University of Groningen

Properties of organic-inorganic hybridsKamminga, Machteld Elizabeth

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Citation for published version (APA):Kamminga, M. E. (2018). Properties of organic-inorganic hybrids: Chemistry, connectivity and confinement.Rijksuniversiteit Groningen.

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8

CHAPTER 8The Role of Hypophosphorous Acid on the Synthesis of

Tin-Based Organic-Inorganic Hybrids

M.E. Kamminga et al., in preparation

Abstract

In this chapter, we synthesized high-quality single crystals of two new compounds: 2,5-dimethylaniline tin iodide organic-inorganic hybrids and 2,5-dimethylaniline triiodide. Weinvestigated the synthesis routes that drive the growth of the different compounds while startingfrom 2,5-dimethylaniline and SnI2. Single-crystal X-ray diffraction reveals that the hybridgrows as a rhombohedral structure, refined in the polar space group R3c, consisting of one-dimensional chains of SnI6-octahedra that share corners and edges to build up a ribbon alongthe [111] direction. The triiodide salt forms a monoclinic structure consisting of linearlycoordinated I3 – units, separated by the organic amines, refined in space group P21/m. Ourfindings give a better understanding of the role of hypophosphorous acid, H3PO2, on theformation of both compounds.

8.1 IntroductionOrganic-inorganic hybrid perovskites, such as CH3NH3PbI3, have attracted growingattention as promising candidates for diverse optoelectronic applications. Thecombination of organic and inorganic components in a single compound leads to aclass of materials that exhibit a large variety of properties. Because of their uniqueoptical [1,2] and excitonic [3,4] properties and electrical [5] and ionic conductivity, [6] variousoptoelectronic applications are reported in literature. These applications include light-emitting diodes, [7,8] lasers, [9,10] photodetectors [11] and efficient planar heterojunctionsolar cell devices. [12–16] However, the best performing organic-inorganic hybrid solarcells are lead-based. Substitution of lead is desired because of its toxicity. [17] Thefeasibility of substituting tin for lead has been studied, [18–21] because they are in thesame group in the periodic table. However, tin has the major disadvantage that Sn2+

can oxidize easily to Sn4+. Still, tin-based hybrid perovskites are reported to haveexcellent mobilities in transistors [22] and can be intentionally or unintentionally dopedto become metallic. [23,24] Furthermore, encapsulation under inert atmosphere allows forthe successful implementation and study of tin-based perovskite solar cell devices. [18,19]

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In this chapter, we synthesize high-quality single crystals of a new tin-based hybridcompound: 2,5-dimethylaniline (abbreviated as 2,5-DMA) tin iodide. The motivationfor synthesizing this compound is that tin-based compounds generally can exhibit goodmobilities and that the introduction of aromatic 2,5-DMA molecules might enhance themobility due to possible π-π stacking. Using single-crystal X-ray diffraction, we findthat 2,5-DMASnI3 grows as a rhombohedral structure, consisting of one-dimensionalchains of SnI6-octahera that share corners and edges to build up a ribbon along the[111] direction. We believe that this compound might have an enhanced exciton lifetime,because the one-dimensional nature supports the separation of charges. Various low-dimensional tin-based hybrid structures have previously been reported. [25] However, wehave synthesized a tin-based hybrid with an alternative structural motif, not reportedbefore.

In addition to the target product, 2,5-DMASnI3, we also successfully synthesizedhigh-quality single crystals of another new compound: 2,5-DMAI3. This triiodide saltforms a monoclinic structure consisting of linearly coordinated I3 – units, separated bythe organic amines. We find that both compounds, 2,5-DMASnI3 and 2,5-DMAI3,form from the same starting compounds. However, we find that the product dependson the experimental conditions. Our findings give a better understanding of the role ofhypophosphorous acid, H3PO2, on the formation of both compounds.

