Chemistry of Cyclo[18]Carbon (C18): A Review.

Carbon-based allotropes are propelling a technological revolution in communication, sensing, and computing, concurrently challenging fundamental theories of the previous century. Nevertheless, the demand for advanced carbon-based materials remains substantial. The crux lies in the efficient and reliable engineering of novel carbon allotrope. Although C18 has undergone theoretical and experimental investigation for an extended period, its preparation and direct observation in the condensed phase occurred only recently through STM/AFM techniques. The distinctive cyclic ring structure and the dual 18-center π delocalization character introduce various uncommon properties to C18, rendering it a subject worthy of in-depth exploration. In this context, this review delves into past developments contributing to the state-of-the-art understanding of C18 and provides insights into how future endeavours can expedite practical applications. Encompassing a broad spectrum, this review comprehensively investigates almost all facets of C18, including geometric characteristics, electron delocalization, bonding nature, aromaticity, reactivity, electronic excitation, UV/Vis spectrum, intermolecular interaction, response to external fields, electron affinity, ionization, and other molecular properties. Moreover, the review also outlines representative strategies for the direct synthesis and characterization of C18 using atom manipulation techniques. Following this, C18-based complexes are summarized, and potential applications in catalysis, electrochemical devices, optoelectronics, and sensing are discussed.

Bispericyclic Ambimodal Dimerization of Pentafulvene: The Origin of Asynchronicity and Kinetic Selectivity of the Endo Transition State.

The propensity of fulvenes to undergo dimerization has long been known, although the in-depth mechanism and electronic behavior during dimerization are still elusive. Herein, we made an attempt to gain insights into the reactivity of pentafulvene for Diels-Alder (DA) and [6 + 4]-cycloadditions via conventional and ambimodal routes. The result emphasizes that pentafulvene dimerization preferentially proceeds through a unique bifurcation mechanism where two DA pathways merge together to produce two degenerate [4 + 2]-cycloadducts from a single TS. Despite the [6 + 4]-cycloadduct being thermodynamically preferred, [4 + 2]-cycloaddition reactions are kinetically driven. Singlet biradicaloid is involved in through-space 6e- delocalization as a secondary orbital interaction that originates asynchronicity and stabilizes the bispericyclic transition state (TS). The transformation of various actively participating intrinsic bonding orbitals (IBOs) unambiguously forecasts the formation of multiple products from a single TS and rationalizes the mechanism of ambimodal reactions that are rather difficult to probe with other analyses. The changes in active IBOs clearly distinguish the conventional reactions from bifurcation reactions and can be employed to characterize and confirm the ambimodal mechanism. This report gains a crucial theoretical insight into the mechanism of bifurcation, the origin of asynchronicity, and electronic behavior in ambimodal TS, which will certainly be of enormous value for future studies.

Orbital and free energy landscape expedition towards the unexplored catalytic realm of aromatically modified FLPs for CO2 sequestration.

The emergence of frustrated Lewis pairs (FLPs) has created a whole new dimension in the development of metal free catalysts for CO2 sequestration. Efforts have been made to enhance the catalytic activity of the FLPs. The aromatic modulation of the catalytic sites has been successfully demonstrated to enhance the activity towards CO2. Although various aromatically modified geminal FLPs have been investigated for CO2 capture, the catalytic space of these FLPs has not been fully resolved yet. Thus, to fulfil the knowledge gap in the understanding of the catalytic behaviour and to extend the concept of aromatically enhanced FLPs, in the present study all the possible combinations of aromatic and antiaromatic modulations of the acidic and basic sites have been proposed and examined using density functional theory based orbital analysis. Further to verify the results obtained from the orbital analysis and to fully explore the catalytic space of the proposed systems, free energy landscapes have been examined using metadynamics simulations. The detailed intrinsic bond orbital (IBO) and principal interacting orbital (PIO) analyses capture crucial details of the reactions. Furthermore, evolution of anisotropy of induced current density (AICD) along the reaction justifies the effect of aromatic/antiaromatic modulation on the catalytic sites. The results show that highly asynchronous mechanisms have been found due to the aromatic/antiaromatic modulations. The simultaneous favourable aromatic/antiaromatic modification on the acidic and basic sites may greatly reduce the CO2 activation barrier. The enhancement of the acidic character of the B atom in the intramolecular FLPs (IFLPs) leads to a thermodynamically more feasible reaction with stable CO2 adducts. The acidic site has been found to play a major role in controlling the kinetics and thermodynamics of the reaction. This study provides valuable insights into the catalytic realm of the aromatically modified FLPs, which can be utilized to design more efficient and specific next-generation FLPs.

