Synthesis, Structural, Linear, and Nonlinear Optical Studies of Inorganic-Organic Hybrid Semiconductors (R-C6H4CHCH3NH3)2PbI4, (R = CH3, Cl).

Synthesis, crystal structure, and optical properties of two-dimensional (2D) layered structurally slightly different inorganic-organic (IO) hybrid semiconductors (R-C6H4C2H4NH3)2PbI4 (R = CH3, Cl) are presented. They are naturally self-assembled systems where two (RNH3)+ moieties are sandwiched between two infinitely extended 2D layers of the [PbI6]4- octahedral network and treated as natural IO multiple quantum wells. While the former compound crystallizes into an orthorhombic system in the Cmc21 space group, the latter crystallizes into a monoclinic system in the space group P21/c. As a thin film, they are well-oriented along the (l00) direction. Both single crystals and thin films show strong room-temperature Mott type exciton features that are highly sensitive to the self-assembly and crystal packing. Linear (one-photon) and nonlinear (two-photon) optical probing of single crystals for exciton photoluminescence imaging and spectral spatial mapping provide deep insight into the layered re-arrangement and structural crumpling due to organic conformation. The strongly confined excitons, within the lowest band gap of inorganic, show distinctly different one- and two-photon excited photoluminescence peaks: free excitons from perfectly aligned 2D self-assembly and energy down-shifted excitons originated from the locally crumpled layered arrangement. Their structural aspects are successfully presented with proper correlation that emphasize various differences in physical and optical properties associated between these novel IO hybrids.

Introduction

Inorganic–organic (IO) naturally self-assembled hybrid systems have been a fascinating subject of interest for many years because of their strong room-temperature excitons with high oscillator strength,[1−3] interesting structural features,[4−11] magnetic properties,[12] and promising electrical aspects.[13−17] The self-assembled systems from the class of (RNH3)2MX4 (R = organic) are special and considered as natural multiple quantum well (MQW) structures where two (RNH3)+ moieties are sandwiched between two infinitely extended layers of [MX6]4– octahedral. These two-dimensional (2D) IO hybrids show strong Mott type excitons within the MX6 network. As a result of dielectric contrast and quantum confinement-related effects,[18] the exciton binding energies are enhanced larger than 200 meV.[19] The band gap tunability can be conveniently achieved from ∼2 to ∼3.2 eV by changing divalent metal halide (MX2) inorganic entities alone (where M = Pb2+, Mn2+, Sn2+, Ge2+, Eu2+, Cu2+, Ni2+, Fe2+, and X = F−, Cl−, Br−, I−).[20−26] Such synergy between inorganic and organic has produced a great promising effect that led to the demonstration of many applications, namely, LEDs,[27] FETs,[28−30] solar cells,[31−38] polariton lasers,[39,56−59] optical switches,[40,41] electro-optical modulators,[42] and so on.