8.2 Experimental Techniques

8.2.1 Crystal Growth of 2,5-Dimethylaniline Tin IodideSingle crystals of 2,5-dimethylaniline tin iodide (2,5-DMASnI3) were grown followingthe synthesis method previously reported by Stoumpos et al. to synthesize methylam-monium tin iodide and formamidinium tin iodide. [20] At first, 2,5-dimethyliodide salt(2,5-DMAI) was synthesized from an equimolar mixture of 2,5-DMA and HI. A syringewas used to slowly add concentrated (57 wt%) aqueous hydriodic acid (Sigma Aldrich;99.95%) to 2,5-DMA (Sigma Aldrich; 99%). The mixture was heated to 70 °C, to removeexcess solvent. The resulting white salt was washed with diethyl ether (Avantor) and driedin air. Subsequently, a 100 mL 3-necked Schlenk flask was charged with 6.8 mL concen-trated (57 wt%) aqueous hydriodic acid (Sigma Aldrich; 99.95%) and 1.7 mL concen-trated (50 wt%) aqueous hypophosphorous acid, H3PO2 (Sigma Aldrich). This mixturewas degassed with argon and kept under an argon atmosphere throughout the experiment.373 mg (1 mmol) SnI2 (Sigma Aldrich; 99%) was added to the flask and dissolved uponheating the mixture to 120 °C using an oil bath, while stirring magnetically. A yellowmixture was obtained. A stoichiometric amount of 1 mmol of 2,5-DMAI salt was addedto the hot solution and dissolved immediately. Next, the solution was kept under contin-uous stirring and slowly evaporated at 120 °C to approximately half its original volume.Then, stirring was discontinued and the mixture was left to cool down to room tempera-ture at a rate of approximately 20 °C/h. Upon cooling, crystals shaped as yellow needleswith a length of approximately 3 mm were obtained. In the rest of this chapter, thissynthesis method is referred to as the Stoumpos method.

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8.3. RESULTS AND DISCUSSION

8.2.2 Crystal Growth of 2,5-Dimethylaniline TriiodideSingle crystals of 2,5-dimethylaniline triiodide (2,5-DMAI3) were grown at roomtemperature, following a modified layered-solution synthesis method previously reportedby Mitzi. [26] In this method, 60 mg (0.16 mmol) SnI2 (Sigma Aldrich; 99%) was addedto 3.0 mL of concentrated (57 wt%) aqueous hydriodic acid (Sigma Aldrich; 99.95%).As SnI2 did not fully dissolve, the precipitate was not transferred into a standard size (18× 150 mm) glass test tube. 3.0 mL of absolute MeOH (Lab-Scan, 99.8%) was carefullyplaced on top of the red-brown SnI2/HI mixture, without mixing the solutions. A sharpinterface was formed between the two layers due to a large difference in densities. 2,5-DMA (Sigma Aldrich; 99%) was added in great excess by adding 15 droplets, using aglass pipette. The test tube was covered with aluminum foil and kept in a fume hood underambient conditions. This method turned out to be very slow. It took more than a monthto grow black/red bar-shaped crystals of up to 3 mm long. Moreover, as we observed thattin was not included in the final product (2,5-DMAI3 was the product formed), we foundthat this elaborate method was not necessary. Simply adding MeOH and 2,5-DMA to thefiltered SnI2/HI mixture, briefly stirring and leaving it in the fume hood under ambientconditions gave the same result. However, using this simplified method, high-qualitycrystals were observed within 24 h. This led us to believe that the evaporation rate wasthe most important parameter. Note that the addition of MeOH is not vital to the formationof 2,5-DMASnI3. It does promote the solubility of the organic component, as it preventsthe formation of 2,5-DMAI. However, the absence of MeOH reduced the evaporationrate, and subsequently, larger crystals of up to around 8 mm long were obtained. Whilewe modified the synthesis method from the original method designed by Mitzi, [26] in therest of this chapter we still refer to this simplified method as the Mitzi method.

8.2.3 X-Ray DiffractionSingle-crystal X-ray diffraction (XRD) measurements were performed using a Bruker D8Venture diffractometer equipped with a Triumph monochromator and a Photon100 areadetector, operating with Mo Kα radiation. A 0.3 mm nylon loop and cryo-oil were usedto mount the crystals. The crystals were cooled with a nitrogen flow from an OxfordCryosystems Cryostream Plus. Data processing was done using the Bruker Apex IIIsoftware and the SHELX97 [27] software was used for structure solution and refinement.