Novel route to enhance the thermo-optical performance of bicyclic diene photoswitches for solar thermal batteries.

Harnessing solar energy by employing chemical photoswitches in molecular solar thermal (MOST) energy storage systems is a topic of appealing research interest. However, incorporating all the features desired for an ideal MOST system in a single photoswitching couple is challenging. Inspired by experimental synthesis, herein we report our attempt to enhance both the thermochemical and photophysical properties in a single-bridged bicyclic diene (BBD)-based photoswitch by elongating the unsaturated bridge with different heteroatomic units. To elucidate the best elongation unit, the energy storage capacity and the TBR barriers were accounted using the DLPNO-CCSD(T) and (8,8)-CASPT2 methods, respectively. The photophysical properties including the absorption onset, excitation wavelengths, and the absorption intensities were extensively investigated with the time-dependent calculations. The result provides information on the most versatile solvent to exhibit the best photoswitching behaviour which is beneficial for real-life energy storage applications. Additionally, the stability and reversibility of the photoswitching system with elongated unsaturated bridges have also been assessed. By means of the studied modification, the storage energy of 158.57 kJ/mol, energy storage density of 1.48 MJ/kg, TBR barrier of 136.36 kJ/mol, and the absorption onset of 305.00 nm is achieved in acetonitrile. These values are substantially higher when compared with the storage energy (96.06 kJ/mol), energy storage density (1.04 MJ/kg), and TBR barrier (121.76 kJ/mol) of prototype NBD/QC in the gas phase. The outcomes render useful insights into the stability and properties of bicyclic diene-based photoswitches having elongated unsaturated bridges and indeed paves the way for the rational design of practical MOST systems.

Origin of structure and stability of M@C18 (M = Cu, Ag, and Au) complexes with D9h point group.

Theoretical predictions and recent experimental studies lead to the discovery of an exciting new member of the carbon allotrope family polyynic cyclo[18]carbon (C18 ). Present investigation aims to probe the structure, stability, and properties of coinage metal (M)@C18 complexes using density functional theory (DFT) calculations. The DFT results unequivocally show that even Cu@C18 , Ag@C18 , and Au@C18 complexes substantially preserve the ground state polyynic structure of C18 . It is also worth to mention that only Au@C18 is a stable D9h structure, however the symmetry is distorted in the case of Cu@C18 and Ag@C18 . Due to computational limitations, in this investigation the M@C18 complexes were scrutinized using the C2v sub abelian group of D9h . The highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) of D9h conformers are a singlet a1 and two same value singlets a1 ⊕ b1 generated from doublet e, respectively. The non-covalent interaction index (NCI), quantum theory of atoms in molecule (QTAIM), and energy decomposition analysis (EDA) vividly explains the interaction between a coinage metal atom and C18 ring. It is found from the results that the stability of Cu@C18 Ag@C18 , and Au@C18 is governed by the attractive electrostatic, orbital and dispersion interaction.

Alanine boronic acid functionalized UiO-66 MOF as a nanoreactor for the conversion of CO2 into formic acid.

The alarming increase in the atmospheric CO2 concertation is a global concern today. Thus, the researchers around the globe are finding ways to decrease the amount of CO2 in the atmosphere. Converting CO2 into valuable chemicals like formic acid is one of the best ways to address this issue, but the stability of the CO2 molecule poses a great challenge in its conversion. To date various metal-based and organic catalysts are available for the reduction of CO2 . Still there is a great need for better, robust and economic catalytic systems and the advent of functionalized nanoreactors based on metal organic frame works (MOF) have opened a new dimension in this field. Thus, in the present work UiO-66 MOF functionalized with alanine boronic acid (AB) have been theoretically investigated for the reaction of CO2 with H2 . The density functional theory (DFT) based calculations were carried out to probe the reaction pathway. The result shows that the proposed nanoreactors can efficiently catalyze the CO2 hydrogenation. Further, the periodic energy decomposition analysis (pEDA) unveils important insights about the catalytic action of the nanoreactor.

Phenothiazine functionalized fulleropyrrolidines: synthesis, charge transport and applications to organic solar cells.

