I am teaching a course titled “Spectroscopic Characterization of Molecules, Materials and Photovoltaics.” The first few lectures were on molecular photophysics and included a thorough introduction of the Jablonski Diagram (For anyone interested, my lecture ppt slides are available here).
The Jablonski diagram, first introduced by Aleksander Jabłoński in 1933, is a graphical depiction of the electronic states of a molecule and the transitions between those states. The y axis of the graph is energy, which increases from the bottom (ground state or S0) to the top (singlet and triplet excited states or Sn and Tn). The transitions between the states—like excitation, internal conversion, fluorescence, intersystem crossing, etc—are depicted as arrows. Because of its simplicity, the Jablonski Diagram is a starting point for many discussions about the events that occur following electronic excitation of a molecule.
I am new to teaching and spent a lot of time thinking about what homework I should give my students to both facilitate learning and gauge their understanding of the content. An idea hit me when I saw Mark Lorch’s revamping of the periodic table to mimic an underground rail system.
We’re all familiar with the most common depiction of the periodic table because it’s hanging in every chemistry classroom on the planet. Yet, there is no inherent physical reason we have to map the elements in that particular way. The underlying motive for this common form is to show the periodic nature of the properties of atoms as defined by their number of protons. But there are many possible ways to fulfill that goal. Non-traditional periodic tables can provide a new perspective on the relationships between atoms that are not obvious in the traditional drawing.
With that concept in mind, I decided to ask my students to rethink the Jablonski diagram.
The exact wording of the assignment was to “draw a Jablonski diagram that includes singlet and triplet excited states.” I was hoping to inspire some creativity so I pointed to the periodic table on the wall and then showed them a number of non-traditional periodic tables. I even said, “If you can express the nuances of the Jablonski Diagram through interpretive dance I would love to see it.”
No one choreographed a dance, but I was still blown away by my students’ response to the assignment. Below are some of their awesome creations.
Here is one, by Tian Zhao, depicting the lowest energy species on top and increasing energy as you go down. I like to think that it is expressing the cyclic nature of the excitation/relaxation process under steady-state conditions.
This next submission, by Hadi Fares, is similar to the Bohr model of atoms and their electron orbitals that show the lowest energy state at the center and energy increasing outward. Unfortunately, this static image does not do justice to the animations he incorporated into the diagram.
This next diagram is similar to the one above but with additional artistic flare involving negative space. This aesthetic was inspired by “Vortex”, a game that Peilu Liu played on her ipod. The image got me thinking about the nodal planes of an orbital and whether or not it possible to graphically depict the likelihood of an electronic transition based on a comparison between the valence orbitals of a given state.
This final drawing, by Daniel Nascimento, really bumps up the information density of the Jablonski Diagram by not only including the energies of states and their transitions but also the approximate timescales of the events as shown on the x axis.
Much to my delight, a few students decided to really take a leap from tradition and make physical models of the Jablonski Diagram. Here is an ~12” tall work of art that was made by Maxime Matras out of aluminum rings—denoting the states (S0, T1, S1 and S2 from bottom to top)—and wires to denote the transitions between states (aluminum = excitation; copper = intersystem crossing, internal conversion, fluorescence and phosphorescence; coiled shavings = non-radiative decay). This one now sits on my desk.
I even received a Jablonski diagram cake. The ground state, first singlet, second singlet and first triplet excited states are depicted in quadrants going clockwise (indicated by candy letters/numbers). The transitions are various colored lines of decorative frosting. What is really clever about this model is that the energy of the states are defined by their height from the pan. That is, the ground state is just a layer of chocolate frosting, the first excited state is a single layer of cake, and the second excited state is two layers. He also made the cake with tonic water in an effort to have it glow under a UV light. Unfortunately, I fear the cooking process destroyed the quinine and with it any possibility of glowing. While not necessarily the most delicious of cakes, it was very creative.
And, finally, is “Jablinko!” This Jablonski diagram is based on the Japanese arcade and/or gambling game known as Pachinko. This photophysics-based game begins by placing a small metal ball into the S0 hole just above the lever. Pulling the lever, or exciting the molecule, shoots a ball to the top of the board into the singlet excited state. The ball (excited state) can fall one of three possible directions, NRD (non-radiative decay), ISC (intersystem crossing) or fluorescence. If it undergoes ISC, the ball can then fall into a potential well representing either non-radiative decay or phosphorescence from the triplet excited state. To top it all off, when the ball falls in to the fluorescence or phosphorescence holes it closes an electrical circuit that turns on a blue or red LED below the potential well. Those colors are the emission wavelengths for fluorescence and phosphorescence from anthracene.
In closing, I’d like to send a special thanks to my students. I have thoroughly enjoyed our time together and will always remember their clever responses to my first assignment.