8.3 Results and DiscussionWe explored two different methods to synthesize 2,5-DMASnI3 (2,5-DMA = 2,5-dimethylaniline) and we found that the two techniques gave rise to different products.Using the Stoumpos method, we obtained the tin-based organic-inorganic hybrids, 2,5-DMASnI3, in the form of yellow needles. With the Mitzi method, we obtained very darkred/black bar-shaped crystals that turned out to be the triiodide salt, 2,5-DMAI3. No tinwas observed in this structure. We used single-crystal XRD to study both structures in

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detail. Moreover, we studied the reaction parameters of both methods to understand whatdrives the formation of one compound over the other. This will be discussed below. Atfirst, both crystal structures are described, see Figures 8.1 and 8.1. Crystallographic andrefinement parameters of both compounds are listed in Table 8.1.

Table 8.1: Crystallographic and refinement parameters of 2,5-DMAI3 and 2,5-DMASnI3. Themeasurements are performed using Mo Kα radiation (0.71073 Å). Full-matrix least squaresrefinement against F2 was carried out using anisotropic displacement parameters. Multi-scanabsorption corrections were performed. Hydrogen atoms were added by assuming a regulartetrahedral coordination to carbon and nitrogen, with equal bond angles and fixed distances.

2,5-DMAI3 2,5-DMASnI3temperature (K) 100(2) 100(2)formula C8H12I3N C8H11I3NSnformula weight (g/mol) 502.89 621.58crystal size (mm3) 0.08×0.10×0.16 0.02×0.06×0.22crystal color black/dark red yellowcrystal system monoclinic rhombohedralspace group P21/m (No. 11) R3c (No. 161)symmetry centrosymmetric non-centrosymmetric (polar)Z 2 6D (calculated) (g/cm3) 2.686 2.900F(000) 452 2208a (Å) 9.3195(8) 17.2991(9)b (Å) 6.6052(6) 17.2991(9)c (Å) 11.0657(9) 17.2991(9)α (°) 90.0 117.373(2)β (°) 114.095(3) 117.373(2)γ (°) 90.0 117.373(2)volume (Å3) 621.82(9) 2143.5(4)µ (mm−1) 7.497 8.980min / max transmission 0.380 / 0.585 0.196 / 0.810θ range (°) 3.08-36.39 2.76-27.22index ranges -13 < h < 13 -24 < h < 24

-9 < k < 9 -24 < k < 24-15 < l < 15 -24 < l < 24

data / restraints / parameters 2051 / 0 / 76 4367 / 1 / 110GooF of F2 1.195 1.200no. total reflections 28187 113765no. unique refelctions 2051 4367no. obs Fo > 4σ (Fo) 1983 3746R1 [Fo > 4σ (Fo)] 0.0197 0.0494R1 [all data] 0.0205 0.0683wR2 [Fo > 4σ (Fo)] 0.0512 0.1128wR2 [all data] 0.0518 0.1347largest peak and hole (e/Å3) 0.48 and -2.71 1.61 and -1.76

2,5-DMASnI3Figure 8.1 shows the crystal structure of 2,5-DMASnI3. As listed in Table 8.1, refinementwas done in the polar space group R3c, using rhombohedral settings. The combinationof single-crystal XRD measurements at 100 K and 300 K, and above room temperaturedifferential scanning calorimetry (DSC) and thermogravimetric analysis (TGA) reveal nophase transitions in 2,5-DMASnI3. 2,5-DMASnI3 maintains space group R3c throughoutthe entire temperature range, before decomposition at around 350 K.