A series of phenothiazine-C60/70 dyads containing fulleropyrrolidine tethered to C-3 position (C60-PTZ and C70-PTZ) or to the heteroatom N-position via either phenyl (C60-Ph-PTZ and C70-Ph-PTZ) or phenoxyethyl linkers (C60-PhOEt-PTZ and C70-PhOEt-PTZ) of the phenothiazine were synthesized and light-induced electron transfer events were explored. Optimized studies suggested that the highest molecular orbital (HOMO) resides on donor phenothiazine moiety while lowest molecular orbital (LUMO) on the acceptor fulleropyrrolidine moiety of the dyads. Optical and electrochemical properties suggested no electronic communication between the donor and acceptor moieties in the ground state. However, steady-state emission studies in solvents of varied polarity, involving selective excitation of C60/C70, disclosed that the emission intensity of C60/C70 was quenched in the dyads in the increasing order, C60/70-PTZ > C60/70-Ph-PTZ > C60/70-PhOEt-PTZ as a consequence of the donor-acceptor distance resulted due to spacer lengths. Also, the emission quenching is more pronounced in polar solvents such as DMF compared to a non-polar solvent, toluene. With the support of parallel electrochemical studies, the emission quenching is attributed to intramolecular photo-induced electron transfer occurring from PTZ to (C60/C70)* generating a radical ion pair, PTZ+⋅-C60-⋅/PTZ+⋅-C70-⋅. Finally, bulk heterojunction (BHJ) solar cells devices inverted fashion prepared by employing the dyads as acceptors, and PTB7 as donor, suggested that the devices prepared from C70 derivatives i.e., PTB7:C70-PTZ and PTB7:C70-PhOEt-PTZ exhibited better power conversion efficiency of 2.66% and 2.15%, respectively over C60 derivatives i.e., PTB7:C60-PTZ and PTB7:C60-PhOEt-PTZ with the efficiencies of 1.80 and 1.72%, respectively. AFM studies revealed that the poor performance of PTB7:C60-PTZ- and PTB7:C60-PhOEt-PTZ-based devices can be ascribed to the lower solubility of the dyads in 1,2-DCB solvent leading to rough morphology.

Structure, stability, and electronic and optical properties of TMDC-coinage metal composites: vertical atomically thin self-assembly of Au clusters.

Composites of metal clusters supported on transition metal dichalcogenides (TMDCs) often provide promising opportunities for applications in nanoelectronics, catalysis, sensing, etc. In the present investigation, a systematic attempt has been made to unveil the structure and stability of coinage M6 clusters supported on TMDC (MoS2 and WS2) monolayers. The more prominent objective is to explore potential candidates that stabilize the two-dimensional (2D) M6 clusters on their surface. Periodic energy decomposition analysis (pEDA) was carried out to probe the various interaction energy (IE) components that govern the stability of the M6 clusters in the composites. Attention has also been devoted to unravelling the electronic and optical properties of these TMDCs/M6 composites. Moreover, ab initio molecular dynamics (AIMD) simulations were performed to scrutinize the dynamic behaviour of Au cluster on WS2 monolayer. The results reveal that the coinage M6 clusters form energetically more stable composites on MoS2 than WS2 monolayer. It is worth mentioning that WS2 promotes the stability of 2D M6 clusters. Inclusion of dispersion correction marginally altered the geometries of the TMDCs/M6 composites but its impact on the IE values was significant. AIMD simulation explicitly emphasizes that the WS2 surface preferentially facilitates the vertical 2D self-assembling of Au atoms and, interestingly, the planarity is mostly retained during the course of simulations. The adsorption of coinage M6 clusters substantially influences the electronic and optical properties of the TMDCs. HSE06 calculation confirms that the decrease in energy gap is more pronounced in MoS2/M6 composites. The outcomes of this study render fundamental insights into the various TMDCs/M6 composites that would certainly be worthwhile probing for diverse practical applications.

PIO and IBO analysis to unravel the hidden details of the CO2 sequestration mechanism of aromatically tempered N/B-based IFLPs.