While the 2D crystal packing from RMX3-type group entities is the most common, the organic moiety (R–NH3)+ conformation within the MX2 network may lead to other dimensionalities depending on the type of the organic moiety, size, shape, and amino group location.[43−48] Recently, it is well explored that the structural influence of the organic moiety in 2D IO hybrids (RNH3)2PbI4 (R = organic) directly influences the Pb–I–Pb crumpling angles and eventually band gap changes which directly reflects in the exciton energies.[23,39−41] In this communication, we have studied the synthesis, crystal structure, and strong room-temperature exciton features in the 2D IO hybrid semiconductors, (CH3–C6H4–C2H4NH3)2PbI4 (MPEPI) and (Cl–C6H4–C2H4NH3)2PbI4 (CEPI) (Table ). For both, the crystallographic, thermal, and optical studies are systematically performed and presented here. A critical comparison of MPEPI and CEPI is reported in this paper, emphasizing significant differences in the exciton features, resulted out of these similar organic moieties conformation within the inorganic network. Here, we focus on linear (one-photon induced, ℏω ≥ Eg) and nonlinear (two-photon-induced, 2ℏω ≥ Eg) optical excitations to study various possible exciton energies that are possible in these 2D-aligned single crystals.[49] One-photon absorption (1PA, ℏω ≥ Eg) due to high absorption coefficients results into quite small penetration depths (∼1/α0),[50] therefore, conventional photoluminescence (1PA-PL) provides information only from the near-surfaces and are influenced by many unwanted effects such as background emission, photo-bleaching, ablation, laser heating effects, and so forth.[51] On the other hand, two-photon absorption (2PA) is a well-known third-order nonlinear effect which utilizes two infrared photons (2ℏω ≥ Eg) and undergoes an electronic transition from the ground state to the excited state through an intermediate virtual state within the band gap.[52] Such below-band gap nonlinear excitation is having a much larger penetration depth and therefore can probe into many nonradiative (dark) and radiative states, that may not be invoked through conventional excitation. Thus, the comparative spectral and spatial imaging studies of 1PA-PL and 2PA-PL help us to analyze how even the slightest structural re-organization shows a significant difference in the optical features.

Table 1

Empirical Names and Chemical Formulae of the Synthesized IO-Hybrids

s. no.	empirical name	chemical formula
1	1-(4-chlorophenyl) ethylammonium tetraiodoplumbate (CEPI)	(Cl–C6H4–C2H4NH3)2 PbI4
2	1-(4-methylphenyl) ethylammonium tetraiodoplumbate (MPEPI)	(CH3–C6H4–C2H4NH3)2 PbI4

Results and Discussion

Crystal Packing and Structural and Thermal Features

CEPI crystallizes into the monoclinic system in the space group P21/c, whereas MPEPI crystallizes into the orthorhombic system in Cmc21 space group. The unit cell dimensions of MPEPI are a = 32.544 Å; b = 9.316 Å; c = 8.6028 Å and α = β = γ = 90°; on the other hand, the cell dimensions of CEPI are a = 16.200 Å; b = 9.2695 Å; c = 8.6314 Å and β = 96.738°. For MPEPI, the Pb–I–Pb bond angle is 147.27°; I–Pb–I bond angles are 84.17, 90.98, 95.86, and 88.2°; Pb–I bond lengths are in the range of 3.159 and 3.336 Å; C–C bond lengths are in the range of 1.350 and 1.536 Å; the C–N bond length is 1.529 Å. In case of CEPI, the Pb–I–Pb bond angle is 153.72°; I–Pb–I bond angles are 93.83, 86.14, 85.48, and 94.52°; Pb–I bond lengths are 3.285 and 3.232 Å; C–C bond lengths are in the range of 1.360–1.511 Å; the C–Cl bond length is 1.747 Å; and the C–N bond length is 1.507 Å. Both the structures consist of 2D alternate layers of PbI6 and organic ammonium. This alternate layer fashion contains a double layer of protonated organic ammonium cations which are being sandwiched between infinitely extending PbI6 sheets as shown in Figure . Layers are infinetely extended by corner-sharing distorted PbI6 octahedra with the neighboring four octahedra through double-bridging iodine atoms. The complete crystallographic data of both MPEPI and CEPI are given in the Supporting Information (Table S1).

Figure 1

(a,b) Crystal packing of MPEPI and CEPI showing layer separation and Pb–I–Pb bond angle, respectively. (Hydrogens are omitted for clarity).

