Figure 1). For the majority of countries that do provide it, it has largely been on a downward trend. Nevertheless, some countries, such as Portugal, which opposes the taxation of aid precisely because it would result in a reduction in available funds for project aid, provide significant levels of budget support: 12.81 percent for Portugal compared with 9.57 percent for New Zealand, 8.5 percent for Russia, and 7.54 percent for Ireland on average between 2000 and 2015 (see Figure 3). As Netherlands and Norway that have decided to stop asking for some 24 These authors test empirically the effect of aid on democracy in 108 recipient countries over the period 1960 to 1999.

ODA tax exemptions (Steel et al., 2018), Ireland is already beginning to forgo tax exemptions on project aid. Great Britain, which does not have an official position, allocated on average 5.3 percent of its aid to budget support between 2000 and 2015.

The purpose of taxing project aid contributes to broaden the tax base by rationalising some tax expenditure. This would lead to an improvement in recipient countries' tax revenue to GDP ratio, easing tax on the "modern formal" sector and even help with the formalisation of developing economies, as tax authorities would then have better knowledge of the turnover of donors' local suppliers, mainly via the simple calculation of Value Added Tax (VAT)

deductibility.

An increase in general budget support would offer vital fiscal leeway to recipient countries to pursue their own development policies in accordance with the principle of appropriation. As donors become taxpayers, it would also strengthen political dialogue in recipient countries over their tax policies within a framework of mutual responsibility and appropriation. ITD published several documents, including draft guidelines for the tax treatment of aid-funded projects prepared by the secretariats of the member organisations (ITD, 2006, 2007). These guidelines were supposed to give rise to a recommendation by the United Nations Economic and Social Council (ECOSOC). However, the draft guidelines were never finalized, and no other document resulting from a global and organised approach to taxing project aid has been made public since 2007. The issue of exemptions for aid-funded projects continues to be raised frequently in discussions on aid funding (for example, at the OECD's annual meeting in Kampala in 2009), without this leading to concrete initiatives (OECD, 2009). 27

25 "To eliminate these inconsistencies and distortions and reduce transaction costs in the administration of Bank-financed projects, Bank policy would be changed to provide Bank financing for the reasonable costs of taxes and duties associated with project expenditures." (World Bank, 2004, p. 11). The authors are not aware of any effective implementation of such a commitment. 26 The ITD is a joint initiative of the European Union Commission, the Inter-American Development Bank, the IMF, the OECD, the World Bank, and the Inter-American Center of Tax Administrations. 27 In 2010, France considered establishing a working task force to study this issue. This working group would have measured the impact of aid taxation and discussed the issue with the various stakeholders before presenting a proposal to the DAC to set in motion a fresh round of discussion.

Today, the major donors say they are ready to forgo aid exemptions, many of them expressing a desire for this to take place as part of a joint initiative, in particular under the aegis of the European Union. Officially, they accept the idea of tax-inclusive project finance, sometimes with some conditions, notably the application of "reasonable" or "efficient" taxation. Broadly speaking, the countries that have honoured the Paris Declaration's commitments by using national PFM systems are generally those that favour the taxation of project aid (see Figure 2). 28 However, despite this change of discourse, the commitments made and a relative consensus on the benefits of a move towards taxing project aid, the status quo remains and foreign aid-funded projects still enjoy taxes and duties exemption. 29

IV. Conclusion

The aim of this note is to participate to the current debate on the taxation of aid. Since the Addis Ababa conference in August 2015, domestic revenue mobilisation is the main tool for developing countries to finance their development and foreign aid is a secondary source of funding. Several developing countries engaged in streamlining their tax system by reducing several tax rates and broadening relative tax base. One privileged tool has been the assessment and the publication of tax expenditures. Tax exemption of ODA affects this effort. Revenue losses are not negligible particularly in fragile States (1 to 3 percent of GDP). Moreover, it induces several distortion costs such as favouring importations over local purchases, increasing the complexity of recipient countries tax systems and the risk of fraud.

Today, donors are more open to the taxation of project aid. Discussion has led to the recognition of changes in the international aid environment, resulting in a (theoretical) warming to the taxation of aid. It has also led to a recommendation for dialogue between donors and recipient countries to tax foreign aid-funded projects. Nevertheless, in practice, little change has taken place. No international guidelines on the taxation of aid projects have been issued, and no organised discussions between donors and recipient countries have been initiated. The coordination of tax policies within the framework of regional integration should nevertheless encourage both donors and recipients to revive dialogue at the regional level. The notion of 28 There are a few exceptions: Romania is opposed but uses national procedures for 90% of its aid; Belgium and Luxembourg are in favor but have done little to meet the commitments of the Paris Declaration. The use of national systems is a signal of confidence among donors in the systems of recipient countries, which may explain donors' stance on the taxation of project aid.

29 Some countries are beginning to forgo exemptions. France has embarked on this process through "debt reduction and development contracts" (C2D), which finance tax-inclusive development projects and programs.

"reasonable taxation" frequently associated with a favourable position on the part of donors remains unspecified if such a notion is definable.

The taxation of foreign aid would improve the consistency of donors regarding their respective aid policy. Donors emphasize that domestic revenue should be the main financing source of development, but they continue in claiming tax exemptions for their projects. Consequent tax expenditures narrow the tax bases of recipient countries and their efforts in mobilizing more revenue. Agreeing on the taxation of their aid would be also a sign of respect from the donors towards the institutions of the recipient countries (in particular, their tax systems) in line with the 2005 Paris Declaration on Aid Effectiveness.

Numerous questions remain unresolved and deserve more in-depth discussion and study, including (but not limited to): What impact would the taxation of project aid by donors have on tax revenue in beneficiary countries? What is the administrative cost (in time and resources) of keeping such a system in place for both donors and recipient countries? Beyond the direct (revenue) and indirect (administrative cost) financial impact of tax exemptions, what policy implications do they entail for both donors and recipients? What initiatives can beneficiary countries undertake to curb the exemptions they grant? How can the reluctance to pay tax be lifted? What would the cost of change be for donor administrations, and does this explain the status quo?