Enhancing the catalytic reactivity of Frustrated Lewis Pairs (FLPs) in various activities such as CO2 activation and sequestration has recently gained interest among researchers around the globe. A recent investigation showed the use of aromaticity as a tool to modulate the catalytic behaviour of FLPs, establishing a whole new dimension in this area. In this work, aromatically tempered N/B-based intramolecular frustrated Lewis Pairs (IFLPs) are proposed for CO2 sequestration. Density functional theory (DFT)-based calculations were carried out to probe the reaction mechanism. The detailed mechanistic investigation was carried out using intrinsic reaction coordinate (IRC), principal interacting orbital (PIO), intrinsic bond orbital (IBO) and natural bonding orbital (NBO) analyses. The results show that aromatic gain in the system at the basic sites lowers the activation barrier, whereas the antiaromatic gain results in increased activation energy. The sequestration mechanism was found to be an asynchronous concerted mechanism, and polar solvents result in higher asynchronicity. This work, for the first time, reports asynchronicity in the catalytic behavior of aromatically tempered IFLPs, which can be crucial to designing better IFLPs for CO2 sequestration.

Electronic and photophysical properties of an atomically thin bowl-shaped beryllene encapsulated inside the cavity of [6]cycloparaphenylene (Ben@[6]CPP).

Exotic metallic nanostructures are being intensely pursued for a myriad of applications, with ultrathin membranes currently at the heart of several investigations. The objective of the present study was to systematically assess the atom-by-atom encapsulation of Be in the molecular nanoring of [6]cycloparaphenylene ([6]CPP). Further, the study aimed to scrutinize the structure, stability, and properties of the encapsulated Ben@[6]CPP systems. The outcomes clearly revealed that [6]CPP enabled the cooperative confinement of atomically thin bowl-shaped beryllene inside its circular cavity. The confinement of Be in [6]CPP generated topologically anisotropic surfaces with distinct interior and exterior charge distributions. The Ben@[6]CPP complexes could render a cationic or anionic nature to Be depending on its neighbouring environment. Thus, the systems may offer a promising opportunity for the synergistic co-adsorption of multiple reactants that are involved in multicomponent reactions. Energy decomposition analysis (EDA) elucidated that the bonding between Be and [6]CPP was partially ionic and covalent in character. The progressive encapsulation of Be atoms inside the cavity of [6]CPP led to a red-shift of the excitation wavelength to the visible region. The calculated optical absorption coefficient was higher than 104 L mol-1 cm-1, which shows promise for diverse optoelectronic applications.

Unprecedented Activation of CO2 by α-Amino Boronic Acids.

Efficient and environmentally benign transformation of carbon dioxide (CO2) into valuable chemicals is mainly obstructed by the lack of suitable catalysts. To date, various catalysts have already been investigated for the conversion of CO2 molecules, but still finding metal-free, simple, and environment-friendly catalysts is a topic of utmost interest among researchers. Thus, in this regard, the present work projects α-amino boronic acids (AABs) as a metal-free and simple catalyst for CO2 activation. The density functional theory (DFT)-based calculations have been carried out to explore the catalytic potential of AABs. The detailed electronic structure analysis of the considered AABs unveils the catalytic similarities with frustrated Lewis pairs (FLPs) in a gas phase. Interestingly, a peculiar catalytic action of AABs has been observed in the presence of solvents. The contrasting catalytic behavior of AABs in solvents has been extensively investigated by employing principal interacting orbital (PIO), intrinsic bond orbital (IBO), and natural bond orbital (NBO) analyses along the reaction paths. The results of the orbital studies provide concrete ground for the observed reaction mechanism. Further, the energetic analysis of the reaction of CO2 with AABs reveals that <5 kcal/mol energy is required for activation in a solvent phase, and the formed adducts are readily active. These observations show that AABs can be considered as an efficient catalyst for CO2 activation.

Boron based intramolecular heterocyclic frustrated Lewis pairs as organocatalysts for CO2 adsorption and activation.

The massive increase in the amount of carbon dioxide (CO2 ) in the atmosphere has led to serious environmental problems. One of the best ways to tackle this problem is the CO2 capture and its utilization as a C1 carbon source for the production of industrially valuable chemicals. But the thermodynamic stability of the CO2 molecule poses a great challenge in its transformation. Since the last two decades, various metal-based and organic catalysts have been developed for the adsorption and activation of CO2 . Among all the catalysts the Frustrated Lewis pairs (FLPs) have been shown great potential in CO2 capture and conversion. Thus, in the present work, Intramolecular Frustrated Lewis pairs (IFLP) based on N-Heterocycles with boron group functionalization at the α-position to N has been theoretically investigated for CO2 activation. Thorough orbital analysis has been carried out to investigate the reactivity of the proposed catalytic systems. The result shows that the considered IFLPs are capable of activating CO2 with minimum energy requirements. The CO2 activation energy range between 8 and 14 kcal/mol. The non-polar solvent was found to be the suitable medium for the reaction. Also, the reversibility of the adducts formed with the IFLPs can be controlled by appropriate substitution at B atom in the IFLPs.