While 2,5-DMASnI3 yields the same structural formula as the cubic ABX3 hybridperovskite structure, with A the monovalent organic cation, B the divalent metal and X the

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8.3. RESULTS AND DISCUSSION

Figure 8.1: Crystal structure of 2,5-DMASnI3. (a) Polyhedral model of the full crystal structure,projected along the [111] direction. (b) A single SnI6-octahedron, showing severe distortion. (c)A single inorganic ribbon. Shading represents the strip of SnI6-octahedra that share edges. Thetotal inorganic ribbon consists of three such edge-sharing strips that are connected through corner-sharing.

halide, the structural motif is very different. As shown in Figure 8.1a, the structure of 2,5-DMASnI3 consists of SnI6-octahedra that form one-dimensional chains along the [111]direction. This one-dimensional nature is also apparent from the shape of the crystals. Thecrystals grow as needles and the longest direction corresponds to the [111] direction. As

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CHAPTER 8. THE ROLE OF HYPOPHOSPHOROUS ACID

shown in Figure 8.1, the SnI6-octahedra are tremendously distorted. The three I – Sn – Iangles deviate from a perfect 180° and obtain values of 171.019(1)°, 166.243(1)° and154.760(1)°, respectively. We believe that this distortion is caused by the interactionbetween the NH3+ group and the Sn2+ lone pair. Figure 8.1 shows a single inorganicribbon. Each ribbon consists of three strips of edge-sharing SnI6-octahedra, that areconnected to each other by corner-sharing. This bonding pattern appears rather unusual.The organic molecules form a herringbone-type structure. Therefore, no π-π stacking isobserved. The ammonium groups are oriented towards the inorganic ribbons, to createhydrogen bonding.

We believe that the polar nature of the structure (space group R3c) might aid chargeseparation. However, we also argue that the band gap is probably quite large as the crystalsare yellow in color. Moreover, our previous work on lead iodide-based hybrids (seeChapter 4) has shown that the dimension and connectivity of the metal halide octahedraplays a significant role in the band gap of the compound. [28] The one-dimensional natureand presence of edge-sharing SnI6-octahedra in 2,5-DMASnI3 then contribute to the largeband gap.

2,5-DMAI3Figure 8.2 shows the crystal structure of 2,5-DMAI3. As listed in Table 8.1, refinementwas done in the monoclinic space group P21/m. The combination of single-crystal XRDmeasurements at 100 K and 300 K, and above room temperature DSC and TGA reveal nophase transitions in 2,5-DMAI3. 2,5-DMAI3 maintains space group P21/m throughoutthe entire temperature range, before decomposition at around 410 K. 2,5-DMAI3 is atriiodide salt, and while SnI2 was one of the starting compounds, no tin was observed inthe product. We used energy-dispersive X-ray spectroscopy (EDAX) measurements toprove the absence of tin. As shown in Figure 8.2, the triiodide complexes are ‘dumbbell’-shaped and not connected to each other by shared atoms. Furthermore, the complexesare highly asymmetric: the I1−I2 and I2−I3 distances are 3.1373(4) Å and 2.779(3) Å,respectively. This large asymmetry means that the I3 – unit exhibits strong ionic character.Furthermore, the triiodide ion deviates significantly from linearity with an I1−I2−I3angle of 175.989(2)°, which is close to the mean value for triiodide ions taken from theCrystallographic Open Database (COD) of 178° (see Table 8.2). [29–33] Moreover, linearand symmetrical I3 – ions are generally associated with large cations, in contrast to theasymmetric bent I3 – anions found with small asymmetric or highly charged cations. [34]

Notably, all organic molecules lie parallel to the (101)-plane and hence, parallel to eachother. The distance between two 2,5-DMA molecules corresponds to half a unit cell, i.e.3.3026(6) Å. The molecules are stacked in an off-set manner, but this still means thatthere is some π-π overlap between adjacent rings.

There are several triiodide salts reported in literature. In Table 8.2 we list a selectionof reported triiodide salts with various organic cations. This list is not complete, butbased on all structures found in the COD. Except for 1-ethylpyridinium and 1,2,4-trimethylpyridinium. No crystallographic information files (CIFs) for these compoundswere deposited in the database, but as the organic molecules have a similar size to 2,5-DMA, we manually constructed CIFs from the structural data presented in the papers. [35]

Note that we only considered fully organic cations. Cations containing for example iron

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8.3. RESULTS AND DISCUSSION

Figure 8.2: Crystal structure of 2,5-DMAI3. The hydrogen atoms of the methyl groups are splitover two positions by symmetry and should be considered illustrative only.