The separation between the infinitely extended lead iodide sheets is found to be 16.92 and 17.19 Å in MPEPI and CEPI, respectively. This difference can be attributed to the difference in the π–π and C–H−π interactions between the phenyl rings having different chemical environments around them. In both the crystal structures, the PbI6 octahedra are slightly tilted because of in-plane Pb–I–Pb bond crumpling. This is observed to be ∼153.7 and ∼147.27° in CEPI and MPEPI, respectively (Figure ). The difference in Pb–I–Pb bond crumpling in the MPEPI is being compressed more compared to the previous one due to difference in the steric effects between the organic ammoniums of both. The double layer of organic in between the lead iodide planes is oriented in different fashions in CEPI and MPEPI. In CEPI-protonated ammonia of two layers connects to opposite sides, and the organic is inclined at an angle of ∼62.42° as in the case of another IO hybrid, DDPI (NH3(CH2)12NH3PbI4, Dodecyl ammonium lead iodide) where the inclination angle is ∼45°.[3] Unlike CEPI, in MPEPI-protonated ammonium connects to the same side and the net alignment of the double layer of the organic ammonium is vertically oriented, but the organics of the upper and bottom layers seem to be crossing each other, whereas in the case of CEPI, they are parallel. A view of the double layer of organic alone is presented in Figure S1. It shows that both MPEPI and CEPI contain each single layer of organic with organics oriented in two different directions (Figure S1a,b). The double-layer view is even more clearly visible when it is seen along the c-axis (Figure S1c,d).

In both CEPI and MPEPI, the bonding between the organic and inorganic layers plays a crucial role in enhancing the mechanical and thermal stability of these materials. The protonated amine of the organic interacts with three iodines through N···H–I hydrogen bonding interaction in right-angled and terminal halide bonding configurations as shown in Figure . This sort of bonding helps in rigid confirmation of the sensitive organic part of the IO hybrid system.

Figure 2

(a,b) Showing terminal halide and right-angled triangle bonding configuration in both MPEPI and CEPI, respectively.

It is interesting to compare with the previous studies[25] of similar hybrids formed by organic moieties, 4-X–C6H4NH2, where X = Cl or CH3O. When chloro-aniline is substituted as the organic moiety, the organic–inorganic hybrid assembles as a 2D-layered network [(Cl–C6H4NH3)2PbI4], whereas for methoxy–aniline, the resultant network is composed of 1D PbI ribbons as [(CH3O–C6H4NH3)2Pb3I8·2H2O]. This signifies that the crystal packing is critically dependent on the nature and the shape of the guest moiety as well as the nature of the substituents.

The thin-film glancing angle X-ray diffraction (GAXRD) pattern shows strong orientation in both MPEPI- and CEPI-layered sheets along (2l00) and (00l) (l = 1,2,3..) directions, respectively, as shown in Figure . The corresponding d-spacing in MPEPI and CEPI are 16.25 and 16.89 Å, respectively, which are the same as observed from the single crystal diffraction analysis. As shown in Figure , the thin-film XRD is coinciding with the observed powder XRD extracted from the crystallographic information.

Figure 3

(a,b) are the thin film glancing angle XRD (GAXRD) of MPEPI and CEPI (blue) and powder XRD (black) extracted from single crystal XRD data with the orientation on the substrate along (0,0,l) and (2l,0,0) (l = 1,2,3...) planes, respectively.

The thermogravimetry analysis (TGA) and the derivative thermogravimetry (DTG) curve of MPEPI (Figure a), the temperature up to which it is stable is observed to be 231 °C, suggests that both are thermally stable up to 240 °C with organic decomposed with a weight loss of 49.95%. After this, only PbI2 remains in the sample. In case of CEPI (Figure b), it is suggested that it is thermally stable up to 240.6 °C. Below this temperature, the intermediate organic is stable and a weight loss of 50.48% (which is 1.893 mg out of 3.75 mg) is observed with further increase in temperature. This weight loss suggests complete decomposition of organic, leaving PbI2 alone behind. The thermal stability suggests that both these hybrids can be used for device operations even up to 200 °C. Beyond this temperature, organic completely decomposes with a weight loss of 52.66% (which is 2.94 mg out of 5.7487 mg), leaving PbI2 behind.

Figure 4

(a,b) shows the TGA (red) and DTG (blue) curves of MPEPI and CEPI, respectively.