  The 2005 Paris Declaration on Aid Effectiveness called on donors to use or strengthen existing national systems with a view to improving recipient countries' appropriation of aid. Despite their commitments, donors' use of recipient countries' Public Finance Management (PFM) systems is still far from systematic (see Figure2). Although not mentioned explicitly in the Paris declaration, tax and customs systems are an important part of PFM systems. Since 2005, the World Bank officially announced first its willingness to pay taxes if the recipient country's taxes are "reasonable." 25 Following this approach, other donors decided to review their position within the framework of the International Tax Dialogue (ITD).26

Table 1 :

 1 Tax revenue losses from the exemption of foreign aid in 2016

	Country	Total tax revenue losses (% GDP)	Indirect tax revenue losses (% GDP)	Aid (USD, const, Millions)	GDP (USD, const, Millions)	Aid (%GDP)	Tax revenue to GDP	Indirect tax revenue to GDP
	Algeria	0,03	0,01	189	199 000	0,09	28,97	7,58
	Armenia	0,44	0,23	256	12 365	2,07	21,11	11,00
	Bangladesh	0,20	0,12	3 760	180 000	2,09	9,71	5,79
	Belize	0,61		34	1 602	2,13	28,73
	Benin	1,06		676	9 634	7,01	15,17
	Bolivia	1,13	0,58	949	27 881	3,40	33,22	16,99
	Brazil	0,00	0,00	272	2 280 000	0,01	30,92	13,83
	Burkina Faso	1,61		890	13 162	6,76	23,81
	Cambodia	0,93	0,54	856	18 216	4,70	19,73	11,56
	Cameroon	0,55	0,30	1 210	36 365	3,33	16,49	9,08
	Chad	0,64		648	12 265	5,28	12,07
	Costa Rica	0,03	0,02	98	48 174	0,20	14,13	8,77
	Dominica	1,26	0,70	19	457	4,15	30,27	16,80
	Dominican Republic	0,02	0,01	118	77 017	0,15	14,75	8,92
	El Salvador	0,11	0,06	151	21 988	0,69	16,37	9,43
	Equatorial Guinea	0,01		7	14 300	0,05	18,70
	Ethiopia	1,13	0,55	4 120	58 320	7,07	16,02	7,77
	Gabon	0,10	0,04	106	19 005	0,56	17,13	7,21
	Georgia	0,80	0,41	446	15 923	2,80	28,52	14,60
	Ghana	0,43	0,25	1 260	50 620	2,50	17,08	10,05
	Grenada	0,15	0,09	6	979	0,58	26,35	15,75
	Guatemala	0,07	0,04	366	52 841	0,69	10,62	6,44
	Guinea	0,69	0,41	457	10 778	4,24	16,23	9,70
	Guinea-Bissau	1,65		113	1 123	10,08	16,40
	Guyana	0,45	0,22	51	2 987	1,71	26,08	12,84
	Honduras	0,64	0,29	441	20 368	2,17	29,41	13,39
	Indonesia	0,00		262	1 090 000	0,02	12,54
	Jordan	2,37	1,08	2 920	31 614	9,24	25,70	11,64
	Kenya	0,75		2 480	58 109	4,26	17,64
	Lebanon	0,56	0,25	1 310	42 719	3,06	18,39	8,14
	Lesotho	2,13	1,13	147	2 912	5,04	42,18	22,33
	Liberia	7,49		622	2 577	24,14	31,03
	Madagascar	1,07	0,58	771	10 802	7,13	14,99	8,17
	Malawi	3,27	1,33	1 520	9 059	16,72	19,53	7,95
	Maldives	0,31	0,17	43	3 914	1,10	28,38	15,33
	Mali	1,76	1,00	1 360	14 147	9,59	18,29	10,47
	Mauritania	1,38		284	5 735	4,96	27,91
	Mauritius	0,02		12	12 898	0,09	20,12
	Mongolia	1,18	0,72	764	12 443	6,14	19,26	11,71
	Morocco	0,41	0,21	1 890	119 000	1,58	26,02	13,17
	Mozambique	3,04	1,25	1 790	15 401	11,63	26,13	10,77
	Myanmar	0,36		1 540	79 496	1,94	18,49
	Namibia	0,40	0,22	187	14 880	1,25	31,58	17,35
	Nepal	1,38	0,80	1 270	21 464	5,91	23,32	13,51
	Paraguay	0,06	0,04	142	36 207	0,39	16,48	9,68
	Philippines	0,01	0,00	157	303 000	0,05	15,16	7,31
	Rwanda	2,97	1,17	1 230	9 354	13,10	22,69	8,95
	Senegal	1,06	0,56	910	22 965	3,96	26,77	14,09
	Seychelles	0,52	0,28	19	1 343	1,38	37,63	20,04
	South Africa	0,09	0,03	1 010	427 000	0,24	36,64	11,75
	Sri Lanka	0,05	0,04	297	82 651	0,36	14,15	10,18
	Sudan	0,11	0,06	840	79 411	1,06	10,01	5,47
	Tajikistan	1,00		317	9 101	3,48	28,80
	Tanzania	0,70	0,38	2 580	49 737	5,19	13,49	7,25
	Thailand	0,01	0,01	250	424 000	0,06	20,78	10,67
	Togo	1,80	1,26	345	5 058	6,81	26,45	18,56
	Tunisia	0,36	0,19	775	49 666	1,56	22,81	12,31
	Uganda	1,03	0,57	2 010	28 579	7,03	14,61	8,04
	Uruguay	0,02	0,01	41	49 616	0,08	25,73	11,38
	Uzbekistan	0,23	0,14	638	63 525	1,00	23,03	13,67
	Zambia	0,67		1 040	27 957	3,71	18,18
	Zimbabwe	0,88	0,49	726	17 986	4,04	21,72	12,26

Sources : Gouvernment Revenue Dataset (UNU-WIDER), Official Development Dataset (OECD).

Note: Empty cells correspond to missing data rergading indirect tax revenue.

This tax revenue does not include external tax revenue in the form of tariff receipts or any levies raised on countries' foreign trade, which have declined.

In 2014, ODA represented only 23% of finance provided by developed countries (OECD data).

Authors' calculation based on UNU-Wider database (https://www.wider.unu.edu/project/government-revenue-dataset).

For example,[START_REF] Mansour | Tax coordination and Competition in the West African Economic and Monetary Union[END_REF] show that WAEMU member states compete mainly via special tax arrangements offered to firms as most member states' taxes and duties are regulated by widely-followed community directives.

Notable sources of tax expenditure are sectoral codes (mining, oil), investment codes, ministerial decrees, and even establishment agreements and ad hoc decisions.

Broadly speaking, there are two types of aid: Project aid, which consists in financing certain public goods or services, in particular infrastructure (roads, bridges, ports), and budget support, which consists of transfers to the government of a developing country. Project aid represents around

70% of all ODA (see Chart 1).

See the Instruction N°196/414/PM/MBRSP portant mesures d'application du régime fiscal des marches publics et projets publics du 13/12/1996.


Bertrand Donnio

Butterfly-like Shape Liquid Crystals Based Fused-Thiophene as Unidimensional Ambipolar Organic Semiconductors with High Mobility

Butterfly-like mesogens based on benzodithiophene, benzodithiophene-4,8-dione and cyclopentadithiophen-4-one core moieties were efficiently synthesized by the Suzuki-Miyaura/Scholl reactions' tandem. Depending on the core nature and radial aliphatic chains number, they selforganize into columnar hexagonal mesophase, possess solvent-gelling ability, emit light in the visible range, and display hole-transport performances. In particular, the mesogen with benzodithiophen-4,8dione core shows ambipolar charge carrier transport with both high electron and hole mobility values.

