Remember those times when we were all amateur stargazers as kids, trying to locate those prized constellations? Well, I was fascinated by stars of the other kind too - the ‘glow in the dark’ kind. Why just stars? The dials on a watch, those pale yellow paints advertised as ‘radium’ paints, and even the creepy yet cool-looking ‘glowing' skeletons all seemed magical. Of course, I know now that the glow is the result of a process called phosphorescence, which involves the emission of light from a slow, radiative de-excitation of electrons from an excited state to the ground state. And, there goes the magic! But, stay with me because something interesting is about to follow.
Let me start by addressing a common misconception that arises from a convenient interchange of two words – fluorescence and phosphorescence. They are both forms of a broader process called photoluminescence. Photoluminescence is a process where a molecule absorbs energetic photons that excite its electrons to higher energy levels and then drop back down to the ground state through different pathways of energy loss. One of the ways to account for this loss of energy is the emission of a photon when the excited electron drops to the ground state. Quantum mechanics tells us that spatially confined particles have discrete energy levels. The lowest energy level is the ground state, and the higher energy levels are referred to as excited states. The ground state is a singlet state, which means the two electrons residing in that state are spin-paired. The excited states can be singlet states or triplet states. In a singlet excited state, one of the electrons from the pair is excited to a higher energy state but retains its original spin. In a triplet state, the excited electron residing in a higher energy level flips its spin, and now assumes the spin of its erstwhile spin-coupled partner. Quantum mechanics also dictates that electronic transitions between two states with different spin multiplicities are theoretically forbidden. In practical terms, the probability of such a transition is very low. Now, an energetic photon incident on a molecule with energy sufficient enough to meet the energy gap between the ground state and an excited state will excite an electron from the ground state to that higher excited state. When this excited electron drops to the ground state, it is accompanied by the release of a photon of energy equivalent to the energy difference between the two states. The given description broadly fits both fluorescence and phosphorescence, but the key difference lies in the lifetimes of emission. Brightly glowing fluorescent materials cease to glow almost immediately when the radiation source is exhausted, whereas phosphorescent materials continue to glow lightly for a longer time. Fluorescence always takes place due to the electronic transition between the first singlet excited state and the singlet ground state, phosphorescence on the other hand involves a spin-forbidden transition from the first excited triplet state, which is the reason why it is a slow and less intense process. This should sufficiently explain why terming those glowing paints as fluorescent might be a misnomer. They are technically phosphorescent. Fun fact - phosphors are defined as substances that exhibit luminescence, and they share the same etymological origins as the word phosphorescence. Both of them are derived from the property of light illumination exhibited by white phosphorous when exposed to oxygen. Ironically, this property is not phosphorescence! Instead, it is called chemiluminescence, since it involves luminescence caused by a chemical species.
The concepts of luminescence are deeply entrenched in my field of interest, which is organic electronics. I am currently working on two molecular electronic systems that work on contrasting principles - organic light-emitting diodes and organic solar cells. Organic light-emitting diodes make use of electric current to emit light, whereas organic solar cells use light to generate electricity. The difference between conventional light-emitting diodes or solar cells and their organic counterparts is the use of conjugated organic molecules for conduction purposes, instead of silicon-based inorganic substances. Conjugated organic molecules have delocalized electrons that can facilitate electrical conduction. The introduction of organic molecules for exhibiting fluorescence and phosphorescence in organic light-emitting diodes, and for absorbing light in organic solar cells has led to a significant simplification in the manufacturing process of these devices. Their fabrication involves the deposition of thin-film organic layers which are solution-processable, low weight, and lend flexibility to the physical structures of the devices. Organic light-emitting diodes are well-known for their applications in digital displays, and the use of organic materials ensures economic production practices.
I was introduced to this field during my second summer internship when I was working on the synthesis and characterization of a particular π-conjugated system called porphyrin. Porphyrins are the macrocycles known to us from the structures of natural molecules like chlorophyll and haemoglobin. While the project itself was purely synthesis-related, the supplementary research papers that I read contained many interesting applications of porphyrin derivatives, and further citation-chasing introduced me to the worlds of organic solar cells and electroluminescent devices. The working of organic solar cells is pretty neat. When light is incident on the organic donor material in the solar cell, it excites electrons situated in the highest occupied molecular orbital (HOMO) to the lowest unoccupied molecular orbital (LUMO). The positively charged vacancy created by the excited electron is called a hole, and it is electrostatically bound to the excited electron. This leads to the creation of a quasiparticle called an exciton, where the excited electron-hole pair is regarded as a single weakly-bound particle. The 'quasi'-ness of this particle is essential to the generation of electricity, as we are about to discover. Connected to the donor material is an organic acceptor material, which is made up of a molecule with typically low-lying LUMO levels. The exciton is split at the donor-acceptor interface when the electron drops from the LUMO level of the donor to the LUMO level of the acceptor, whereas the hole remains confined to the HOMO level of the donor molecule. The transport of the separated hole and electron to their respective electrodes is facilitated by selective carrier blocking layers that allow charged carriers of one kind to pass through them and block carriers of the other kind. This motion of electrons and holes leads to the generation of electricity. The construction of an organic solar cell is more layered, but this elementary description should suffice for now as we move on to organic light-emitting diodes. The simplest organic light-emitting diode consists of an organic layer sandwiched between two electrodes. When a potential difference is applied across the terminals, electrons flow from the cathode to the LUMO level of the organic material that is near the cathode and are collected at the HOMO level, near the anode. The excited LUMO electrons are bound to the holes in the HOMO level, thus forming excitons. Except, in this case, the exciton pairs are not broken by physical separation, but by physical recombination. The excited electron falls to the ground state (HOMO) where it combines with the hole, and let there be light, it says! As expected, the loss in the energy of the electron manifests as an emitted photon.
While all this sounds great on paper, one must take into account the limitations of organic materials. The efficiencies of organic electronic devices are less than that of silicon semiconductors or fancier devices that make use of expensive metals like iridium or platinum. Besides, organic materials are susceptible to degradation by water, oxygen, and light, which leads to the radical dissociation of the molecules, and the heavier metal electrodes cause morphological defects in the organic thin-films. This is where chemists come in, to develop more robust organic frameworks that can improve the efficiencies of the devices while also maintaining the stability of the material.
I was set to begin work on the development of these specialized organic molecules, but the prevailing global situation put a damper on all lab activities. Thus, I had to settle for a review project instead. Review projects involve an extensive literature survey to highlight the developments in the respective fields and lay some groundwork before jumping into the experimental aspects. Lab work and reading scientific articles go hand-in-hand, but doing a reading project or a review project makes matters easier when the actual experimental work starts. It is important to keep oneself updated about modern research in one’s field of interest. Oftentimes, independent research activities involve building upon existing ideas and finding ways of bettering the work that someone else has done. So, it is critical to stay in touch with developments in the scientific world.
On a personal note, I have always aspired to contribute to solving a real-world problem through my research activities. The need for developing novel energy devices to achieve sustainability in energy production comes across as a pressing issue in modern times. In a way, it was this realization that drew me to my current field of interest, and I believe we can revolutionize the way we harvest energy through the effective utilization of organic materials. I call it the 'Fluoresce to Flourish' revolution, one which hopefully sees us achieve total energy autarky.
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