Optical Absorption and PL Features of IO Hybrid Thin Films

These 2D IO hybrids are naturally self-assembled into alternative stacks of the PbI6 infinitely extended network, where organic moieties are conformed within the layer spacing (Figure ). Typically, the inorganic layers are of ∼6 Å thickness and the organic spacers are 16–18 Å, depending on the shape/size of the organic moiety. This layered structure is considered to be natural MQWs, wherein the band gap contrast values between the inorganic and organic layers are ∼3 and ∼6 eV, forming “well” and “barrier”, respectively. Figure a,b shows strong room-temperature absorption and PL spectra of both the MPEPI and CEPI thin films, respectively. The absorption spectrum of CEPI shows a broad peak at ∼381 nm and a strong absorption peak at ∼498 nm. The former peak is attributed to the charge transfer between the HOMO of organic to that of the conduction band of inorganic. The sharp and narrow absorption (∼498 nm) is attributed to the Mott type exciton, confined within the inorganic lowest band gap. The corresponding exciton PL peak is found to be at ∼510 nm with a fwhm of ∼16 nm. In the case of MPEPI, the exciton strong absorption peak is at 494 nm and the corresponding PL peak is found to be at 506 nm with a fwhm of 18 nm.

Figure 5

(a,b) shows the absorption and PL spectra of MPEPI and CEPI thin films respectively.

As mentioned in the previous sections, the optical exciton features in these 2D-layered IO hybrids are strongly influenced by the characteristic features of the organic moiety, which eventually results into structural rearrangements of the PbI network. From Figure , it is evident that each lead (Pb) atom is connected to six iodine (I), four bridging iodines to build up the network extension, and two terminal iodines bonded to organic through hydrogen bonding. Thus, the Pb–I–Pb bond angle is an indicative of corner-sharing of PbI6 octahedra that are distorted. In our previous communications,[23,40] a strong correlation between the Pb–I–Pb angles to that of exciton energies is established. It was observed that there is systematic correlation between structural rearrangements of the PbI octahedral extended network to that of the associated band gap. In these 2D naturally self-assembled quantum wells, the lowest band gap is from the inorganic network (“well”); therefore, any structural crumpling in the Pb–I–Pb extended network is directly reflected on the electronic band structure. It has been established from both the experimental and electronic band structure calculations that more crumpling of the PbI network (deviation from planar structure 180°) results into a higher band gap (and corresponding exciton energies).[23,40] In the present case, the Pb–I–Pb angles are observed to be ∼153.7 and ∼147.27° for CEPI and MPEPI, respectively (Figure ). Therefore, it is obvious that the more crumpled (MPEPI) system shows UV side shift compared to the less crumpled CEPI system. If we compare with other 2D IO hybrids published previously,[23] (4-ClC6H4NH3)2PbI4 (4-chloroanilinium tetraiodoplumbate, CAPI) is having a relatively more crumpling Pb–I–Pb angle (143.01°) with the corresponding PL peak at 484 nm, while (C6H5C2H4NH3)2PbI4 (phenyl ethylammonium tetraiodoplumbate, PAPI) is having a more planar arrangement with the Pb–I–Pb angle (155.43°) with red-end emission at 523 nm.

High-Resolution PL Imaging and Spectral Spatial Mapping of IO Hybrid Crystals

Apart from structural variations, excitons from these 2D IO hybrids are highly sensitive to thickness. Typically, 2D alignment sustains to a thickness around 120 nm, beyond which crystal packing gets crumpled because of heaviness. Therefore, probing the crystal would be of worth to see the effect of crystal packing. However, probing with conventional excitation, above the band gap (ℏω ≥ Eg, 1PA-PL), is limited by the penetration depth (1/α0) which is few 100 s of nanometers, whereas, nonlinear pumping, that is, two- (or more) photon excitation (2ℏω ≥ Eg, 2PA-PL) probes much deeper depths and gives a comprehensive idea of several excitons that are clearly PbI network order-/disorder-dependent.[23,24]