Introduction

In recent decades, great advances in the chemistry of carbon-and heteroatom-based π-conjugated systems have contributed substantially to the emergence of organic electronics and optoelectronics research domains, [1] as evidenced by the large quantity of synthesized semiconducting molecular and macromolecular systems for a variety of technological and medical applications. Among the numerous synthetic methods now available in the chemistry of conjugated systems toolbox, Suzuki-Miyaura cross-coupling [2] and Scholl aromatic oxidative cyclo-dehydrogenation [3] are two of the very powerful methodologies and now used mostly ubiquitously in the construction of 2-dimensional nanographenes and polycyclic heteroatomic semiconducting materials. [3,4,5] π-Conjugated liquid crystals[6] (π-LCs) are versatile, everlasting hot-topic of functional materials. They are able to spontaneously selforganize into fluid mesophases of low dimensionality, [7] possess outstanding electronic properties, and are in principle inexpensive to process in solution to form aligned, self-healing films, of interest in many potential sensorand display-based industrial applications. In particular, conjugated discotic LCs (π-DLCs), [8] consisting of a discotic polycyclic aromatic hydrocarbon (PAH) core structure chemically bonded to at least six peripheral alkyl chains, are being extensively studied as possible one-dimensional organic semiconductors. [9] Such π-DLCs indeed spontaneously self-assemble into infinite onedimensional columns by face-to-face π-π stacking, thus providing unique molecular conductive channels for electrons hopping, insulated from each other by an aliphatic continuum. [10] As a type of soft, dynamic, anisotropic electronic conductors, π-DLCs appear as promising self-organized (macro) molecular semiconductors for applications in electronic devices such as organic field effect transistors (OFETs), [8,9,10] organic light-emitting diodes (OLEDs) [11] and organic solar cells (OSCs). [12] Columnar mesophases are primarily obtained by the self-assembly of highly symmetrical disc-shaped molecules such as the archetypical polycyclic aromatics of the benzoid family e.g. triphenylenes, [13] hexabenzocoronenes, [14] or the heterocyclic porphyrine and phthalocycanine derivatives. [15,16] They can also be induced by other molecular systems with shapes deviating substantially from a disc such as, just to cite but a few examples, tapered, [17] polycatenar, [18] star- [19] and bent-shaped [20] mesogens. However, the former systems possess generally higher charge carrier transport properties [21] than in columns made of these non-discoid mesogens, [22] whose small size of their aromatic cores with limited conjugation and π-overlap, associated with rotational symmetry around the long axis, may be detrimental to the optimization of molecular interactions and stacking. To improve the mobility of charges in discotic systems, research attentions have focused on modifying the periphery (nature and type of chains) and/or changing the molecular-core nature, symmetry and size. [10,23] Beside disc-like molecules, there have been recently few reports on board-like molecules, pseudo-discs also possessing large extended π-systems and being able to stack, exhibiting high electronic charge transport mobility competing with those from conventional DLCs. [24] Structurally related butterfly-like shape molecules specifically made from fused-thiophene aromatics also stack up into columnar mesophases, and were found to exhibit attractive optical properties as well as enhanced electron-conducting properties. [25][26][27] Sulphur-containing heterocyclic aromatics are electron-rich and extended π-orbitals benefit largely to enhanced π-π stacking, and charges hop smoothly among positionally ordered molecules. [28][29][30] Furthermore, rotational freedom and conformational flexibility may lead to lower mesophases' stability and narrower temperature ranges, [25,31] while conformationally fixed π-conjugated scaffolds display in contrast very stable columnar mesophases with high phase transition temperatures, that can act as high temperature organic semiconductors in addition to promote self-assembly in supramolecular organic gels (Figure 1). [27] Lately, we have successfully combined both the Suzuki-Miyaura coupling and the Scholl cyclo-dehydrogenation model reactions to synthesize, in few efficient steps, several new mesogens yielding columnar liquid crystalline semiconductors. [25][26][27]31] Our exploration of original molecular systems and the understanding of structure-activity relationships lent us to construct novel butterfly-shaped molecules based on fused-thiophene with different π-extended scaffolds (Figure 1). Chemical design of organic semiconductors is an important research subject, as it can help tuning the materials light absorption and emission, their stability in ambient condition, and the semiconducting p-or n-type, or both (ambipolar) character. For this study, three different electroactive cores were considered, namely the electron-rich benzodithiophene (BDT), and both electron-deficient benzodithiophene-4,8-dione (BDTD) and cyclopentadithiophen-4-one (CDTO), respectively. The target diphenanthrene-fused compounds described here were obtained in rather satisfying overall yields by this effective tandem reactions methodology. The results show that, depending on the nature of the core and chains, these lipophilic butterfly-shaped fused-thiophene molecules, both before and after annulation, self-organize into supramolecular columnar mesophases with high thermal stability and wide mesomorphic ranges, form fibrillary gels in various solvents, are luminescent and display high charge carrier mobility values.

Results and Discussion

Design, synthesis and characterization The extension of aromatic cores and conjugation towards nanographene-like discoid structures can result in more efficient π-π stacking that helps stabilizing columnar mesophases, and thus promoting high charge carrier mobility. [10,12,31] Furthermore, the S atom has extended p-orbitals and larger atomic radius, and when piled face-to-face, sulphurcontaining aromatic cycles (usually fused thiophenes) possess strong intermolecular interactions. Such interactions between disc-shaped aromatic mesogens can additionally be tuned by the number of alkyl groups chemically attached to the periphery of the rigid core or/and by specific polar groups inserted within the central aromatic core. [32][33][34] We report in this work a robust and efficient synthesis of several new series of columnar mesophase semiconductors having a butterfly shape, and the investigation of the impact of electrondeficient/rich cores and length and number of alkyl chains on their photophysical and electronic properties. The target butterfly-shaped molecules were obtained by conventional Pd(PPh3)4-catalyzed Suzuki-Miyaura cross-coupling and intramolecular Scholl FeCl3catalyzed aromatic oxidative reactions, from commercially available reagents. In brief, the bromation of various accessible dithiophene-containing polyaromatic cores e.g. 4,8-dialkoxybenzo [ ']dithiophen-4-one (CDTO) respectively yielded the corresponding tetrabromo-substituted dithiophene arenes (B1-4, Scheme S1). Four identical 2-(3,4-bis(alkyloxy) phenyl)-4,4,5,5-tetramethyl-1,3,2-dioxaborolane fragments (6a-6e, Scheme 1) were then bonded to these various bromide-rich cores by the Suzuki-Miyaura coupling reaction to lead the corresponding tetraaryl dithieno-arene derivatives (1a-4e, Scheme 1). The desired annulated diphenanthrene-fused derivatives (Scheme 1) were obtained by direct intramolecular FeCl3-oxidative cyclo-dehydrogenation reaction. The high efficiency of the intramolecular oxidative coupling is to be emphasized: four C-H bonds were cleaved and two C-C bonds formed during the one-pot synthesis of the diphenanthrene derivatives, without the formation of regio-isomers or other side products, as confirmed, when applicable, by NMR. Lipophilic compounds BDTa-c, with ten or more diverging chains, exhibited excellent solubility in organic solvents, and were solely purified by silica gel column chromatography. However, the ketone derivatives BDTDd and CDTOc-e with 8 peripheral alkyls chains showed lower solubility in organic solvents and could only be purified by crystallization in dry hot toluene. All the new precursors and annulated compounds but the poorly soluble diphenanthrene derivatives BDTDd and CDTOc-e were characterized by 1 H and 13 C NMR spectroscopy (Figures S17-S30). In the case of the highly soluble BDTa-c compounds, the cyclo-dehydrogenation was neatly confirmed by 1 H NMR: they provided much simpler spectra than their tetraaryl parents, consisting of four well-defined singlet signals in the 7-10 ppm range corresponding each to the remaining four different types of aromatic protons of the πconjugated core. All the target diphenanthrenes were further characterized by high resolution mass spectra (Figures S31-S37) and elemental analysis. These analyses were in good agreement with the proposed chemical structures. Thermal and structural properties The good chemical stability of the precursory tetraaryls and diphenanthrenes was primarily confirmed during their isolation, purification and characterization. Thermal gravimetrical analysis (TGA) (Figure S1, Table S1) revealed that the decomposition temperatures, Tdec, are higher than 300°C for essentially all of them, with no or little effect of the core stiffening upon annulation or chains topology and length; the highest decomposition onset was as high as 395°C, probing the excellent thermal stability of these fused-thiophene molecules. The nature of the core slightly affects the thermal stability. For the first series of compounds based on the BDT core (1a-2c and BDTa-c), congested by two central protruding side chains, Tdec lied in the range between 311 to 341°C, whereas those of the compounds with BDTD and CDTO cores were increased by ca 45-50°C. The lower thermal stability of the BDT derivatives may be related to the presence of the two central protruding chains. Actually, a two-step decomposition process was observed for these derivatives: a first decomposition event at ca. 300°C, which could be attributed to thermal dealkylation of the central benzene ring, which was followed by a second weight-loss occurring slightly below 400°C, fully consistent with the onset of the dealkylation of the eight remaining alkoxy groups, as perfectly superimposed with the decomposition curves of the BDTD and CDTO compounds.