or arsenic are excluded from this list. [36,37] Almost all triiodide salts found in the COD arepublished as structure reports. The triiodide salts are often reported as undesired productsformed as byproduct or instead of a target product. Here we show successful growthof both the triiodide salt and the target product, 2,5-DMASnI3, and argue why eitherof the two components form, depending on the experimental conditions. This adds tothe understanding why triiodide salts can form under certain conditions. Furthermore, asmost triiodide salts are reported as structure reports, not much is said about the properties.As shown in Table 8.2, a few compounds have π-π stacking and the compound we presenthere is one of them. The combination of the dark color (see Figure 8.3a) and π-π stackingin our structure led us to believe that the conduction can be large. However, the ioniccharacter of the I3 – unit expects us to believe that the conductivity can be reduced bylimited charge transfer. Investigation of mobilities and theoretical analysis are part offuture research.

As shown in Table 8.2, the appearance of 2,5-DMAI3 is rather dark, indicating arelatively small band gap. The absorption spectrum is shown in Figure 8.3b. The spectrumappears to show some excitonic absorption at around 720 nm. While we argue that theband gap will be in the order of 1.55 eV, it is difficult to extract the exact band gap dueto absorbance that extends beyond 1.55 eV (800 nm). Notably, no emission could bedetected, indicating the likelihood of an indirect band gap.

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CHAPTER 8. THE ROLE OF HYPOPHOSPHOROUS ACID

Table8.2:

Appearance,

I1−I2−

I3angle

andpresence

ofπ-π

interactionof

selectedtriiodide

saltscontaining

anorganic

cation.This

tabledoes

notcoveralltriiodide

saltsin

literature,butisbased

inthe

crystalstructuresfound

inthe

Crystallographic

Open

Database

(CO

D). [29–33]

Note

thatthe

π-π

interactiondistance

isdefined

asthe

distancebetw

eenthe

planesofthe

two

rings,alongthe

stackingdirection.

Thism

eansthatany

off-setstacking

isignored.

organiccation

appearanceI1−

I2−I3

angle(°)

π-πinteraction

a

2,5-dimethylaniline

red/blackbar

175.989(2)yes,3.303

Å2-am

inopyridin-1-ium[38]

orangeplate

176.017(9)yes,3.423

Å(E

)-2-[4-(dimethylam

ino)styryl]-1]methylpyridinium

[39]orange

needle180.0

yes,3.306Å

4-(4-pyridyl)pyridinium[40]

yellow/brow

nprism

176.443(13)yes,3.759

Åtrans-4-[p-(N

,N-diethylam

ino)styryl]-N-m

ethylpyridinium[41]

darkred

prism177.50(2)

yes,3.585Å

4-tert-butylpyridinium[42]

redblock

177.55(3)no

1,3-bis(2,6-diisopropylphenyl)-4,5-dihydro-1H-im

idazol-3-ium[43]

brown

block178.309(18)

no1,4-dim

ethylpyridinium[44]

brown

plate180.0

no6,6,9,9-tetram

ethyl-1,2,5,6,9,9a-hexahydroimidazo[2,1-d][1,2,5]dithiazepin-1-ium

[45]brow

nprism

178.05(4)no

dihydrobis(methylam

ine)borate[46]

red/purpleneedle

179.232(15)no

2-(2-pyridyl)pyridinium[34]

brown

needle177.88(5)

no1-ethylpyridinium

[35]red/black

block177.70(2)

no1,2,4-trim

ethylpyridinium[35]

red/blackblock

179.76(2)no

1,2,3,5-tetramethyl-1H

-pyrazol-2-ium[47]

redprism

177.099(12)no

2,5-dibromopyrazinium

[48]yellow

block180.0

no

aD

istancebetw

eenthe

planesofthe

aromatic

rings.

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8

8.3. RESULTS AND DISCUSSION

Figure 8.3: (a) Photograph of 2,5-DMAI3 single crystals. (b) Absorption spectrum of 2,5-DMAI3single crystal, showing excitonic absorption at around 720 nm.