Figure presents the 1PA-PL and 2PA-PL spectra of CEPI crystals. Here, for 1PA-PL excitations, 400 nm CW and 400 nm fs1 lasers and for 2PA-PL 800 nm, fs1 laser is used. The deconvoluted 1PA-PL spectra for both CW and fs1 400 nm excitations (Figure a,b) show three types of emission peaks: the strong and narrow dominant peak at 512 nm (fwhm ≈ 15 nm), a shoulder at 541 nm (fwhm ≈ 20 nm), and a broad weak shoulder at 585 nm (fwhm > 60 nm). The strong emission features at 512 nm is attributed to free excitons (PLFE) which originates from the near-top surface. The shoulder peak at 541 nm is attributed to crumpled excitons (PLCE) and the broad emission at 585 nm is for defect induced broad emission (PLdef), respectively. In general, both the spectra (Figure a,b) are dominated by the strong green emission at 512 nm and a shoulder at 541 nm.

Figure 6

(a) shows 1PA-PL spectra of CEPI crystal when excited by 400 nm CW laser. The spectra are deconvoluted into free exciton (PLFE) at 512 nm, crumpled exciton (PLCE) at 541 nm, and defect-induced emission (PLdef) at 585 nm. Similarly, (b,c) shows 1PA-PL and 2PA-PL spectra under 400 and 800 nm (from fs1 laser) excitations, respectively.

Figure 7

(a) (i–iv) 1PA-PL crystal image and spatial mapping of CEPI crystal under 400 nm CW excitation (at power < 1 mW). (b) (i–iv) 1PA-PL crystal image and spatial mapping of CEPI crystal under 400 nm fs1 excitation at P = 400 μW, I = 0.1 GW/cm2. (c) (i–iv) 2PA-PL crystal image and spatial mapping of CEPI crystal under 800 nm fs1 excitation at average power = 50 mW (I = 4 GW/cm2). Spatial intensity images correspond to the identified peak positions (see Figure ).

However, in 2PA-PL (excited by 800 nm fs1 laser), the spectrum shows dominance of PLCE (at ∼541 nm) with a shoulder peak at ∼570 nm (PLdef). To probe more into the details, we have also recorded spatial and spectral imaging of CEPI crystals for 1PA and 2PA excitations (Figure ) with a modified microscope. As one can see in Figure , the crystal PL images show intense green color appearance when excited by 400 nm (1PA-PL) or 800 nm (2PA-PL) lasers, which is the characteristic of their highly emitting nature. Spatial spectral mapping was also performed for CEPI crystal and peak intensities of respective peak position mappings are presented in (ii–iv). In these soft 2D materials, three types of emission centers are co-existing: (a) high-lying free-excitons (PLFE), (b) energy down-shifted crumpled excitons (PLCE), and (c) defect-induced emission (PLdef). However, observation of these emissions is strongly dependent on excitation intensities and pulse widths, apart from material-related properties such as crystal packing, thickness, temperature, and so forth.[23,24,60,61] The peak maxima spatial maps corresponding to free excitons (PLFE ≈ 512 nm), crumpled excitons (PLCE ≈ 541 nm), and defect-induced emission (PLdef ≈ 585 nm) are shown in Figure . The 2PA-PL spectral spatial mapping under 800 nm fs1 excitation shows dominating emission from crumpled excitons (PLCE) along with relatively low-intense shoulder from defect-induced emission (PLdef) [Figure c(ii–iv)]. This confirms that one-photon excitation probes free excitons (1PA-PL) related to top few nearly perfectly aligned layers, whereas two-photon excitation probes overall crystal (because of larger penetration depth), and the 2PA-PL is dominated by red-end PLCE, which is related to excitons confined within the low-lying thickness-distorted (crumpled) bond angles within the (PbI4)2– network. Throughout the crystal, the presence of free-exciton emission (PLFE) is almost insignificant. This clearly supports that under the two-photon excitation, the free-exciton level acts as a nonradiative center to facilitate the emission from low-lying crumpled exciton level (PLCE). In general, one-photon excitation resembles typical optical absorption, whereas the two-photon excitation process represents only those possible based on nonlinear two-(or more) photon excitation selection rules. Also, other nonlinearities (such as exciton–exciton annihilation, two-photon absorption, free-carrier absorption, nonlinear refractive index, and so forth.) possibly affect the excitation/deactivation processes. In other 2D direct band gap materials, several diverse radiative (bright) and nonradiative (dark) site exciton states are reported and discussed in detail.[60,61] In principle, under conventional excitation and room-temperature conditions, such exciton states are undetectable and easily dissociate and broaden because of their small binding energies. Essentially, the Mott type excitons from these soft 2D-layered hybrids are highly sensitive to the self-assembly process, inorganic network distortions, and thickness and interlayer distortions. Therefore, the downshifted 2PA-PL is from the excitons related to the deep-down thickness-distorted (crumpled) bond angles within the (PbI4)2– network.