The liquid crystal properties of the precursors, 1a-4e, and of the diphenanthrene derivatives were first studied by polarized optical microscopy (POM, Figures 2 andS2) and differential scanning calorimetry (DSC, Figure S3, Table S2): 9 compounds out of the 14 synthesized thiophene-containing derivatives (3d/4c-e, BDTc/BDTDd/CDTOc-e) are mesomorphous. Representative POM textures revealed birefringent pseudo-focal conic domains (Figure 2a,c) and large area homeotropic domains with long linear defects (Figure 2b, d-f), both features pointing to Colhex mesophases.

The combination of 10 or 12 surrounding alkyl chain fragments and 4 σ-bonded phenyl rings with free rotational freedom within the small electron-rich aromatic BDT core, i. e. compounds 1a, 2b and 2c, was obviously detrimental to the induction of mesomorphism, as all melted in the isotropic liquid below 100°C without showing any mesophase; furthermore, crystallization occurred with some supercooling effect (DSC). Modifying the electronic property of the central aromatic core from BDT to the electron-deficient BDTD and CDTO led to the systematic induction of mesomorphism: all the tetraaryls 3d/ 4c-e exhibit a Colhex mesophase, as easily recognized by POM (growing birefringent monodomains and homeotropic zones, Figure 2a,b). Tetra-aryl 3d, possessing two central carbonyl groups in the core, displays a broad mesophase between 42 to 167°C, whereas the clearing temperatures of the compounds with only one carbonyl group, series 4, drastically dropped down to ca. 110°C (108°C for 4d with same chain-length as 3d), likely due to the slighter deviation from planarity as compared to 3d (DFT, Table S11). The first two homologues of this series (4c, 4d) appeared to show the so-called double-clearing behaviour, a rare phenomenon in low-molecular weight LCs though frequently observed in oligomeric and polymeric LCs. This singularity, later confirmed by S/WAXS experiments (see below), will not be further discussed as not relevant in this study, but may be due to the high flexibility of the four pending aryl groups freezing the whole molecule in specific conformations, leading first to the melting of the hexagonal lattice into a first isotropic liquid (no birefringence detected by POM), I', made of short-range, uncorrelated, columnar aggregates, followed by their complete destruction and isotropization into the second isotropic liquid, I. This behaviour is progressively lost with chain-length increase. The observation of mesomorphism in these tetraaryl thiophene molecules suggests that the electron-deficient core strongly promotes molecular stacking and columnar mesophases' formation, similarly to some other structurally related systems reported in the literature. [35,36] Recall in particular that the analogous dithienothiophene derivatives DTT, [27] whose S-atom is replaced by the carbonyl group (4d), exhibits only a monotropic and over a narrower mesophase range (from 67 to 26°C on cooling). S2). Cr: crystal phase; I, I': isotropic liquids(see text); M: pre-organized phase; Colhex: hexagonal columnar phase.

The thermal and self-organizational behaviour change dramatically after oxidative aromatic cyclization (Scholl reaction). The resulting diphenanthrene molecules exhibit pronounced one-dimensional columnar stacking, as shown in the optical textures by the presence of long linear defect features and large homeotropic zones (Figures 2c-f andS2), and broader mesophase ranges significantly shifted to higher temperatures compared to their non-annulated counterparts. Specifically, for the electrondeficient systems, with BDTD and CDTO cores, a Colhex mesophase is systematically formed from ca. 122 to 259°C, and from ca. 100 to 266, 244 and 224°C, respectively, likely due to both core stiffening (MD, DFT-Figures S38-S40, Table S11) and conjugation extension, thus to enhanced intermolecular interactions (stronger π-π stacking and electrostatic interactions). The maximum mesophase extension of the CDTO series is observed for the dodecyl homolog (Figure 3). As for the diphenanthrenes with the electron-rich BDT core, mesomorphism was induced only for the derivative with the longer linear chains, showing an enantiotropic room temperature Colhex mesophase (-13 Colhex 80°C). So mesomorphism in this system clearly depends on the balance between the lateral chains' length to compensate for the detrimental role played by the two central chains. Rigidity is thus not the sole criteria for mesomorphism induction. The assignment of the Colhex mesophase made by POM was unambiguously confirmed by S/WAXS (small-and wide-angle X-ray scattering) measurements. The X-ray patterns of both tetraaryls and annulated mesomorphous compounds, recorded at various temperatures in the mesophases (Figures 4 andS4), display indistinctly the characteristic scattering signals in the wide-angle range, hch and hπ, emerging respectively from lateral distances between molten chains (hch~4.3-4.6 Å) and from the stacking between the juxtaposed mesogenic cores (hπ ~3.3-3.7 Å); let us remark that this latter signal is present for most compounds but with a variable intensity, indicative of the more or less extension of the stacking ordering range. In the small-angle range, the patterns exhibit an intense, first-order reflection (10), and two or three weaker, higher-order reflections, indexed as (11), ( 20) and ( 21) of a hexagonal lattice, respectively, according to spacing ratios of 1:√3:√4:√7 (Figure 4). The presence of several sharp reflections (hk) reveals the long-range ordering of the two-dimensional lattice and of well-defined interfaces characterizing the efficient segregation between the non-miscible parts, i. e. the supramolecular columns, resulting from the regular stacking of the mesogenic cores localized at the nodes of the lattice, and the molten aliphatic continuum between the columns.