Enhanced Selectivity of the Propylene Epoxidation Reaction on a Cu Monolayer Surface via Eley-Rideal Mechanism.

The aerobic oxidation of propylene to selectively achieve propylene oxide (PO) is a challenging reaction in catalysis. Therefore, an active catalyst which shows enhanced PO selectivity is extremely desired. In the present investigation, an attempt has been made to explore the catalytic activity of a mono-atomically thin two-dimensional (2D) hexagonal (HX) Cu layer for selective propylene epoxidation using molecular O2 with the aid of density functional theory calculations. The results reveal that the conversion of propylene to PO via Eley-Rideal mechanism is an exoergic and barrierless reaction on the O2 pre-adsorbed Cu monolayer. The Pauli energy component plays a decisive role for barrierless reaction whereas the electrostatic and orbital contribution governs the energetic stability of PO. Car-Parrinello molecular dynamics (CPMD) simulation reinforces the outcomes of climbing image nudged elastic band (CI-NEB) calculation. Further, the formation of oxametallacycle OMC-2 (0.47 eV) is kinetically favourable over OMC-1 (0.87 eV) and AHS (0.50 eV) on O pre-adsorbed 2D HX Cu. Interestingly, the energy barrier for the conversion of OMC-2 to PO (0.70 eV) is considerably low in comparison with the acetone formation (0.90 eV). Therefore, it is worth to mention that the 2D HX Cu surface provides a promising platform for selective propylene epoxidation.

Dissociative Adsorption of H2 S on Li(110) Surface Using Density Functional Theory Calculations and Car-Parrinello Molecular Dynamics Simulations.

The information concerning dissociative adsorption of H2 S on Li surface is inadequate and the mechanistic insight for its complete dissociation is yet to be explored. The present investigation aims to scrutinize the dissociative adsorption of H2 S on Li(110) surface using density functional theory calculations. The climbing image nudged elastic band calculation was employed to unveil the relative energy profiles for S-H dissociation. To elucidate the components of interaction energy responsible for stabilizing the adsorbed moieties on the surface, periodic energy decomposition analysis was performed. A Car-Parrinello molecular dynamics (CPMD) simulation was performed to understand the dynamic behaviour of H2 S on Li(110). Results vividly demonstrates: (i) partially dissociated product with perpendicular S-H is comparatively stable than the parallel SH, (ii) completely dissociated moieties H/H/S are the most stable among all, (iii) dissociation of first S-H is barrierless and the second S-H dissociation is a low energy barrier reaction, (iv) complete dissociation of H2 S occurs in a stepwise manner, (v) orbital and electrostatic contributions of the interaction energy plays a vital role in stabilizing the dissociated moieties, and (vi) stepwise dissociation of H2 S was further reinforced by CPMD.

Novel Insight into the Molecular Frustration of IFLPs Based on Boron-Functionalized Pyrimidines for CO2 Sequestration.

The emergence of frustrated Lewis pairs (FLPs) as organocatalysts for CO2 sequestration has opened a new dimension in this area. To date, various inter- and intramolecular frustrated Lewis pairs have been experimentally and theoretically explored for CO2 activation, still there is plenty of room for new insights into FLPs. Thus, in the present study intramolecular frustrated Lewis pairs (IFLPs) based on boron-functionalized pyrimidines have been proposed for CO2 activation and computationally investigated to gain new insights into the molecular frustration. The extensive natural bond orbital (NBO) analysis unveils an interesting relationship between the orbital charge transfer and the activity. The result shows that the greater the charge transfer between the acidic and basic sites, the higher will be the frustration in the molecule. Also, the presence of atoms bearing a lone pair attached to the acidic site relieves the frustration by charge transfer. Based on the orbital charge transfer, the predicted activity order for the proposed IFLPs is supported by the energetics for the reaction of CO2 with the IFLPs. Further, the activation strain analysis (ASA) provides a different viewpoint about the reactivities of the IFLPs and highlights the importance of the geometrical structure of the catalyst. Furthermore, the ab initio molecular dynamics (AIMD) uplights the reversibility of the formed products.