Similarly, we have recorded 1PA-PL and 2PA-PL crystal spectra and respective high-resolution PL images and spectral spatial mappings (Figure ) for MPEPI. Similar to the case of CEPI, the 1PA-PL spectra clearly demonstrate free excitons (PLFE ≈ 503 nm), while the 2PA-PL is dominated by crumpled excitons (PLCE ≈ 531 nm) (Figure ). The spectral peak position differences between CEPI and MPEPI, as per the crystal structure variation, are consistent as discussed previously. It is to be noted that the spectral/mapping/images of crystal bits are of variable surface heights. Therefore, the spectral deviations are correspondingly different. In general, the locations of PLFE are quite different from PLCE. Throughout the crystal, the defect emission (PLdef) is relatively weak but uniformly present throughout the crystal area.

Figure 8

(a) (i–iv) 1PA-PL crystal image and spatial mapping of MPEPI crystal under 400 nm CW excitation at power < 1 mW. (b) (i–iv) 1PA-PL crystal image and spatial mapping of MPEPI crystal under 400 nm fs1 excitation at P = 400 μW, I = 0.1 GW/cm2. (c) (i–iv) 2PA-PL crystal image and spatial mapping of MPEPI crystal under 800 nm fs1 excitation at average power = 50 mW (I = 4 GW/cm2). Spatial intensity images correspond to the identified peak positions (see Figure ).

Figure 9

(a) shows 1PA-PL spectra of the MPEPI crystal when excited by 400 nm CW laser. The spectra are deconvoluted into free-exciton (PLFE) at 503 nm, crumpled exciton (PLCE) at 531 nm, and defect-induced emission (PLdef) at 560 nm. Similarly (b,c) show 1PA-PL and 2PA-PL spectra under 400 and 800 nm fs1 excitation, respectively [for 400 fs1, P = 400 μW, (I = 0.1 GW/cm2). For 800 nm fs1, the average power is 50 mW (I = 4 GW/cm2)].

In conventional excitation (one-photon), PL is directly related to the absorption coefficient (α0), following the Beer–Lambert law as IPL ∝ I0e(−α where I0 is the excitation intensity and t is the thickness, whereas in the case of femtosecond pulsed laser excitation, such large excitation intensities (in the order of GW/cm2) and the resultant PL intensities behave nonlinearly and the 2PA-PL intensity (I2PL) is related as I2PL ∝ I0e[−(α. Here, the laser intensity threshold limited third order nonlinear absorption (β) is related to two- (or multi) photon absorption, saturation of absorption and excited state absorption, and so on.[52] Further, during the femtosecond pulse excitation, the population of excited states is relatively fast enough and radiative, and nonradiative processes are critically dependent on the pulse duration and the excited state lifetimes.