As for DTT compounds previously reported, [27] the insertion of phenylene (BDT and BDTD) or cyclopentanone (CDTO) molecular units between the two thiophene moieties confers an elliptical shape to the aromatic rigid core. The depletion of chains in the central parts of BDTD/CDTO, may foster stronger interactions between consecutive mesogens. A rough estimation deduced by molecular modelling (Figures S38-S40) gives an elliptical core aspect ratio of around 2 (i. e. ratio of the diameters at the equator and at the poles, Table 1 and Figure 5). The emergence of the hexagonal symmetry however implies that the columns must have an averaged circular cross-section area, causing a homogeneous distribution of the aliphatic chains at the periphery. Despite this structural core anisotropy, the shape of the core-to-periphery interface of the columns can be averaged to a cylinder through the continuous orientational change of the elliptical mesogens along the stacking direction, with, if applicable, some additional tilt of the cores with respect to the plane symmetry to guarantee this cylindrical symmetry, and if necessary to increase the core-chain interface. [37] This way, the space-demanding radiating alkyl chains can be distributed over the whole stack periphery. The elongated shape of the cores also implies, for identical cross-sections' areas that the contact area between cores and radiating alkyl chains is at local-range larger than the statistical cylinder area (sell versus scyl in Table 1, Figure 5). The values of the molecular slice thickness, hmol (Table 1) deduced experimentally, confirms the necessity of a tilt for BDTc/BDTDd cores due to substantial shift from hπ (hmol>hπ), whereas these values for compounds CDTOc-e are similar to the stacking distance (hmol≈hπ), implying a quasi-non-tilted stacking of the cores along the columnar axis. For the latter compounds, the area needed by the peripheral chains is also compatible with the interface area offered by the nontilted stacked mesogens, since the ratio qcyl (Table 1) between required and available areas does not deviate from unity; [38] the chains are therefore fully stretched in the plane and densely packed in this case, allowing for easy orientational changes between neighbouring molecules within and between columns, which as a result equally distribute the aliphatic chains in the periphery of the columns. The slight divergence of the ratio from unity for the other derivatives is in agreement with the steric hindrance from the additional lateral chains (BDT series) or the two carbonyl groups (BDTD series), which compromise a perfect orthogonal stacking into columns in order to manage the distribution of the appending chains or carbonyl fragments evenly, respectively. [38] These models (Figure 5) are also in agreement with the relative mesophase stability (i. e. temperature range) observed in these series. As seen above, the tetraaryl precursors are also mesomorphous, although providing that the core is devoid of the two central chains as in 2. Compounds 3d and 4c-e show a Colhex phase but at lower temperature and over smaller ranges than their annulated congeners. Their lattice parameters (Figure 4) are slightly increased with respect to their more constrained annulated congeners due to the slight in-plane expansion of the free aryl substituents. These compounds adopt an overall circular shape with the phenyl groups lying out of plane i.e. a propeller conformation.[30, 35,36] Such a flexible configuration allows a more optimum stacking by their alternated and shifted intercalation in the periphery and greater freedom in the chains' arrangement. The area needed by the peripheral chains is also compatible with the interface area offered by this type of stacking, since the ratio qcyl (Table S4) of both areas does not deviate excessively from unity. This suggests as above an almost equitable distribution of the aliphatic chains in the periphery of the columns. Photophysical properties As anticipated from the presence of the thiophene central core, all the compounds exhibit strong light absorption and demonstrate also for some of them photo-induced light emission. Based on the optical absorption and emission spectra, the resulting optical HOMO-LUMO energy gaps (EFO) were calculated. The optical energy gap (Eopt) was determined from the onset of the long-wavelength absorption edge (λonset) from the solution UV/vis spectra. In Table 2 are summarized the optical properties of all derivatives. UV/vis absorption spectra were recorded in THF solutions with concentration of 1×10 -5 molL -1 (Figures 6a andS5). Broad absorption bands with two distinct maxima at 293, 346 nm for 1a-2c and at 302, 382 nm for BDTa-c, respectively, were seen. Generally, this band can be assigned to π-π* transitions of the conjugated aromatic moieties. As expected, a red shift of the absorption occurred in the extended π-conjugated systems. In detail, the most prominent peak with maximum absorption is shifted by 9 nm, from 293 nm (ɛ, 3.61×10 -4 M -1 cm -1 ) in 1a to 302 nm (ɛ, 9.41×10 -4 M -1 cm -1 ) for its annulated congener BDTa, with an increasing of the absorption intensity (molar coefficient). No significant change in absorption was observed upon changing the length and number of peripheral chains and of the solvent (Figure S5). The absorption spectra of BDTD derivatives (3d and BDTDd) and CDTO derivatives (4d and CDTOd) exhibited the general feature consisting of one strong intense absorption band in the UV region, which is attributed to π-π* transitions as like the OR-substituted/S-bridged central aromatic derivatives, and a broad but less intense absorption band in the visible region. All of these derivatives could be regarded as quadrupolar donor-acceptor-donor molecules, [19a] in which BDTD/ CDTO-cores are strong electron acceptors in contrast to the electron donor character of the substituted aryl derivatives. Such systems show a moderately intense absorption band in their UV/vis spectra, which can be assigned to intramolecular charge transfer (ICT) between donor moiety and acceptor unit. Because both of the BDTD and CDTO moieties possess substantial quinoidal character within the conjugated backbone, it allowed for stable electron delocalization consistent with the broad absorption band exhibited in these derivatives. On the other hand, a significant bathochromic shift in the absorption edge was observed when the central aromatic ring substituents were changed from alkoxy group (OR) to carbonyl group (C=O). For instance, the S0-S1 transition peak shifted from 378 nm in 2c by 70 nm to 448 nm in 3d and by 183 nm to 561 nm in 4d. Similar bathochromic shift of 120 nm and 233 nm were observed in the aryl fused system (λmax: 531 nm in BDTDd and 644 nm in CDTOd as compared with 411 nm in BDTc). Such observations can be attributed to the smaller band gap caused by the extension of the πconjugated skeleton upon the introduction of the C=O group, which is represented by a red shift of the maximum absorption wavelength (Figure 6).

Emission properties of 1a/2b-c and BDTa-c were studied both in solution and film state (Figure 6b-c), while a primary photoluminescent (PL) peak at 437 nm with two equally spaced shoulder (416 and 464 nm) for 1a/2b-c and two separated peaks (420 and 445 nm) as well as weak shoulder at 473 nm for BDTa-c were observed in THF solution (1×10 -5 M). As for the fluorescence emission in molten film state, all derivatives but BDTb, with a disappearing shoulder at long wavelength, exhibited similar peak conditions as in solution and obvious maxima red-shifted by ca. 10 nm to up to 36 nm compared to only 4 to 7 nm in 1a-2c. This result can be explained by considering the stacking motif within the aggregation process. The four aryl groups bonded on 1a/2b-c are more turbulent with respect to their planar core due to steric constraints, while the annulated planar molecules BDTa-c aggregate in thin film state and have stronger intermolecular interactions. Compounds 3d/4d, BDTDd/CDTOd were however found non-emissive, as the photo-induced intramolecular electron transfer resulted in the quenching of the emission in both solutions and films.