As stated above, the formation of either 2,5-DMASnI3 or 2,5-DMAI3 directly dependson the synthesis method used. In Table 8.3 we list the differences is experimentalconditions between the two methods. To understand which difference induces thedifferent products, we investigate the effect of all experimental conditions: temperature,state of organic precursor, solvent and atmosphere.

Table 8.3: Outline of both synthesis methods.

Stoumpos method [20] Mitzi method [26]temperature 120° 25°state of organic precursor 2,5-DMAI (s) 2,5-DMA (l)solvents HI and H3PO2 HI (and MeOH)atmosphere argon ambientobtained compound 2,5-DMASnI3 2,5-DMAI3

Effect of TemperatureWe grew 2,5-DMAI3 at room temperature, while we grew 2,5-DMASnI3 at elevatedtemperature. To investigate the effect of temperature, we performed both synthesismethods at the alternate temperatures. The Mitzi method appears to be successful at 120°C. The same product was obtained. However, the crystals contained more imperfectionsand were smaller in size. We reason that this is caused by the rapid evaporation rate at120 °C, which negatively influences the crystal quality. This argument is similar to theargument about the necessity of adding MeOH to the reaction mixture. This is discussedin the Section 8.2. Conversely, the Stoumpos method does not work at room temperature.The main problem here is that the relatively large amount of SnI2 does not fully dissolve atroom temperature, and SnI2 was the only product obtained. Possibly, the optimal relativeconcentrations of organic and inorganic components were not reached or the hybrid didnot nucleate due to the relatively easy growth on existing SnI2 particles. Furthermore, wedissolved both products in EtOH and allowed them to recrystallize at ambient conditions.This appeared successful for both 2,5-DMAI3 and 2,5-DMASnI3. Consequently, both

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products are stable and the driving force for formation of either of the two componentslies in the initial formation of the product. Thus, we find that the elevated temperature isimportant for the growth of 2,5-DMASnI3, but as 2,5-DMAI3 can also form at the sametemperature, this is not the crucial difference between the two methods.

Effect of the State of the Organic PrecursorIn the Stoumpos method, 2,5-DMA is added as a pre-made 2,5-DMAI salt, while in theMitzi method, 2,5-DMA is directly used. Here we explored the necessity of forming the2,5-DMAI salt before continuation of the synthesis process. We found that this step can beremoved from the synthesis procedure. The salt immediately dissolves in HI and addingthe salt, instead of the solution, only adds more iodine to the reaction mixture, while thereis already a great excess of HI. For the Mitzi method, the use of pre-made 2,5-DMAI salt,instead of 2,5-DMA as solution, also did not influence the product formed. This methodis also based on an excess of HI, therefore no change is observed. Thus, the state of theorganic component does not influence the formation of the final product of either of thesynthesis methods.

Effect of Atmosphere and Hypophosphorous AcidDespite the fact that both 2,5-DMAI3 and 2,5-DMASnI3 are stable in air for at least 24hours, both synthesis methods are performed under different environmental conditions.The Stoumpos method was performed under inert atmosphere, while the Mitzi methodwas performed under ambient conditions. Notably, when the alternate atmosphere wasused, both synthesis methods failed. The Stoumpos method failed in ambient atmosphere,as no 2,5-DMASnI3 was formed. The Mitzi method also failed to give any product underinert environment. However, when the reaction mixture was exposed to air, 2,5-DMAI3formed within one hour. This leads to the conclusion that oxygen plays a crucial role in theformation of the triiodide salt. While Sn2+ easily oxidizes to Sn4+, neither was observedin the final product 2,5-DMAI3. Notably, HI is also not very stable in air. The iodide ions,I– , can be oxidized to iodine, I2. This is the reason why H3PO2 should be introduced. [49]