Conclusions

Two new 2D IO-layered hybrid semiconductors (R–C6H4C2H4NH3)2PbI4 (R = CH3, Cl) are successfully synthesized and their crystal structures have been analyzed. Both are self-assembled into alternative stacks of a two-dimensionally extended PbI6 network, where the two organic moieties are interdigitized as spacers. These single crystals are thermally much stable for more than 200 °C. As a virtue of dielectric contrast and quantum confinement-related effects, these 2D IO hybrid semiconductors show strong room-temperature Mott type exciton PL with high binding energies greater than 200 meV and the order of magnitude greater than their parent PbI2 (∼23 meV). In the thin-film form, they are perfectly oriented along the c-axis. The strong room-temperature exciton emission features are highly sensitive to the local environment and crystal packing. The spectral, imaging, and spatial mapping studies of single crystals revealed many interesting details about their self-assembly, crystal packing, and local environment. The comparison between the linear and nonlinear optical excitation imaging and spatial mapping of single crystal studies reveal the fact that two different types of excitons are present in these systems. The conventional one-photon absorption-induced PL (UV-excited, Eexe = ℏω ≥ Eg, 1PA-PL) probing is restricted by shorter penetration depths (1/α0) and reveals free-exciton emission originated from perfectly aligned 2D self-assembly from the top few layers, whereas the two-photon absorption-related excitation (infrared excited, Eexe = 2ℏω ≥ Eg, 2PA-PL) probes much deeper depths of the crystal and demonstrates entirely different excitons, which are energy down-shifted excitons, originated from the locally crumpled layered arrangement. The room-temperature exciton features and one- and two-photon imaging/spatial mapping studies further pave way to explore these stable 2D IO semiconductors for many new optoelectronic applications.

Experimental Section

MPEPI and CEPI are prepared by conventional solution processing techniques.[23,25,48] In both the cases, stoichiometric quantities of organic [1-(4-methylphenyl) ethylamine or 1-(4-chlorophenyl) ethylamine] and lead (II) iodide (PbI2) were taken in HI (55%) separately and stirred thoroughly to get clear solutions. These two solutions were mixed together slowly with constant stirring. The resultant precipitation is separated and dried properly. Both MPEPI and CEPI are dissolved in the acetonitrile solvent to get a saturated solution. Slow evaporation of saturated solution left dark yellow-colored crystal platelets and pale yellow rodlike crystals for CEPI and MPEPI, respectively. Suitable crystals are selected for single crystal analysis and optical studies.

Single crystal X-ray diffraction studies were carried out on BRUKER AXS SMART-APEX diffractometer with a CCD area detector (Mo Kα = 0.71073 Å, monochromator = graphite).[53−55] The thin films of MPEPI and CEPI are prepared by the spin-coating technique (typically at 3000 rpm resulting in the uniform film thickness of 120 nm), dissolving crystals in the acetonitrile solvent at a temperature slightly below room temperature. These thin films are used for glancing angle X-ray Diffraction (GAXRD) using Cu Kα (1.5418 Å) radiation. Absorption spectra are recorded using a Shimadzu UV-VIS-NIR3600 spectrometer. Thermo gravimetry (TG) was carried out between 25 and 800 °C at a scan rate of 5 °C/min under N2 atmosphere using Al2O3 as the reference material. One- and two-photon absorption-induced photoluminescence (1PA-PL and 2PA-PL) was performed using 400 nm CW diode laser and high-intensity femtosecond Ti:Sapphire mode-locked oscillator laser (Spectra-Physics; MaiTai, tunable between 690 and 1040 nm, repetition rate = 84 MHz and pulse duration = 120 fs.). For 1PA-PL excitation, the sources are either from 400 nm CW diode laser or 400 nm (frequency doubled 800 nm) fs1 laser. High-resolution PL images, PL spatial spectral mapping of CEPI and MPEPI single crystals were carried out using an optical upright microscope (BX51, Olympus) by using above lasers. Spatial mapping was carried out by a controlled motorized X–Y stage attached with the microscope. The fiber optic-coupled spectrometer (Ocean Optics; QEPro) is used to collect the spectra through appropriate high and low pass filters.