DFT calculations Density function theory (DFT) method was used to calculate the frontier molecular orbitals (FMO) distribution, energy levels and gaps for the methoxy homologs of these π-conjugated molecular scaffolds (Figures 7 andS43-S44).

The calculated HOMO-LUMO energy levels and gaps are all distinctive. The BDT electron-rich molecule has low-lying HOMO (-5.30 eV), and high-lying LUMO (-1.90 eV), and a consequently wide energy gap (3.40 eV), while the electron-deficient diketone BDTD exhibits lowered HOMO energy level (-5.67 eV) and much lowered LUMO level (-3.57 eV), giving a narrower energy gap of 2.10 eV. The calculated FMO of the ketone derivative CDTOd lie in between of the former ones, with a HOMO level at -5.23 eV and a LUMO level at -3.02 eV, and a comparable energy gap of 2.21 eV. The DFT results agreed rather well with the measured UV-Vis absorption and photoluminescence spectra (Table 2). Compared with the tetraaryl molecules 2c, 3d and 4d, the corresponding diphenanthrenes, possessing more extended πconjugating systems, displayed lowered LUMO and narrower energy gaps, which were evidenced by the red-shifted observed in the absorption and emission spectra. The polar molecules BDTDd and CDTOd had different HOMO/LUMO wave function distributions, implying intramolecular charge transfer (ICT) and explaining theoretically the quenching of fluorescence. At last, DFT results further predict the potential ambipolar charge carrier property of the polar compounds, as it will be discussed in TOF photoconductivity part. Gelling behavior Polycyclic aromatic hydrocarbons consisting of large cores and multiple peripheral alkyl chains have strong tendency to form supramolecular aggregation or gels via π-π stacking, the flexible alkyl chains not only providing van der Waals interactions to facilitate self-association but also imparting favourable solubility characteristics to control the balance between aggregation and dissolution in solution.

[39] The gelling phenomenon of giant π-conjugated aromatics with long alkyl chain is an obvious demonstration of the anisotropic property of π-π stacking and solubility of the alkyl part. The gelation ability of these various diphenyl-fused derivatives BDTa,c, BDTDd and CDTOd was therefore evaluated in various organic solvents, polar and non-polar, protic and non-protic. A fixed amount of the sample was dissolved in different solvents by heating until complete dissolution and then were slowly cooled down. The formation of the organogels was confirmed by the "stable to inversion of the test tube" method (Figures 8 andS6). The results are summarized in Table 3. Surprisingly, neither BDTa nor BDTc are able to gel any of the solvents considered, they are just highly soluble in all of the tested ones. In contrast, CDTOd exhibits good gel formation behaviour in hexane, cyclohexane, toluene, dichloromethane, trichloromethane, tetrahydrofuran. BDTDd, having a lower solubility, is only able gel cyclohexane, toluene, trichloromethane, tetrahydrofuran, and is partially dissolved in hexane and dichloromethane. Further, we noticed that the phase transitions of the organogels were thermally reversible even after several heating and cooling cycles, while BDTDd and CDTOd formed opaque gels in various solvents (in cyclohexane BDTDd formed a transparent gel). The critical gel concentrations (CGC) were estimated for both BDTDd and CDTOd. CGC values lie in the range of 0.11 to 0.30 wt%, indicating that they can be considered as supergelators (i.e. materials capable of gelling solvents with a concentration lesser than 1 wt%).[39] In addition, these gelators exhibit a significant structure and solvent dependency. For example, the CGC value of previously reported structurally related dithienothiophene derivatives [27] in toluene was 2.5 mg/ mL, while those of BDTDd and CDTOd dropped to 1.5 and 1.0 mg/mL, respectively. Likewise, the value of CDTOd and DTT[27] were 1.5 and 2.0 mg/mL in hexane, respectively. In the case of BDTa and BDTc, the nonformation of gel may be imparted to both the electron-rich character of the core and the presence of two alkyl chains, branched or linear, irradiating from the central core, that strongly diminish the efficiency of staking through π-π interactions as well as filling the available space.

The improved gelation efficiency can be attributed to the presence of additional dipole-dipole interaction introduced by ketone group, while π-π interactions remain the primary driving force to form gel. [39] Besides, the nature of the solvent also had an effect on CGC: CDTOd showed a CGC value of 4.5 mg/mL in CHCl3, 2.0 mg/mL in CH2Cl2 and 1.5 mg/mL in THF as compared to 1.0 mg/mL in toluene. Furthermore, the CGC value of BDTDd decreased from 3.0 to 1.5 and 1.0 mg/mL in CHCl3, THF and cyclohexane, respectively, as the polarity of solvent decreased. Therefore, the strong solvent dependency of gelation may be attributed to stronger π-π interactions in non-polar hydrocarbon solvent than in polar solvent, as reported in other discotic mesogens.[39] In summary, the mechanical strength of the organogels is dependent on gelator-gelator interactions: here mainly π-π and dipoledipole interactions, and to a lesser extend to the gelator-solvent interactions.

To gain a direct insight into the self-assembled gel microstructures, xerogels of gelators BDTDd/CDTOd obtained from various solvents or by precipitation of the non-gelator BDTc from mixed solvents e.g. trichloromethane as the "good" solvent and ethanol as the "poor" solvent, were studied by scanning electron microscopy (SEM, Figures 8 andS7). As revealed by SEM, representative morphologies of the self-assemblies of BDTc and gelators BDTDd/CDTOd showed densely packed 3D networks, which resulted in the solvent immobilization. [40] Specifically, xerogels of BDTDd, obtained from cyclohexane, formed a closely packed 3D networks composed of wide, straight fibrous bundles (Figure 8b). A similar structure but composed of thinner interlocked and more flexible ribbons was observed in the case of CDTOd in THF and toluene (Figures 8d andS7h). Besides, a xerogel morphology with two interlocked 3D structures was observed for BDTDd in THF (Figure 8c) and in toluene (Figure S7c). However, the self-assembled morphology of BDTc aggregated in a mixed solvent containing good/poor solvents exhibited coarse, short fibrillary rods (Figure 8a), in which the structure with open features cannot form rings or holes leading to difficult solvent fixation, thereby in agreement with the non-gelling behaviour of BDTc. Similar fibrous morphologies were revealed for CDTOd in hexane, dichloromethane and chloroform (Figure S7). Photoconductivity Self-organized discotic columnar nanostructures consisting of π-extended polycyclic aromatic hydrocarbons can be envisaged as potentially microscopic supramolecular cables for charge carrier hopping highways. Transient photocurrent time-of-flight (TOF) technique is a unique, accurate, direct, and simple method to measure charge carrier mobility rate of organic semiconductors with a very long charge hopping pathway. [41] Further, by this method, both electron and hole mobility rates can be measured independently; the charge carriers drift a distance of 15 to 20 μm along the ππ overlapping columns stacked by thousands of discotic molecules, very similar to their configurations in electronic devices.