Excitation-Wavelength-Dependent Light-Induced Electron Transfer and Twisted Intramolecular Charge Transfer in N,N-Bis(4'-tert-butylbiphenyl-4-yl)aniline Functionalized Borondipyrromethenes.

A series of bis(4'-tert-butylbiphenyl-4-yl)aniline (BBA) functionalized borondipyrromethene (BODIPY) dyads, Dyads 1-3, containing the BBA group tethered to BODIPY moiety either directly or through a phenyl or alkynyl phenyl spacers are synthesized, and the light-mediated charge transfer within the chromophores has been systematically investigated. The crystal structure of Dyad-1 showed a tilt of 44.2° between the BODIPY and BBA molecular planes and intermolecular C-H···π interactions with these moieties. Cyclic voltammetric and computational studies showed that the BBA moiety can act as the electron donor (D) and BODIPY as the electron acceptor (A) and the optical absorption studies revealed that an increase in the conjugation of the linker from Dyad-1 to Dyad-2 resulted in bathochromic shifts. Steady-state fluorescence studies involving photoexcitation of the BBA moiety at 326 nm resulted in the decrease in fluorescence intensity of the BBA, indicating the possibility of sequential occurrence of faster photoinduced energy transfer (PEnT) followed by the photoinduced electron transfer (PET) or solely PET within the dyads, and the driving forces of the charge separation were calculated to be exothermic in all of the employed solvents. Parallel time-resolved fluorescence experiments involving the excitation of BBA moiety also supported the occurrence of charge separation in these dyads. Interestingly, excitation of the BODIPY moiety of Dyad-1 and Dyad-2 at 490 nm in solvents of increasing polarity leads to a red-shifted BODIPY emission with weakened intensity. This spectral behavior indicated the occurrence of emission from the locally excited (LE) state in nonpolar solvents, whereas formation of an LE state followed by the rotation of the chromophores at the D-A bond leads to a low energy twisted intramolecular charge transfer state (TICT), resulting in a charge-separated state BBA+•-BODIPY-• in polar solvents. Furthermore, the hydrophobicity studies involving the solutions of dyads in admixtures of polar tetrahydrofuran (THF) and nonpolar hexanes revealed that when the fraction of hexanes in these mixtures is increased, the emission of BODIPY moiety was observed to be blue-shifted and exhibited enhanced intensity supporting the occurrence of TICT in these dyads.

Prediction of the [4 + 2]- and [5 + 4]-cycloaddition reactions in zig-zag carbon nanotubes via an ambimodal transition state: density functional theory calculations.

A unique type of chemical reaction known as an ambimodal reaction has drawn tremendous attention owing to its intriguing feature of forming multiple (two or more) products from the same (single) transition state. In contrast to conventional reactions, bifurcation of the potential energy surface takes place in ambimodal reactions. Density functional theory (DFT) based calculations were performed to probe the Diels-Alder (DA) cycloaddition reactions of various carbon nanotubes (CNTs) with 1,3-butadiene. The present investigation reveals the possibility of ambimodal transition state formation on a potential energy surface (PES) corresponding to an unusual [5 + 4]-cycloadduct along with the conventional [4 + 2]-cycloadduct. The ground state of the [5 + 4]-cycloadduct obtained from butadiene and the H-terminated CNTs is a triplet (3T) state, but on the other hand the [4 + 2]-cycloadduct is a singlet (1S) state. The [5 + 4]-adduct is energetically more stable in comparison with the [4 + 2]-adduct. The possibility of the formation of the [5 + 4]-adduct is validated using frontier molecular orbitals. The length of the nanotube significantly influences the overall kinetics and thermodynamics of the reaction.

Heterojunction hybrid devices from vapor phase grown MoS2.

We investigate a vertically-stacked hybrid photodiode consisting of a thin n-type molybdenum disulfide (MoS2) layer transferred onto p-type silicon. The fabrication is scalable as the MoS2 is grown by a controlled and tunable vapor phase sulfurization process. The obtained large-scale p-n heterojunction diodes exhibit notable photoconductivity which can be tuned by modifying the thickness of the MoS2 layer. The diodes have a broad spectral response due to direct and indirect band transitions of the nanoscale MoS2. Further, we observe a blue-shift of the spectral response into the visible range. The results are a significant step towards scalable fabrication of vertical devices from two-dimensional materials and constitute a new paradigm for materials engineering.