H3PO2 is a reducing agent that brings I2 back to I– . Consequently, the 2,5-DMASnI3 canbe formed with the Stoumpos method. In case of the Mitzi method, the presence of ambientair and lack of any reducing agent oxidizes a significant amount of I– to I2, which in turnreacts with I– to form the reactive triiodide complex: I3 – . This triiodide easily reactswith the organic component to form the triiodide salt, 2,5-DMAI3. Thus, the additionof H3PO2 is crucial for the synthesis of the 2,5-DMASnI3 hybrid. However, H3PO2 isnot the only experimental requirement for the synthesis of the hybrid. As stated above,the reaction temperature is crucial for the formation of the hybrid as well. Additionalexperiments showed that adding H3PO2 to the reaction mixture used in the Mitzi method(25 vol%) still produced 2,5-DMAI3 when exposed to air. The main difference is thatit took significantly longer to grow the crystals. Thus, in order to grow 2,5-DMASnI3hybrids, H3PO2, inert atmosphere and elevated temperatures is required. Leaving out anyof these three conditions does not give any product or gives 2,5-DMAI3. In order to grow2,5-DMAI3, the key experimental condition is the absence of inert atmosphere.

Peculiar to the Mitzi method is the fact that no triiodide salt forms when differentorganic molecules are used. We have tried this method with several organic moieties,

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8.3. RESULTS AND DISCUSSION

including benzylammonium and 2-thiophenemethylammonium, but none gave anyproduct. As shown in Table 8.2, several organic triiodide salts have been successfullymade, but we believe that it is not possible to implement every organic cation. We thinkthat 2,5-DMA has a more favorable size and shape for inclusion in a triiodide salt. Whilebenzylammonium and 2-thiophenemethylammonium have a relatively long shape withrespect to the ammonium group, 2,5-DMA has a more wide shape. The widest span in2,5-DMA is between the two methyl groups and corresponds to 5.8545(3) Å. Notably, thespan of the I3 – complex in 2,5-DMAI3 is 5.9088(3) Å, which is very similar. We thinkthat growth of 2,5-DMAI3 is favorable over some other XI3 salts, with X being an organiccation, as both building blocks are similar in size.

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8.4 ConclusionsIn conclusion, we have synthesized and investigated the crystal structures of twonew compounds: 2,5-dimethylaniline tin iodide organic-inorganic hybrid and 2,5-dimethylaniline triiodide. Starting from 2,5-dimethylaniline and SnI2, we haveinvestigated the experimental conditions that drive the formation of the hybrid and thetriiodide salt. Our findings reveal that the hybrid only grows at elevated temperatures,under inert atmosphere and with the addition of hypophosphorous acid, H3PO2. Leavingout any of these three conditions does not give any product or an alternative compound.Crucial for the growth of the triiodide salt is the absence of inert atmosphere. As HI isnot very stable in air, the iodide ions, I– , can easily oxidize to iodine, I2. This happensin presence of ambient air and lack of any reducing agent, such as H3PO2. As a result,a significant amount of I2 will be formed, which can react with I– , to form the reactivetriiodide complex: I3 – . This triiodide complex then easily reacts with the organic moietyto form the triiodide salt. Our result shows an alternative structural motif for organic-inorganic hybrids with structural formula ABX3. Moreover, our findings add to theunderstanding of how experimental conditions drive the formation of different productswhile starting from the same precursors.

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8.4. CONCLUSIONS

The Role of Hypophosphorous Acid on the Synthesis of Tin-Based Organic-Inorganic HybridsM.E. Kamminga, M. Gélvez-Rueda, S. Maheshwari, I.S. van Droffelaar, J. Baas, G.R.Blake, F.C. Grozema & T.T.M. Palstra, in preparation.

Author contributions: M.E.K. and T.T.M.P. conceptualized and designed the experi-ments. M.E.K. and I.S.v.D. performed the experiments with assistance of J.B. M.E.K.and G.R.B. studied the structural phase transition in great detail. M.G.-R., S.M. andF.C.G. performed pulse-radiolysis time-resolved microwave conductivity measurementsand calculations (not in this thesis). M.E.K., G.R.B. and T.T.M.P. discussed the overallconclusions of the work. M.E.K. composed the manuscript. Everybody reviewed themanuscript and was involved in the final discussions.

Acknowledgments: M.E.K. was supported by The Netherlands Organisation for ScientificResearch NWO (Graduate Programme 2013, No. 022.005.006). We thank H.-H. Fang forthe absorption measurements and S. Faraji for stimulating discussions.

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