We have investigated the photoconducting properties of three representatives of these butterflyshaped fused-thiophene columnar mesogens by using the aforementioned TOF technique, and the results are showed in Figures 9 and10, and in more detail in supplementary information (Figures S8-S16 and Table S5-S10). The transient photo current decay curves are all in good shape and nondispersive (Figure 9), demonstrating the high purity of these molecular semiconductors and their excellent homeotropic alignment behaviour and thermal stability in ITO cells.

The isotropic liquid state of 4d at 110°C displays low hole mobility, i.e. 3.1×10 -5 cm -2 V -1 s -1 (Figure 10a). On cooling, passing in the second liquid (I', see above), the hole mobility substantially increases by a factor of 5 to 1.7×10 -4 cm -2 V -1 s -1 . Then, it increases to 7.9-4x10 -4 cm -2 V -1 s -1 in the Colhex phase, corresponding to the same range of the well-known triphenylene discogens. [42] This rather low value reflects the formation of relatively short-range ordered 1D π-stacked nanostructures in the Colhex phase, as confirmed by S/WAXS (weak hπ signal, Figure S4). Of interest, the photoconducting mobility of 4d was completely reversible in successive heating and cooling runs, demonstrating that this butterfly-shaped cyclopentadithiophen-4-one core is a good semiconducting columnar mesogen. As expected, the π-extended annulated congener CDTOd displays higher hole mobility rate than 4d, in the interval 1.5-2.1×10 -3 cm -2 V -1 s -1 over the entire mesomorphic range in the cooling cycle (Figure 10b). Below 90°C, the sample starts to crystallize in the LC cell (Figure S11), however the detected hole transport value exhibits some increase. This was attributed to the formation of poly-crystalline domains at the initial crystallizing with close intermolecular π-π stacking and slow molecular dynamics, therefore increasing mobility. [43] Further, the cooling run curve shows a slightly higher hole mobility than during the heating run (Figure S12).

The other π-extended mesogen, BDTDd, exhibits even higher hole as well as electron mobility rates, with μh=4.0×10 -3 cm -2 V -1 s -1 and μe=5.1×10 -3 cm -2 V -1 s -1 at 220°C, respectively (Figure 10c). The electron transport behaviour exhibits a stable value of (5.1-6.6)×10 -3 cm -2 V -1 s -1 between 110-220°C, while the hole mobility rate μh lies in the range of 4.0-4.5×10 -3 cm -2 V -1 s -1 , before declining to 2.2×10 -3 cm -2 V -1 s -1 below 160°C. The charge mobility of BDTDd in the Colhex mesophase reveals an important feature: both electrons and holes are transported by these materials, with μe higher than μh. Therefore, the annulated π-extended benzodithiophene-4,8-dione core can be considered as an ambipolar mesogenic semiconductor. It is noteworthy that such high ambipolar TOF drift mobility is uncommon in dynamic columnar mesophases.

In summary, these novel butterfly-shaped fused-thiophene columnar mesogens with polar cores (BDTD and CDTO) show outstanding charge carrier transport properties largely due to their larger πelectron systems. The results can be interpreted by the Marcus theory: larger π-conjugated aromatics possess lower reorganization energies and higher electronic transfer integrals and therefore cause higher rates of charge hopping. [44] The trend observed is in good agreement with the stacking values measured by S/WAXS (hπ, Figure S4), whose most intense signal, which indicates a regular and longrange stacking, is observed for BDTDd, whereas it is weaker, and thus corresponds to a shorter stacking range for CDTOd, and almost imperceptible for 4d. Unfortunately, compounds with the electron-rich BDT core did not display satisfactory photocurrent decay curves for the drift time readout, the laser-induced photo-oxidation of the aromatic core might cause the failure of TOF measurement for this molecule. S2).

Usually, three techniques are applied for the measurements of charge carrier mobility (μ) in columnar LC materials: pulse-radiolysis/flash photolysis time-resolved microwave conductivity (PR/FP-TRMC), time-of-flight photoconductivity, and space-charge limited conductivity (SCLC).[6] SCLC is very sensitive to ionic impurities of the sample-cell interface and to the history of sample thermal electrical-field annealing, and the measured conduction from the I/V curve of a same sample may surprisingly vary by 5 to 6 orders of magnitude. [6] The TRMC technique measures the short-range, ideal charge transportation of LC samples, usually yielding high μ values. On the other hand, TOF technique measures the long-range (10-20 μm) charge carrier drift mobility, which is sensitive to ionic impurities and sample alignment in LC cells. Thus, from the above, comparison of LC conducting values measured from these different techniques may not be rational nor appropriate.

In this context, the results obtained here compared quite well with those of other columnar LC thiophene derivatives reported in the literature, and thus are quite relevant. Indeed, one of the highest TOF mobility was reported for a thiophene-fused phthalocyanine-Zn complex (10 -2 cm -2 V -1 s -1 ), [16] whereas a X-shaped pyrene-oligothiophenes [45] showed mobility in the range from 10 -4 to 10 -5 cm -2 V -1 s -1 , but could be largely improved to 10 -3 cm -2 V -1 s -1 when mixed in a physical gel blend. Let us as well mention some ambipolar octupolar π-conjugated columnar LCs with a mobility in the range 10 -2 -10 -3 cm -2 V -1 s -1 .[19] For comparison, with PR-TRMC technique, a propeller-shaped fused oligothiophene columnar mesogen displayed a short-range mobility of 0.18 cm -2 V -1 s -1 , [30] while its long-range TOF mobility was 0.02 cm -2 V -1 s -1 , one order of magnitude lowered. Similarly, an H-bonded benzotristhiophene derivative was reported to exhibit a mobility of 0.02 cm -2 V -1 s -1 , while TOF mobility was found in the range of 10 -3 cm -2 V -1 s -1 . [46] The most important factors in determining conductivity of LC semiconductors remain thus essentially the extension of their π-conjugated aromatic core and the order degree of stacking in the selforganized mesophases in order to maximize orbital overlaps. Columnar mesogens with enlarged πconjugating cores, such as hexabenzocoronenes, [47] phthalocyanines, [48] and dibenzocoronene diimides, [49] indeed display very high conductivities. These new butterfly-shaped thiophene-fused discogens, as well as some reported previously by us, [25][26][27] stand out therefore as effective concurrents, and can be foreseen as promising candidates for future applications in thin film electronic devices.

Conclusions

We have demonstrated an efficient synthetic process to access new thiophene-fused butterfly-shaped columnar liquid crystals from accessible starting materials. The tandem of reactions used, Suzuki-Miyaura cross-coupling and intramolecular Scholl aromatic oxidative cyclo-dehydrogenation, respectively, proceeded in high overall yields. As expected, the electronic and stereo effects of the molecules have significant impact on their self-organizational behaviours, photophysical properties, gelation and semiconducting properties. For the tetraphenyl-substituted thiophene-containing aromatic hydrocarbons, electron-deficient (ketone 4d and quinone 3d) systems exhibit advantageous effect on the formation and stabilization of Colhex mesophases by enhanced intermolecular interactions, while the electron-rich systems (1 and 2) with 10 or 12 alkyl chains do not show any mesomorphism. The intramolecular oxidative aromatic cyclization permitted the extension and hardening of the π-conjugated systems by annulating the peripheral phenyls, therefore affording materials showing broader Colhex mesophases and higher phase transition temperatures. The electronwithdrawing ketone and diketone (quinone) functional groups separate the molecules into electrondonating and electron-accepting segments. Thus, the molecules based on CDTO and BDTD display the classical intramolecular charge transfer (ICT) UV/vis absorption band, which consequently quenches the fluorescence emission, while molecules based on BDT (and the previously reported DTT) exhibit blue light fluorescence emission in both solutions and films. On the other hand, the latter display enhanced solubility thus failed to gel any organic solvents, whereas the electron-deficient molecules with less alkoxy chains but stronger π-π intermolecular interactions were found to be supergelators. Finally, the charges transport mobility in their columnar mesophases was investigated by TOF transient photocurrent measurement. The electron-deficient molecular centres greatly enhance the n-type characteristic and promote strong stabilization of the Colhex mesophase, thereby providing efficient 1D electron-transporting channels with relatively important charge carrier mobility values (μh of 4.5×10 -3 cm -2 V -1 s -1 and μe of 6.6×10 -3 cm -2 V -1 s -1 for the ambipolar BDTDd and μh of 2.1×10 -3 cm -2 V -1 s -1 for CDTOd). We believe that more exciting mesogenic semiconducting materials, with improved mobility values, may be easily attainable by this highly reliable synthetic methodology from a wide range of available carbon-and heteroatom-based extended π-conjugating molecular scaffolds in the near future. This should allow the exploration of new molecular designs and paradigms of practical interest for the study and elaboration of various types of organic electronic devices.

Experimental Section

Experimental methods, synthesis and characterization, TGA, POM, DSC, SAXS, TOF, MD, DFT, 1 H and 13 C NMR, HRMS are given in the supporting information. Supporting information for this article is available on the WWW under https://doi.org/10.1002/asia.202100173

Figure 1 .

 1 Figure 1. Board-like and butterfly-shaped fused-thiophene columnar liquid crystalline semiconductors synthesized by successive Suzuki (bonds in red) and Scholl (bonds in blue) reactions: DT, [25] BTBT, [26] DTT, [27] BDT, BDTD and CDTO, this work (R and R' are alkyl chains, CnH2n+1).

  1,2-b:4,5b']dithiophene (BDT), benzo[1,2-b:4,5-b']dithiophene-4,8-dione (BDTD) and 4H-cyclopenta[2,1-b:3,4-

  b

Scheme 1 .

 1 Scheme 1. General synthetic scheme and nomenclature of the various tetraaryls and corresponding diphenanthrene butterfly-shaped fused-thiophene LC semiconductors: 4,8-dialkoxybenzo[1,2-b:4,5b']dithiophene (BDT) core for 1a-2c and BDTa-c; benzo[1,2-b:4,5-b']dithiophene-4,8-dione (BDTD) core for 3d and BDTDd; 4H-cyclopenta[2,1-b:3,4-b']dithiophen-4-one (CDTO) core for 4c-e and CDTOc-e.

Figure 2 .

 2 Figure 2. Representative polarized optical photomicrographs of (a): 3d at 100°C; (b): 4d at 85 °C; (c): BDTc at 50°C; (d): BDTDd at 235°C; (e): CDTOd at 140°C; (f): CDTOe at 127°C, on cooling from isotropic liquid with rate of 5°C/min. Additional POM images in Figure S2.

Figure 3 .

 3 Figure 3. Thermal and thermotropic mesophase diagram of 1a-4e and corresponding diphenanthrenes (DSC curves are shown in Figure S3, and numerical data gathered in TableS2). Cr: crystal phase; I, I': isotropic liquids(see text); M: pre-organized phase; Colhex: hexagonal columnar phase.

Figure 4 .

 4 Figure 4. Top: Small-angle X-ray scattering (S/WAXS) patterns of mesophases for the three representative diphenanthrene mesogens (recorded on cooling at various temperatures); Bottom: Evolution of the hexagonal lattice parameter with structure and temperature.

Figure 5 .

 5 Figure 5. Schematic representation of the three butterfly-like mesogens; the elongated shape of the aromatic core roughly approximates an ellipse of aspect ratio close to 2, offering an interface area per peripheral chain sell (bright green contour line); the continuous orientational changes of the stacked (tilted or non-tilted) cores, perpendicular to the columnar axis, distribute the space-demanding radiating alkyl chains over the whole stack periphery and design average cylindrical columns of reduced statistical interface area scyl; cylindrical columns arranging in hexagonal lattices of parameter a and with molecular slice thickness hmol compared to π-stacking distance, hπ.

Figure 6 .

 6 Figure 6. Absorption and photoluminescence spectra. (a): UV/vis absorption spectra of 2c/3d/4d and corresponding diphenanthrenes; Fluorescence emission spectra in solutions and films of (b): benzodithiophene 1a/2b-c derivatives, λex=350 nm; (c): and corresponding BDTa-c derivatives, λex=380 nm.

Figure 7 .

 7 Figure 7. Comparison of frontier molecular orbitals and HOMO-LUMO, energy levels and gaps calculated for the methoxy homologs 2-OMe, BDT-OMe, 3-OMe, BDTD-OMe, 4-OMe and CDTO-OMe (from left to right).

Figure 8 .

 8 Figure 8. Scanning electronic microscopy (SEM) images of xerogels or crystals: (a), BDTc crystals in CHCl3/EtOH good/poor solvent; (b,c), BDTDd xerogels in cyclohexane and THF, respectively; (d) CDTOd xerogels in THF. Other SEM images are shown in Figure S7.

Figure 9 .

 9 Figure 9. Transient photocurrent decay curves measured at various electric field of (2-5)×10 4 Vcm -1 on cooling. (a): hole, 4d at 70 °C, LC cell=17.0 μm; (b): hole, CDTOd at 180°C, LC cell=16.4 μm; (c): hole, BDTDd at 220°C; (d): electron, BDTDd at 220°C, LC cell=18.6 μm. The insets show log-log plot and thetransit times were labelled.

Figure 10 .

 10 Figure 10. Temperature dependence of charge carrier mobility. (a) 4d, (b) CDTOd and (c) BDTDd on cooling run (, μe; , μh). DSC determined transition temperatures and corresponding phase states were denoted by straight lines (see TableS2).

Acknowledgement

This research was financially supported by the National Natural Science Foundation of China (NSFC, Fund numbers: 21772135, 51773140, 51973135). BD thanks the CNRS and University of Strasbourg for support.

Author Information

[a] T. Ma

Author Contribution

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

Conflict of Interest

The authors declare no conflict of interest.

Keywords: Mesogen • Columnar • Luminescence • Semiconductor • Gel